finally 浅谈

近日做程序时突发奇想：try ... finally ... 中的 finally 会在返回前执行（在XXX书上看到的，忘了书名了），是于做了以下测试

public class FinallyTest 
{
	public static void main(String args[])
	{
		System.out.println("return: " + new FinallyTest().getString());
	}
	
	public String getString()
	{
		String returnString = null;
		try
		{
			returnString = "this string will be return.";
			return returnString;		
		}finally
		{			
			System.out.println("execute finally...");
			System.out.println("before clean returnString's value: " + returnString);
			returnString = null;
			System.out.println("after  clean returnString's value: " + returnString);
			System.out.println("execute finally end.");
		}		
	}
}

测试结题：

execute finally...
before clean returnString's value: this string will be return.
after  clean returnString's value: null
execute finally end.
return: this string will be return.

如果返回前执行，那么返回的字符串应该为空。但从结果来看，似乎出人的意料之外。
考虑原因：返回前将返回的结果拷贝了。

于是做了以下修改：

public class FinallyTest 
{
	public static void main(String args[])
	{
		System.out.println("return: " + new FinallyTest().getMessage().message);
	}
	
	public Message getMessage()
	{
		Message returnMessage = new Message();
		try
		{
			returnMessage.message = "message";
			return returnMessage;		
		}finally
		{			
			System.out.println("execute finally...");
			System.out.println("before clean returnString's value: " + returnMessage.message);
			returnMessage.message = null;
			System.out.println("after  clean returnString's value: " + returnMessage.message);
			System.out.println("execute finally end.");
		}		
	}
}

class Message
{
	public String message = "message";
}

结果：

execute finally...
before clean returnString's value: message
after  clean returnString's value: null
execute finally end.
return: null

从结果来看，只调用了对象的引用，而对象没有被拷贝。
看来，如所下用 finally 统一处理一般数据类型还是免谈了。

public class FinallyTest 
{
	public static void main(String args[])
	{
		System.out.println("return: " + new FinallyTest().getInt());
	}
	
	public int getInt()
	{
		int returnInt = 0;
		try
		{
			returnInt = 1;
			throw new RuntimeException("...");
		}catch(Exception e)
		{
			returnInt = 2;
			return returnInt;
		}finally
		{			
			System.out.println("execute finally...");
			System.out.println("before switch returnInt's value: " + returnInt);
			switch(returnInt)
			{
				case 1:
					returnInt = 0;
					break;
				case 2:
					returnInt = 3;
					break;
				default:
					returnInt = 4;
			}
			System.out.println("after  switch returnInt's value: " + returnInt);
			System.out.println("execute finally end.");
		}		
	}
}

评论

1 楼 liwenso 2010-07-29  

不明白为什么要出现对象拷贝

而你想要的结果又是什么样子的？