Java字节码揭秘——第三部分-优快云博客

本文链接：https://blog.youkuaiyun.com/iteye_1364/article/details/82069766

本文通过一个简单的Java异常处理示例，展示了try-catch-finally结构的工作原理，并深入探讨了其对应的字节码指令及异常表，揭示了JVM如何执行异常处理逻辑。

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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)