Spring bean為什么需要依賴注入

更新時(shí)間：2021年07月16日 10:22:34 作者：寧在春

本篇文章主要介紹了Spring依賴注入的三種方式小結(jié)，小編覺(jué)得挺不錯(cuò)的，現(xiàn)在分享給大家，也給大家做個(gè)參考。一起跟隨小編過(guò)來(lái)看看吧

具體步驟：

1.創(chuàng)建一個(gè)maven項(xiàng)目 spring-day1-constructor

2.導(dǎo)入依賴

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <!--這里是java 版本號(hào)-->
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
        <!--這里是方便版本控制-->
        <spring.version>5.3.1</spring.version>
        <lombok.version>1.18.20</lombok.version>
        <junit.version>4.12</junit.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-beans</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring.version}</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>${lombok.version}</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>${junit.version}</version>
        </dependency>
    </dependencies>

3.工程項(xiàng)目結(jié)構(gòu)

在這里插入圖片描述

樣例1：

1.創(chuàng)建一個(gè)Student類

public class Student {
    private Long number;
    private String name;
    private String school;
    public void setNumber(Long number) {
        this.number = number;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void setSchool(String school) {
        this.school = school;
    }
    public Student() {
    }
    public Student(Long number, String name, String school) {
        this.number = number;
        this.name = name;
        this.school = school;
    }
    @Override
    public String toString() {
        return "Student{" +
                "number=" + number +
                ", name='" + name + '\'' +
                ", school='" + school + '\'' +
                '}';
    }
}

寫一個(gè)配置文件

<?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">
    <!--這里是根據(jù)構(gòu)造函數(shù)內(nèi)的順序往里面注入-->
    <bean id="s1" class="com.crush.pojo.Student">
       <constructor-arg index="0" value="12"/>
        <constructor-arg index="1" value="wyh"/>
        <constructor-arg index="2" value="北大"/>
    </bean>
    <!--這里是根據(jù)構(gòu)造函數(shù)中的 類型來(lái)進(jìn)行注入 -->
    <bean id="s2" class="com.crush.pojo.Student">
        <constructor-arg type="java.lang.Long" value="123"/>
        <constructor-arg type="java.lang.String" value="crush"/>
        <constructor-arg type="java.lang.String" value="浙江大學(xué)"/>
    </bean>
</beans>

3.測(cè)試

   @org.junit.Test
    public void testStudent(){
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
        Student student = applicationContext.getBean("s2", Student.class);
        System.out.println(student);
    }

樣例2：

1.創(chuàng)建Teacher類

public class Teacher {
    private String name;
    private String school;
    private List<Student> studentList;
    private Map<String,String> map;
    private Set<String> set;
    public Teacher(String name, String school, List<Student> studentList, Map<String, String> map, Set<String> set) {
        this.name = name;
        this.school = school;
        this.studentList = studentList;
        this.map = map;
        this.set = set;
    }
    @Override
    public String toString() {
        return "Teacher{" +
                "name='" + name + '\'' +
                ", school='" + school + '\'' +
                ", studentList=" + studentList +
                ", map=" + map +
                ", set=" + set +
                '}';
    }
}public class Teacher {
    private String name;
    private String school;
    private List<Student> studentList;
    private Map<String,String> map;
    private Set<String> set;
    public Teacher(String name, String school, List<Student> studentList, Map<String, String> map, Set<String> set) {
        this.name = name;
        this.school = school;
        this.studentList = studentList;
        this.map = map;
        this.set = set;
    }
    @Override
    public String toString() {
        return "Teacher{" +
                "name='" + name + '\'' +
                ", school='" + school + '\'' +
                ", studentList=" + studentList +
                ", map=" + map +
                ", set=" + set +
                '}';
    }
}

2.beans.xml

<bean id="teacher" class="com.crush.pojo.Teacher">
    <constructor-arg index="0" value="xxx"/>
    <constructor-arg index="1" value="北京大學(xué)"/>
    <constructor-arg index="2" >
        <list>
            <ref bean="s1"/>
            <ref bean="s2"/>
        </list>
    </constructor-arg>
    <constructor-arg index="3">
        <map>
            <entry key="k1" value="xiaowang"/>
        </map>
    </constructor-arg>
    <constructor-arg index="4">
        <set>
            <value>1</value>
            <value>2</value>
        </set>
    </constructor-arg>
</bean>

3.測(cè)試

    @org.junit.Test
    public void testTeacher(){
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
        Teacher teacher  = applicationContext.getBean("teacher", Teacher.class);
        System.out.println(teacher);
    }

Spring單例模式和原型模式

一、單例模式

Spring默認(rèn)是單例模式的。

以Student的那個(gè)樣例1 為例。 scope=“singleton” 加上這么一個(gè)設(shè)置當(dāng)然默認(rèn)也是它。

bean id="s1" class="com.crush.pojo.Student" scope="singleton">
    <constructor-arg index="0" value="12"/>
    <constructor-arg index="1" value="wyh"/>
    <constructor-arg index="2" value="北大"/>
</bean>

這個(gè)時(shí)候我們來(lái)進(jìn)行測(cè)試

    @org.junit.Test
    public void testStudent(){
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
        Student student1 = applicationContext.getBean("s1", Student.class);
        Student student2 = applicationContext.getBean("s1", Student.class);
        // 并且如果我們對(duì)其中一個(gè)做了修改 ，其余也會(huì)跟著一起被修改
        // 可以看到我們只修改了一個(gè)
        student1.setSchool("夢(mèng)中的學(xué)校");
        System.out.println(student1);
        System.out.println(student2);
        System.out.println(student1==student2);
    }

在這里插入圖片描述

二、原型模式

我們還是以**Student來(lái)做例子講解 ** 注意：我們把原來(lái)設(shè)置改成了 scope=“prototype” 也就是原型模式

<!--這里是根據(jù)構(gòu)造函數(shù)中的 類型來(lái)進(jìn)行注入 -->
<bean id="s2" class="com.crush.pojo.Student" scope="prototype">
    <constructor-arg type="java.lang.Long" value="123"/>
    <constructor-arg type="java.lang.String" value="crush"/>
    <constructor-arg type="java.lang.String" value="浙江大學(xué)"/>
</bean>

接著測(cè)試

    @org.junit.Test
    public void testStudent(){
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
        Student student1 = applicationContext.getBean("s2", Student.class);
        Student student2 = applicationContext.getBean("s2", Student.class);
        // 并且如果我們對(duì)其中一個(gè)做了修改 ，其余也會(huì)跟著一起被修改
        // 可以看到我們只修改了一個(gè)
        student1.setSchool("夢(mèng)中的學(xué)校");
        System.out.println(student1);
        System.out.println(student2);
        System.out.println(student1==student2);
    }

在這里插入圖片描述

思考為什么需要依賴注入

為什么我們以前用一個(gè)對(duì)象 new一下就好了，但用了Spring 之后，反而還需要寫

這樣一段代碼再去獲取勒？明明感覺(jué)更麻煩啦丫？用這個(gè)又有什么樣的好處呢？

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("beans.xml");
Student student1 = applicationContext.getBean("s2", Student.class);