log4j源码学习

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logging service

说明

分析的版本是1.2.17,跟最新的log4j2可能有较大出入.下载地址
解压缩后,直接导入成maven工程即可.

代码结构

Log4J核心的类：

  • Logger:最核心的类, Most logging operations, except configuration, are done through this class.
  • Level:日志级别
  • Appender:决定日志输出的目的地
  • Layout:日志格式

测试代码

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import org.apache.log4j.BasicConfigurator;
import org.apache.log4j.Logger;

public class Test {

    public static Logger logger = Logger.getLogger(Test.class);

    public static void main(String[] args) throws Exception {
        BasicConfigurator.configure();
        logger.debug("Hello world.");
        logger.info("What a beatiful day.");
    }

}

输出:

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0 [main] DEBUG log4jtest.Test  - Hello world.
0 [main] INFO log4jtest.Test  - What a beatiful day.

这是最简单的测试代码,不需要配置文件.

配置文件加载

更多情况下,我们需要使用自己的配置文件,那配置文件又是怎么加载的呢?其实也很简单,在启动的时候org.apache.log4j.LogManager中的static方法会加载log4j的配置文件,如果指定了log4j.configuration,则从指定的文件加载,否则就先读取log4j.xml格式的配置文件,如果没有再读取log4j.properties的文件,如果需要测试此特性,可以在运行的时候加上-Dlog4j.configuration=myLog4jConfig.properties即可. 

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static {
    // By default we use a DefaultRepositorySelector which always returns 'h'.
    Hierarchy h = new Hierarchy(new RootLogger((Level) Level.DEBUG));
    repositorySelector = new DefaultRepositorySelector(h);
    /** Search for the properties file log4j.properties in the CLASSPATH. */
    String override = OptionConverter.getSystemProperty(DEFAULT_INIT_OVERRIDE_KEY, null);

    // if there is no default init override, then get the resource
    // specified by the user or the default config file.
    if (override == null || "false".equalsIgnoreCase(override)) {

        String configurationOptionStr = OptionConverter.getSystemProperty(DEFAULT_CONFIGURATION_KEY, null);
        String configuratorClassName = OptionConverter.getSystemProperty(CONFIGURATOR_CLASS_KEY, null);
        URL url = null;
        // if the user has not specified the log4j.configuration
        // property, we search first for the file "log4j.xml" and then
        // "log4j.properties"
        if (configurationOptionStr == null) {
            url = Loader.getResource(DEFAULT_XML_CONFIGURATION_FILE);
            if (url == null) {
                url = Loader.getResource(DEFAULT_CONFIGURATION_FILE);
            }
        } else {
            try {
                url = new URL(configurationOptionStr);
            } catch (MalformedURLException ex) {
                // so, resource is not a URL:
                // attempt to get the resource from the class path
                url = Loader.getResource(configurationOptionStr);
            }
        }
        // If we have a non-null url, then delegate the rest of the
        // configuration to the OptionConverter.selectAndConfigure
        // method.
        if (url != null) {
            LogLog.debug("Using URL [" + url + "] for automatic log4j configuration.");
            try {
                OptionConverter.selectAndConfigure(url, configuratorClassName, LogManager.getLoggerRepository());
            } catch (NoClassDefFoundError e) {
                LogLog.warn("Error during default initialization", e);
            }
        } else {
            LogLog.debug("Could not find resource: [" + configurationOptionStr + "].");
        }
    } else {
        LogLog.debug("Default initialization of overridden by " + DEFAULT_INIT_OVERRIDE_KEY + "property.");
    }
}

不管是从哪个入口进入,最终会调用org.apache.log4j.spi.Configurator.doConfigure()方法.

Spring中的初始化

在很多情况下是结合spring使用的,在web.xml中的配置文件如下:

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<context-param>
    <param-name>log4jConfigLocation</param-name>
    <param-value>classpath:log4j.xml</param-value>
</context-param>

<listener>
    <listener-class>org.springframework.web.util.Log4jConfigListener</listener-class>
</listener>

从源代码中可以看出就是调用log4j的PropertyConfigurator.configure方法. 

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public static void initLogging(String location) throws FileNotFoundException {
    String resolvedLocation = SystemPropertyUtils.resolvePlaceholders(location);
    URL url = ResourceUtils.getURL(resolvedLocation);
    if (ResourceUtils.URL_PROTOCOL_FILE.equals(url.getProtocol()) && !ResourceUtils.getFile(url).exists()) {
      throw new FileNotFoundException("Log4j config file [" + resolvedLocation + "] not found");
    }

    if (resolvedLocation.toLowerCase().endsWith(XML_FILE_EXTENSION)) {
      DOMConfigurator.configure(url);
    }
    else {
      PropertyConfigurator.configure(url);
    }
}

如何工作

当我们调用logger.info("msg")的时候,到底是怎么发生的呢?
拿最简单的org.apache.log4j.ConsoleAppender举例子

第一步:org.apache.log4j.Category.info(Object)

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public void info(Object message) {
        if (repository.isDisabled(Level.INFO_INT)) 
           return;
        if (Level.INFO.isGreaterOrEqual(this.getEffectiveLevel())) 
           forcedLog(FQCN, Level.INFO, message, null);
    }

第二步:一直会调用父类中的c.aai.appendLoopOnAppenders(event);,除非把c.additive设置成false,默认为true

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protected void forcedLog(String fqcn, Priority level, Object message, Throwable t) {
    callAppenders(new LoggingEvent(fqcn, this, level, message, t));
}
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);
    }
}

第三步:

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public int appendLoopOnAppenders(LoggingEvent event) {
    int size = 0;
    Appender appender;
    if (appenderList != null) {
        size = appenderList.size();
        for (int i = 0; i < size; i++) {
            appender = (Appender) appenderList.elementAt(i);
            appender.doAppend(event);
        }
    }
    return size;
}

第四步:调用真正的logger的doAppend方法

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// Reminder: the nesting of calls is:
//
//    doAppend()
//      - check threshold
//      - filter
//      - append();
//        - checkEntryConditions();
//        - subAppend();
  public void append(LoggingEvent event) {
      if (!checkEntryConditions()) {
          return;
      }
      subAppend(event);
  }
//由于ConsoleAppender没有继承subAppend方法,所以实际就是调用了WriterAppender的subAppend方法
    protected void subAppend(LoggingEvent event) {
      this.qw.write(this.layout.format(event));
      if (layout.ignoresThrowable()) {
          String[] s = event.getThrowableStrRep();
          if (s != null) {
              int len = s.length;
              for (int i = 0; i < len; i++) {
                  this.qw.write(s[i]);
                  this.qw.write(Layout.LINE_SEP);
              }
          }
      }
      if (shouldFlush(event)) {
          this.qw.flush();
      }
  }
  //其中的qw由ConsoleAppender初始化
  public void activateOptions() {
      if (follow) {
          if (target.equals(SYSTEM_ERR)) {
             setWriter(createWriter(new SystemErrStream()));
          } else {
             setWriter(createWriter(new SystemOutStream()));
          }
      } else {
          if (target.equals(SYSTEM_ERR)) {
             setWriter(createWriter(System.err));
          } else {
             setWriter(createWriter(System.out));
          }
      }

      super.activateOptions();
}

通过这几个步骤就能把日志打印出来了.