Java字节码揭秘——第三部分

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。

The Java Virtual Machine Specification(2nd Edition) JVM规范(第二版)