3、Spring之入門案例

3.1、創建module

3.1.1、右擊project，創建新module

3.1.2、選擇maven

3.1.3、設置module名稱和路徑

3.1.4、module初始狀態

3.1.5、配置打包方式和依賴

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.rain</groupId>
    <artifactId>spring_helloword</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <dependencies>
        <!-- 基於Maven依賴傳遞性，導入spring-context依賴即可導入當前所需所有jar包 -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.3.1</version>
        </dependency>
        <!-- junit測試 -->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

3.2、示例

3.2.1、創建組件類

package org.rain.spring.component;

/**
 * @author liaojy
 * @date 2023/7/25 - 19:17
 */
public class HelloWord {

    public void sayHello(){
        System.out.println("Hello,Spring!");
    }
}

3.2.2、創建Sprig配置文件

++++++++++++++++++++++++++++++++++++分割線++++++++++++++++++++++++++++++++++++

註意：因為會通過自定義代碼指定Spring配置文件，所以Spring配置文件名可以是任意的；
但當整合ssm後，就不能通過通過自定義代碼指定Spring配置文件，因此文件名有硬性要求。

<?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標簽：配置交給Spring的IOC容器管理的對象
            id屬性：設置bean的唯一標識
            class屬性：設置bean所對應類型的全類名
    -->
    <bean id="helloWord" class="org.rain.spring.component.HelloWord"></bean>

</beans>

3.2.3、創建測試類

如圖所示，獲取到了IOC容器和容器中對應的bean組件，併成功調用了該bean組件的方法

package org.rain.spring.test;

import org.junit.Test;
import org.rain.spring.component.HelloWord;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * @author liaojy
 * @date 2023/7/25 - 19:39
 */
public class HelloWordTest {

    @Test
    public void testHelloWord(){
        //獲取IOC容器
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        //獲取IOC容器中的bean
        HelloWord helloWord = (HelloWord)applicationContext.getBean("helloWord");
        helloWord.sayHello();
    }
}

3.3、獲取bean的三種方式

3.3.1、根據id獲取

由於 id 屬性是 bean 的唯一標識，所以根據 bean 標簽的 id 屬性可以精確獲取到一個組件對象；但也存在類型轉換問題，具體見上一小節。

3.3.2、根據類型獲取（最常用）

如圖所示，根據類型獲取bean，則不存在類型轉換問題

    @Test
    public void testHelloWord(){
        //獲取IOC容器
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        //獲取IOC容器中的bean
        HelloWord helloWord = applicationContext.getBean(HelloWord.class);
        helloWord.sayHello();
    }

註意：如下圖所示，當根據類型獲取bean時，要求IOC容器中指定類型的bean有且只能有一個；否則會報錯

org.springframework.beans.factory.NoUniqueBeanDefinitionException: 
No qualifying bean of type 'org.rain.spring.component.HelloWord' available: 
expected single matching bean but found 2: helloWord,helloWordtwo

+++++++++++++++++++++++++++++++++++分割線+++++++++++++++++++++++++++++++++++

3.3.3、根據id和類型獲取

    @Test
    public void testHelloWord(){
        //獲取IOC容器
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        //獲取IOC容器中的bean
        HelloWord helloWord = applicationContext.getBean("helloWord",HelloWord.class);
        helloWord.sayHello();
    }

3.3.4、重要擴展

如果組件類實現了介面，則根據介面類型可以獲取 bean，前提是IOC容器中實現該介面的組件類型的bean有且只能有一個

+++++++++++++++++++++++++++++++++++分割線+++++++++++++++++++++++++++++++++++

+++++++++++++++++++++++++++++++++++分割線+++++++++++++++++++++++++++++++++++

+++++++++++++++++++++++++++++++++++分割線+++++++++++++++++++++++++++++++++++

    @Test
    public void testHelloWord(){
        //獲取IOC容器
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
        //根據介面類型獲取IOC容器中的bean
        HelloWord helloWord = (HelloWord) applicationContext.getBean(Hello.class);
        helloWord.sayHello();
    }