DynamoDBMapper.java

/*
 * Copyright 2011-2013 Amazon Technologies, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 *
 *    http://aws.amazon.com/apache2.0
 *
 * This file is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES
 * OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and
 * limitations under the License.
 */
package com.amazonaws.services.dynamodbv2.datamodeling;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.text.ParseException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Random;

import com.amazonaws.AmazonClientException;
import com.amazonaws.AmazonServiceException;
import com.amazonaws.AmazonWebServiceRequest;
import com.amazonaws.http.AmazonHttpClient;
import com.amazonaws.services.dynamodbv2.AmazonDynamoDB;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapperConfig.ConsistentReads;
import com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapperConfig.SaveBehavior;
import com.amazonaws.services.dynamodbv2.model.AttributeValue;
import com.amazonaws.services.dynamodbv2.model.AttributeValueUpdate;
import com.amazonaws.services.dynamodbv2.model.BatchGetItemRequest;
import com.amazonaws.services.dynamodbv2.model.BatchGetItemResult;
import com.amazonaws.services.dynamodbv2.model.BatchWriteItemRequest;
import com.amazonaws.services.dynamodbv2.model.BatchWriteItemResult;
import com.amazonaws.services.dynamodbv2.model.ComparisonOperator;
import com.amazonaws.services.dynamodbv2.model.Condition;
import com.amazonaws.services.dynamodbv2.model.ConditionalCheckFailedException;
import com.amazonaws.services.dynamodbv2.model.DeleteItemRequest;
import com.amazonaws.services.dynamodbv2.model.DeleteRequest;
import com.amazonaws.services.dynamodbv2.model.ExpectedAttributeValue;
import com.amazonaws.services.dynamodbv2.model.GetItemRequest;
import com.amazonaws.services.dynamodbv2.model.GetItemResult;
import com.amazonaws.services.dynamodbv2.model.KeysAndAttributes;
import com.amazonaws.services.dynamodbv2.model.PutItemRequest;
import com.amazonaws.services.dynamodbv2.model.PutRequest;
import com.amazonaws.services.dynamodbv2.model.QueryRequest;
import com.amazonaws.services.dynamodbv2.model.QueryResult;
import com.amazonaws.services.dynamodbv2.model.ScanRequest;
import com.amazonaws.services.dynamodbv2.model.ScanResult;
import com.amazonaws.services.dynamodbv2.model.Select;
import com.amazonaws.services.dynamodbv2.model.UpdateItemRequest;
import com.amazonaws.services.dynamodbv2.model.WriteRequest;
import com.amazonaws.util.VersionInfoUtils;

/**
 * Object mapper for domain-object interaction with DynamoDB.
 * <p>
 * To use, annotate domain classes with the annotations found in the
 * com.amazonaws.services.dynamodbv2.datamodeling package. A minimal example:
 *
 * <pre>
 * &#064;DynamoDBTable(tableName = &quot;TestTable&quot;)
 * public class TestClass {
 *
 *     private Long key;
 *     private double rangeKey;
 *     private Long version;
 *
 *     private Set&lt;Integer&gt; integerSetAttribute;
 *
 *     &#064;DynamoDBHashKey
 *     public Long getKey() {
 *         return key;
 *     }
 *
 *     public void setKey(Long key) {
 *         this.key = key;
 *     }
 *
 *     &#064;DynamoDBRangeKey
 *     public double getRangeKey() {
 *         return rangeKey;
 *     }
 *
 *     public void setRangeKey(double rangeKey) {
 *         this.rangeKey = rangeKey;
 *     }
 *
 *     &#064;DynamoDBAttribute(attributeName = &quot;integerSetAttribute&quot;)
 *     public Set&lt;Integer&gt; getIntegerAttribute() {
 *         return integerSetAttribute;
 *     }
 *
 *     public void setIntegerAttribute(Set&lt;Integer&gt; integerAttribute) {
 *         this.integerSetAttribute = integerAttribute;
 *     }
 *
 *     &#064;DynamoDBVersionAttribute
 *     public Long getVersion() {
 *         return version;
 *     }
 *
 *     public void setVersion(Long version) {
 *         this.version = version;
 *     }
 * }
 * </pre>
 * <p>
 * Save instances of annotated classes to DynamoDB, retrieve them, and delete
 * them using the {@link DynamoDBMapper} class, as in the following example.
 *
 * <pre>
 * DynamoDBMapper mapper = new DynamoDBMapper(dynamoDBClient);
 * Long hashKey = 105L;
 * double rangeKey = 1.0d;
 * TestClass obj = mapper.load(TestClass.class, hashKey, rangeKey);
 * obj.getIntegerAttribute().add(42);
 * mapper.save(obj);
 * mapper.delete(obj);
 * </pre>
 * <p>
 * When using the save, load, and delete methods, {@link DynamoDBMapper} will
 * throw {@link DynamoDBMappingException}s to indicate that domain classes are
 * incorrectly annotated or otherwise incompatible with this class. Service
 * exceptions will always be propagated as {@link AmazonClientException}, and
 * DynamoDB-specific subclasses such as {@link ConditionalCheckFailedException}
 * will be used when possible.
 * <p>
 * This class is thread-safe and can be shared between threads. It's also very
 * lightweight, so it doesn't need to be.
 *
 * @see DynamoDBTable
 * @see DynamoDBHashKey
 * @see DynamoDBRangeKey
 * @see DynamoDBAutoGeneratedKey
 * @see DynamoDBAttribute
 * @see DynamoDBVersionAttribute
 * @see DynamoDBIgnore
 * @see DynamoDBMarshalling
 * @see DynamoDBMapperConfig
 */
public class DynamoDBMapper {

    private final AmazonDynamoDB db;
    private final DynamoDBMapperConfig config;
    private static final DynamoDBReflector reflector = new DynamoDBReflector();
    /** The max back off time for batch write */
    private static final long MAX_BACKOFF_IN_MILLISECONDS = 1000 * 3;

    /**
     * User agent for requests made using the {@link DynamoDBMapper}.
     */
    private static final String USER_AGENT = DynamoDBMapper.class.getName() + "/" + VersionInfoUtils.getVersion();

    /**
     * Constructs a new mapper with the service object given, using the default
     * configuration.
     *
     * @param dynamoDB
     *            The service object to use for all service calls.
     * @see DynamoDBMapperConfig#DEFAULT
     */
    public DynamoDBMapper(AmazonDynamoDB dynamoDB) {
        this(dynamoDB, DynamoDBMapperConfig.DEFAULT);
    }

    /**
     * Constructs a new mapper with the service object and configuration given.
     *
     * @param dynamoDB
     *            The service object to use for all service calls.
     * @param config
     *            The default configuration to use for all service calls. It can
     *            be overridden on a per-operation basis.
     */
    public DynamoDBMapper(AmazonDynamoDB dynamoDB, DynamoDBMapperConfig config) {
        this.db = dynamoDB;
        this.config = config;
    }

    /**
     * Loads an object with the hash key given and a configuration override.
     * This configuration overrides the default provided at object construction.
     *
     * @see DynamoDBMapper#load(Class, Object, Object, DynamoDBMapperConfig)
     */
    public <T extends Object> T load(Class<T> clazz, Object hashKey, DynamoDBMapperConfig config) {
        return load(clazz, hashKey, null, config);
    }

    /**
     * Loads an object with the hash key given, using the default configuration.
     *
     * @see DynamoDBMapper#load(Class, Object, Object, DynamoDBMapperConfig)
     */
    public <T extends Object> T load(Class<T> clazz, Object hashKey) {
        return load(clazz, hashKey, null, config);
    }

    /**
     * Loads an object with a hash and range key, using the default
     * configuration.
     *
     * @see DynamoDBMapper#load(Class, Object, Object, DynamoDBMapperConfig)
     */
    public <T extends Object> T load(Class<T> clazz, Object hashKey, Object rangeKey) {
        return load(clazz, hashKey, rangeKey, config);
    }

    /**
     * Returns an object whose keys match those of the prototype key object given,
     * or null if no such item exists.
     *
     * @param keyObject
     *            An object of the class to load with the keys values to match.
     *
     * @see DynamoDBMapper#load(Object, DynamoDBMapperConfig)
     */
    public <T extends Object> T load(T keyObject) {
        return load(keyObject, this.config);
    }

