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ObjectGraphTraverser.java
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ObjectGraphTraverser.java
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/**
* Copyright (C) 2022 Czech Technical University in Prague
* <p>
* This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public
* License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later
* version.
* <p>
* This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
* warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
* details. You should have received a copy of the GNU General Public License along with this program. If not, see
* <http://www.gnu.org/licenses/>.
*/
package cz.cvut.kbss.jsonld.serialization.traversal;
import cz.cvut.kbss.jsonld.common.BeanAnnotationProcessor;
import cz.cvut.kbss.jsonld.common.BeanClassProcessor;
import cz.cvut.kbss.jsonld.common.IdentifierUtil;
import cz.cvut.kbss.jsonld.exception.MissingIdentifierException;
import java.lang.reflect.Field;
import java.util.*;
/**
* Traverses the provided object graph, visiting each instance and its fields, notifying visitors of these encounters.
* <p>
* Each object is visited only once, so circular references are not a problem.
* <p>
* The traversal algorithm is depth-first in nature.
*/
public class ObjectGraphTraverser {
private final SerializationContextFactory serializationContextFactory;
private InstanceVisitor visitor;
private final InstanceTypeResolver typeResolver = new InstanceTypeResolver();
private boolean requireId = false;
private final Map<Object, String> knownInstances = new IdentityHashMap<>();
public ObjectGraphTraverser(SerializationContextFactory serializationContextFactory) {
this.serializationContextFactory = serializationContextFactory;
}
public void setVisitor(InstanceVisitor visitor) {
this.visitor = Objects.requireNonNull(visitor);
}
public void removeVisitor() {
this.visitor = null;
}
public void traverse(Object instance) {
Objects.requireNonNull(instance);
traverse(serializationContextFactory.create(instance));
}
public void traverse(SerializationContext<?> ctx) {
Objects.requireNonNull(ctx);
assert visitor != null;
if (ctx.getValue() instanceof Collection) {
traverseCollection((SerializationContext<? extends Collection<?>>) ctx);
} else {
traverseSingular(ctx);
}
}
private void traverseCollection(SerializationContext<? extends Collection<?>> ctx) {
openCollection(ctx);
for (Object item : ctx.getValue()) {
if (item == null) {
continue;
}
traverseSingular(serializationContextFactory.create(item, ctx));
}
closeCollection(ctx);
}
void traverseSingular(SerializationContext<?> ctx) {
if (ctx.getValue() == null) {
return;
}
final boolean shouldTraverse = visitInstance(ctx);
if (!shouldTraverse) {
return;
}
if (isIndividual(ctx)) {
visitIndividual(ctx);
return;
}
final boolean firstEncounter = !knownInstances.containsKey(ctx.getValue());
openInstance(ctx);
visitIdentifier(ctx);
if (firstEncounter) {
visitTypes(ctx);
serializeFields(ctx);
serializePropertiesField(ctx);
}
closeInstance(ctx);
}
private static boolean isIndividual(SerializationContext<?> ctx) {
return BeanClassProcessor.isIdentifierType(ctx.getValue().getClass()) || ctx.getValue().getClass().isEnum();
}
private void serializeFields(SerializationContext<?> ctx) {
final Object instance = ctx.getValue();
final List<Field> fieldsToSerialize =
orderAttributesForSerialization(BeanAnnotationProcessor.getSerializableFields(instance),
BeanAnnotationProcessor.getAttributeOrder(instance.getClass()));
for (Field f : fieldsToSerialize) {
if (shouldSkipFieldSerialization(f)) {
continue;
}
Object value = BeanClassProcessor.getFieldValue(f, instance);
final SerializationContext<?> fieldCtx = serializationContextFactory.createForAttribute(f, value, ctx);
visitAttribute(fieldCtx);
}
}
private boolean shouldSkipFieldSerialization(Field f) {
return BeanAnnotationProcessor.isInstanceIdentifier(f) || BeanAnnotationProcessor.isPropertiesField(
f) || BeanAnnotationProcessor.isTypesField(f);
}
private List<Field> orderAttributesForSerialization(List<Field> fields, String[] ordering) {
final List<Field> result = new ArrayList<>(fields.size());
for (String item : ordering) {
final Iterator<Field> it = fields.iterator();
while (it.hasNext()) {
final Field f = it.next();
if (f.getName().equals(item)) {
it.remove();
result.add(f);
break;
}
}
}
result.addAll(fields);
return result;
}
private void serializePropertiesField(SerializationContext<?> ctx) {
final Object instance = ctx.getValue();
if (!BeanAnnotationProcessor.hasPropertiesField(instance.getClass())) {
return;
}
final Field propertiesField = BeanAnnotationProcessor.getPropertiesField(instance.getClass());
final Object value = BeanClassProcessor.getFieldValue(propertiesField, instance);
if (value == null) {
return;
}
assert value instanceof Map;
new PropertiesTraverser(this)
.traverseProperties(
serializationContextFactory.createForProperties(propertiesField, (Map<?, ?>) value, ctx));
}
public boolean visitInstance(SerializationContext<?> ctx) {
return visitor.visitObject(ctx);
}
public void visitIndividual(SerializationContext<?> ctx) {
visitor.visitIndividual(ctx);
}
public void openInstance(SerializationContext<?> ctx) {
if (!BeanClassProcessor.isIdentifierType(ctx.getValue().getClass())) {
final String identifier = resolveIdentifier(ctx.getValue());
knownInstances.put(ctx.getValue(), identifier);
}
visitor.openObject(ctx);
}
private String resolveIdentifier(Object instance) {
final Optional<Object> extractedId = BeanAnnotationProcessor.getInstanceIdentifier(instance);
if (!extractedId.isPresent() && requireId) {
throw MissingIdentifierException.create(instance);
}
return extractedId.orElseGet(() -> knownInstances.containsKey(instance) ? knownInstances.get(instance) :
IdentifierUtil.generateBlankNodeId()).toString();
}
public void closeInstance(SerializationContext<?> ctx) {
visitor.closeObject(ctx);
}
public void visitIdentifier(SerializationContext<?> ctx) {
final Object identifier = ctx.getValue();
final Class<?> idCls = identifier.getClass();
final String id;
final SerializationContext<String> idContext;
id = resolveIdentifier(identifier);
idContext = serializationContextFactory.createForIdentifier(
BeanAnnotationProcessor.getIdentifierField(idCls).orElse(null), id, ctx);
knownInstances.put(identifier, id);
visitor.visitIdentifier(idContext);
}
public void visitTypes(SerializationContext<?> ctx) {
final Object instance = ctx.getValue();
final Set<String> resolvedTypes = typeResolver.resolveTypes(instance);
assert !resolvedTypes.isEmpty();
final SerializationContext<Set<String>> typesContext = serializationContextFactory.createForTypes(
BeanAnnotationProcessor.getTypesField(instance.getClass()).orElse(null), resolvedTypes, ctx);
visitor.visitTypes(typesContext);
}
public void visitAttribute(SerializationContext<?> ctx) {
visitor.visitAttribute(ctx);
}
public void openCollection(SerializationContext<? extends Collection<?>> ctx) {
visitor.openCollection(ctx);
}
public void closeCollection(SerializationContext<?> ctx) {
visitor.closeCollection(ctx);
}
public void setRequireId(boolean requireId) {
this.requireId = requireId;
}
}