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Spring bean为什么需要依赖注入

作者：宁在春　　发布时间：2022-01-24 11:07:21　

标签：Spring,bean,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>

<maven.compiler.source>11</maven.compiler.source>
<maven.compiler.target>11</maven.compiler.target>

<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">

<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>

<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

<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);

来源：https://blog.csdn.net/weixin_45821811/article/details/117675372

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