    /**
     * Returns an object whose keys match those of the prototype key object given,
     * or null if no such item exists.
     *
     * @param keyObject
     *            An object of the class to load with the keys values to match.
     * @param config
     *            Configuration for the service call to retrieve the object from
     *            DynamoDB. This configuration overrides the default given at
     *            construction.
     */
    public <T extends Object> T load(T keyObject, DynamoDBMapperConfig config) {
        @SuppressWarnings("unchecked")
        Class<T> clazz = (Class<T>) keyObject.getClass();

        config = mergeConfig(config);

        String tableName = getTableName(clazz, config);

        GetItemRequest rq = new GetItemRequest();

        Map<String, AttributeValue> key = getKey(keyObject, clazz);

        rq.setKey(key);
        rq.setTableName(tableName);
        rq.setConsistentRead(config.getConsistentReads() == ConsistentReads.CONSISTENT);


        GetItemResult item = db.getItem(applyUserAgent(rq));
        Map<String, AttributeValue> itemAttributes = item.getItem();
        if ( itemAttributes == null ) {
            return null;
        }

        return marshallIntoObject(clazz, itemAttributes);
    }

    /**
     * Returns a key map for the key object given.
     *
     * @param keyObject
     *            The key object, corresponding to an item in a dynamo table.
     */
    @SuppressWarnings("unchecked")
    private <T> Map<String, AttributeValue> getKey(T keyObject) {
        return getKey(keyObject, (Class<T>)keyObject.getClass());
    }

    private <T> Map<String, AttributeValue> getKey(T keyObject, Class<T> clazz) {
        Map<String, AttributeValue> key = new HashMap<String, AttributeValue>();
        for (Method keyGetter : reflector.getKeyGetters(clazz)) {
            Object getterResult = safeInvoke(keyGetter, keyObject);
            AttributeValue keyAttributeValue = getSimpleAttributeValue(keyGetter, getterResult);
            if (keyAttributeValue == null) {
                throw new DynamoDBMappingException("Null key found for " + keyGetter);
            }
            key.put(reflector.getAttributeName(keyGetter), keyAttributeValue);
        }

        if ( key.isEmpty() ) {
            throw new DynamoDBMappingException("Class must be annotated with " + DynamoDBHashKey.class + " and "
                    + DynamoDBRangeKey.class);
        }
        return key;
    }


    /**
     * Returns an object with the given hash key, or null if no such object
     * exists.
     *
     * @param clazz
     *            The class to load, corresponding to a DynamoDB table.
     * @param hashKey
     *            The key of the object.
     * @param rangeKey
     *            The range key of the object, or null for tables without a
     *            range key.
     * @param config
     *            Configuration for the service call to retrieve the object from
     *            DynamoDB. This configuration overrides the default given at
     *            construction.
     */
    public <T extends Object> T load(Class<T> clazz, Object hashKey, Object rangeKey, DynamoDBMapperConfig config) {
        config = mergeConfig(config);
        T keyObject = createKeyObject(clazz, hashKey, rangeKey);
        return load(keyObject, config);
    }

    /**
     * Creates a key prototype object for the class given with the single hash and range key given.
     */
    private <T> T createKeyObject(Class<T> clazz, Object hashKey, Object rangeKey) {
        T keyObject = null;
        try {
            keyObject = clazz.newInstance();
        } catch ( Exception e ) {
            throw new DynamoDBMappingException("Failed to instantiate class", e);
        }
        boolean seenHashKey = false;
        boolean seenRangeKey = false;
        for ( Method getter : reflector.getKeyGetters(clazz) ) {
            if ( getter.isAnnotationPresent(DynamoDBHashKey.class) ) {
                if ( seenHashKey ) {
                    throw new DynamoDBMappingException("Found more than one method annotated with "
                            + DynamoDBHashKey.class + " for class " + clazz
                            + ". Use load(Object) for tables with more than a single hash and range key.");
                }
                seenHashKey = true;
                safeInvoke(reflector.getSetter(getter), keyObject, hashKey);
            } else if ( getter.isAnnotationPresent(DynamoDBRangeKey.class) ) {
                if ( seenRangeKey ) {
                    throw new DynamoDBMappingException("Found more than one method annotated with "
                            + DynamoDBRangeKey.class + " for class " + clazz
                            + ". Use load(Object) for tables with more than a single hash and range key.");
                }
                seenRangeKey = true;
                safeInvoke(reflector.getSetter(getter), keyObject, rangeKey);
            }
        }
        if ( !seenHashKey ) {
            throw new DynamoDBMappingException("No method annotated with " + DynamoDBHashKey.class + " for class "
                    + clazz + ".");
        } else if ( rangeKey != null && !seenRangeKey ) {
            throw new DynamoDBMappingException("No method annotated with " + DynamoDBRangeKey.class + " for class "
                    + clazz + ".");
        }
        return keyObject;
    }

    /**
     * Returns a map of attribute name to EQ condition for the key prototype
     * object given.
     */
    private Map<String, Condition> getHashKeyEqualsConditions(Object obj) {
        Map<String, Condition> conditions = new HashMap<String, Condition>();
        for ( Method getter : reflector.getKeyGetters(obj.getClass()) ) {
            if ( getter.isAnnotationPresent(DynamoDBHashKey.class) ) {
                conditions.put(
                        reflector.getAttributeName(getter),
                        new Condition().withComparisonOperator(ComparisonOperator.EQ).withAttributeValueList(
                                getSimpleAttributeValue(getter, safeInvoke(getter, obj, (Object[])null))));
            }
        }
        return conditions;
    }

    /**
     * Returns the table name for the class given.
     */
    private <T> String getTableName(Class<T> clazz, DynamoDBMapperConfig config) {
        DynamoDBTable table = reflector.getTable(clazz);
        String tableName = table.tableName();
        if ( config.getTableNameOverride() != null ) {
            if ( config.getTableNameOverride().getTableName() != null ) {
                tableName = config.getTableNameOverride().getTableName();
            } else {
                tableName = config.getTableNameOverride().getTableNamePrefix() + tableName;
            }
        }

        return tableName;
    }

    /**
     * Creates and fills in the attributes on an instance of the class given
     * with the attributes given.
     * <p>
     * This is accomplished by looking for getter methods annotated with an
     * appropriate annotation, then looking for matching attribute names in the
     * item attribute map.
     *
     * @param clazz
     *            The class to instantiate and hydrate
     * @param itemAttributes
     *            The set of item attributes, keyed by attribute name.
     */
    public <T> T marshallIntoObject(Class<T> clazz, Map<String, AttributeValue> itemAttributes) {
        T toReturn = null;
        try {
            toReturn = clazz.newInstance();
        } catch ( InstantiationException e ) {
            throw new DynamoDBMappingException("Failed to instantiate new instance of class", e);
        } catch ( IllegalAccessException e ) {
            throw new DynamoDBMappingException("Failed to instantiate new instance of class", e);
        }

        if ( itemAttributes == null || itemAttributes.isEmpty() )
            return toReturn;

        itemAttributes = untransformAttributes(clazz, itemAttributes);

        for ( Method m : reflector.getRelevantGetters(clazz) ) {
            String attributeName = reflector.getAttributeName(m);
            if ( itemAttributes.containsKey(attributeName) ) {
                setValue(toReturn, m, itemAttributes.get(attributeName));
            }
        }

        return toReturn;
    }

    /**
     * Marshalls the list of item attributes into objects of type clazz
     *
     * @see DynamoDBMapper#marshallIntoObject(Class, Map)
     */
    public <T> List<T> marshallIntoObjects(Class<T> clazz, List<Map<String, AttributeValue>> itemAttributes) {
        List<T> result = new ArrayList<T>();
        for (Map<String, AttributeValue> item : itemAttributes) {
            result.add(marshallIntoObject(clazz, item));
        }
        return result;
    }

    /**
     * Sets the value in the return object corresponding to the service result.
     */
    private <T> void setValue(final T toReturn, final Method getter, AttributeValue value) {

        Method setter = reflector.getSetter(getter);
        ArgumentUnmarshaller unmarhsaller = reflector.getArgumentUnmarshaller(toReturn, getter, setter);
        unmarhsaller.typeCheck(value, setter);

