使用 return 的 Try-finally 块不会引发异常

Question

在这段代码中，main 方法的 Catch 没有捕获运行时异常。 finally块执行完后，本该进入main的异常块，但是没有。

 class FinallyDemo {
    static int m1(){
        try{
            System.out.println("Inside m1");
            throw new RuntimeException("hi");
        }
        finally {
            System.out.println("m1 finally");
            return 5;
        }
    }
    public static void main(String[] args) {
        try{
            System.out.println(m1());
        }catch (Exception e){
            System.out.println("main caught: "+ e);
        }
    }
}

输出：

Inside m1
m1 finally
5

Answer 1

（如评论中所述，编写的代码甚至无法编译，但可以在不改变问题核心的情况下修复。）

it should've gone to the exception block of main, but it doesn't.

不，它的行为完全符合规范 section 14.20.2。那里有很多路径，但这里应用的路径是：

...

If execution of the try block completes abruptly because of a throw of a value V

...

If the run-time type of V is not assignment compatible with a catchable exception class of any catch clause of the try statement, then the finally block is executed

...

If the finally block completes abruptly for reason S, then the try statement completes abruptly for reason S (and the throw of value V is discarded and forgotten).

根据 section 14.1，return 语句算作“突然完成”。所以整个 try/finally 语句由于“return 有一个值”的原因突然完成，就好像没有抛出异常一样。

如果您希望异常传播到 try/finally 语句之外，请不要在 finally 块中使用 return 语句。

Answer 2

这是 class 的字节码。

static int m1();
    descriptor: ()I
    flags: ACC_STATIC
    Code:
      stack=3, locals=1, args_size=0
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Inside m1
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: new           #5                  // class java/lang/RuntimeException
        11: dup
        12: ldc           #6                  // String hi
        14: invokespecial #7                  // Method java/lang/RuntimeException."<init>":(Ljava/lang/String;)V
        17: athrow
        18: astore_0
        19: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        22: ldc           #8                  // String m1 finally
        24: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        27: iconst_5
        28: ireturn
      Exception table:
         from    to  target type
             0    19    18   any

查看此程序后。

public class FinallyDemo {
    static void m1(){
        try{
            System.out.println("Inside m1");
            throw new RuntimeException("hi");
        }
        finally {
            System.out.println("m1 finally");
        }
    }
    public static void main(String[] args) {
        try{
            m1();
        }catch (Exception e){
            System.out.println("main caught: "+ e);
        }
    }
}

字节码。

 static void m1();
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=3, locals=1, args_size=0
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Inside m1
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: new           #5                  // class java/lang/RuntimeException
        11: dup
        12: ldc           #6                  // String hi
        14: invokespecial #7                  // Method java/lang/RuntimeException."<init>":(Ljava/lang/String;)V
        17: athrow
        18: astore_0
        19: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        22: ldc           #8                  // String m1 finally
        24: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        27: aload_0
        28: athrow
      Exception table:
         from    to  target type
             0    19    18   any

在我看来，athrow本质上是一个跳转语句。很明显，在finally中加入了一条return语句，编译时省略了athrow语句，因为确定不可达

使用 return 的 Try-finally 块不会引发异常

Try-finally block with return doesn't rise exception

java

exception

try-finally