log4j源码阅读

基于log4j1.2.17的源代码阅读

org.apache.log4j.xml.DOMConfigurator 类是log4j的xml配置文件初始化类

org.apache.log4j.PropertyConfigurator 类是log4j的properties形式配置文件的初始化类

今天看的是前者

  /**
     A static version of {@link #doConfigure(String, LoggerRepository)}.  */
  static
  public
  void configure(String filename) throws FactoryConfigurationError {
    new DOMConfigurator().doConfigure(filename,
				      LogManager.getLoggerRepository());
  }

一般我们都是讲log4j.xml配置文件放到与src同目录下，方便在class path 的当前路径进行查找（不能放置到src目录里面，不然打包之后也会被打包到类目录下）

 public
  void doConfigure(final String filename, LoggerRepository repository) {
    ParseAction action = new ParseAction() {
          public Document parse(final DocumentBuilder parser) throws SAXException, IOException {
              return parser.parse(new File(filename));
          }
          public String toString() {
              return "file [" + filename + "]";
          }
    };
    doConfigure(action, repository);
  }

repositroy的作用目前还没有看到

先看下是如何解析xml和初始化相关对象的。

最终会调用DOMConfigurator 类当中的方法

/**
     Used internally to configure the log4j framework by parsing a DOM
     tree of XML elements based on <a
     href="doc-files/log4j.dtd">log4j.dtd</a>.

  */
  protected
  void parse(Element element) {

    String rootElementName = element.getTagName();

    if (!rootElementName.equals(CONFIGURATION_TAG)) {
      if(rootElementName.equals(OLD_CONFIGURATION_TAG)) {
	LogLog.warn("The <"+OLD_CONFIGURATION_TAG+
		     "> element has been deprecated.");
	LogLog.warn("Use the <"+CONFIGURATION_TAG+"> element instead.");
      } else {
	LogLog.error("DOM element is - not a <"+CONFIGURATION_TAG+"> element.");
	return;
      }
    }

    String debugAttrib = subst(element.getAttribute(INTERNAL_DEBUG_ATTR));

    LogLog.debug("debug attribute= \"" + debugAttrib +"\".");
    // if the log4j.dtd is not specified in the XML file, then the
    // "debug" attribute is returned as the empty string.
    if(!debugAttrib.equals("") && !debugAttrib.equals("null")) {
      LogLog.setInternalDebugging(OptionConverter.toBoolean(debugAttrib, true));
    } else {
      LogLog.debug("Ignoring " + INTERNAL_DEBUG_ATTR + " attribute.");
    }

      //
      //   reset repository before configuration if reset="true"
      //       on configuration element.
      //
    String resetAttrib = subst(element.getAttribute(RESET_ATTR));
    LogLog.debug("reset attribute= \"" + resetAttrib +"\".");
    if(!("".equals(resetAttrib))) {
         if (OptionConverter.toBoolean(resetAttrib, false)) {
             repository.resetConfiguration();
         }
    }

    String confDebug = subst(element.getAttribute(CONFIG_DEBUG_ATTR));
    if(!confDebug.equals("") && !confDebug.equals("null")) {
      LogLog.warn("The \""+CONFIG_DEBUG_ATTR+"\" attribute is deprecated.");
      LogLog.warn("Use the \""+INTERNAL_DEBUG_ATTR+"\" attribute instead.");
      LogLog.setInternalDebugging(OptionConverter.toBoolean(confDebug, true));
    }

    String thresholdStr = subst(element.getAttribute(THRESHOLD_ATTR));
    LogLog.debug("Threshold =\"" + thresholdStr +"\".");
    if(!"".equals(thresholdStr) && !"null".equals(thresholdStr)) {
      repository.setThreshold(thresholdStr);
    }

    //Hashtable appenderBag = new Hashtable(11);

    /* Building Appender objects, placing them in a local namespace
       for future reference */

    // First configure each category factory under the root element.
    // Category factories need to be configured before any of
    // categories they support.
    //
    String   tagName = null;
    Element  currentElement = null;
    Node     currentNode = null;
    NodeList children = element.getChildNodes();
    final int length = children.getLength();

    for (int loop = 0; loop < length; loop++) {
      currentNode = children.item(loop);
      if (currentNode.getNodeType() == Node.ELEMENT_NODE) {
	currentElement = (Element) currentNode;
	tagName = currentElement.getTagName();

	if (tagName.equals(CATEGORY_FACTORY_TAG) || tagName.equals(LOGGER_FACTORY_TAG)) {
	  parseCategoryFactory(currentElement);
	}
      }
    }

    for (int loop = 0; loop < length; loop++) {
      currentNode = children.item(loop);
      if (currentNode.getNodeType() == Node.ELEMENT_NODE) {
	currentElement = (Element) currentNode;
	tagName = currentElement.getTagName();

	if (tagName.equals(CATEGORY) || tagName.equals(LOGGER)) {
	  parseCategory(currentElement);
	} else if (tagName.equals(ROOT_TAG)) {
	  parseRoot(currentElement);
	} else if(tagName.equals(RENDERER_TAG)) {
	  parseRenderer(currentElement);
    } else if(tagName.equals(THROWABLE_RENDERER_TAG)) {
        if (repository instanceof ThrowableRendererSupport) {
            ThrowableRenderer tr = parseThrowableRenderer(currentElement);
            if (tr != null) {
                ((ThrowableRendererSupport) repository).setThrowableRenderer(tr);
            }
        }
    } else if (!(tagName.equals(APPENDER_TAG)
            || tagName.equals(CATEGORY_FACTORY_TAG)
            || tagName.equals(LOGGER_FACTORY_TAG))) {
        quietParseUnrecognizedElement(repository, currentElement, props);
    }
      }
    }
  }

通过分析这个方法我们可以看到

1）log4j.xml 中的根tag必须开头是log4j:configuration

如果不是，将会返回错误，并且同时检查是否是老的标签

OLD_CONFIGURATION_TAG ------configuration 如果是的话就打出warn日志，不是打出error日志
2）
在Log4j的源代码当中我们也发现了很多Log日志 ，在Log4j还没有加载完成的时候，利用的是什么打印的呢？
import org.apache.log4j.helpers.LogLog; 利用的就是这个内部帮助类，当我们在log4j.xml的根tag上标注debug=“true”的时候，就会发现这些log日志也打出了，主要是log4j的初始化日志，在quartz当中也可以看到类似的初始化日志