        Object argument;
        try {
            argument = unmarhsaller.unmarshall(value);
        } catch ( IllegalArgumentException e ) {
            throw new DynamoDBMappingException("Couldn't unmarshall value " + value + " for " + setter, e);
        } catch ( ParseException e ) {
            throw new DynamoDBMappingException("Error attempting to parse date string " + value + " for "+ setter, e);
        }

        safeInvoke(setter, toReturn, argument);
    }

    /**
     * Returns an {@link AttributeValue} corresponding to the getter and return
     * result given, treating it as a non-versioned attribute.
     */
    private AttributeValue getSimpleAttributeValue(final Method getter, final Object getterReturnResult) {
        if ( getterReturnResult == null )
            return null;

        ArgumentMarshaller marshaller = reflector.getArgumentMarshaller(getter);
        return marshaller.marshall(getterReturnResult);
    }

    /**
     * Saves the object given into DynamoDB, using the default configuration.
     *
     * @see DynamoDBMapper#save(Object, DynamoDBMapperConfig)
     */
    public <T extends Object> void save(T object) {
        save(object, config);
    }

    /**
     * Saves an item in DynamoDB. The service method used is determined by the
     * {@link DynamoDBMapperConfig#getSaveBehavior()} value, to use either
     * {@link AWSDynamoDB#putItem(PutItemRequest)} or
     * {@link AWSDynamoDB#updateItem(UpdateItemRequest)}. For updates, a null
     * value for an object property will remove it from that item in DynamoDB.
     * For puts, a null value will not be passed to the service. The effect is
     * therefore the same, except when the item in DynamoDB contains attributes
     * that aren't modeled by the domain object given.
     *
     * @param object
     *            The object to save into DynamoDB
     * @param config
     *            The configuration to use, which overrides the default provided
     *            at object construction.
     */
    public <T extends Object> void save(T object, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        @SuppressWarnings("unchecked")
        Class<? extends T> clazz = (Class<? extends T>) object.getClass();
        String tableName = getTableName(clazz, config);

        Map<String, AttributeValueUpdate> updateValues = new HashMap<String, AttributeValueUpdate>();
        Map<String, ExpectedAttributeValue> expectedValues = new HashMap<String, ExpectedAttributeValue>();

        boolean forcePut = config.getSaveBehavior() == SaveBehavior.CLOBBER;
        List<ValueUpdate> inMemoryUpdates = new LinkedList<ValueUpdate>();

        Collection<Method> keyGetters = reflector.getKeyGetters(clazz);

        /*
         * Determine if there are any auto-assigned keys to assign. If so, force
         * a put and assign the keys.
         */
        boolean hashKeyGetterFound = false;
        for ( Method method : keyGetters ) {
            Object getterResult = safeInvoke(method, object);
            if ( getterResult == null && reflector.isAssignableKey(method) ) {
                forcePut = true;
            }
            if ( method.isAnnotationPresent(DynamoDBHashKey.class) ) {
                hashKeyGetterFound = true;
            }
        }
        if ( !hashKeyGetterFound ) {
            throw new DynamoDBMappingException("No " + DynamoDBHashKey.class + " annotation found in class " + clazz);
        }

        Map<String, AttributeValue> key = new HashMap<String, AttributeValue>();

        /*
         * First handle keys
         */
        for ( Method method : keyGetters ) {
            Object getterResult = safeInvoke(method, object);
            String attributeName = reflector.getAttributeName(method);

            if ( getterResult == null && reflector.isAssignableKey(method) ) {
                AttributeValue newVersionValue = getAutoGeneratedKeyAttributeValue(method, getterResult);
                key.put(attributeName, newVersionValue);

                if ( forcePut ) {
                    updateValues.put(attributeName,
                            new AttributeValueUpdate().withAction("PUT").withValue(newVersionValue));
                    inMemoryUpdates.add(new ValueUpdate(method, newVersionValue, object));

                    if ( config.getSaveBehavior() != SaveBehavior.CLOBBER ) {
                        // Add an expect clause to make sure that the item
                        // doesn't already exist, since it's supposed to be new
                        ExpectedAttributeValue expected = new ExpectedAttributeValue();
                        expected.setExists(false);
                        expectedValues.put(attributeName, expected);
                    }
                }
            } else {
                AttributeValue currentValue = getSimpleAttributeValue(method, getterResult);
                if ( currentValue == null ) {
                    throw new DynamoDBMappingException("Null or empty value for key: " + method);
                }

                key.put(attributeName, currentValue);

                if ( forcePut ) {
                    updateValues.put(attributeName, new AttributeValueUpdate().withValue(currentValue)
                            .withAction("PUT"));
                }
            }
        }

        /*
         * Next construct an update for every non-key property
         */
        for ( Method method : reflector.getRelevantGetters(clazz) ) {

            // Skip any key methods, since they are handled separately
            if ( keyGetters.contains(method) )
                continue;

            Object getterResult = safeInvoke(method, object);
            String attributeName = reflector.getAttributeName(method);

            /*
             * If this is a versioned field, update it
             */
            if ( reflector.isVersionAttributeGetter(method) ) {
                if ( config.getSaveBehavior() != SaveBehavior.CLOBBER ) {
                    // First establish the expected (current) value for the
                    // update call
                    ExpectedAttributeValue expected = new ExpectedAttributeValue();

                    // For new objects, insist that the value doesn't exist.
                    // For existing ones, insist it has the old value.
                    AttributeValue currentValue = getSimpleAttributeValue(method, getterResult);
                    expected.setExists(currentValue != null);
                    if ( currentValue != null ) {
                        expected.setValue(currentValue);
                    }
                    expectedValues.put(attributeName, expected);
                }

                AttributeValue newVersionValue = getVersionAttributeValue(method, getterResult);
                updateValues
                        .put(attributeName, new AttributeValueUpdate().withAction("PUT").withValue(newVersionValue));
                inMemoryUpdates.add(new ValueUpdate(method, newVersionValue, object));
            }

            /*
             * Otherwise apply the update value for this attribute.
             */
            else  {
                AttributeValue currentValue = getSimpleAttributeValue(method, getterResult);
                if ( currentValue != null ) {
                    updateValues.put(attributeName, new AttributeValueUpdate().withValue(currentValue)
                            .withAction("PUT"));
                } else if ( config.getSaveBehavior() != SaveBehavior.CLOBBER ) {
                    updateValues.put(attributeName, new AttributeValueUpdate().withAction("DELETE"));
                }
            }
        }

        /*
         * Do a put or an update, according to the configuration. For a put (not
         * the default), we need to munge the data type.
         */
        if ( config.getSaveBehavior() == SaveBehavior.CLOBBER || forcePut ) {
            db.putItem(applyUserAgent(new PutItemRequest().withTableName(tableName)
                    .withItem(transformAttributes(clazz, convertToItem(updateValues)))
                    .withExpected(expectedValues)));
        } else {
            db.updateItem(applyUserAgent(new UpdateItemRequest().withTableName(tableName).withKey(key)
                    .withAttributeUpdates(transformAttributeUpdates(clazz, key, updateValues)).withExpected(expectedValues)));
        }

        /*
         * Finally, after the service call has succeeded, update the in-memory
         * object with new field values as appropriate.
         */
        for ( ValueUpdate update : inMemoryUpdates ) {
            update.apply();
        }
    }

    /**
     * Deletes the given object from its DynamoDB table.
     */
    public void delete(Object object) {
        delete(object, this.config);
    }

    /**
     * Deletes the given object from its DynamoDB table.
     *
     * @param config
     *            Config override object. If {@link SaveBehavior#CLOBBER} is
     *            supplied, version fields will not be considered when deleting
     *            the object.
     */
    public <T> void delete(T object, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        @SuppressWarnings("unchecked")
        Class<T> clazz = (Class<T>) object.getClass();

        String tableName = getTableName(clazz, config);

        Map<String, AttributeValue> key = getKey(object, clazz);

        /*
         * If there is a version field, make sure we assert its value. If the
         * version field is null (only should happen in unusual circumstances),
         * pretend it doesn't have a version field after all.
         */
        Map<String, ExpectedAttributeValue> expectedValues = new HashMap<String, ExpectedAttributeValue>();
        if ( config.getSaveBehavior() != SaveBehavior.CLOBBER ) {
            for ( Method method : reflector.getRelevantGetters(clazz) ) {

                if ( reflector.isVersionAttributeGetter(method) ) {
                    Object getterResult = safeInvoke(method, object);
                    String attributeName = reflector.getAttributeName(method);

