Logback: the reliable, generic, fast and flexible logging framework.
Copyright (C) 1999-2015, QOS.ch. All rights reserved.
This program and the accompanying materials are dual-licensed under
either the terms of the Eclipse Public License v1.0 as published by
the Eclipse Foundation
  or (per the licensee's choosing)
under the terms of the GNU Lesser General Public License version 2.1
as published by the Free Software Foundation.
/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.classic.sift;

import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.sift.AbstractDiscriminator;
import ch.qos.logback.core.util.OptionHelper;

import java.util.Map;

MDCBasedDiscriminator essentially returns the value mapped to an MDC key. If
the said value is null, then a default value is returned.

Both Key and the DefaultValue are user specified properties.
Author:
Ceki Gülcü/**
 * MDCBasedDiscriminator essentially returns the value mapped to an MDC key. If
 * the said value is null, then a default value is returned.
 * <p/>
 * <p>Both Key and the DefaultValue are user specified properties.
 *
 * @author Ceki G&uuml;lc&uuml;
 */
public class MDCBasedDiscriminator extends AbstractDiscriminator<ILoggingEvent> {

    private String key;
    private String defaultValue;

    
Return the value associated with an MDC entry designated by the Key
property. If that value is null, then return the value assigned to the
DefaultValue property.
/**
     * Return the value associated with an MDC entry designated by the Key
     * property. If that value is null, then return the value assigned to the
     * DefaultValue property.
     */
    public String getDiscriminatingValue(ILoggingEvent event) {
        // http://jira.qos.ch/browse/LBCLASSIC-213
        Map<String, String> mdcMap = event.getMDCPropertyMap();
        if (mdcMap == null) {
            return defaultValue;
        }
        String mdcValue = mdcMap.get(key);
        if (mdcValue == null) {
            return defaultValue;
        } else {
            return mdcValue;
        }
    }

    @Override
    public void start() {
        int errors = 0;
        if (OptionHelper.isEmpty(key)) {
            errors++;
            addError("The \"Key\" property must be set");
        }
        if (OptionHelper.isEmpty(defaultValue)) {
            errors++;
            addError("The \"DefaultValue\" property must be set");
        }
        if (errors == 0) {
            started = true;
        }
    }

    public String getKey() {
        return key;
    }

    public void setKey(String key) {
        this.key = key;
    }

    
See Also:
setDefaultValue(String)
Returns:
/**
     * @return

     * @see #setDefaultValue(String)
     */
    public String getDefaultValue() {
        return defaultValue;
    }

    
The default MDC value in case the MDC is not set for mdcKey. 

 For example, if Key is set to the value "someKey", and the MDC is not set for "someKey", then this appender will use the default value, which you can set with the help of this method. 
Params:
defaultValue – /**
     * The default MDC value in case the MDC is not set for
     * {@link #setKey(String) mdcKey}.
     * <p/>
     * <p> For example, if {@link #setKey(String) Key} is set to the value
     * "someKey", and the MDC is not set for "someKey", then this appender will
     * use the default value, which you can set with the help of this method.
     *
     * @param defaultValue
     */
    public void setDefaultValue(String defaultValue) {
        this.defaultValue = defaultValue;
    }
}