本文来探讨一下依赖注入的三种实现方式是哪三种，Spring的支持又怎么样。

首先，依赖注入（DI）的实现方式有三种：设值注入、构造方法注入、接口注入，下面来看看这三种注入方式在Spring中的支持怎么样。

1、设值注入。

①、新建一个项目

②、新建一个User类，实现setter、getter方法

package com.bean;

public class User {
    String username;
    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;
    }
    
}

③、新建一个接口，名字为UserDAO，里面包含一个save方法，将信息存入数据库

package com.dao;

import com.bean.User;

public interface UserDAO {
    public void save(User user);
}

④、新建一个UserDAO的实现类UserDAOImpl，实现save方法，模拟数据库存入

package com.dao;

import com.bean.User;

public class UserDAOImpl implements UserDAO {

    @Override
    public void save(User user) {
        // TODO Auto-generated method stub
        System.out.println(user.getUsername()+"被存储");
    }
    
}

⑤、为了不将直接操作数据库的对象UserDAO暴露给用户，需添加service层，在数据库操作前后添加业务逻辑。
先实现业务逻辑接口UserService:

package com.bean;

public interface UserService {
    public void add(User user);
}

然后再写这个接口的实现类UserServiceImpl:

package com.bean;

import com.dao.UserDAO;

public class UserServiceImpl implements UserService {

    private UserDAO userDAO;
    
    public UserDAO getUserDAO() {
        return userDAO;
    }
    
    //设值注入 为UserDAO的注入做准备
    public void setUserDAO(UserDAO userDAO) {
        this.userDAO = userDAO;
    }
    //实现负责业务逻辑的add方法
    @Override
    public void add(User user) {
        // TODO Auto-generated method stub
        userDAO.save(user);
    }
    
}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:context="http://www.springframework.org/schema/context" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:tx="http://www.springframework.org/schema/tx" xmlns:p="http://www.springframework.org/schema/p" xmlns:util="http://www.springframework.org/schema/util" xmlns:jdbc="http://www.springframework.org/schema/jdbc"
    xmlns:cache="http://www.springframework.org/schema/cache"
    xsi:schemaLocation="
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/tx
    http://www.springframework.org/schema/tx/spring-tx.xsd
    http://www.springframework.org/schema/jdbc
    http://www.springframework.org/schema/jdbc/spring-jdbc-3.1.xsd
    http://www.springframework.org/schema/cache
    http://www.springframework.org/schema/cache/spring-cache-3.1.xsd
    http://www.springframework.org/schema/aop
    http://www.springframework.org/schema/aop/spring-aop.xsd
    http://www.springframework.org/schema/util
    http://www.springframework.org/schema/util/spring-util.xsd">
<!-- 配置数据访问类，实例名称为u -->
    <bean id="u" class="com.dao.UserDAOImpl">
    </bean>
    <bean id="user" class="com.bean.User">
    </bean>
    
    <!-- 设值注入 配置业务逻辑实现类，实例名称为userService-->
    <bean id="userService" class="com.bean.UserServiceImpl">
        <!-- 在这里实现注入，注入实例名称为u的实例到该实例的userDAO属性 -->
        <property name="userDAO">
            <ref bean="u" />
        </property>
    </bean>
</beans>

 //设值注入测试
 public static void setValDI(){
 //初始化容器
 ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
 UserServiceImpl service = (UserServiceImpl)ctx.getBean("userService");
 User u=(User)ctx.getBean("user");
 u.setPassword("123456");
 u.setUsername("test");
 service.add(u);
 }

package com.bean;

import com.dao.UserDAO;

public class UserServiceImpl implements UserService {

    private UserDAO userDAO;
    
    public UserDAO getUserDAO() {
        return userDAO;
    }
    
    public void setUserDAO(UserDAO userDAO) {
        this.userDAO = userDAO;
    }
    
    //如果需要构造方法注入 则这里必须写上构造方法
    public UserServiceImpl(UserDAO userDAO) {
        super();
        this.userDAO = userDAO;
    }

    @Override
    public void add(User user) {
        // TODO Auto-generated method stub
        userDAO.save(user);
    }
    
}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:context="http://www.springframework.org/schema/context" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:tx="http://www.springframework.org/schema/tx" xmlns:p="http://www.springframework.org/schema/p" xmlns:util="http://www.springframework.org/schema/util" xmlns:jdbc="http://www.springframework.org/schema/jdbc"
    xmlns:cache="http://www.springframework.org/schema/cache"
    xsi:schemaLocation="
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/tx
    http://www.springframework.org/schema/tx/spring-tx.xsd
    http://www.springframework.org/schema/jdbc
    http://www.springframework.org/schema/jdbc/spring-jdbc-3.1.xsd
    http://www.springframework.org/schema/cache
    http://www.springframework.org/schema/cache/spring-cache-3.1.xsd
    http://www.springframework.org/schema/aop
    http://www.springframework.org/schema/aop/spring-aop.xsd
    http://www.springframework.org/schema/util
    http://www.springframework.org/schema/util/spring-util.xsd">
    <bean id="u" class="com.dao.UserDAOImpl">
    </bean>
    <bean id="userService" class="user.bean.UserServiceImpl">
        <!-- 构造注入 -->
        <constructor-arg>
            <ref bean="u"/>
        </constructor-arg>
    </bean>
</beans>