                    ExpectedAttributeValue expected = new ExpectedAttributeValue();
                    AttributeValue currentValue = getSimpleAttributeValue(method, getterResult);
                    expected.setExists(currentValue != null);
                    if ( currentValue != null )
                        expected.setValue(currentValue);
                    expectedValues.put(attributeName, expected);
                    break;
                }
            }
        }

        db.deleteItem(applyUserAgent(new DeleteItemRequest().withKey(key).withTableName(tableName).withExpected(expectedValues)));
    }

    /**
     * Deletes the objects given using one or more calls to the
     * {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)} API.
     *
     * @see DynamoDBMapper#batchWrite(List, List, DynamoDBMapperConfig)
     */
    public List<FailedBatch> batchDelete(List<? extends Object> objectsToDelete) {
        return batchWrite(Collections.emptyList(), objectsToDelete, this.config);
    }

    /**
     * Deletes the objects given using one or more calls to the
     * {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)} API.
     *
     * @see DynamoDBMapper#batchWrite(List, List, DynamoDBMapperConfig)
     */
    public List<FailedBatch> batchDelete(Object... objectsToDelete) {
        return batchWrite(Collections.emptyList(), Arrays.asList(objectsToDelete), this.config);
    }

    /**
     * Saves the objects given using one or more calls to the
     * {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)} API.
     *
     * @see DynamoDBMapper#batchWrite(List, List, DynamoDBMapperConfig)
     */
    public List<FailedBatch> batchSave(List<? extends Object> objectsToSave) {
        return batchWrite(objectsToSave, Collections.emptyList(), this.config);
    }

    /**
     * Saves the objects given using one or more calls to the
     * {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)} API.
     *
     * @see DynamoDBMapper#batchWrite(List, List, DynamoDBMapperConfig)
     */
    public List<FailedBatch> batchSave(Object... objectsToSave) {
        return batchWrite(Arrays.asList(objectsToSave), Collections.emptyList(), this.config);
    }

    /**
     * Saves and deletes the objects given using one or more calls to the
     * {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)} API.
     *
     * @see DynamoDBMapper#batchWrite(List, List, DynamoDBMapperConfig)
     */
    public List<FailedBatch> batchWrite(List<? extends Object> objectsToWrite, List<? extends Object> objectsToDelete) {
        return batchWrite(objectsToWrite, objectsToDelete, this.config);
    }

    /**
     * Saves and deletes the objects given using one or more calls to the
     * {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)} API.
     *
     * @param objectsToWrite
     *            A list of objects to save to DynamoDB. No version checks are
     *            performed, as required by the
     *            {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)}
     *            API.
     * @param objectsToDelete
     *            A list of objects to delete from DynamoDB. No version checks
     *            are performed, as required by the
     *            {@link AmazonDynamoDB#batchWriteItem(BatchWriteItemRequest)}
     *            API.
     * @param config
     *            Only {@link DynamoDBMapperConfig#getTableNameOverride()} is
     *            considered; if specified, all objects in the two parameter
     *            lists will be considered to belong to the given table
     *            override.
     * @return A list of failed batches which includes the unprocessed items and
     *         the exceptions causing the failure.
     */
    public List<FailedBatch> batchWrite(List<? extends Object> objectsToWrite, List<? extends Object> objectsToDelete, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        List<FailedBatch> totalFailedBatches = new LinkedList<FailedBatch>();

        HashMap<String, List<WriteRequest>> requestItems = new HashMap<String, List<WriteRequest>>();

        List<ValueUpdate> inMemoryUpdates = new LinkedList<ValueUpdate>();
        for ( Object toWrite : objectsToWrite ) {
            Class<?> clazz = toWrite.getClass();
            String tableName = getTableName(clazz, config);

            Map<String, AttributeValue> attributeValues = new HashMap<String, AttributeValue>();

            // Look at every getter and construct a value object for it
            for ( Method method : reflector.getRelevantGetters(clazz) ) {
                Object getterResult = safeInvoke(method, toWrite);
                String attributeName = reflector.getAttributeName(method);

                AttributeValue currentValue = null;
                if ( getterResult == null && reflector.isAssignableKey(method) ) {
                    currentValue = getAutoGeneratedKeyAttributeValue(method, getterResult);
                    inMemoryUpdates.add(new ValueUpdate(method, currentValue, toWrite));
                } else {
                    currentValue = getSimpleAttributeValue(method, getterResult);
                }

                if ( currentValue != null ) {
                    attributeValues.put(attributeName, currentValue);
                }
            }

            if ( !requestItems.containsKey(tableName) ) {
                requestItems.put(tableName, new LinkedList<WriteRequest>());
            }

            requestItems.get(tableName).add(
                    new WriteRequest().withPutRequest(new PutRequest().withItem(transformAttributes(clazz, attributeValues))));
        }

        for ( Object toDelete : objectsToDelete ) {
            Class<?> clazz = toDelete.getClass();

            String tableName = getTableName(clazz, config);

            Map<String, AttributeValue> key = getKey(toDelete);

            if ( !requestItems.containsKey(tableName) ) {
                requestItems.put(tableName, new LinkedList<WriteRequest>());
            }

            requestItems.get(tableName).add(
                    new WriteRequest().withDeleteRequest(new DeleteRequest().withKey(key)));
        }

        // Break into chunks of 25 items and make service requests to DynamoDB
        while ( !requestItems.isEmpty() ) {
            HashMap<String, List<WriteRequest>> batch = new HashMap<String, List<WriteRequest>>();
            int i = 0;
            Iterator<Entry<String, List<WriteRequest>>> tableIter = requestItems.entrySet().iterator();
            while ( tableIter.hasNext() && i < 25 ) {
                Entry<String, List<WriteRequest>> tableRequest = tableIter.next();
                batch.put(tableRequest.getKey(), new LinkedList<WriteRequest>());
                Iterator<WriteRequest> writeRequestIter = tableRequest.getValue().iterator();
                while ( writeRequestIter.hasNext() && i++ < 25 ) {
                    WriteRequest writeRequest = writeRequestIter.next();
                    batch.get(tableRequest.getKey()).add(writeRequest);
                    writeRequestIter.remove();
                }

                // If we've processed all the write requests for this table,
                // remove it from the parent iterator.
                if ( !writeRequestIter.hasNext() ) {
                    tableIter.remove();
                }
            }

            List<FailedBatch> failedBatches = writeOneBatch(batch);
            if (failedBatches != null) {
                totalFailedBatches.addAll(failedBatches);

                // If contains throttling exception, we do a backoff
                if (containsThrottlingException(failedBatches)) {
                    try {
                        Thread.sleep(1000 * 2);
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                        throw new AmazonClientException(e.getMessage(), e);
                    }
                }
            }
        }



        // Once the entire batch is processed, update assigned keys in memory
        for ( ValueUpdate update : inMemoryUpdates ) {
            update.apply();
        }

        return totalFailedBatches;
    }

    /**
     * Process one batch of requests(max 25). It will divide the batch if
     * receives request too large exception(the total size of the request is beyond 1M).
     */
    private List<FailedBatch> writeOneBatch(Map<String, List<WriteRequest>> batch) {

        List<FailedBatch> failedBatches = new LinkedList<FailedBatch>();
        Map<String, List<WriteRequest>> firstHalfBatch = new HashMap<String, List<WriteRequest>>();
        Map<String, List<WriteRequest>> secondHalfBatch = new HashMap<String, List<WriteRequest>>();
        FailedBatch failedBatch = callUntilCompletion(batch);

        if (failedBatch != null) {
            // If the exception is request entity too large, we divide the batch
            // into smaller parts.

            if (failedBatch.getException() instanceof AmazonServiceException
            && AmazonHttpClient.isRequestEntityTooLargeException((AmazonServiceException) failedBatch.getException())) {

                // If only one item left, the item size must beyond 64k, which
                // exceedes the limit.

                if (computeFailedBatchSize(failedBatch) == 1) {
                    failedBatches.add(failedBatch);
                } else {
                    divideBatch(batch, firstHalfBatch, secondHalfBatch);
                    failedBatches.addAll(writeOneBatch(firstHalfBatch));
                    failedBatches.addAll(writeOneBatch(secondHalfBatch));
                }

            } else {
                failedBatches.add(failedBatch);
            }

        }
        return failedBatches;
    }

    /**
     * Check whether there are throttling exception in the failed batches.
     */
    private boolean containsThrottlingException (List<FailedBatch> failedBatches) {
        for (FailedBatch failedBatch : failedBatches) {
           Exception e = failedBatch.getException();
            if (e instanceof AmazonServiceException
                    && AmazonHttpClient.isThrottlingException((AmazonServiceException) e)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Divide the batch of objects to save into two smaller batches. Each contains half of the elements.
     */
    private void divideBatch(Map<String, List<WriteRequest>> batch, Map<String, List<WriteRequest>> firstHalfBatch, Map<String, List<WriteRequest>> secondHalfBatch) {
        for (String key : batch.keySet()) {

