Log4J blocked 问题

链接转载：http://www.cnblogs.com/suxuan/p/5041420.html

"qtp1056944384-232" prio=10 tid=0x00007f54900d0800 nid=0x63b3 waiting for monitor entry [0x00007f54492d0000]
   java.lang.Thread.State: BLOCKED (on object monitor)
	at org.apache.log4j.Category.callAppenders(Category.java:205)
	- waiting to lock <0x00000007e81c4830> (a org.apache.log4j.spi.RootLogger)
	at org.apache.log4j.Category.forcedLog(Category.java:391)
	at org.apache.log4j.Category.log(Category.java:856)
	at org.slf4j.impl.Log4jLoggerAdapter.info(Log4jLoggerAdapter.java:368)

 总计有200多个log4j的线程在等待锁"0x00000007e81c4830"，而这把锁被谁持有呢？通过搜索，找到以下dump信息：

"qtp1056944384-218" prio=10 tid=0x00007f54800bb800 nid=0x63a5 runnable [0x00007f544a0de000]
   java.lang.Thread.State: RUNNABLE
	at java.net.SocketOutputStream.socketWrite0(Native Method)
	at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:109)
	at java.net.SocketOutputStream.write(SocketOutputStream.java:141)
	at net.logstash.log4j.SocketAppender.append(SocketAppender.java:190)
	at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
	- locked <0x00000007e8210868> (a net.logstash.log4j.SocketAppender)
	at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
	at org.apache.log4j.Category.callAppenders(Category.java:206)
	- locked <0x00000007e81c4830> (a org.apache.log4j.spi.RootLogger)
	at org.apache.log4j.Category.forcedLog(Category.java:391)
	at org.apache.log4j.Category.log(Category.java:856)
	at org.slf4j.impl.Log4jLoggerAdapter.info(Log4jLoggerAdapter.java:368)

为什么log4j会出现几百个线程等待一个锁的问题呢？log4j的Category.callAppenders源码：

  /**
     Call the appenders in the hierrachy starting at
     this.  If no appenders could be found, emit a
     warning.

     
This method calls all the appenders inherited from the
     hierarchy circumventing any evaluation of whether to log or not
     to log the particular log request.

     @param event the event to log.  */
public void callAppenders(LoggingEvent event) {
    int writes = 0;

    for(Category c = this; c != null; c=c.parent) {
      // Protected against simultaneous call to addAppender, removeAppender,...
      synchronized(c) {
        if(c.aai != null) {
            writes += c.aai.appendLoopOnAppenders(event);
        }
        if(!c.additive) {
            break;
        }
      }
    }

    if(writes == 0) {
      repository.emitNoAppenderWarning(this);
    }
  }

og4j版本1.x中，使用的是古老的synchronized(this)，所有线程共用一个Category，而它通过log4j.properties指定。 同一个Category下的线程打log时，需要进行全局同步，因此它的效率会很低，log4j 1.x版不适合高并发的场景。

为了杜绝这样的问题，后续需要吸取教训：

1. 尽量减少不必要的日志，在成熟的接口上，关闭日志输出，这样有利于提高效率。

2. 替换底层log的实现类，不再使用log4j 1.x，使用logback（推荐）或者新的log4j 2.x版本。

最后，附带两篇关于log4j 1.x中，日志系统死锁的分析（我们最开始怀疑是这个问题）：

https://bz.apache.org/bugzilla/show_bug.cgi?id=50213

http://javaeesupportpatterns.blogspot.com/2012/09/log4j-thread-deadlock-case-study.html

---------------

slf4j jar sources 下载： http://www.slf4j.org/download.html

log4j jar sources 下载: http://archive.apache.org/dist/logging/log4j/2.7/

log4j2 配置：

   http://www.cnblogs.com/morvenhuang/p/3958086.html

   官方配置：http://logging.apache.org/log4j/2.x/manual/configuration.html#AutomaticConfiguration

log4j和log4j2 分别配置http://josh-persistence.iteye.com/blog/2205021