蛋微疼的"Finally block does not complete normally"

今天用try-finally的时候，整出一个“finally block does not complete normally”的警告。想了半天也没明白什么一个情况。

 

可以看下面的代码， return 写在finally块就会得到警告！解决方案： return放到finally外面。

 

但是，java里面不是可以保证finally一定会执行的么，为什么不可以在finally块做return？？？

 

细细看道来：

 

debug一下这个函数，就会惊讶的发现， 里面抛出的异常会被finally吃掉。 这也就是为什么会被警告的原因。

	 @SuppressWarnings("finally")
	 private boolean isReturnWithinFinally() {
	 try {
	 if (true) throw new RuntimeException();
	 } finally {
	 return true; // This hides the exception
	 }
	 }

 

那么，下面这样会不会ok呢？先把异常处理

	public static void main(String[] args) {
		try{
			throw new RuntimeException();
		}catch(Exception e) {
			//
		}
		finally {
			return;
		}
	}

结论是：依旧不行。java里面的异常分为可不获和不可捕获两类，即便使用到catch块，也会导致非捕获的错误被finally吃掉。

 

因此，return一定要放到finally外面。

 

===1.java异常简介

类Throwable有两个直接子类：Error和Exception。Error类对象（如动态连接错误等），由Java虚拟 机生成并抛弃（通常，Java程序不对这类 异常进行处理） ；Exception类对象是Java程序处理或抛弃的对象。它有各种不同的子类分别对应于不同类型的例外。其中类RuntimeException代表运行时由Java虚拟机生成的 异常，如算术运算异常ArithmeticException(由除 0错等导致)、数组越界例外ArrayIndexOutOfBoundsException等；其它则为非运行时异常，如输入输出例外 IOException等。Java编译器要求 Java程序必须捕获或声明所有的非运行时异常 ，但对运行时异常 可以不做处理。

 

附赠示例代码一个：

 

 

=============================================================

2.参考文章：A Tale of Bad Exception Handling in Finally Blocks in Java。

老外开篇第一段文字，写的非常实在。于是我就坚持看完了全文，也找到了这个问题的答案

 

"There is always something more to learn . That was the lesson for me last week when I learned something new about the Java programming language, despite having used professionally it for almost 10 years."

评论

4 楼 sw1982 2010-09-27  

第2个例子有问题，自己写的，和

3 楼 beneo 2010-09-27  

第二个的例子应该是

    private static boolean isReturnWithinFinally() {
        try {
            if (true)
                throw new RuntimeException("what the F");
        } finally {
             return true; // This hides the exception
        }
    }

2 楼 beneo 2010-09-27  

引用

14.1 Normal and Abrupt Completion of Statements

If all the steps are carried out as described, with no indication of abrupt completion, the statement is said to complete normally. However, certain events may prevent a statement from completing normally:
The break (§14.15), continue (§14.16), and return (§14.17) statements cause a transfer of control that may prevent normal completion of statements that contain them.
Evaluation of certain expressions may throw exceptions from the Java virtual machine; these expressions are summarized in §15.6. An explicit throw (§14.18) statement also results in an exception. An exception causes a transfer of control that may prevent normal completion of statements.

这上面说的是break, continue, return都 may prevent normal completion of statements that contain them

举例子

    private static boolean isReturnWithinFinally() {
        try {
            if (true)
                throw new RuntimeException("what the F");
        } finally {
        }

        return true;
    }

这个finally里面是完全执行

    private static boolean isReturnWithinFinally() {
        try {
            if (true)
                throw new RuntimeException("what the F");
        } finally {
        }

        return true;
    }

这个就不是。。。。

*那么，在finally的return就破坏了throw E的条件，所以就不throw E。。

*还有一个问题，JLS上面写的是可以，那也就是说，其他的VM实现，表现出来的结果不一样。。

1 楼 beneo 2010-09-27  

我觉得你给的理由不是很充分

NPE，或者任何Exception子类都不是ERROR

问题应该在，为什么在fianlly里面的return，为什么会导致异常栈不能抛出。


根据
http://java.sun.com/docs/books/jls/third_edition/html/statements.html#236078

引用

A try statement can throw an exception type E iff either:

The try block can throw E and E is not assignable to any catch parameter of the try statement and either no finally block is present or the finally block can complete normally; or
Some catch block of the try statement can throw E and either no finally block is present or the finally block can complete normally; or
A finally block is present and can throw E.

但是，JVM怎么实现这个功能的，我就找不出来了