java 7中再谈try catch

  java 7中的try catch除了之前谈到的新特性外，本文简单来例子小结下，其实还是有不少地方要注意的，首先看一个典型的代码：

先来两个异常类：

 

public class ExceptionA extends Exception{
    public ExceptionA(String message){
        super(message);
    }
}
public class ExceptionB extends Exception{
    public ExceptionB(String message){
        super(message);
    }
}

再创建一个资源类oldresource，如下：

public class OldResource{
    public void doSomeWork(String work) throws ExceptionA{
        System.out.println("Doing: "+work);
        throw new ExceptionA("Exception occured while doing work");
    }
    public void close() throws ExceptionB{
        System.out.println("Closing the resource");
        throw new ExceptionB("Exception occured while closing");
    }
}

我们开始使用之：

public class OldTry {
    public static void main(String[] args) {
        OldResource res = null;
        try {
            res = new OldResource();
            res.doSomeWork("Writing an article");
        } catch (Exception e) {
            System.out.println("Exception Message: "+
                      e.getMessage()+" Exception Type: "+e.getClass().getName());
        } finally{
            try {
                res.close();
            } catch (Exception e) {
                System.out.println("Exception Message: "+
                         e.getMessage()+" Exception Type: "+e.getClass().getName());
            }
        }
    }
}

看输出：
   Doing: Writing an article
Exception Message: Exception occured while doing work Exception Type: javaapplication4.ExceptionA
Closing the resource
Exception Message: Exception occured while closing Exception Type: javaapplication4.ExceptionB

再来看java 7中的新写法，代码如下：

public class NewResource implements AutoCloseable{
    String closingMessage;
 
    public NewResource(String closingMessage) {
        this.closingMessage = closingMessage;
    }
 
    public void doSomeWork(String work) throws ExceptionA{
        System.out.println(work);
        throw new ExceptionA("Exception thrown while doing some work");
    }
    public void close() throws ExceptionB{
        System.out.println(closingMessage);
        throw new ExceptionB("Exception thrown while closing");
    }
 
    public void doSomeWork(NewResource res) throws ExceptionA{
        res.doSomeWork("Wow res getting res to do work");
    }
}

  在JAVA 7中，要自动在try catch中利用到新特性，想不写那么多东西就关闭资源，则可以编写实现 AutoCloseable类的，则都可以利用该特性了。
在主程序中调用：
 

public class TryWithRes {
    public static void main(String[] args) {
        try(NewResource res = new NewResource("Res1 closing")){
            res.doSomeWork("Listening to podcast");
        } catch(Exception e){
            System.out.println("Exception: "+
		     e.getMessage()+" Thrown by: "+e.getClass().getSimpleName());
        }
    }
}

输出结果为：
  Listening to podcast
Res1 closing
Exception: Exception thrown while doing some work Thrown by: ExceptionA

大家可以思考下为什么这样输出，在新特性中，资源的自动关闭调用了close(),而
NewResource res = new NewResource("Res1 closing")){
已经为closingMessage赋值了，而最后的Exception e是输出了，suprred掉了
exception a和exception b的输出。

再看一个多层嵌套的try catch例子
 

public class TryWithRes {
    public static void main(String[] args) {
        try(NewResource res = new NewResource("Res1 closing");
            NewResource res2 = new NewResource("Res2 closing")){
            try(NewResource nestedRes = new NewResource("Nestedres closing")){
                nestedRes.doSomeWork(res2);
            }
        } catch(Exception e){
            System.out.println("Exception: "+
		    e.getMessage()+" Thrown by: "+e.getClass().getSimpleName());
        }
 
    }
}

输出：
  Wow res getting res to do work
Nestedres closing
Res2 closing
Res1 closing
Exception: Exception thrown while doing some work Thrown by: ExceptionA

可以看到，后声明的资源上被最先CLOSE掉的，这里各自原先的exception都被supressed掉了。还可以用e.getSuppressed() 把屏蔽掉的exception都放来，比如

public class TryWithRes {
    public static void main(String[] args) {
        try(NewResource res = new NewResource("Res1 closing");
            NewResource res2 = new NewResource("Res2 closing")){
            try(NewResource nestedRes = new NewResource("Nestedres closing")){
                nestedRes.doSomeWork(res2);
            }
        } catch(Exception e){
            System.out.println("Exception: "+
		    e.getMessage()+" Thrown by: "+e.getClass().getSimpleName());
            if (e.getSuppressed() != null){
                for (Throwable t : e.getSuppressed()){
                    System.out.println(t.getMessage()+
                               " Class: "+t.getClass().getSimpleName());
                }
            }
        }
 
    }
}

输出显示：
Wow res getting res to do work
Nestedres closing
Res2 closing
Res1 closing
Exception: Exception thrown while doing some work Thrown by: ExceptionA
Exception thrown while closing Class: ExceptionB
Exception thrown while closing Class: ExceptionB
Exception thrown while closing Class: ExceptionB