Spring DI 简单案例

Posted 2022-11-27 GoldenaArcher

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Spring DI 简单案例

本案例中使用 XML 文档配置进行 Dependency Injection 的实现。

Spring 容器的主要功能包括：

创建和管理对象（Inversion of Control，IoC）
注入对象的依赖（Dependency Injection，DI）

这个案例中其他的代码沿用了 Spring IoC 简单案例部分代码。

本案例中需要两个功能：

提供每日锻炼（已经在 IoC 中实现的功能）
提供每日幸运语

这一部分需要实现一个新的帮助函数（依赖）：ForuneService

Spring 中提供了好几种依赖注入的方法，主要的有三种：

构造函数注入 construction injection
setter 注入 setter injection
注解中使用 auto wiring

本案例中会使用构造函数和 setter 注入进行实现。

构造函数注入

开发过程：

定义依赖 interface 和实现类

FortuneService interface：

package springdemo;

public interface FortuneService 
    public String getFortune();

新的实现类：

package springdemo;

public class HappyFortuneService implements FortuneService 

    @Override
    public String getFortune() 
        return "Today is your lucky day";

修改之前的 interface：

package springdemo;

public interface Coach 
    public String getDailyWorkout();
    
    public String getDailyFortune();

package springdemo;

public class BaseballCoach implements Coach 

    @Override
    public String getDailyWorkout() 
        return "Spend 30 minutes on batting practice.";
    

    @Override
    public String getDailyFortune() 
        return null;

package springdemo;

public class TrackCoach implements Coach 

    @Override
    public String getDailyWorkout() 
        return "Run a hrad 5k.";
    

    @Override
    public String getDailyFortune() 
        return null;

在实现类中创造一个构造函数让 Spring 实现注入

package springdemo;

public class TrackCoach implements Coach 

    // define a private field for the dependency
    private FortuneService fortuneService;
    
    // define a constructor for dependency injection
    public TrackCoach(FortuneService fortuneService) 
        this.fortuneService = fortuneService;
    
    
    @Override
    public String getDailyWorkout() 
        return "Run a hrad 5k.";
    

    @Override
    public String getDailyFortune() 
        // use my fortuneService to get a fortune
        return fortuneService.getFortune();

在 Spring 配置文件中注入依赖

    <bean id="myFortune" class="springdemo.HappyFortuneService"></bean>
    <!-- id is like an alias -->
    <bean id="myCoach" class="springdemo.TrackCoach">
        <constructor-arg ref="myFortune" />
    </bean>

这时候运行结果如下：

XML 中的内容最终在 Spring 内部转化为对应的 Java 代码，创建对应的对象和传入参数：

HappyFortuneService myFortuneService = new HappyFortuneService();
BaseballCoach myCoach = new BaseballCoach(myFortuneService);

Setter 注入

这里会创建一个新的类以及新的主函数进行实现。

开发流程：

在类中创建 setter 进行依赖注入

package springdemo;

public class CricketCoach implements Coach 
    private FortuneService fortuneService;

    public FortuneService getFortuneService() 
        return fortuneService;
    

    public void setFortuneService(FortuneService fortuneService) 
        System.out.println("CricketCoach: inside setter method");
        this.fortuneService = fortuneService;
    

    public CricketCoach() 
        System.out.println("CricketCoach: inside no-arg constructor");
    

    @Override
    public String getDailyWorkout() 
        return "Practice fast bowling for 15 minutes";
    

    @Override
    public String getDailyFortune() 
        return fortuneService.getFortune();

在 Spring 配置文件中完成依赖注入

<!-- define the dependency -->
<bean id="myFortuneService" class="springdemo.HappyFortuneService"></bean>

<bean id="myCricketCoach" class="springdemo.CricketCoach">
	<!-- set up setter injection -->
	<!-- name must be identical to the property in object -->
	<property name="fortuneService" ref="myFortuneService" />
</bean>

对应新的 Main 函数：

package springdemo;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SetterDemoApp 

    public static void main(String[] args) 
        // load the spring configuration file
        ClassPathXmlApplicationContext context = 
                new ClassPathXmlApplicationContext("applicationContext.xml");
                
        // retrieve bean from spring container
        Coach theCoach = context.getBean("myCricketCoach", Coach.class);
                
        // call methods on the bean
        System.out.println(theCoach.getDailyWorkout());
                
        // call new method for fortune
        System.out.println(theCoach.getDailyFortune());
                
        // close the context
        context.close();

运行结果：

注入字面值

Spring 也可以通过配置文件注入字面值。

开发过程：

在类中创建 setter 进行依赖注入

新增邮箱和队伍属性：

package springdemo;

public class CricketCoach implements Coach 
    private FortuneService fortuneService;
    
    private String emailAddress;

    private String team;

    public FortuneService getFortuneService() 
        return fortuneService;
    

    public CricketCoach() 
        System.out.println("CricketCoach: inside no-arg constructor");
    
    
    public void setFortuneService(FortuneService fortuneService) 
        System.out.println("CricketCoach: inside setter method");
        this.fortuneService = fortuneService;
    
    
    public String getEmailAddress() 
        return emailAddress;
    

    public void setEmailAddress(String emailAddress) 
        this.emailAddress = emailAddress;
    

    public String getTeam() 
        return team;
    

    public void setTeam(String team) 
        this.team = team;
    

    @Override
    public String getDailyWorkout() 
        return "Practice fast bowling for 15 minutes";
    

    @Override
    public String getDailyFortune() 
        return fortuneService.getFortune();

在 Spring 配置文件中完成依赖注入

    <bean id="myCricketCoach" class="springdemo.CricketCoach">
        <!-- set up setter injection -->
        <!-- name must be identical to the property in object -->
        <property name="fortuneService" ref="myFortuneService" />
        <property name="emailAddress" value="test@test.com"></property>
        <property name="team" value="Yankee"></property>
    </bean>

更新主函数：

package springdemo;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SetterDemoApp 

    public static void main(String[] args) 
        // load the spring configuration file
        ClassPathXmlApplicationContext context = 
                new ClassPathXmlApplicationContext("applicationContext.xml");
                
        // retrieve bean from spring container
        CricketCoach theCoach = context.getBean("myCricketCoach", CricketCoach.class);
                
        // call methods on the bean
        System.out.println(theCoach.getDailyWorkout());
                
        // call new method for fortune
        System.out.println(theCoach.getDailyFortune());
        
        System.out.println(theCoach.getEmailAddress());
        System.out.println(theCoach.getTeam());
                
        // close the context
        context.close();

运行结果：

通过配置文件注入字面值

除了直接在 Spring 配置文件中注入字面值之外，Spring 还能通过在 Spring 配置文件中读取其他配置文件的方式，更加动态地获取字面值。

开发流程如下：

新建属性文件

foo.email=email@properties.com
foo.team=Team From Properties File

在 Spring 配置文件中加载属性文件

🚰属性文件

<!-- load the properties files: sport.properties -->
<context:property-placeholder location="classpath:sport.properties" />

    <bean id="myCricketCoach" class="springdemo.CricketCoach">
	<!-- set up setter injection -->
	<!-- name must be identical to the property in object -->
	<property name="fortuneService" ref="myFortuneService" />
	
	<!-- <property name="emailAddress" value="test@test.com"></property> -->
	<!-- <property name="team" value="Yankee"></property> -->
	<property name="emailAddress" value="$foo.email" />
	<property name="team" value="$foo.team" />
	
</bean>

运行结果如下：

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