依赖注入的实现方式：设值注入和构造方法注入

SSH开发时，一般将项目分成3层：web层、service层和dao层。开发的基本流程是先定义dao接口，然后实现该接口，在定义同类型的service接口，再实现service接口（此时使用dao接口注入），接着再从web层去调用service层。

dao层完成的是底层的数据库操作，service层则完成纯粹的业务逻辑，该层中的数据操作部分是通过注入的dao层来实现的。service层的作用是将从dao层取得的数据做更贴近业务的实现，dao层则只实现对数据的增、删、改、查操作。使用这种分层方式更有利于项目的扩展和维护。

一、设值注入

设值注入是指IoC容器使用setter方法注入被依赖的实例。通过调用无参构造器或无参static工厂方法实例化bean之后，调用该bean的setter方法，即可实现基于setter的DI。

1. 新建项目，配置运行环境（Build Path中加入spring和其它依赖的jar包）
2. User类，作为被保存的模型对象
   
   

   package com.deciphering.model;
   
   public class User {
       private String username;
       private String password;
   
       public String getUsername() {
           return username;
       }
       public void setUsername(String username) {
           this.username = username;
       }
       public String getPassword() {
           return password;
       }
       public void setPassword(String password) {
           this.password = password;
       }
   }

   User.java

3. User类的接口UserDAO（针对接口编程的好处是UserDAO可以有多个不同的实现类，分别用来操作不同的数据库）
   
   

   package com.deciphering.dao;
   import com.deciphering.model.User;
   
   public interface UserDAO {
       public void save(User user);
   }

   UserDAO

4. UserDAO的实现类UserDAOImpl（简化处理）
   
   

   package com.deciphering.dao.impl;
   
   import com.deciphering.dao.UserDAO;
   import com.deciphering.model.User;
   
   public class UserDAOImpl implements UserDAO {
   
       public void save(User user) {
   
           System.out.println(user.getUsername() + " saved in Oracle!");
           //System.out.println(user.getUsername() + " saved in DB2!");
           //System.out.println(user.getUsername() + " saved in mysql!");
   
       }
   
   }

   UserDAOImpl

5. 业务逻辑接口UserService
   
   

   package com.deciphering.service;
   import com.deciphering.dao.UserDAO;
   import com.deciphering.model.User;
   
   public class UserServiceImpl {
   
       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 UserServiceImpl(UserDAO userDAO) {
           super();
           this.userDAO = userDAO;
       }
       */
   }

   UserService

6. 业务逻辑接口的实现类UserServiceImpl

   package com.deciphering.service;
   import com.deciphering.dao.UserDAO;
   import com.deciphering.model.User;
   
   public class UserServiceImpl implements 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;
       }
   
   }

7. Spring的配置文件beans.xml

   <?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.deciphering.dao.impl.UserDAOImpl">
     </bean>
   
     <bean id="userService" class="com.deciphering.service.UserServiceImpl">
          <property name="userDAO">
              <ref bean="u"/>
          </property> 
   
     </bean>
   </beans>

8. 测试类
   
   Test

   package com.deciphering.service;
   import org.junit.Test;
   import org.springframework.context.ApplicationContext;
   import org.springframework.context.support.ClassPathXmlApplicationContext;
   
   import com.deciphering.model.User;
   import com.deciphering.service.UserServiceImpl;
   
   public class UserServiceTest {
   
       @Test
       public void testAdd() throws Exception {
   
           ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
           UserServiceImpl service = (UserServiceImpl)ctx.getBean("userService");
   
           User u = new User();
   
           u.setUsername("测试用户");
           u.setPassword("123456");
   
           service.add(u);
       }
   
   }

二、构造方法注入

构造方法注入是指IoC容器使用构造方法来注入被依赖的实例。基于构造器的DI通过调用带参数的构造方法来实现，每个参数代表着一个依赖。

在上面的项目文件里，做少量的修改，即可变为构造方法注入。

1. 修改UserServiceImpl类，在使用构造方法注入时，必须显式的给出待参数的构造方法。

   package com.deciphering.service;
   import com.deciphering.dao.UserDAO;
   import com.deciphering.model.User;
   
   public class UserServiceImpl {
   
       private UserDAO userDAO;  
   
       public void add(User user) {
           userDAO.save(user);
       }
       public UserDAO getUserDAO() {
           return userDAO;
       }
       public void setUserDAO(UserDAO userDAO) {
           this.userDAO = userDAO;
       }
   //定义构造方法
       public UserServiceImpl(UserDAO userDAO) {
           super();
           this.userDAO = userDAO;
       }
   
   }

2. 修改beans.xml文件，加粗代码为修改部分

   <?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.deciphering.dao.impl.UserDAOImpl">
     </bean>
   
     <bean id="userService" class="com.deciphering.service.UserServiceImpl">
           <!--
          <property name="userDAO">
              <ref bean="u"/>
          </property>
          -->
    //使用构造方法注入，为UserService实例注入u实例
          <constructor-arg>
              <ref bean="u"/>
          </constructor-arg>
   
     </bean>
   
   </beans>

文件结构：