log4j 异步配置

最近一段时间，在跟进一个项目的性能优化时，发现当系统qps达到一定程度时，会有大量线程阻塞在写日志的阶段，打印线程栈，阻塞信息如下：

"pool-60-thread-7" #820 prio=5 os_prio=0 tid=0x00007f67fc007000 nid=0x3b4 waiting for monitor entry [0x00007f6747bf9000]
   java.lang.Thread.State:BLOCKED (on object monitor)
        at org.apache.log4j.Category.callAppenders(Category.java:204)
        - waiting to lock <0x00000000f9768ef0> (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:177)
        at org.apache.http.impl.conn.Wire.wire(Wire.java:77)
        at org.apache.http.impl.conn.Wire.input(Wire.java:115)

经过查阅，发现这是log4j同步写入导致的，结合jvisualvm,发现日志的写入占用CPU时间也不少。究其原因，主要是log4j在同步写入时，会做同步，发生问题的代码片段如下，即：

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);
    }
  }

我们的log配置采用的properties文件的方式进行的，这种方式经过加载后，默认的就是同步方式，加载过程如：

File logConfig = new File("log4j.properties");
if (logConfig.exists()) {
PropertyConfigurator.configure(logConfig.toString());
}

如要采用异步方式，需要采用xml的方式配置，并用DOMConfigurator加载配置。
异步和同步的主要区别在于，异步会设置一个缓冲区，当缓冲区写满会，才会刷新到文件，没有同步竞争的环节，效率更高。
接下来，贴一个log4j.xml的配置，备忘。

<log4j:configuration xmlns:log4j='http://jakarta.apache.org/log4j/' >

    <appender name="console" class="org.apache.log4j.DailyRollingFileAppender">
        <param name="Target" value="System.out"/>
        <param name="Threshold" value="ERROR"/>
        <param name="Append" value="true"/>
        <param name="encoding" value="UTF-8"/>
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d %5p %c:%L - %m%n"/>
        layout>
    appender>

    <appender name="debugLog" class="org.apache.log4j.DailyRollingFileAppender">
        <param name="File" value="../logs/debug.log" />
        <param name="Append" value="true" />
        <param name="BufferSize" value="8192" />
        <param name="ImmediateFlush" value="false" />
        <param name="BufferedIO" value="true" />
        <param name="Threshold" value="INFO" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d %5p %c:%L - %m%n" />
        layout>
        <param name="encoding" value="UTF-8" />
    appender>

    <appender name="errorLog" class="org.apache.log4j.DailyRollingFileAppender">
        <param name="File" value="../logs/error.log" />
        <param name="Append" value="true" />
        <param name="Threshold" value="ERROR" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d %5p %c:%L - %m%n" />
        layout>
        <param name="encoding" value="UTF-8" />
    appender>


    <appender name="async" class="org.apache.log4j.AsyncAppender">
        <appender-ref ref="debugLog"/>
    appender>

    <category name="asyncLog" additivity="false">
        <priority value="info" />
        <appender-ref ref="async" />
    category >

    
    <root>
        <priority value ="info"/>
        <appender-ref ref="async"/>
    root>
log4j:configuration>

注意其中缓冲区的设置，异步appender的设置，以及root日志的设置。
最后，加载日志的代码：

File logConfig = new File("log4j.xml");
        if (logConfig.exists()) {
            Document doc = null;
            try {
                System.out.println("User config " + logConfig);
                doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(logConfig);
                DOMConfigurator.configure(doc.getDocumentElement());
            } catch (Exception e) {
                System.out.println("read log.xml error ,exit -1");
                e.printStackTrace();
                System.exit(-1);
            }
        }