            List<WriteRequest> requests = batch.get(key);

            List<WriteRequest> firstHalfRequests = requests.subList(0, requests.size() / 2);

            List<WriteRequest> secondHalfRequests = requests.subList(requests.size() / 2, requests.size());

            firstHalfBatch.put(key, firstHalfRequests);

            secondHalfBatch.put(key, secondHalfRequests);

        }

    }

    /**
     * Count the total number of unprocessed items in the failed batch.
     */

    private int computeFailedBatchSize(FailedBatch failedBatch) {

        int count = 0;

        for (String tableName : failedBatch.getUnprocessedItems().keySet()) {
            count += failedBatch.getUnprocessedItems().get(tableName).size();
        }
        return count;
    }

    /**
     * Continue trying to process the batch until it finishes or an exception
     * occurs.
     */

    private FailedBatch callUntilCompletion(Map<String, List<WriteRequest>> batch) {
        BatchWriteItemResult result = null;
        int retries = 0;
        FailedBatch failedBatch = null;
        while (true) {
            try {
                result = db.batchWriteItem(new BatchWriteItemRequest().withRequestItems(batch));
            } catch (Exception e) {
                failedBatch = new FailedBatch();
                failedBatch.setUnprocessedItems(batch);
                failedBatch.setException(e);
                return failedBatch;
            }
            retries++;
            batch = result.getUnprocessedItems();
            if (batch.size() > 0) {
                pauseExponentially(retries);
            } else {
                break;
            }
        }
        return failedBatch;
    }

    /**
     * Retrieves multiple items from multiple tables using their primary keys.
     *
     * @see DynamoDBMapper#batchLoad(List, DynamoDBMapperConfig)
     */
    public Map<String, List<Object>> batchLoad(List<Object> itemsToGet) {
        return batchLoad(itemsToGet, this.config);
    }

    /**
     * Retrieves multiple items from multiple tables using their primary keys.
     *
     * @param itemsToGet
     *            Key objects, corresponding to the class to fetch, with their
     *            primary key values set.
     * @param config
     *            Only {@link DynamoDBMapperConfig#getTableNameOverride()} and
     *            {@link DynamoDBMapperConfig#getConsistentReads()} are
     *            considered.
     */
    public Map<String, List<Object>> batchLoad(List<Object> itemsToGet, DynamoDBMapperConfig config) {
        config = mergeConfig(config);
        boolean consistentReads = (config.getConsistentReads() == ConsistentReads.CONSISTENT);

        if ( itemsToGet == null || itemsToGet.isEmpty() ) {
            return new HashMap<String, List<Object>>();
        }

        Map<String, KeysAndAttributes> requestItems = new HashMap<String, KeysAndAttributes>();
        Map<String, Class<?>> classesByTableName = new HashMap<String, Class<?>>();
        Map<String, List<Object>> resultSet = new HashMap<String, List<Object>>();
        int count = 0;

        for ( Object keyObject : itemsToGet ) {
            Class<?> clazz = keyObject.getClass();

            String tableName = getTableName(clazz, config);
            classesByTableName.put(tableName, clazz);

            if ( !requestItems.containsKey(tableName) ) {
                requestItems.put(
                        tableName,
                        new KeysAndAttributes().withConsistentRead(consistentReads).withKeys(
                                new LinkedList<Map<String, AttributeValue>>()));
            }

            requestItems.get(tableName).getKeys().add(getKey(keyObject));

            // Reach the maximum number which can be handled in a single batchGet
            if ( ++count == 100 ) {
                processBatchGetRequest(classesByTableName, requestItems, resultSet);
                requestItems.clear();
                count = 0;
            }
        }

        if ( count > 0 ) {
            processBatchGetRequest(classesByTableName, requestItems, resultSet);
        }

        return resultSet;
    }

    /**
     * Retrieves the attributes for multiple items from multiple tables using
     * their primary keys.
     * {@link AmazonDynamoDB#batchGetItem(BatchGetItemRequest)} API.
     *
     * @see DynamoDBMapper#batchLoad(Map, List, DynamoDBMapperConfig)
     */
    public Map<String, List<Object>> batchLoad(Map<Class<?>, List<KeyPair>> itemsToGet) {
        return batchLoad(itemsToGet, this.config);
    }

    /**
     * Retrieves multiple items from multiple tables using their primary keys.
     * Valid only for tables with a single hash key, or a single hash and range
     * key. For other schemas, use
     * {@link DynamoDBMapper#batchLoad(List, DynamoDBMapperConfig)}
     *
     * @param itemsToGet
     *            Map from class to load to list of primary key attributes.
     * @param config
     *            Only {@link DynamoDBMapperConfig#getTableNameOverride()} and
     *            {@link DynamoDBMapperConfig#getConsistentReads()} are
     *            considered.
     */
    public Map<String, List<Object>> batchLoad(Map<Class<?>, List<KeyPair>> itemsToGet, DynamoDBMapperConfig config) {

        List<Object> keys = new ArrayList<Object>();
        if ( itemsToGet != null ) {
            for ( Class<?> clazz : itemsToGet.keySet() ) {
                if ( itemsToGet.get(clazz) != null ) {
                    for ( KeyPair keyPair : itemsToGet.get(clazz) ) {
                        keys.add(createKeyObject(clazz, keyPair.getHashKey(), keyPair.getRangeKey()));
                    }
                }
            }
        }

        return batchLoad(keys, config);
    }

    private void processBatchGetRequest(Map<String, Class<?>> classesByTableName,
                                        Map<String, KeysAndAttributes> requestItems,
                                        Map<String, List<Object>> resultSet) {

        BatchGetItemResult batchGetItemResult = null;
        BatchGetItemRequest batchGetItemRequest = new BatchGetItemRequest();
        batchGetItemRequest.setRequestItems(requestItems);
        do {
            if ( batchGetItemResult != null ) {
                batchGetItemRequest.setRequestItems(batchGetItemResult.getUnprocessedKeys());
            }

            batchGetItemResult = db.batchGetItem(batchGetItemRequest);
            Map<String, List<Map<String, AttributeValue>>> responses = batchGetItemResult.getResponses();
            for ( String tableName : responses.keySet() ) {
                List<Object> objects = null;
                if ( resultSet.get(tableName) != null ) {
                    objects = resultSet.get(tableName);
                } else {
                    objects = new LinkedList<Object>();
                }

                for ( Map<String, AttributeValue> item : responses.get(tableName) ) {
                    objects.add(marshallIntoObject(classesByTableName.get(tableName), item));
                }

                resultSet.put(tableName, objects);
            }
            // To see whether there are unprocessed keys.
        } while ( batchGetItemResult.getUnprocessedKeys() != null && batchGetItemResult.getUnprocessedKeys().size() > 0 );

    }

    /**
     * Swallows the checked exceptions around Method.invoke and repackages them
     * as {@link DynamoDBMappingException}
     */
    private Object safeInvoke(Method method, Object object, Object... arguments) {
        try {
            return method.invoke(object, arguments);
        } catch ( IllegalAccessException e ) {
            throw new DynamoDBMappingException("Couldn't invoke " + method, e);
        } catch ( IllegalArgumentException e ) {
            throw new DynamoDBMappingException("Couldn't invoke " + method, e);
        } catch ( InvocationTargetException e ) {
            throw new DynamoDBMappingException("Couldn't invoke " + method, e);
        }
    }

    private final class ValueUpdate {

        private Method method;
        private AttributeValue newValue;
        private Object target;

        public ValueUpdate(Method method, AttributeValue newValue, Object target) {
            this.method = method;
            this.newValue = newValue;
            this.target = target;
        }

        public void apply() {
            setValue(target, method, newValue);
        }
    }

    /**
     * Converts the {@link AttributeValueUpdate} map given to an equivalent
     * {@link AttributeValue} map.
     */
    private Map<String, AttributeValue> convertToItem(Map<String, AttributeValueUpdate> putValues) {
        Map<String, AttributeValue> map = new HashMap<String, AttributeValue>();
        for ( Entry<String, AttributeValueUpdate> entry : putValues.entrySet() ) {
            /*
             * AttributeValueUpdate allows nulls for its values, since they are
             * semantically meaningful. AttributeValues never have null values.
             */
            if ( entry.getValue().getValue() != null )
                map.put(entry.getKey(), entry.getValue().getValue());
        }
        return map;
    }

