JAVA进阶－－线程运行栈信息的获取讲解

        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)

        ...

*/

public LocationInfo(Throwable t, String fqnOfCallingClass) {

String s;

…

t.printStackTrace(pw);

s = sw.toString();

sw.getBuffer().setLength(0);

…. // 这里的代码省略

}

[/code]

这里我们可以看到整体的实现思路。

首先，t.printStackTrace(pw); 获得stack trace字符串。这个t是 new Throwable()的结果。用户程序调用Log4J方法之后，Log4J自己又进行了4次调用，然后才获得了 t = new 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)

那么，往下走4行，就可以回到用户程序本身的调用信息：

at callers.fully.qualified.className.methodName(FileName.java:74)

这一行里面，类名、方法名、文件名、行号等信息全有了。解析这一行，就可以获得需要的所有信息。

三、JDK1.4 Log的相关实现
Log4J大获成功，Sun决定在JDK1.4中引入这个Log功能。

为了免去解析StackTrace字符串的麻烦，JDK1.4引入了一个新的类，StackTraceElement。

public final class StackTraceElement implements java.io.Serializable {

// Normally initialized by VM (public constructor added in 1.5)

private String declaringClass;

private String methodName;

private String fileName;

private int lineNumber;

可以看到，恰好包括类名、方法名、文件名、行号等信息。

我们来看JDK1.4 Log的相关实现。

LocationInfo.java 的infoCaller方法（推算调用者）

// Private method to infer the callers class and method names

private void inferCaller() {

…

// Get the stack trace.

StackTraceElement stack[] = (new Throwable()).getStackTrace();

// First, search back to a method in the Logger class.

…. // 这里的代码省略

// 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"))

// Weve found the relevant frame.

… // 这里的代码省略

}

// We haven found a suitable frame, so just punt. This is

// OK as we are only committed to making a "best effort" here.

}

从注释中就可以看出实现思路。过程和Log4J异曲同工。只是免去了解析字符串的麻烦。

四、Log4J 1.3 alpha的相关实现
既然JDK1.4中引入了StackTraceElement类，Log4J也要与时俱进。LocationInfo类也有了相应的变化。

/**

Instantiate location information based on a Throwable. We

expect the Throwable t, to be in the format

        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)

        ...

However, we can also deal with JIT compilers that "lose" the

location information, especially between the parentheses.

*/

public LocationInfo(Throwable t, String fqnOfInvokingClass) {

if(PlatformInfo.hasStackTraceElement()) {

StackTraceElementExtractor.extract(this, t, fqnOfInvokingClass);

} else {

LegacyExtractor.extract(this, t, fqnOfInvokingClass);

}

}

可以看到，Log4J首先判断Java平台是否支持StackTraceElement，如果是，那么用StackTraceElementExtractor，否则使用原来的LegacyExtractor。

下面来看StackTraceElementExtractor.java

/**

* A faster extractor based on StackTraceElements introduced in JDK 1.4.

*

* The present code uses reflection. Thus, it should compile on all platforms.

*

* @author Martin Schulz

* @author Ceki Gülcü

*

*/

public class StackTraceElementExtractor {

protected static boolean haveStackTraceElement = false;

private static Method getStackTrace = null;

private static Method getClassName = null;

private static Method getFileName = null;

private static Method getMethodName = null;

private static Method getLineNumber = null;

…. // 以下代码省略

可以看到，Log4J 1.3仍然兼容JDK1.3，而且为JDK1.4也做了相应的优化。

五、JDK1.5的Thread Stack Trace
JDK1.5在Thread类里面引入了getStackTrace()和getAllStackTraces()两个方法。这下子，我们不用 (new Throwable()).getStackTrace ();可以调用

Thread.getCurrentThread().getStackTrace()来获得当前线程的运行栈信息。不仅如此，只要权限允许，还可以获得其它线程的运行栈信息。

/**

* Returns an array of stack trace elements representing the stack dump

* of this thread. This method will return a zero-length array if

* this thread has not started or has terminated.

* If the returned array is of non-zero length then the first element of

* the array represents the top of the stack, which is the most recent

* method invocation in the sequence. The last element of the array

* represents the bottom of the stack, which is the least recent method

* invocation in the sequence.

*

*

If there is a security manager, and this thread is not

* the current thread, then the security managers

* checkPermission method is called with a

* RuntimePermission("getStackTrace") permission

* to see if its ok to get the stack trace.

*

*

Some virtual machines may, under some circumstances, omit one

* or more stack frames from the stack trace. In the extreme case,

* a virtual machine that has no stack trace information concerning

* this thread is permitted to return a zero-length array from this

* method.

*

* @return an array of StackTraceElement,

* each represents one stack frame.

*

* @since 1.5

*/

public StackTraceElement[] getStackTrace() {

if (this != Thread.currentThread()) {

// check for getStackTrace permission

SecurityManager security = System.getSecurityManager();

if (security != null) {

security.checkPermission(

SecurityConstants.GET_STACK_TRACE_PERMISSION);

}

}

if (!isAlive()) {

return EMPTY_STACK_TRACE;

}

Thread[] threads = new Thread[1];

threads[0] = this;

StackTraceElement[][] result = dumpThreads(threads);

return result[0];

}

/**

* Returns a map of stack traces for all live threads.

*

* @since 1.5

*/

public static Map getAllStackTraces() {

// check for getStackTrace permission

// Get a snapshot of the list of all threads

}

六、总结
从总的发展趋势来看，JDK不仅提供越来越多、越来越强的功能，而且暴露给用户的控制方法越来越多，越来越强大。