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本文主要研究一下LogbackMDCAdapter
MDCAdapter
org/slf4j/spi/MDCAdapter.java
代码语言：javascript复制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
代码语言：javascript复制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
代码语言：javascript复制 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
代码语言：javascript复制 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
代码语言：javascript复制 @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
代码语言：javascript复制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。
