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。

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