MDC.java

/**
 * Copyright (c) 2004-2011 QOS.ch
 * All rights reserved.
 *
 * Permission is hereby granted, free  of charge, to any person obtaining
 * a  copy  of this  software  and  associated  documentation files  (the
 * "Software"), to  deal in  the Software without  restriction, including
 * without limitation  the rights to  use, copy, modify,  merge, publish,
 * distribute,  sublicense, and/or sell  copies of  the Software,  and to
 * permit persons to whom the Software  is furnished to do so, subject to
 * the following conditions:
 *
 * The  above  copyright  notice  and  this permission  notice  shall  be
 * included in all copies or substantial portions of the Software.
 *
 * THE  SOFTWARE IS  PROVIDED  "AS  IS", WITHOUT  WARRANTY  OF ANY  KIND,
 * EXPRESS OR  IMPLIED, INCLUDING  BUT NOT LIMITED  TO THE  WARRANTIES OF
 * MERCHANTABILITY,    FITNESS    FOR    A   PARTICULAR    PURPOSE    AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 * OF CONTRACT, TORT OR OTHERWISE,  ARISING FROM, OUT OF OR IN CONNECTION
 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 */
package org.slf4j;

import java.io.Closeable;
import java.util.Map;

import org.slf4j.helpers.NOPMDCAdapter;
import org.slf4j.helpers.BasicMDCAdapter;
import org.slf4j.helpers.Util;
import org.slf4j.impl.StaticMDCBinder;
import org.slf4j.spi.MDCAdapter;

/**
 * This class hides and serves as a substitute for the underlying logging
 * system's MDC implementation.
 * 
 * <p>
 * If the underlying logging system offers MDC functionality, then SLF4J's MDC,
 * i.e. this class, will delegate to the underlying system's MDC. Note that at
 * this time, only two logging systems, namely log4j and logback, offer MDC
 * functionality. For java.util.logging which does not support MDC,
 * {@link BasicMDCAdapter} will be used. For other systems, i.e. slf4j-simple
 * and slf4j-nop, {@link NOPMDCAdapter} will be used.
 *
 * <p>
 * Thus, as a SLF4J user, you can take advantage of MDC in the presence of log4j,
 * logback, or java.util.logging, but without forcing these systems as
 * dependencies upon your users.
 * 
 * <p>
 * For more information on MDC please see the <a
 * href="http://logback.qos.ch/manual/mdc.html">chapter on MDC</a> in the
 * logback manual.
 * 
 * <p>
 * Please note that all methods in this class are static.
 * 
 * @author Ceki G&uuml;lc&uuml;
 * @since 1.4.1
 */
public class MDC {

    static final String NULL_MDCA_URL = "http://www.slf4j.org/codes.html#null_MDCA";
    static final String NO_STATIC_MDC_BINDER_URL = "http://www.slf4j.org/codes.html#no_static_mdc_binder";
    static MDCAdapter mdcAdapter;

    /**
     * An adapter to remove the key when done.
     */
    public static class MDCCloseable implements Closeable {
        private final String key;

        private MDCCloseable(String key) {
            this.key = key;
        }

        public void close() {
            MDC.remove(this.key);
        }
    }

    private MDC() {
    }

    /**
     * As of SLF4J version 1.7.14, StaticMDCBinder classes shipping in various bindings
     * come with a getSingleton() method. Previously only a public field called SINGLETON 
     * was available.
     * 
     * @return MDCAdapter
     * @throws NoClassDefFoundError in case no binding is available
     * @since 1.7.14
     */
    private static MDCAdapter bwCompatibleGetMDCAdapterFromBinder() throws NoClassDefFoundError {
        try {
            return StaticMDCBinder.getSingleton().getMDCA();
        } catch (NoSuchMethodError nsme) {
            // binding is probably a version of SLF4J older than 1.7.14
            return StaticMDCBinder.SINGLETON.getMDCA();
        }
    }

    static {
        try {
            mdcAdapter = bwCompatibleGetMDCAdapterFromBinder();
        } catch (NoClassDefFoundError ncde) {
            mdcAdapter = new NOPMDCAdapter();
            String msg = ncde.getMessage();
            if (msg != null && msg.contains("StaticMDCBinder")) {
                Util.report("Failed to load class \"org.slf4j.impl.StaticMDCBinder\".");
                Util.report("Defaulting to no-operation MDCAdapter implementation.");
                Util.report("See " + NO_STATIC_MDC_BINDER_URL + " for further details.");
            } else {
                throw ncde;
            }
        } catch (Exception e) {
            // we should never get here
            Util.report("MDC binding unsuccessful.", e);
        }
    }