    /**
     * Gets the attribute value object corresponding to the
     * {@link DynamoDBVersionAttribute} getter, and its result, given. Null
     * values are assumed to be new objects and given the smallest possible
     * positive value. Non-null values are incremented from their current value.
     */
    private AttributeValue getVersionAttributeValue(final Method getter, Object getterReturnResult) {
        ArgumentMarshaller marshaller = reflector.getVersionedArgumentMarshaller(getter, getterReturnResult);
        return marshaller.marshall(getterReturnResult);
    }

    /**
     * Returns an attribute value corresponding to the key method and value given.
     */
    private AttributeValue getAutoGeneratedKeyAttributeValue(Method getter, Object getterResult) {
        ArgumentMarshaller marshaller = reflector.getAutoGeneratedKeyArgumentMarshaller(getter);
        return marshaller.marshall(getterResult);
    }

    /**
     * Scans through an Amazon DynamoDB table and returns the matching results as
     * an unmodifiable list of instantiated objects, using the default configuration.
     *
     * @see DynamoDBMapper#scan(Class, DynamoDBScanExpression, DynamoDBMapperConfig)
     */
    public <T> PaginatedScanList<T> scan(Class<T> clazz, DynamoDBScanExpression scanExpression) {
        return scan(clazz, scanExpression, config);
    }

    /**
     * Scans through an Amazon DynamoDB table and returns the matching results as
     * an unmodifiable list of instantiated objects. The table to scan is
     * determined by looking at the annotations on the specified class, which
     * declares where to store the object data in Amazon DynamoDB, and the scan
     * expression parameter allows the caller to filter results and control how
     * the scan is executed.
     * <p>
     * Callers should be aware that the returned list is unmodifiable, and any
     * attempts to modify the list will result in an
     * UnsupportedOperationException.
     * <p>
     * The unmodifiable list returned is lazily loaded when possible, so calls
     * to DynamoDB will be made only as needed.
     *
     * @param <T>
     *            The type of the objects being returned.
     * @param clazz
     *            The class annotated with DynamoDB annotations describing how
     *            to store the object data in Amazon DynamoDB.
     * @param scanExpression
     *            Details on how to run the scan, including any filters to apply
     *            to limit results.
     * @param config
     *            The configuration to use for this scan, which overrides the
     *            default provided at object construction.
     * @return An unmodifiable list of the objects constructed from the results
     *         of the scan operation.
     * @see PaginatedScanList
     */
    public <T> PaginatedScanList<T> scan(Class<T> clazz, DynamoDBScanExpression scanExpression, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        ScanRequest scanRequest = createScanRequestFromExpression(clazz, scanExpression, config);

        ScanResult scanResult = db.scan(applyUserAgent(scanRequest));
        return new PaginatedScanList<T>(this, clazz, db, scanRequest, scanResult);
    }

    /**
	 * Scans through an Amazon DynamoDB table on logically partitioned segments
	 * in parallel and returns the matching results in one unmodifiable list of
	 * instantiated objects, using the default configuration.
	 *
	 * @see DynamoDBMapper#parallelScan(Class, DynamoDBScanExpression,int,
	 *      DynamoDBMapperConfig)
	 */
    public <T> PaginatedParallelScanList<T> parallelScan(Class<T> clazz, DynamoDBScanExpression scanExpression, int totalSegments) {
        return parallelScan(clazz, scanExpression, totalSegments, config);
    }

	/**
	 * Scans through an Amazon DynamoDB table on logically partitioned segments
	 * in parallel. This method will create a thread pool of the specified size,
	 * and each thread will issue scan requests for its assigned segment,
	 * following the returned continuation token, until the end of its segment.
	 * Callers should be responsible for setting the appropriate number of total
	 * segments. More scan segments would result in better performance but more
	 * consumed capacity of the table. The results are returned in one
	 * unmodifiable list of instantiated objects. The table to scan is
	 * determined by looking at the annotations on the specified class, which
	 * declares where to store the object data in Amazon DynamoDB, and the scan
	 * expression parameter allows the caller to filter results and control how
	 * the scan is executed.
	 * <p>
	 * Callers should be aware that the returned list is unmodifiable, and any
	 * attempts to modify the list will result in an
	 * UnsupportedOperationException.
	 * <p>
	 * The unmodifiable list returned is lazily loaded when possible, so calls
	 * to DynamoDB will be made only as needed.
	 *
	 * @param <T>
	 *            The type of the objects being returned.
	 * @param clazz
	 *            The class annotated with DynamoDB annotations describing how
	 *            to store the object data in Amazon DynamoDB.
	 * @param scanExpression
	 *            Details on how to run the scan, including any filters to apply
	 *            to limit results.
	 * @param totalSegments
	 *            Number of total parallel scan segments.
	 *            <b>Range: </b>1 - 4096
	 * @param config
	 *            The configuration to use for this scan, which overrides the
	 *            default provided at object construction.
	 * @return An unmodifiable list of the objects constructed from the results
	 *         of the scan operation.
	 * @see PaginatedParallelScanList
	 */
    public <T> PaginatedParallelScanList<T> parallelScan(Class<T> clazz, DynamoDBScanExpression scanExpression, int totalSegments, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        // Create hard copies of the original scan request with difference segment number.
        List<ScanRequest> parallelScanRequests = createParallelScanRequestsFromExpression(clazz, scanExpression, totalSegments, config);
        ParallelScanTask parallelScanTask = new ParallelScanTask(this, db, parallelScanRequests);

        return new PaginatedParallelScanList<T>(this, clazz, db, parallelScanTask);
    }

    /**
     * Scans through an Amazon DynamoDB table and returns a single page of matching
     * results. The table to scan is determined by looking at the annotations on
     * the specified class, which declares where to store the object data in AWS
     * DynamoDB, and the scan expression parameter allows the caller to filter
     * results and control how the scan is executed.
     *
     * @param <T>
     *            The type of the objects being returned.
     * @param clazz
     *            The class annotated with DynamoDB annotations describing how
     *            to store the object data in Amazon DynamoDB.
     * @param scanExpression
     *            Details on how to run the scan, including any filters to apply
     *            to limit results.
     * @param config
     *            The configuration to use for this scan, which overrides the
     *            default provided at object construction.
     */
    public <T> ScanResultPage<T> scanPage(Class<T> clazz, DynamoDBScanExpression scanExpression, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        ScanRequest scanRequest = createScanRequestFromExpression(clazz, scanExpression, config);

        ScanResult scanResult = db.scan(applyUserAgent(scanRequest));
        ScanResultPage<T> result = new ScanResultPage<T>();
        result.setResults(marshallIntoObjects(clazz, scanResult.getItems()));
        result.setLastEvaluatedKey(scanResult.getLastEvaluatedKey());

        return result;
    }

    /**
     * Scans through an Amazon DynamoDB table and returns a single page of matching
     * results.
     *
     * @see DynamoDBMapper#scanPage(Class, DynamoDBScanExpression, DynamoDBMapperConfig)
     */
    public <T> ScanResultPage<T> scanPage(Class<T> clazz, DynamoDBScanExpression scanExpression) {
        return scanPage(clazz, scanExpression, this.config);
    }

    /**
     * Queries an Amazon DynamoDB table and returns the matching results as an
     * unmodifiable list of instantiated objects, using the default
     * configuration.
     *
     * @see DynamoDBMapper#query(Class, DynamoDBQueryExpression,
     *      DynamoDBMapperConfig)
     */
    public <T> PaginatedQueryList<T> query(Class<T> clazz, DynamoDBQueryExpression<T> queryExpression) {
        return query(clazz, queryExpression, config);
    }

