自定义Exception

DaoException，ServiceException都是自定义异常类，
DaoException继承自ServiceException，ServiceException继承自Exception类。

package com.wei.exception;

public class DaoException extends ServiceException{
    
    public DaoException(){
        super("dao层异常");
    }
    
    public DaoException(String msg){
        super(msg);
    }
    
    public DaoException(Exception e){
        super("dao层异常",e);
    }
    
    public DaoException(String msg , Exception e){
        super(msg,e);
    }
}

package com.wei.dao.impl;

// class created by wwei 2011-12-11 下午01:32:45 

public class DaoImpl {
    public void testDaoExce() throws DaoException{
        try {
            User user = null;
            user.printUser();  //空值调用方法，必报异常java.lang.NullPointerException
        } catch (Exception e) {
            throw new DaoException("dao层testDaoExce方法异常",e);
        }
    }
}

package com.wei.exception;

// class created by wwei 2011-12-11 下午02:15:47 

public class ServiceException extends Exception{
    public ServiceException(){
        super("service层异常");
    }
    
    public ServiceException(String msg){
        super(msg);
    }
    
    public ServiceException(Exception e){
        super("service层异常",e);
    }
    
    public ServiceException(String msg , Exception e){
        super(msg,e);
    }
}

package com.wei.service.impl;

// class created by wwei 2011-12-11 下午02:16:59 

public class ServiceImpl {

    DaoImpl daoImpl = new DaoImpl();
    
    public void testServiceExce(int num) throws ServiceException{
            try {
                int a = 2/num;     //参数num若为0，报异常java.lang.ArithmeticException: / by zero
            } catch (Exception e) {
                throw new ServiceException("num为零,零不能做除数！",e);
            }
            daoImpl.testDaoExce();
    }
}

package com.wei.test;

// class created by wwei 2011-12-11 下午01:32:45 

public class Test {
    public static void main(String[] args) {
        ServiceImpl service = new ServiceImpl();
        try {
            int num = 0;
            service.testServiceExce(num);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

当num为0时，报的是Service层的错。

com.wei.test.ServiceException: num为零,零不能做除数！
    at com.wei.test.ServiceImpl.testServiceExce(ServiceImpl.java:13)
    at com.wei.test.Test.main(Test.java:10)
Caused by: java.lang.ArithmeticException: / by zero
    at com.wei.test.ServiceImpl.testServiceExce(ServiceImpl.java:11)
    ... 1 more

当num不为0时，报的是dao层的错

com.wei.test.DaoException: dao层testDaoExce方法异常
    at com.wei.test.DaoImpl.testDaoExce(DaoImpl.java:11)
    at com.wei.test.ServiceImpl.testServiceExce(ServiceImpl.java:15)
    at com.wei.test.Test.main(Test.java:10)
Caused by: java.lang.NullPointerException
    at com.wei.test.DaoImpl.testDaoExce(DaoImpl.java:9)
    ... 2 more