java 7中再談try catch

java 7中的try catch除了之前談到的新特性外，本文簡單來例子小結下，其實還是有不少地方要注意的，首先看一個典型的代碼：

先來兩個異常類：

 

Java代碼

1. public class ExceptionA extends Exception{   
2.     public ExceptionA(String message){   
3.         super(message);   
4.     }   
5. }   
6. public class ExceptionB extends Exception{   
7.     public ExceptionB(String message){   
8.         super(message);   
9.     }   
10. }  

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

再建立一個資源類oldresource，如下： Java代碼

1.   
2. public class OldResource{   
3.     public void doSomeWork(String work) throws ExceptionA{   
4.         System.out.println("Doing: "+work);   
5.         throw new ExceptionA("Exception occured while doing work");   
6.     }   
7.     public void close() throws ExceptionB{   
8.         System.out.println("Closing the resource");   
9.         throw new ExceptionB("Exception occured while closing");   
10.     }   
11. }  

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");    }}

我們開始使用之： Java代碼

1. public class OldTry {   
2.     public static void main(String[] args) {   
3.         OldResource res = null;   
4.         try {   
5.             res = new OldResource();   
6.             res.doSomeWork("Writing an article");   
7.         } catch (Exception e) {   
8.             System.out.println("Exception Message: "+   
9.                       e.getMessage()+" Exception Type: "+e.getClass().getName());   
10.         } finally{   
11.             try {   
12.                 res.close();   
13.             } catch (Exception e) {   
14.                 System.out.println("Exception Message: "+   
15.                          e.getMessage()+" Exception Type: "+e.getClass().getName());   
16.             }   
17.         }   
18.     }   
19. }  

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());            }        }    }}

看輸出：
   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

再來看java 7中的新寫法，代碼如下： Java代碼

1. public class NewResource implements AutoCloseable{   
2.     String closingMessage;   
3.     
4.     public NewResource(String closingMessage) {   
5.         this.closingMessage = closingMessage;   
6.     }   
7.     
8.     public void doSomeWork(String work) throws ExceptionA{   
9.         System.out.println(work);   
10.         throw new ExceptionA("Exception thrown while doing some work");   
11.     }   
12.     public void close() throws ExceptionB{   
13.         System.out.println(closingMessage);   
14.         throw new ExceptionB("Exception thrown while closing");   
15.     }   
16.     
17.     public void doSomeWork(NewResource res) throws ExceptionA{   
18.         res.doSomeWork("Wow res getting res to do work");   
19.     }   
20. }  

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，要自動在try catch中利用到新特性，想不寫那麼多東西就關閉資源，則可以編寫實現 AutoCloseable類的，則都可以利用該特性了。

在主程式中調用：
 

Java代碼

1. public class TryWithRes {   
2.     public static void main(String[] args) {   
3.         try(NewResource res = new NewResource("Res1 closing")){   
4.             res.doSomeWork("Listening to podcast");   
5.         } catch(Exception e){   
6.             System.out.println("Exception: "+   
7.              e.getMessage()+" Thrown by: "+e.getClass().getSimpleName());   
8.         }   
9.     }   
10. }  

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());        }    }}

輸出結果為：
  Listening to podcast
Res1 closing
Exception: Exception thrown while doing some work Thrown by: ExceptionA

大家可以思考下為什麼這樣輸出，在新特性中，資源的自動關閉調用了close(),而
NewResource res = new NewResource("Res1 closing")){
已經為closingMessage賦值了，而最後的Exception e是輸出了，suprred掉了
exception a和exception b的輸出。

再看一個多層嵌套的try catch例子
 

Java代碼

1. public class TryWithRes {   
2.     public static void main(String[] args) {   
3.         try(NewResource res = new NewResource("Res1 closing");   
4.             NewResource res2 = new NewResource("Res2 closing")){   
5.             try(NewResource nestedRes = new NewResource("Nestedres closing")){   
6.                 nestedRes.doSomeWork(res2);   
7.             }   
8.         } catch(Exception e){   
9.             System.out.println("Exception: "+   
10.             e.getMessage()+" Thrown by: "+e.getClass().getSimpleName());   
11.         }   
12.     
13.     }   
14. }  

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());        }     }}

輸出：
  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，後聲明的資源上被最先CLOSE掉的，這裡各自原先的exception都被supressed掉了。還可以用e.getSuppressed() 把屏蔽掉的exception都放來，比如
Java代碼

1. public class TryWithRes {   
2.     public static void main(String[] args) {   
3.         try(NewResource res = new NewResource("Res1 closing");   
4.             NewResource res2 = new NewResource("Res2 closing")){   
5.             try(NewResource nestedRes = new NewResource("Nestedres closing")){   
6.                 nestedRes.doSomeWork(res2);   
7.             }   
8.         } catch(Exception e){   
9.             System.out.println("Exception: "+   
10.             e.getMessage()+" Thrown by: "+e.getClass().getSimpleName());   
11.             if (e.getSuppressed() != null){   
12.                 for (Throwable t : e.getSuppressed()){   
13.                     System.out.println(t.getMessage()+   
14.                                " Class: "+t.getClass().getSimpleName());   
15.                 }   
16.             }   
17.         }   
18.     
19.     }   
20. }  

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());                }            }        }     }}

輸出顯示：
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