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Log能够把代码运行时间，类名，方法名，还有信息，全部都打印出来。
一个直观的例子，每次启动Tomcat（缺省配置）的时候。一般可以看到
Jul 9, 2004 11:22:29 AM org.apache.struts.util.PropertyMessageResources <init>
INFO: Initializing, config='org.apache.webapp.admin.ApplicationResources', returnNull=true
Jul 9, 2004 11:22:41 AM org.apache.coyote.http11.Http11Protocol start
INFO: Starting Coyote HTTP/1.1 on port 8080

时间，类名，方法名，信息都打印了出来。
那么，log是如何获取调用自身的这个类和这个方法名的呢？

后面给出的代码是JDK1.4的源代码，和Log4J的源代码。说明其实现原理。
获得调用类，和方法名，就是需要获得当前运行栈的结构。
new Throwable().getStackTrace() 会返回当前运行栈的结构层次。
利用这种原理，可以获得整个运行栈的调用关系。

JDK1.4的java.util.logging包， 通过Throwable.getStackTrace()方法实现的。
// Get the stack trace.
StackTraceElement stack[] = (new Throwable()).getStackTrace();

完整的代码在JDK1.4的源代码里面，java.util.logging.LogRecord类的inferCaller方法。
Java代码

// Private method to infer the caller's class and method names private void inferCaller() { needToInferCaller = false; // Get the stack trace. StackTraceElement stack[] = (new Throwable()).getStackTrace(); // First, search back to a method in the Logger class. int ix = 0; while (ix < stack.length) { StackTraceElement frame = stack[ix]; String cname = frame.getClassName(); if (cname.equals("java.util.logging.Logger")) { break; } ix++; } // Now search for the first frame before the "Logger" class. while (ix < stack.length) { StackTraceElement frame = stack[ix]; String cname = frame.getClassName(); if (!cname.equals("java.util.logging.Logger")) { // We've found the relevant frame. setSourceClassName(cname); setSourceMethodName(frame.getMethodName()); return; } ix++; } // We haven't found a suitable frame, so just punt. This is // OK as we are only commited to making a "best effort" here. } // Private method to infer the caller's class and method names private void inferCaller() { needToInferCaller = false; // Get the stack trace. StackTraceElement stack[] = (new Throwable()).getStackTrace(); // First, search back to a method in the Logger class. int ix = 0; while (ix < stack.length) { StackTraceElement frame = stack[ix]; String cname = frame.getClassName(); if (cname.equals("java.util.logging.Logger")) { break; } ix++; } // Now search for the first frame before the "Logger" class. while (ix < stack.length) { StackTraceElement frame = stack[ix]; String cname = frame.getClassName(); if (!cname.equals("java.util.logging.Logger")) { // We've found the relevant frame. setSourceClassName(cname); setSourceMethodName(frame.getMethodName()); return; } ix++; } // We haven't found a suitable frame, so just punt. This is // OK as we are only commited to making a "best effort" here. }

Log4j有异曲同工之妙。
org.apache.log4j.spi.LocationInfo类。
先用Throwable.printStackTrace()方法把Exception信息打印到一个字符串里。
然后按行分析这个字符串。抽出调用类和方法。参见LocationInfo类的构造函数。

Java代码
/** Instantiate location information based on a Throwable. We expect the Throwable <code>t</code>, to be in the format <pre> java.lang.Throwable ... at org.apache.log4j.PatternLayout.format(PatternLayout.java:413) at org.apache.log4j.FileAppender.doAppend(FileAppender.java:183) at org.apache.log4j.Category.callAppenders(Category.java:131) at org.apache.log4j.Category.log(Category.java:512) at callers.fully.qualified.className.methodName(FileName.java:74) ... </pre> <p>However, we can also deal with JIT compilers that "lose" the location information, especially between the parentheses. */ public LocationInfo(Throwable t, String fqnOfCallingClass) /** Instantiate location information based on a Throwable. We expect the Throwable <code>t</code>, to be in the format <pre> java.lang.Throwable ... at org.apache.log4j.PatternLayout.format(PatternLayout.java:413) at org.apache.log4j.FileAppender.doAppend(FileAppender.java:183) at org.apache.log4j.Category.callAppenders(Category.java:131) at org.apache.log4j.Category.log(Category.java:512) at callers.fully.qualified.className.methodName(FileName.java:74) ... </pre> <p>However, we can also deal with JIT compilers that "lose" the location information, especially between the parentheses. */ public LocationInfo(Throwable t, String fqnOfCallingClass)

e.printStackTrace()把Exception发生当时的整个运行栈结构展开，打印出来。
Log4J就是分析这个打印结果，获得所有的调用层次。

关于直接获取调用类名的方法。
我们来看sun.reflect.Reflection的getCallerClass()方法的说明。

Java代码

/** Returns the class of the method <code>realFramesToSkip</code> frames up the stack (zero-based), ignoring frames associated with java.lang.reflect.Method.invoke() and its implementation. The first frame is that associated with this method, so <code>getCallerClass(0)</code> returns the Class object for sun.reflect.Reflection. Frames associated with java.lang.reflect.Method.invoke() and its implementation are completely ignored and do not count toward the number of "real" frames skipped. */ public static native Class getCallerClass(int realFramesToSkip); /** Returns the class of the method <code>realFramesToSkip</code> frames up the stack (zero-based), ignoring frames associated with java.lang.reflect.Method.invoke() and its implementation. The first frame is that associated with this method, so <code>getCallerClass(0)</code> returns the Class object for sun.reflect.Reflection. Frames associated with java.lang.reflect.Method.invoke() and its implementation are completely ignored and do not count toward the number of "real" frames skipped. */ public static native Class getCallerClass(int realFramesToSkip);

这是一个native方法。原理也是根据StackFrame（运行栈）获取相应类的信息。这个方法直接返回一个Class名字，直接有效。参数realFramesToSkip用来选取你所需要的Stack层次，所以，你可以用这个方法获得任何层次的上的调用类名。

Throwable.getStackTrace()也是一个native方法。原理也是根据StackFrame（运行栈）获取相应类的信息。返回一个StackTraceElement[]。
StackTraceElement类在JDK1.4的java.lang的包里面。里面包含丰富的信息，非常适合Debug。
StackTraceElement类有如下方法：
getFileName(),getLineNumber(), getClassName(), getMethodName()。