聊聊LogbackMDCAdapter

序

本文主要研究一下LogbackMDCAdapter

MDCAdapter

org/slf4j/spi/MDCAdapter.java

public interface MDCAdapter {

    /**
     * Put a context value (the <code>val</code> parameter) as identified with
     * the <code>key</code> parameter into the current thread's 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>If the current thread does not have a context map it is created as a side
     * effect of this call.
     */
    public void put(String key, String val);

    /**
     * Get the context identified by the <code>key</code> parameter.
     * The <code>key</code> parameter cannot be null.
     * 
     * @return the string value identified by the <code>key</code> parameter.
     */
    public String get(String key);

    /**
     * Remove the context identified by the <code>key</code> parameter.
     * The <code>key</code> parameter cannot be null. 
     * 
     * <p>
     * This method does nothing if there is no previous value 
     * associated with <code>key</code>.
     */
    public void remove(String key);

    /**
     * Clear all entries in the MDC.
     */
    public void 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 Map<String, String> 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 
     * parameter must only contain keys and values of type String.
     * 
     * Implementations must support null valued map passed as parameter.
     * 
     * @param contextMap must contain only keys and values of type String
     * 
     * @since 1.5.1
     */
    public void setContextMap(Map<String, String> contextMap);
    
    /**
     * Push a value into the deque(stack) referenced by 'key'.
     *      
     * @param key identifies the appropriate stack
     * @param value the value to push into the stack
     * @since 2.0.0
     */
    public void pushByKey(String key, String value);
    
    /**
     * Pop the stack referenced by 'key' and return the value possibly null.
     * 
     * @param key identifies the deque(stack)
     * @return the value just popped. May be null/
     * @since 2.0.0
     */
    public String popByKey(String key);

    /**
     * Returns a copy of the deque(stack) referenced by 'key'. May be null.
     * 
     * @param key identifies the  stack
     * @return copy of stack referenced by 'key'. May be null.
     * 
     * @since 2.0.0
     */
    public Deque<String>  getCopyOfDequeByKey(String key);
    

    /**
     * Clear the deque(stack) referenced by 'key'. 
     * 
     * @param key identifies the  stack
     * 
     * @since 2.0.0
     */
    public void clearDequeByKey(String key);
    
}

slf4j定义了MDCAdapter接口，该接口定义了put、get、remove、clear、getCopyOfContextMap、setContextMap、pushByKey、popByKey、getCopyOfDequeByKey、clearDequeByKey方法

LogbackMDCAdapter

ch/qos/logback/classic/util/LogbackMDCAdapter.java

public class LogbackMDCAdapter implements MDCAdapter  {


    // BEWARE: Keys or values placed in a ThreadLocal should not be of a type/class
    // not included in the JDK. See also https://jira.qos.ch/browse/LOGBACK-450

    final ThreadLocal<Map<String, String>> readWriteThreadLocalMap = new ThreadLocal<Map<String, String>>();
    final ThreadLocal<Map<String, String>> readOnlyThreadLocalMap = new ThreadLocal<Map<String, String>>();
    private final ThreadLocalMapOfStacks threadLocalMapOfDeques = new ThreadLocalMapOfStacks();

    //......

}      

LogbackMDCAdapter实现了MDCAdapter接口，它基于readWriteThreadLocalMap、readOnlyThreadLocalMap、threadLocalMapOfDeques来实现

readWriteThreadLocalMap

    public void put(String key, String val) throws IllegalArgumentException {
        if (key == null) {
            throw new IllegalArgumentException("key cannot be null");
        }
        Map<String, String> current = readWriteThreadLocalMap.get();
        if (current == null) {
            current = new HashMap<String, String>();
            readWriteThreadLocalMap.set(current);
        }

        current.put(key, val);
        nullifyReadOnlyThreadLocalMap();
    }
    
    @Override
    public String get(String key) {
        Map<String, String> hashMap = readWriteThreadLocalMap.get();

        if ((hashMap != null) && (key != null)) {
            return hashMap.get(key);
        } else {
            return null;
        }
    }

    @Override
    public void remove(String key) {
        if (key == null) {
            return;
        }

        Map<String, String> current = readWriteThreadLocalMap.get();
        if (current != null) {
            current.remove(key);
            nullifyReadOnlyThreadLocalMap();
        }
    }

