Spring之依赖注入（DI）

Dependency Injection简称DI，依赖注入的作用就是在Spring框架创建对象的时候，将对象的属性注入Bean中。依赖注入一般有三种实现方式，一种是提供构造方法注入；另一种是通过属性Setter方法进行注入；还有P命名空间进行注入。

 

1）使用构造方法进行注入:

 

实体类代码：

public class User {
    private String userName;
    private String userAge;
    private String userPsw;
    //无参构造方法
    public User() {
    }
    //有参构造
    public User(String userName, String userAge) {
        this.userName = userName;
        this.userAge = userAge;
    }
    //set与get方法
    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getUserAge() {
        return userAge;
    }

    public void setUserAge(String userAge) {
        this.userAge = userAge;
    }

    public String getUserPsw() {
        return userPsw;
    }

    public void setUserPsw(String userPsw) {
        this.userPsw = userPsw;
    }
    @Override
    public String toString() {
        return "User{" +
                "userName='" + userName + '\'' +
                ", userAge='" + userAge + '\'' +
                ", userPsw='" + userPsw + '\'' +
                '}';
    }
}

Beans中xml文件的书写：

 

?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="user" class="com.yhd.pojo.User">
        <constructor-arg index="0" value="不好喝的柚子茶"></constructor-arg>
        <constructor-arg index="1" value="22"></constructor-arg>
    </bean>
</beans>

 

测试代码：

import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class userText {
    @Test
    public void text(){
        ApplicationContext context =new  ClassPathXmlApplicationContext("res/beans.xml");
        User user = context.getBean("user", User.class);
        System.out.println(user);
    }
}

运行结果：

注：因为我们没有对userPsw属性注入值，所有输出的值为默认值null

 

2）使用setter方法注入

 

Bean的Xml代码：

 

</bean>
    <bean id="user" class="com.yhd.pojo.User">
        <property name="userName" value="不好喝的柚子茶"></property>
        <property name="userAge" value="18"></property>
        <property name="userPsw" value="123456"></property>
    </bean>

 

测试代码：

import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class userText {
    @Test
    public void text(){
        ApplicationContext context =new  ClassPathXmlApplicationContext("res/beans.xml");
        User user = context.getBean("user", User.class);
        System.out.println(user);
    }
}

运行截图：

1）使用P命名空间进行注入（底层实现是setter方法注入）

 

添加命名空间：

 xmlns:p="http://www.springframework.org/schema/p" 

Bean的xml代码：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="user" class="com.yhd.pojo.User" p:userName="不好喝的柚子茶" p:userAge="23" p:userPsw="666666"></bean>
</beans>

测试代码：

import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class userText {
    @Test
    public void text(){
        ApplicationContext context =new  ClassPathXmlApplicationContext("res/beans.xml");
        User user = context.getBean("user", User.class);
        System.out.println(user);
    }
}

运行截图：