【转帖】Java字节码揭秘(三)

原帖地址:http://blog.csdn.net/BU_BetterYou/archive/2008/06/18/2560972.aspx

接上一部份：


以下是一个简单的异常处理例子：


public class ExFun

{

        public static void main(String[] args)

        {

               try

               {

                       System.out.println("In try block");

                       throw new Exception();

               }

               catch (java.io.IOException ioEx)

               {

                       System.out.println("In catch IOException block");

                       System.out.println(ioEx);

               }

               catch (Exception ex)

               {

                       System.out.println("In catch Exception block");

                       System.out.println(ex);

               }

               finally

               {

                       System.out.println("In finally block");

               }

        }

}


字节码为：


Compiled from "ExFun.java"

public class ExFun extends java.lang.Object

        SourceFile: "ExFun.java"

        minor version: 0

        major version: 50

        Constant pool:

               (snipped for simplicity)

{

public ExFun();

        (snipped for simplicity)



public static void main(java.lang.String[]);

        Code:

               Stack=2, Locals=3, Args_size=1

               0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               3: ldc #3; //String In try block

               5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

               8: new #5; //class java/lang/Exception

               11: dup

               12: invokespecial #6; //Method java/lang/Exception."<init>":()V

               15: athrow

               16: astore_1

               17: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               20: ldc #8; //String In catch IOException block

                22: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

               25: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               28: aload_1

               29: invokevirtual #9; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V

               32: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               35: ldc #10; //String In finally block

               37: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

               40: goto 81

               43: astore_1

               44: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               47: ldc #11; //String In catch Exception block

               49: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

               52: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               55: aload_1

               56: invokevirtual #9; //Method java/io/PrintStream.println:(Ljava/lang/Object;)V

               59: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               62: ldc #10; //String In finally block

               64: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

               67: goto 81

               70: astore_2

               71: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;

               74: ldc #10; //String In finally block

               76: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

               79: aload_2

               80: athrow

               81: return

        Exception table:

               from to target type

               0 16 16 Class java/io/IOException

               0 16 43 Class java/lang/Exception

               0 32 70 any

               43 59 70 any

               70 71 70 any

        LineNumberTable: (snipped)

        StackMapTable: (snipped)

}


“try”块内的操作符是在0~16的范围内，Java源代码的“catch”块是被编在异常(Exception)表的开始两行(0(来自)~16(目标)~16(类型) java.io.IOException，0~6~43 java.lang.Exception)，catch的操作符偏移量分别是16和43。也需要注意的是，异常表的最后三行表明了方法的其他地方抛出异常的情况(在异常块内部也一样)，必须跳转到操作符70处来进行finally块的处理。而且需要注意的是，每个catch块都会对应一个finally块，所以finally块在字节码中重复出现了两次。这只是一个简单的例子，让我们看见了javac编译产生的字节码输出。

JVM字节码集合有212个操作符，46个保留字为后续扩展使用。完全的集合可以通过JVM规范来了解。