Spring DI 依赖注入有几种方式？

Spring 实例化 bean 的方式

• Set注入

• 构造器注入

• 静态工厂注入

• 实例化工厂注入

案例实操

Set注入

xml配置（同时spring也提供了对于基本数据类型的set注入方式）

<?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.xsd">
​
 <bean id="userDao" class="com.xxx.demo.UserDao"></bean>
 <!-- setter注入 -->
 <bean id="userService" class="com.xxx.demo.UserService">
 <!--ref是对于外部bean对象引用，与被引用的bean对象的id保持一致-->
 <property name="userDao" ref="userDao"></property>
 </bean>
</beans>

UserDao.java

public class UserDao {
 public String userLogin() {
 return "我是UserDao中的userLogin()的方法";
 }
}

UserService.java

public class UserService {
 //一定要提供属性的setter方法
 private UserDao userDao;
 
 public void userlogin() {
 String res=userDao.userLogin();
 System.out.println(res);
 }
​
 public void setUserDao(UserDao userDao) {
 this.userDao = userDao;
 }
}

App.java

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
​
public class App {
 public static void main(String[] args) {
 ApplicationContext applicationContext=new ClassPathXmlApplicationContext("spring-config.xml");
 UserService userService=applicationContext.getBean("userService", UserService.class);
 userService.userlogin();
 }
}

构造器注入

xml配置（也提供对于基本数据类型、字符串等值的注入）

<?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.xsd">
​
<bean id="userDao" class="com.xxx.demo.UserDao"></bean>

<bean id="userServiceV2" class="com.xxx.demo.UserServiceV2">
<constructor-arg index="0" ref="userDao"></constructor-arg>
<constructor-arg index="1" value="印度三哥"></constructor-arg>
</bean>
</beans>

构造器注入有三种形式：

index属性为参数顺序，如果只有一个参数index可以不设置。

name属性根据构造器中属性的名字。

type属性，是根据构造器中属性的类型来匹配的。如果相同类型属性不唯一，注入的属性按照顺序注入进来。

UserServiceV2.java类提供构造函数

/**
 * 实现构造器注入
 * @author Best Liu
 *
 */
public class UserServiceV2 {
 private UserDao userDao;
 private String name;
 public void userlogin() {
 String res=userDao.userLogin();
 System.out.println(res);
 System.out.println(name);
 }
 public UserServiceV2(UserDao userDao,String name) {
 super();
 this.userDao = userDao;
 this.name = name;
 }
}

静态工厂注入

xml配置

<?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.xsd">

<bean id="userDao01" class="com.xxx.demo.StaticFactory" factory-method="createuserDao"></bean>
<bean id="userService01" class="com.xxx.demo.UserService">
<property name="userDao" ref="userDao01"></property>
</bean>
</beans>

StaticFactory.java

public class StaticFactory {
 public static UserDao createuserDao(){
 return new UserDao();
 }
}

UserService.java

public class UserService {
 private UserDao userDao;
 
 public void userlogin() {
 String res=userDao.userLogin();
 System.out.println(res);
 }
​
 public void setUserDao(UserDao userDao) {
 this.userDao = userDao;
 }
}

tips:静态工厂注入就是IoC静态工厂和DI的setter注入，将需要注入的属性对象利用静态工厂创建出来.

2.4 实例化工厂

xml配置

<?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.xsd">
​

<bean id="instanceFactory" class="com.xxx.demo.InstanceFactory"></bean>
<bean id="userDao3" factory-bean="instanceFactory" factory-method="createUserDao"></bean>
<bean id="userService02" class="com.xxx.demo.UserService">
<property name="userDao" ref="userDao3"></property>
</bean>
</beans>

InstanceFactory.java

public class InstanceFactory {
 public UserDao createUserDao(){
 return new UserDao();
 }
}

tips:重点掌握set，构造器注入，工厂方式了解即可，实际开发中基本使用set方式注入bean。

扩展

循环依赖的问题的产生

Bean通过构造器注入，之间彼此相互依赖对方导致bean无法实例化。

注入的选择：开发项目中set方式注入首选

使用构造注入可以在构建对象的同时一并完成依赖关系的建立，对象一建立则所有的一切也就准备好了，但如果要建立的对象关系很多，使用构造注入会在构建函数上留下一长串的参数，且不易记忆，这时使用Set注入会是个不错的选择。

使用Set注入可以有明确的名称，可以了解注入的对象会是什么，像setxxx()这样的名称比记忆Constructor上某个参数的位置代表某个对象更好。

xml配置

<?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.xsd">
<bean id="goodsService" class="com.xxx.demo.GoodsService">

<property name="userService" ref="userService"></property>
</bean>
<bean id="userService" class="com.xxx.demo.UserService">

<property name="goodsService" ref="goodsService"></property>
</bean>
</beans>

GoodsService.java

public class GoodsService {
 private UserService userService;
 /*public GoodsService(UserService userService) {
 super();
 this.userService = userService;
 }*/
 public void setUserService(UserService userService) {
 this.userService = userService;
 }
}

UserService.java

public class UserService {
 private GoodsService goodsService;
/* public UserService(GoodsService goodsService) {
 super();
 this.goodsService = goodsService;
 }
*/
 public void setGoodsService(GoodsService goodsService) {
 this.goodsService = goodsService;
 } 
}