Spring bean为什么需要依赖注入

具体步骤：

1.创建一个maven项目 spring-day1-constructor

2.导入依赖

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <!--这里是java 版本号-->
        <maven.compiler.source>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
        <!--这里是方便版本控制-->
        <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.工程项目结构

样例1：

1.创建一个Student类

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 + '\'' +
                '}';
    }
}

写一个配置文件

<?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">
    <!--这里是根据构造函数内的顺序往里面注入-->
    <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>
    <!--这里是根据构造函数中的 类型来进行注入 -->
    <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="浙江大学"/>
    </bean>
</beans>

3.测试

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

样例2：

1.创建Teacher类

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="北京大学"/>
    <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.测试

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

Spring单例模式和原型模式

一、单例模式

Spring默认是单例模式的。

以Student的那个样例1 为例。 scope=“singleton” 加上这么一个设置 当然默认也是它。

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>

这个时候我们来进行测试

    @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);
        // 并且如果我们对其中一个做了修改 ，其余也会跟着一起被修改
        // 可以看到我们只修改了一个
        student1.setSchool("梦中的学校");
        System.out.println(student1);
        System.out.println(student2);
        System.out.println(student1==student2);
    }

二、原型模式

我们还是以**Student来做例子讲解 ** 注意：我们把原来设置改成了 scope=“prototype” 也就是原型模式

<!--这里是根据构造函数中的 类型来进行注入 -->
<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="浙江大学"/>
</bean>

接着测试

    @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);
        // 并且如果我们对其中一个做了修改 ，其余也会跟着一起被修改
        // 可以看到我们只修改了一个
        student1.setSchool("梦中的学校");
        System.out.println(student1);
        System.out.println(student2);
        System.out.println(student1==student2);
    }

思考 为什么需要依赖注入

为什么我们以前用一个对象 new一下就好了，但用了Spring 之后，反而还需要写

这样一段代码再去获取勒？明明感觉更麻烦啦丫？用这个又有什么样的好处呢？

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

总结

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