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)