    /**
     * Put a diagnostic context value (the <code>val</code> parameter) as identified with the
     * <code>key</code> parameter into the current thread's diagnostic context map. The
     * <code>key</code> parameter cannot be null. The <code>val</code> parameter
     * can be null only if the underlying implementation supports it.
     * 
     * <p>
     * This method delegates all work to the MDC of the underlying logging system.
     *
     * @param key non-null key 
     * @param val value to put in the map
     * 
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static void put(String key, String val) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key parameter cannot be null");
        }
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.put(key, val);
    }

    /**
     * Put a diagnostic context value (the <code>val</code> parameter) as identified with the
     * <code>key</code> parameter into the current thread's diagnostic context map. The
     * <code>key</code> parameter cannot be null. The <code>val</code> parameter
     * can be null only if the underlying implementation supports it.
     *
     * <p>
     * This method delegates all work to the MDC of the underlying logging system.
     * <p>
     * This method return a <code>Closeable</code> object who can remove <code>key</code> when
     * <code>close</code> is called.
     *
     * <p>
     * Useful with Java 7 for example :
     * <code>
     *   try(MDC.MDCCloseable closeable = MDC.putCloseable(key, value)) {
     *     ....
     *   }
     * </code>
     *
     * @param key non-null key
     * @param val value to put in the map
     * @return a <code>Closeable</code> who can remove <code>key</code> when <code>close</code>
     * is called.
     *
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static MDCCloseable putCloseable(String key, String val) throws IllegalArgumentException {
        put(key, val);
        return new MDCCloseable(key);
    }

    /**
     * Get the diagnostic context identified by the <code>key</code> parameter. The
     * <code>key</code> parameter cannot be null.
     * 
     * <p>
     * This method delegates all work to the MDC of the underlying logging system.
     *
     * @param key  
     * @return the string value identified by the <code>key</code> parameter.
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static String get(String key) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key parameter cannot be null");
        }

        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        return mdcAdapter.get(key);
    }

    /**
     * Remove the diagnostic context identified by the <code>key</code> parameter using
     * the underlying system's MDC implementation. The <code>key</code> parameter
     * cannot be null. This method does nothing if there is no previous value
     * associated with <code>key</code>.
     *
     * @param key  
     * @throws IllegalArgumentException
     *           in case the "key" parameter is null
     */
    public static void remove(String key) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key parameter cannot be null");
        }

        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.remove(key);
    }

    /**
     * Clear all entries in the MDC of the underlying implementation.
     */
    public static void clear() {
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.clear();
    }

    /**
     * Return a copy of the current thread's context map, with keys and values of
     * type String. Returned value may be null.
     * 
     * @return A copy of the current thread's context map. May be null.
     * @since 1.5.1
     */
    public static Map<String, String> getCopyOfContextMap() {
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        return mdcAdapter.getCopyOfContextMap();
    }

    /**
     * Set the current thread's context map by first clearing any existing map and
     * then copying the map passed as parameter. The context map passed as
     * parameter must only contain keys and values of type String.
     * 
     * @param contextMap
     *          must contain only keys and values of type String
     * @since 1.5.1
     */
    public static void setContextMap(Map<String, String> contextMap) {
        if (mdcAdapter == null) {
            throw new IllegalStateException("MDCAdapter cannot be null. See also " + NULL_MDCA_URL);
        }
        mdcAdapter.setContextMap(contextMap);
    }

    /**
     * Returns the MDCAdapter instance currently in use.
     * 
     * @return the MDcAdapter instance currently in use.
     * @since 1.4.2
     */
    public static MDCAdapter getMDCAdapter() {
        return mdcAdapter;
    }

}