关于java:Spring-DI-依赖注入有几种方式

Spring 实例化 bean 的形式

• Set 注入
• 结构器注入
• 动态工厂注入
• 实例化工厂注入

案例实操

Set 注入

xml 配置（同时 spring 也提供了对于根本数据类型的 set 注入形式）

<?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 是对于内部 bean 对象援用，与被援用的 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() 的办法";
    }
} 

UserService.java

public class UserService {
    // 肯定要提供属性的 setter 办法
    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();}
} 

结构器注入

xml 配置（也提供对于根本数据类型、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>
    <!-- 结构器注入 -->
    <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> 

结构器注入有三种模式：

index 属性为参数程序，如果只有一个参数 index 能够不设置。

name 属性依据结构器中属性的名字。

type 属性，是依据结构器中属性的类型来匹配的。如果雷同类型属性不惟一，注入的属性依照程序注入进来。

UserServiceV2.java 类提供构造函数

/**
 * 实现结构器注入
 * @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;
    }
} 

动态工厂注入

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">
    <!-- 动态工厂注入 -->
    <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: 动态工厂注入就是 IoC 动态工厂和 DI 的 setter 注入，将须要注入的属性对象利用动态工厂创立进去.

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">

    <!-- 实例化工厂 -->
    <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: 重点把握 set，结构器注入，工厂形式理解即可，理论开发中根本应用 set 形式注入 bean。

扩大

循环依赖的问题的产生

Bean 通过结构器注入，之间彼此相互依赖对方导致 bean 无奈实例化。

注入的抉择：开发我的项目中 set 形式注入首选

应用结构注入能够在构建对象的同时一并实现依赖关系的建设，对象一建设则所有的所有也就筹备好了，但如果要建设的对象关系很多，应用结构注入会在构建函数上留下一长串的参数，且不易记忆，这时应用 Set 注入会是个不错的抉择。

应用 Set 注入能够有明确的名称，能够理解注入的对象会是什么，像 setxxx() 这样的名称比记忆 Constructor 上某个参数的地位代表某个对象更好。

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">
        <!-- <constructor-arg index="0" ref="userService"></constructor-arg> -->
        <property name="userService" ref="userService"></property>
    </bean>
    <bean id="userService" class="com.xxx.demo.UserService">
        <!-- <constructor-arg index="0" ref="goodsService"></constructor-arg> -->
        <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;}    
}