异常处理

package ExceptSourceCode;
public class EmbededFinally {
    public static void main(String args[]) {
        int result;
        try {
            System.out.println("in Level 1");
            try {
                System.out.println("in Level 2");
                // result=100/0;  //Level 2
                try {
                    System.out.println("in Level 3");
                    result=100/0;  //Level 3
                }
                catch (Exception e) {
                    System.out.println("Level 3:" + e.getClass().toString());
                }
                finally {
                    System.out.println("In Level 3 finally");
                }
                // result=100/0;  //Level 2
            }
            catch (Exception e) {
                System.out.println("Level 2:" + e.getClass().toString());
            }
            finally {
                System.out.println("In Level 2 finally");
            }
            // result = 100 / 0;  //level 1
        }
        catch (Exception e) {
            System.out.println("Level 1:" + e.getClass().toString());
        }
        finally {
             System.out.println("In Level 1 finally");
        }
    }
}

输出：

in Level 1
in Level 2
in Level 3
Level 3:class java.lang.ArithmeticException
In Level 3 finally
In Level 2 finally
In Level 1 finally

执行了第三层的catch和后续的finally中代码，

当把第三层catch注释掉后

package ExceptSourceCode;
public class EmbededFinally {
    public static void main(String args[]) {
        int result;
        try {
            System.out.println("in Level 1");
            try {
                System.out.println("in Level 2");
                // result=100/0;  //Level 2
                try {
                    System.out.println("in Level 3");
                    result=100/0;  //Level 3
                }
                finally {
                    System.out.println("In Level 3 finally");
                }
                // result=100/0;  //Level 2
            }
            catch (Exception e) {
                System.out.println("Level 2:" + e.getClass().toString());
            }
            finally {
                System.out.println("In Level 2 finally");
            }
            // result = 100 / 0;  //level 1
        }
        catch (Exception e) {
            System.out.println("Level 1:" + e.getClass().toString());
        }
        finally {
             System.out.println("In Level 1 finally");
        }
    }
}

输出：

in Level 1
in Level 2
in Level 3
In Level 3 finally
Level 2:class java.lang.ArithmeticException
In Level 2 finally
In Level 1 finally

所以，try catch嵌套，内层不能捕获时，会考虑外层内否捕获，内层能捕获，则外层catch不执行。

-------------------------------

package ExceptSourceCode;
public class SystemExitAndFinally {
    public static void main(String[] args) {
        try {
            System.out.println("in main");
            throw new Exception("Exception is thrown in main");
            //System.exit(0);
        } catch (Exception e) {
            System.out.println(e.getMessage());
            System.exit(0);
        } finally {
            System.out.println("in finally");
        }
    }
}

输出：

in main
Exception is thrown in main

该代码中finally没有被执行，因为在catch中有System.exit(0);当执行到这条语句时，JVM关闭，程序停止。

 

一个方法可以申明抛出多个异常

package ExceptSourceCode;
import java.io.*;
public class ThrowMultiExceptionsDemo {
    public static void main(String[] args)
    {
        try {
            throwsTest();
        }
        catch(IOException e) {
            System.out.println("捕捉异常");
        }
    }
    private static void throwsTest()  throws ArithmeticException,IOException {
        System.out.println("这只是一个测试");
        // 程序处理过程假设发生异常
         throw new IOException();
        //throw new ArithmeticException();
    }
}

一个子类的throws子句抛出的异常，不能是其基类同名方法抛出的异常对象的父类。

package ExceptSourceCode;
import java.io.*;
public class OverrideThrows
{
    public void test()throws IOException
    {
        FileInputStream fis = new FileInputStream("a.txt");
    }
}
class Sub extends OverrideThrows
{
    //如果test方法声明抛出了比父类方法更大的异常,比如Exception
    //则代码将无法编译……
    public void test() throws FileNotFoundException
    {
        //...
    }
}

 

自定义异常通常选择直接派生自Exception

Class MyException extends Exception { … }

在合适的地方使用throw语句抛出自定义异常对象

Class MyClass {
　　void someMethod(){
　　　　if (条件) throw new MyException();
　　}
}