java 异常处理机制及说明。

又抄袭了一篇文章，其实就是想保存到自己的博客中而已，文章出处：http://www.cnblogs.com/LilianChen/p/4639471.html

1. 如何捕获异常

try

{

可能会出现异常的代码段；

}

catch(异常类型名 处理该异常对象)

{

异常处理代码段；

}

 1 import java.io.*;
 2
 3 public class TryCatchTest {
 4
 5     public static void main(String[] args) {
 6         File file = new File("abc.txt");
 7         int a[] = {1, 2};
 8
 9         try
10         {
11             System.out.println(3/0);
12         }
13         catch(ArithmeticException e1)
14         {
15             System.out.println("3/0: ");
16             System.out.println("This is ArithmeticException");
17         }
18
19         try
20         {
21             System.out.println(a[2]);
22         }
23         catch(ArrayIndexOutOfBoundsException e2)
24         {
25             System.out.println("a[2] is out of Array: ");
26             System.out.println("This is ArrayIndexOutOfBoundsException");
27         }
28
29         try
30         {
31             BufferedReader input = new BufferedReader(new FileReader(file));
32         }
33         catch (FileNotFoundException e3)
34         {
35             System.out.println("abc.txt is not found: ");
36             System.out.println("This is FileNotFoundException");
37         }
38         catch(IOException e)
39         {
40             System.out.println("This is IOException");
41         }
42
43     }
44
45 }

3/0: 
This is ArithmeticException
a[2] is out of Array: 
This is ArrayIndexOutOfBoundsException
abc.txt is not found: 
This is FileNotFoundException

2. 如何抛出异常

编写代码过程中，如果不想在这段代码中捕捉和处理一个可能出现的异常，那么就需要将这个异常传递出去，传递给调用它的方法去处理该异常。这个时候就需要使用throw 和throws

• throws语句在方法声明中使用，抛出异常
• throw语句在方法体内部使用，抛出异常

注意： 方法体中若使用了throw语句抛出异常，则必须在该方法声明中，采用throws语句来声明该方法体中抛出的异常，同时，throws语句声明抛出的异常，必须是方法体中throw语句抛出的异常或该异常的父类。

 1 import java.io.*;
 2
 3 public class ThrowTest {
 4
 5     public void throwTest1() throws ArithmeticException
 6     {
 7         System.out.println(3/0);
 8     }
 9
10     public void throwTest2() throws ArrayIndexOutOfBoundsException
11     {
12         int a[] ={1,2};
13         System.out.println(a[2]);
14     }
15
16     public void throwTest3() throws FileNotFoundException
17     {
18         File file=new File("abc.txt");
19         new BufferedReader(new FileReader(file));
20     }
21
22     public void throwTest4() throws FileNotFoundException
23     {
24         throw new FileNotFoundException("abc.txt");
25     }
26
27     public static void main(String[] args) {
28         ThrowTest throwTest=new ThrowTest();
29
30         try
31         {
32             throwTest.throwTest1();
33         }
34         catch (ArithmeticException e1)
35         {
36             System.out.println("3/0: ");
37             System.out.println("This is ArithmeticException");
38         }
39
40         try
41         {
42             throwTest.throwTest2();
43         }
44         catch(ArrayIndexOutOfBoundsException e2)
45         {
46             System.out.println("a[2] is out of Array: ");
47             System.out.println("This is ArrayIndexOutOfBoundsException");
48         }
49
50         try
51         {
52             throwTest.throwTest3();
53         }
54         catch (FileNotFoundException e3)
55         {
56             System.out.println("abc.txt is not found: ");
57             System.out.println("This is FileNotFoundException");
58         }
59
60         try
61         {
62             throwTest.throwTest4();
63         }
64         catch (FileNotFoundException e3)
65         {
66             System.out.println("abc.txt is not found: ");
67             System.out.println("This is FileNotFoundException");
68         }
69
70     }
71
72 }

3/0: 
This is ArithmeticException
a[2] is out of Array: 
This is ArrayIndexOutOfBoundsException
abc.txt is not found: 
This is FileNotFoundException
abc.txt is not found: 
This is FileNotFoundException

3. 自定义异常

建立自己的异常类，要做的只是根据需要，从Exception类或者从Exception类的子类中继承出需要的类。习惯上，会经常为每一个异常类，提供一个默认的和一个包含详细信息的构造器。需要注意的是，自定义异常类，必须由程序员使用throw语句抛出。

 1 public class MyException {
 2
 3     public static void main(String[] args) {
 4         String str="2abcde";
 5
 6         try
 7         {
 8             char c=str.charAt(0);
 9             if(c<'a'||c>'z'||c<'A'||c>'Z')
10                 throw new FirstLetterException();
11         }
12         catch (FirstLetterException e)
13         {
14             System.out.println("This is FirstLetterException");
15         }
16
17     }
18
19 }
20
21 class FirstLetterException extends Exception{
22     public FirstLetterException()
23     {
24         super("The first char is not a letter");
25     }
26
27     public FirstLetterException(String str)
28     {
29         super(str);
30     }
31 }

This is FirstLetterException

 1 public class MyException {
 2
 3     public static void main(String[] args) throws FirstLetterException{
 4         throw new FirstLetterException();
 5     }
 6 }
 7
 8 class FirstLetterException extends Exception{
 9     public FirstLetterException()
10     {
11         super("The first char is not a letter");
12     }
13
14     public FirstLetterException(String str)
15     {
16         super(str);
17     }
18 }

Exception in thread "main" FirstLetterException: The first char is not a letter
at MyException.main(MyException.java:5)

4. 使用finally语句

在使用try...catch语句是，若try语句中的某一句出现异常情况，那么这部分try语句段中，从出现异常的语句开始，之后的所有语句都不会被执行，直到这部分try语句段结束。

但是在很多情况下，希望无论是否出现异常，某些语句都需要被执行。那么就可以把这部分代码放在finally语句段中，即使try或catch语句段中含有return语句，程序都会在异常抛出后先执行finally语句段，除非try或catch语句段中执行System.exit()方法，或者是出现Error错误，finally语句才不会被执行而退出程序。

 1 import java.io.*;
 2
 3 public class FinallyTest {
 4
 5     public static void main(String[] args) {
 6         File file=null;
 7         BufferedReader input=null;
 8         file=new File("abc.txt");
 9
10         try
11         {
12             input=new BufferedReader(new FileReader(file));
13         }
14         catch(FileNotFoundException e)
15         {
16             System.out.print("abc.txt is not found: ");
17             System.out.println("This is FileNotFoundException");
18         }
19         finally
20         {
21             System.out.println("This is finally code part.");
22         }
23
24     }
25
26 }

abc.txt is not found: This is FileNotFoundException
This is finally code part.