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."":()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。