用了这么久的Log4j这次倒下了，而且官方也还没有给出解决方案。

描述：tomcat 经过一天多时间的访问，出现了hang ，使用 Jstack 查看堆栈后，发现现成 blocked ，主要是 Log4j 的ROOTlock了。

"http-nio-2180-exec-9" #35 daemon prio=5 os_prio=0 tid=0x0000000021a8b000 nid=0x12a8 runnable [0x000000002e5ee000]
   java.lang.Thread.State: RUNNABLE
    at java.io.FileOutputStream.writeBytes(Native Method)
    at java.io.FileOutputStream.write(Unknown Source)
    at java.io.BufferedOutputStream.write(Unknown Source)
    - locked <0x00000006c0cf7380> (a java.io.BufferedOutputStream)
    at java.io.PrintStream.write(Unknown Source)
    - locked <0x00000006c0cf7360> (a java.io.PrintStream)
    at org.apache.tomcat.util.log.SystemLogHandler.write(SystemLogHandler.java:172)
    at sun.nio.cs.StreamEncoder.writeBytes(Unknown Source)
    at sun.nio.cs.StreamEncoder.implFlushBuffer(Unknown Source)
    at sun.nio.cs.StreamEncoder.implFlush(Unknown Source)
    at sun.nio.cs.StreamEncoder.flush(Unknown Source)
    - locked <0x00000006c0eb2428> (a java.io.OutputStreamWriter)
    at java.io.OutputStreamWriter.flush(Unknown Source)
    at org.apache.log4j.helpers.QuietWriter.flush(QuietWriter.java:59)
    at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:324)
    at org.apache.log4j.WriterAppender.append(WriterAppender.java:162)
    at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
    - locked <0x00000006c0eaf3b8> (a org.apache.log4j.ConsoleAppender)
    at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
    at org.apache.log4j.Category.callAppenders(Category.java:206)
    - locked <0x00000006c0ea85c8> (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.apache.commons.logging.impl.Log4JLogger.debug(Log4JLogger.java:155)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:861)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:85)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:617)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:518)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1091)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:668)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1527)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1484)
    - locked <0x00000006c27752a8> (a org.apache.tomcat.util.net.NioChannel)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

被锁住的线程堆栈：

"http-nio-2180-exec-10" #36 daemon prio=5 os_prio=0 tid=0x0000000021a8c000 nid=0xcfc waiting for monitor entry [0x000000002e49e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at org.apache.log4j.Category.callAppenders(Category.java:204)
    - waiting to lock <0x00000006c0ea85c8> (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.apache.commons.logging.impl.Log4JLogger.debug(Log4JLogger.java:155)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:861)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:966)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:857)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:842)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:85)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:617)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:518)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1091)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:668)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1527)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1484)
    - locked <0x00000006c2723a10> (a org.apache.tomcat.util.net.NioChannel)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)

查了很多资料，比较全的是这个老兄回答的：

http://stackoverflow.com/questions/3537870/production-settings-file-for-log4j/

第一个回答的截取在这里，备份下，解决方案他也提到了，不过只是减缓，没有完全解决。

Log4j 1.2 is vulnerable to deadlocks when toString() produces nested logging.

See old aged still unresolved issues like Log4J can create deadlock conditions (concurrent package donation) and Deadlock with RollingFileAppender.

It also has performance-killing lock synchronization issues under heavy concurrent load. LikeCategory callAppenders synchronization causes java.lang.Thread.State: BLOCKED and Move org.apache.log4j.Category to reentrant read/write locks.

Even AsyncAppender is not free of excessive locks: AsyncAppender.doAppend() does not need to be synchronized and Deadlock in 1.2.15 caused by AsyncAppender and ThrowableInformation classes. Also beware of AsyncAppender overflow.

One caveat is to always constrain root category level to at least INFO or higher. This would prevent logging calls from acquiring unnecessary locks mentioned in the above issues. Just limiting appender threshold is not sufficient as it is taken into account later. See explanation with publish/subscribe analogy. Nested categories of interest can be individually given lower priorities if needed.