    /**
     * Queries an Amazon DynamoDB table and returns the matching results as an
     * unmodifiable list of instantiated objects. The table to query is
     * determined by looking at the annotations on the specified class, which
     * declares where to store the object data in Amazon DynamoDB, and the query
     * expression parameter allows the caller to filter results and control how
     * the query is executed.
     * <p>
     * When the query is on any local secondary index, callers should be aware that
     * the returned object(s) will only contain item attributes that are projected
     * into the index. All the other unprojected attributes will be saved as type
     * default values.
     * <p>
     * Callers should also be aware that the returned list is unmodifiable, and any
     * attempts to modify the list will result in an
     * UnsupportedOperationException.
     * <p>
     * The unmodifiable list returned is lazily loaded when possible, so calls
     * to DynamoDB will be made only as needed.
     *
     * @param <T>
     *            The type of the objects being returned.
     * @param clazz
     *            The class annotated with DynamoDB annotations describing how
     *            to store the object data in Amazon DynamoDB.
     * @param queryExpression
     *            Details on how to run the query, including any conditions on
     *            the key values
     * @param config
     *            The configuration to use for this query, which overrides the
     *            default provided at object construction.
     * @return An unmodifiable list of the objects constructed from the results
     *         of the query operation.
     * @see PaginatedQueryList
     */
    public <T> PaginatedQueryList<T> query(Class<T> clazz, DynamoDBQueryExpression<T> queryExpression, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        QueryRequest queryRequest = createQueryRequestFromExpression(clazz, queryExpression, config);

        QueryResult queryResult = db.query(applyUserAgent(queryRequest));
        return new PaginatedQueryList<T>(this, clazz, db, queryRequest, queryResult);
    }

    /**
     * Queries an Amazon DynamoDB table and returns a single page of matching
     * results. The table to query is determined by looking at the annotations
     * on the specified class, which declares where to store the object data in
     * Amazon DynamoDB, and the query expression parameter allows the caller to
     * filter results and control how the query is executed.
     *
     * @see DynamoDBMapper#queryPage(Class, DynamoDBQueryExpression, DynamoDBMapperConfig)
     */
    public <T> QueryResultPage<T> queryPage(Class<T> clazz, DynamoDBQueryExpression<T> queryExpression) {
        return queryPage(clazz, queryExpression, this.config);
    }

    /**
     * Queries an Amazon DynamoDB table and returns a single page of matching
     * results. The table to query is determined by looking at the annotations
     * on the specified class, which declares where to store the object data in
     * Amazon DynamoDB, and the query expression parameter allows the caller to
     * filter results and control how the query is executed.
     *
     * @param <T>
     *            The type of the objects being returned.
     * @param clazz
     *            The class annotated with DynamoDB annotations describing how
     *            to store the object data in AWS DynamoDB.
     * @param queryExpression
     *            Details on how to run the query, including any conditions on
     *            the key values
     * @param config
     *            The configuration to use for this query, which overrides the
     *            default provided at object construction.
     */
    public <T> QueryResultPage<T> queryPage(Class<T> clazz, DynamoDBQueryExpression<T> queryExpression, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        QueryRequest queryRequest = createQueryRequestFromExpression(clazz, queryExpression, config);

        QueryResult scanResult = db.query(applyUserAgent(queryRequest));
        QueryResultPage<T> result = new QueryResultPage<T>();
        result.setResults(marshallIntoObjects(clazz, scanResult.getItems()));
        result.setLastEvaluatedKey(scanResult.getLastEvaluatedKey());

        return result;
    }

    /**
     * Evaluates the specified scan expression and returns the count of matching
     * items, without returning any of the actual item data, using the default configuration.
     *
     * @see DynamoDBMapper#count(Class, DynamoDBScanExpression, DynamoDBMapperConfig)
     */
    public int count(Class<?> clazz, DynamoDBScanExpression scanExpression) {
        return count(clazz, scanExpression, config);
    }

    /**
     * Evaluates the specified scan expression and returns the count of matching
     * items, without returning any of the actual item data.
     * <p>
     * This operation will scan your entire table, and can therefore be very
     * expensive. Use with caution.
     *
     * @param clazz
     *            The class mapped to a DynamoDB table.
     * @param scanExpression
     *            The parameters for running the scan.
     * @param config
     *            The configuration to use for this scan, which overrides the
     *            default provided at object construction.
     * @return The count of matching items, without returning any of the actual
     *         item data.
     */
    public int count(Class<?> clazz, DynamoDBScanExpression scanExpression, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        ScanRequest scanRequest = createScanRequestFromExpression(clazz, scanExpression, config);
        scanRequest.setSelect(Select.COUNT);

        // Count scans can also be truncated for large datasets
        int count = 0;
        ScanResult scanResult = null;
        do {
            scanResult = db.scan(applyUserAgent(scanRequest));
            count += scanResult.getCount();
            scanRequest.setExclusiveStartKey(scanResult.getLastEvaluatedKey());
        } while (scanResult.getLastEvaluatedKey() != null);

        return count;
    }


    /**
     * Evaluates the specified query expression and returns the count of matching
     * items, without returning any of the actual item data, using the default configuration.
     *
     * @see DynamoDBMapper#count(Class, DynamoDBQueryExpression, DynamoDBMapperConfig)
     */
    public <T> int count(Class<T> clazz, DynamoDBQueryExpression<T> queryExpression) {
        return count(clazz, queryExpression, config);
    }

    /**
     * Evaluates the specified query expression and returns the count of
     * matching items, without returning any of the actual item data.
     *
     * @param clazz
     *            The class mapped to a DynamoDB table.
     * @param scanExpression
     *            The parameters for running the scan.
     * @param config
     *            The mapper configuration to use for the query, which overrides
     *            the default provided at object construction.
     * @return The count of matching items, without returning any of the actual
     *         item data.
     */
    public <T> int count(Class<T> clazz, DynamoDBQueryExpression<T> queryExpression, DynamoDBMapperConfig config) {
        config = mergeConfig(config);

        QueryRequest queryRequest = createQueryRequestFromExpression(clazz, queryExpression, config);
        queryRequest.setSelect(Select.COUNT);

        // Count queries can also be truncated for large datasets
        int count = 0;
        QueryResult queryResult = null;
        do {
            queryResult = db.query(applyUserAgent(queryRequest));
            count += queryResult.getCount();
            queryRequest.setExclusiveStartKey(queryResult.getLastEvaluatedKey());
        } while (queryResult.getLastEvaluatedKey() != null);

        return count;
    }

    /**
     * Merges the config object given with the one specified at construction and
     * returns the result.
     */
    private DynamoDBMapperConfig mergeConfig(DynamoDBMapperConfig config) {
        if ( config != this.config )
            config = new DynamoDBMapperConfig(this.config, config);
        return config;
    }

    private ScanRequest createScanRequestFromExpression(Class<?> clazz, DynamoDBScanExpression scanExpression, DynamoDBMapperConfig config) {
        ScanRequest scanRequest = new ScanRequest();
        scanRequest.setTableName(getTableName(clazz, config));
        scanRequest.setScanFilter(scanExpression.getScanFilter());
        scanRequest.setLimit(scanExpression.getLimit());
        scanRequest.setExclusiveStartKey(scanExpression.getExclusiveStartKey());

        return scanRequest;
    }

    private List<ScanRequest> createParallelScanRequestsFromExpression(Class<?> clazz, DynamoDBScanExpression scanExpression, int totalSegments, DynamoDBMapperConfig config) {
    	if (totalSegments < 1)
			throw new IllegalArgumentException("Parallel scan should have at least one scan segment.");
    	List<ScanRequest> parallelScanRequests= new LinkedList<ScanRequest>();
        for (int segment = 0; segment < totalSegments; segment++) {
        	ScanRequest scanRequest = createScanRequestFromExpression(clazz, scanExpression, config);
        	parallelScanRequests.add(scanRequest.withSegment(segment).withTotalSegments(totalSegments));
        }
        return parallelScanRequests;
    }

    private <T> QueryRequest  createQueryRequestFromExpression(Class<T> clazz, DynamoDBQueryExpression<T> queryExpression, DynamoDBMapperConfig config) {
        QueryRequest queryRequest = new QueryRequest();
        queryRequest.setConsistentRead(queryExpression.isConsistentRead());
        queryRequest.setTableName(getTableName(clazz, config));
        queryRequest.setIndexName(queryExpression.getIndexName());

        Map<String, Condition> keyConditions = getHashKeyEqualsConditions(queryExpression.getHashKeyValues());

