Log4j2 AsyncAppenderEventDispatcher线程冻结，疑似死锁？是否为误报？

告警信息 #

YourKit检测到Log4j2的AsyncAppenderEventDispatcher线程存在潜在死锁，告警栈如下：

Potential deadlock: frozen threads found

It seems that the following threads have not changed their stack for more than 10 seconds.
These threads are possibly (but not necessarily!) in a deadlock or hung.

+--------------------------------------------------------------------------------------------------------------------------------+
|                                                              Name                                                              |
+--------------------------------------------------------------------------------------------------------------------------------+
|  +---Log4j2-AsyncAppenderEventDispatcher-2-Async Frozen for at least 9m 2s <Ignore a false positive>                           |
|    |                                                                                                                           |
|    +---jdk.internal.misc.Unsafe.park(boolean, long) Unsafe.java (native)                                                       |
|    |                                                                                                                           |
|    +---java.util.concurrent.locks.LockSupport.park(Object) LockSupport.java:194                                                |
|    |                                                                                                                           |
|    +---java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await() AbstractQueuedSynchronizer.java:2081      |
|    |                                                                                                                           |
|    +---java.util.concurrent.ArrayBlockingQueue.take() ArrayBlockingQueue.java:417                                              |
|    |                                                                                                                           |
|    +---org.apache.logging.log4j.core.appender.AsyncAppenderEventDispatcher.dispatchAll() AsyncAppenderEventDispatcher.java:71  |
|    |                                                                                                                           |
|    +---org.apache.logging.log4j.core.appender.AsyncAppenderEventDispatcher.run() AsyncAppenderEventDispatcher.java:63          |
+--------------------------------------------------------------------------------------------------------------------------------+

Log4j2 配置 #

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="INFO" packages="com.mycompany.logging">

    <Appenders>

        <RollingFile name="RollingJson"
                     fileName="logs/application.log.json" filePattern="logs/application.log.json.%i"
                     bufferedIO="true" bufferSize="1024">
            <MyCompanyJsonLayout/>
            <Policies>
                <SizeBasedTriggeringPolicy size="1MB"/>
            </Policies>
            <DefaultRolloverStrategy max="100"/>
        </RollingFile>

        <Async name="Async">
            <AppenderRef ref="RollingJson"/>
        </Async>

    </Appenders>

    <Loggers>
        <Root level="${env:LOG_LEVEL:-INFO}">
            <AppenderRef ref="Async"/>
        </Root>
    </Loggers>

</Configuration>

告警分析与排查建议 #

这大概率是误报 #

从线程栈可以看出，线程卡在ArrayBlockingQueue.take()方法上——这是Log4j2 AsyncAppender的正常 idle 行为：当异步队列中没有待处理的日志事件时，Dispatcher线程会阻塞等待新的日志事件进入队列，并非死锁或挂起。

YourKit的告警逻辑是基于线程栈长时间未变化触发的，但这种阻塞等待是设计预期的，不属于异常情况。

进一步排查确认 #

• 检查对应时间段的应用日志输出情况：如果这段时间应用确实没有生成日志，那可以直接确认是误报；如果有日志生成但未被写入文件，再深入排查
• 检查AsyncAppender的队列配置：默认队列容量为1024，可确认是否存在队列满导致的生产者阻塞（但当前栈是消费者线程在take()，所以队列空的可能性更大）
• 验证自定义MyCompanyJsonLayout的逻辑：如果Layout在处理日志时存在阻塞或死锁，会影响日志处理，但当前线程栈显示Dispatcher并未在处理日志，而是等待队列事件，因此该点优先级较低

内容的提问来源于stack exchange，提问作者sp00m