    @Override
    public void clear() {
        readWriteThreadLocalMap.set(null);
        nullifyReadOnlyThreadLocalMap();
    }

    private void nullifyReadOnlyThreadLocalMap() {
        readOnlyThreadLocalMap.set(null);
    }  

    public void setContextMap(Map contextMap) {
        if (contextMap != null) {
            readWriteThreadLocalMap.set(new HashMap<String, String>(contextMap));
        } else {
            readWriteThreadLocalMap.set(null);
        }
        nullifyReadOnlyThreadLocalMap();
    }      

put、get、remove、clear、setContextMap都是基于readWriteThreadLocalMap，同时修改操作会同时调用nullifyReadOnlyThreadLocalMap，将readOnlyThreadLocalMap设置为null

getCopyOfContextMap

    public Map getCopyOfContextMap() {
        Map<String, String> readOnlyMap = getPropertyMap();
        if (readOnlyMap == null) {
            return null;
        } else {
            return new HashMap<String, String>(readOnlyMap);
        }
    }

    public Map<String, String> getPropertyMap() {
        Map<String, String> readOnlyMap = readOnlyThreadLocalMap.get();
        if (readOnlyMap == null) {
            Map<String, String> current = readWriteThreadLocalMap.get();
            if (current != null) {
                final Map<String, String> tempMap = new HashMap<String, String>(current);
                readOnlyMap = Collections.unmodifiableMap(tempMap);
                readOnlyThreadLocalMap.set(readOnlyMap);
            }
        }
        return readOnlyMap;
    }    

getCopyOfContextMap方法通过getPropertyMap获取，如果不为null则新创建HashMap返回；getPropertyMap先从readOnlyThreadLocalMap读取，如果readOnlyThreadLocalMap为null则从readWriteThreadLocalMap拷贝一份unmodifiableMap，并设置到readOnlyThreadLocalMap

threadLocalMapOfDeques

    @Override
    public void pushByKey(String key, String value) {
        threadLocalMapOfDeques.pushByKey(key, value);
    }

    @Override
    public String popByKey(String key) {
        return threadLocalMapOfDeques.popByKey(key);
    }

    @Override
    public Deque<String> getCopyOfDequeByKey(String key) {
        return threadLocalMapOfDeques.getCopyOfDequeByKey(key);
    }

    @Override
    public void clearDequeByKey(String key) {
        threadLocalMapOfDeques.clearDequeByKey(key);
    }

pushByKey、popByKey、getCopyOfDequeByKey、clearDequeByKey均是基于threadLocalMapOfDeques，它是ThreadLocalMapOfStacks类型

ThreadLocalMapOfStacks

org/slf4j/helpers/ThreadLocalMapOfStacks.java

public class ThreadLocalMapOfStacks {

    // BEWARE: Keys or values placed in a ThreadLocal should not be of a type/class
    // not included in the JDK. See also https://jira.qos.ch/browse/LOGBACK-450

    final ThreadLocal<Map<String, Deque<String>>> tlMapOfStacks = new ThreadLocal<>();

    public void pushByKey(String key, String value) {
        if (key == null)
            return;

        Map<String, Deque<String>> map = tlMapOfStacks.get();

        if (map == null) {
            map = new HashMap<>();
            tlMapOfStacks.set(map);
        }

        Deque<String> deque = map.get(key);
        if (deque == null) {
            deque = new ArrayDeque<>();
        }
        deque.push(value);
        map.put(key, deque);
    }

    public String popByKey(String key) {
        if (key == null)
            return null;

        Map<String, Deque<String>> map = tlMapOfStacks.get();
        if (map == null)
            return null;
        Deque<String> deque = map.get(key);
        if (deque == null)
            return null;
        return deque.pop();
    }

    public Deque<String> getCopyOfDequeByKey(String key) {
        if (key == null)
            return null;

        Map<String, Deque<String>> map = tlMapOfStacks.get();
        if (map == null)
            return null;
        Deque<String> deque = map.get(key);
        if (deque == null)
            return null;

        return new ArrayDeque<String>(deque);
    }
    
    /**
     * Clear the deque(stack) referenced by 'key'. 
     * 
     * @param key identifies the  stack
     * 
     * @since 2.0.0
     */
    public void clearDequeByKey(String key) {
        if (key == null)
            return;

        Map<String, Deque<String>> map = tlMapOfStacks.get();
        if (map == null)
            return;
        Deque<String> deque = map.get(key);
        if (deque == null)
            return;
        deque.clear();
    }

}

ThreadLocalMapOfStacks是slf4j定义的，基于ThreadLocal<Map<String, Deque<String>>>实现的

小结

slf4j定义了MDCAdapter接口，该接口定义了put、get、remove、clear、getCopyOfContextMap、setContextMap、pushByKey、popByKey、getCopyOfDequeByKey、clearDequeByKey方法；LogbackMDCAdapter实现了MDCAdapter接口，它基于readWriteThreadLocalMap、readOnlyThreadLocalMap、threadLocalMapOfDeques来实现，其中put、get、remove、clear、setContextMap都是基于readWriteThreadLocalMap，pushByKey、popByKey、getCopyOfDequeByKey、clearDequeByKey均是基于threadLocalMapOfDeques。