        Map<String, Condition> rangeKeyConditions = queryExpression.getRangeKeyConditions();
        if (null != rangeKeyConditions) {
        	processRangeKeyConditions(clazz, queryRequest, rangeKeyConditions);
        	keyConditions.putAll(rangeKeyConditions);
        }

        queryRequest.setKeyConditions(keyConditions);
        queryRequest.setScanIndexForward(queryExpression.isScanIndexForward());
        queryRequest.setLimit(queryExpression.getLimit());
        queryRequest.setExclusiveStartKey(queryExpression.getExclusiveStartKey());

        return queryRequest;
    }

    /**
     * Utility method for checking the validity of the range key condition, and also it will try to infer the index name if the query is using any index range key.
     */
    private void processRangeKeyConditions(Class<?> clazz, QueryRequest queryRequest, Map<String, Condition> rangeKeyConditions) {
    	/**
         * Exception if conditions on multiple range keys are found.
         */
        if (rangeKeyConditions.size() > 1) {
        	// The current DynamoDB service only supports queries using hash key equal condition
        	// plus ONE range key condition.
        	// This range key could be either the primary key or any index key.
        	throw new AmazonClientException("Conditions on multiple range keys ("
        			+ rangeKeyConditions.keySet().toString()
        			+ ") are found in the query. DynamoDB service only accepts up to ONE range key condition.");
        }
        String assignedIndexName = queryRequest.getIndexName();
        for (String rangeKey : rangeKeyConditions.keySet()) {
        	/**
        	 * If it is a primary range key, checks whether the user has specified
        	 * an unnecessary index name.
        	 */
        	if (rangeKey.equals(reflector.getPrimaryRangeKeyName(clazz))) {
        		if ( null != assignedIndexName )
        			throw new AmazonClientException("The range key ("
        					+ rangeKey + ") in the query is the primary key of the table, not the range key of index ("
        					+ assignedIndexName + ").");
        	}
        	else {
                List<String> annotatedIndexNames = reflector.getIndexNameByIndexRangeKeyName(clazz, rangeKey);
            	/**
            	 * If it is an index range key,
            	 * 		check whether the provided index name matches the @DynamoDBIndexRangeKey annotation,
            	 * 		or try to infer the index name according to @DynamoDBIndexRangeKey annotation
            	 * 			if it is not provided in the query.
            	 */
                if ( null != annotatedIndexNames) {
            		if (null == assignedIndexName) {
            			// infer the index name if the range key is used only in one index
            			if ( 1 == annotatedIndexNames.size()) {
            				queryRequest.setIndexName(annotatedIndexNames.get(0));
            			} else {
            				throw new AmazonClientException("Please specify which index to be used for this query. "
            						+ "(Choose from " + annotatedIndexNames.toString() + ").");
            			}
            		} else {
            			// check whether the provided index name in the query matches the @DyanmoDBIndexRangeKey annotation
            			if ( !annotatedIndexNames.contains(assignedIndexName)) {
            				throw new AmazonClientException(assignedIndexName
            						+ " is not annotated as an index in the @DynamoDBIndexRangeKey annotation on "
            						+ rangeKey + "(Choose from " + annotatedIndexNames.toString() + ").");
            			}
            		}
                }
                else {
            		throw new AmazonClientException("The range key used in the query (" + rangeKey + ") is not annotated with " +
            				"either @DynamoDBRangeKey or @DynamoDBIndexRangeKey in class (" + clazz.getName() + ").");
            	}
        	}
        }
    }
    /**
     * By default, just calls {@link #untransformAttributes(String, String, Map)}.
     */
    protected Map<String, AttributeValue> untransformAttributes(Class<?> clazz, Map<String, AttributeValue> attributeValues) {
        Method hashKeyGetter = reflector.getHashKeyGetter(clazz);
        String hashKeyName = reflector.getAttributeName(hashKeyGetter);
        Method rangeKeyGetter = reflector.getRangeKeyGetter(clazz);
        String rangeKeyName = rangeKeyGetter == null ? null : reflector.getAttributeName(rangeKeyGetter);
        return untransformAttributes(hashKeyName, rangeKeyName, attributeValues);
    }

    /**
     * Transforms the attribute values after loading from DynamoDb.
     * Only ever called by {@link #untransformAttributes(Class, Map)}.
     * By default, returns the attributes unchanged.
     *
     * @param hashKey the attribute name of the hash key
     * @param rangeKey the attribute name of the range key (or null if there is none)
     * @param attributeValues
     * @return the decrypted attributes
     */
    protected Map<String, AttributeValue> untransformAttributes(String hashKey, String rangeKey,
            Map<String, AttributeValue> attributeValues) {
        return attributeValues;
    }

    /**
     * By default, just calls {@link #transformAttributes(String, String, Map)}.
     * @param clazz
     * @param attributeValues
     * @return the decrypted attribute values
     */
    protected Map<String, AttributeValue> transformAttributes(Class<?> clazz, Map<String, AttributeValue> attributeValues) {
        Method hashKeyGetter = reflector.getHashKeyGetter(clazz);
        String hashKeyName = reflector.getAttributeName(hashKeyGetter);
        Method rangeKeyGetter = reflector.getRangeKeyGetter(clazz);
        String rangeKeyName = rangeKeyGetter == null ? null : reflector.getAttributeName(rangeKeyGetter);
        return transformAttributes(hashKeyName, rangeKeyName, attributeValues);
    }

    /**
     * Transform attribute values prior to storing in DynamoDB.
     * Only ever called by {@link #transformAttributes(Class, Map)}.
     * By default, returns the attributes unchanged.
     *
     * @param hashKey the attribute name of the hash key
     * @param rangeKey the attribute name of the range key (or null if there is none)
     * @param attributeValues
     * @return the encrypted attributes
     */
    protected Map<String, AttributeValue> transformAttributes(String hashKey, String rangeKey,
            Map<String, AttributeValue> attributeValues) {
        return attributeValues;
    }

    private Map<String, AttributeValueUpdate> transformAttributeUpdates(Class<?> clazz,
            Map<String, AttributeValue> keys,
            Map<String, AttributeValueUpdate> updateValues) {
        Map<String, AttributeValue> item = convertToItem(updateValues);

        HashSet<String> keysAdded = new HashSet<String>();
        for (Map.Entry<String, AttributeValue> e : keys.entrySet()) {
            if (!item.containsKey(e.getKey())) {
                keysAdded.add(e.getKey());
                item.put(e.getKey(), e.getValue());
            }
        }
        boolean hashKeyAdded = false;
        boolean rangeKeyAdded = false;
        String hashKey = reflector.getAttributeName(reflector.getHashKeyGetter(clazz));
        if (!item.containsKey(hashKey)) {
            item.put(hashKey, keys.get(hashKey));
            hashKeyAdded = true;
        }

        item = transformAttributes(clazz, item);

        // Remove the keys if we added them before.
        for (String key : keysAdded) {
            item.remove(key);
        }

        for(String key: item.keySet()) {
            if (updateValues.containsKey(key)) {
                updateValues.get(key).getValue()
                    .withB(item.get(key).getB())
                    .withBS(item.get(key).getBS())
                    .withN(item.get(key).getN())
                    .withNS(item.get(key).getNS())
                    .withS(item.get(key).getS())
                    .withSS(item.get(key).getSS());
            } else {
                updateValues.put(key, new AttributeValueUpdate(item.get(key), "PUT"));
            }
        }
        return updateValues;
    }

    private void pauseExponentially(int retries) {
        if (retries == 0) {
            return;
        }

        Random random = new Random();
        long delay = 0;
        long scaleFactor = 500 + random.nextInt(100);
        delay = (long) (Math.pow(2, retries) * scaleFactor);
        delay = Math.min(delay, MAX_BACKOFF_IN_MILLISECONDS);

        try {
            Thread.sleep(delay);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            throw new AmazonClientException(e.getMessage(), e);
        }
    }

    static <X extends AmazonWebServiceRequest> X applyUserAgent(X request) {
        request.getRequestClientOptions().addClientMarker(USER_AGENT);
        return request;
    }

    /**
     * The return type of batchWrite, batchDelete and batchSave. It contains the information about the unprocessed items
     * and the exception causing the failure.
     *
     */
    public static class FailedBatch {

        private Map<String, java.util.List<WriteRequest>> unprocessedItems;

        private Exception exception;

        public void setUnprocessedItems(Map<String, java.util.List<WriteRequest>> unprocessedItems) {
            this.unprocessedItems = unprocessedItems;
        }

        public Map<String, java.util.List<WriteRequest>> getUnprocessedItems() {
            return unprocessedItems;
        }

        public void setException(Exception excetpion) {
            this.exception = excetpion;
        }

        public Exception getException() {
            return exception;
        }

    }
}