尚学堂Spring视频教程（二）：Spring控制反转

用Spring来实现IOC

  在上节中我们自定义了一个接口BeanFactory和类ClassPathXmlApplicationContext来模拟Spring，其实它们在Spring中确实是存在的，下面我们具体来看看Spring的控制反转是如何操作的

  其他代码一样，只是配置文件和单元测试的代码有点不同，注意引用其他bean配置的是"ref"属性

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

  <bean id="u" class="com.bjsxt.dao.impl.UserDAOImpl">
  </bean>    
  <bean id="userService" class="com.bjsxt.service.UserService">
      <property name="userDAO" ref="u" />
          
  </bean>
  

</beans>

package com.bjsxt.service;
import org.junit.Test;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.core.io.ClassPathResource;

import com.bjsxt.model.User;

//Dependency Injection
//Inverse of Control
public class UserServiceTest {

    @Test
    public void testAdd() throws Exception {        
        BeanFactory ctx = new ClassPathXmlApplicationContext("beans.xml");
        
        
        UserService service = (UserService)ctx.getBean("userService");
        
        
        User u = new User();
        u.setUsername("zhangsan");
        u.setPassword("zhangsan");
        service.add(u);
    }

}

  注：ClassPathXmlApplicationContext继承ApplicationContext，ApplicationContext又继承BeanFactory，所以也可以这样写ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");推荐使用这种写法。其构造函数也可以传入String数组，适用于多个配置文件的情况

三种注入方式（setter，构造函数，接口）

  setter已经见过了，接口注入很少用，故此忽略，下面来看一下构造函数注入，在UserService中添加一个构造函数初始化UserDAO属性

package com.bjsxt.service;
import com.bjsxt.dao.UserDAO;
import com.bjsxt.model.User;



public class UserService {
        

    private UserDAO userDAO;  
    
    public void add(User user) {
        userDAO.save(user);
    }
    public UserDAO getUserDAO() {
        return userDAO;
    }
    public void setUserDAO(UserDAO userDAO) {
        this.userDAO = userDAO;
    }
    
    public UserService(UserDAO userDAO) {
        super();
        this.userDAO = userDAO;
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

  <bean id="u" class="com.bjsxt.dao.impl.UserDAOImpl">
  </bean>
    
  <bean id="userService" class="com.bjsxt.service.UserService">
      <!-- 
      <property name="userDAO" ref="u" />
       -->
       <constructor-arg>
           <ref bean="u"/>
       </constructor-arg>
  </bean>
  

</beans>

  注意用的是constructor-arg标签，如果构造函数有多个参数，就按照顺序写，或者指定index

Bean的Scope属性

  配置文件中bean有个scope属性，如果配置为singleton，表示该bean只有一个实例，如果配置为prototype，每次取出该bean都会创建一个新的对象。bean的scope属性配置为singleton和prototype，在下面的代码中会分别输出true和false

package com.bjsxt.service;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.bjsxt.model.User;

//Dependency Injection
//Inverse of Control
public class UserServiceTest {

    @Test
    public void testAdd() throws Exception {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        
        
        UserService service = (UserService)ctx.getBean("userService");
        UserService service2 = (UserService)ctx.getBean("userService");
        
        System.out.println(service == service2);
        
        
        User u = new User();
        u.setUsername("zhangsan");
        u.setPassword("zhangsan");
        service.add(u);
    }

}