Log信息获取调用类名和调用方法名的实现原理

恰好看到关于log的讨论。想起以前调查的一个问题。整理出来，希望对大家能有所帮助。

Sun JDK 源代码下载 http://wwws.sun.com/software/communitysource/
先注册并登录到“Sun Community Source Licensing”，然后下载J2SE（几十兆）或者J2EE（几百兆）。

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()。