注入一个被注入的类答案

【问题标题】：Inject into a being injected class注入一个被注入的类
【发布时间】：2017-04-19 05:25:46
【问题描述】：

我在我的应用程序中使用 Google guice 进行依赖注入。我有这样的类结构，

示例取自here

这是我的界面，

package com.journaldev.di.services;

@ImplementedBy(EmailService.class)
public interface MessageService {

    boolean sendMessage(String msg, String receipient);
}

我将在这里实现

package com.journaldev.di.services;

import javax.inject.Singleton;

//import com.google.inject.Singleton;

@Singleton
public class EmailService implements MessageService {

    public boolean sendMessage(String msg, String receipient) {
        //some fancy code to send email
        System.out.println("Email Message sent to "+receipient+" with message="+msg);
        return true;
    }
}

如果我在这里注入 EmailService。

package com.journaldev.di.consumer;

import javax.inject.Inject;

import com.journaldev.di.services.MessageService;

public class MyApplication {

    private MessageService service;

    @Inject
    public void setService(MessageService svc){
        this.service=svc;
    }

    public boolean sendMessage(String msg, String rec){
        //some business logic here
        return service.sendMessage(msg, rec);
    }
}

如果假设我的 EmailService 类看起来像这样，

package com.journaldev.di.services;

import javax.inject.Singleton;

//import com.google.inject.Singleton;

@Singleton
public class EmailService implements MessageService {
    public EmailService(int someValue) {
         FancyEmailService fancyEmailService = new FancyEmailService(someValue);
    }
    public boolean sendMessage(String msg, String receipient) {
        fancyEmailService.doSomething();
        System.out.println("Email Message sent to "+receipient+" with message="+msg);
        return true;
    }
}

为了测试上面的 EmailService 代码，我需要注入 FancyEmailService 而不是从构造函数中实例化。如何将 FancyEmailService 注入 EmailService 代码？并且仍然能够将 EmailService 注入 MyApplication。

【问题讨论】：

标签： java dependency-injection guice

【解决方案1】：

你需要看看这些 - https://github.com/google/guice/wiki/InstanceBindings 和 https://github.com/google/guice/wiki/ProvidesMethods

public class GuiceTestModule extends AbstractModule {

    @Override
    protected void configure() {
        bind(Integer.class)
        .annotatedWith(Names.named("fancy"))
        .toInstance(10);
    }

    @Provides
    FancyEmailService fancyEmailServiceProvider(@Named("fancy") int i){
        return new FancyEmailService(i);
    }

}

我的 FancyEmailService 和 EMailService 如下所示。

public class FancyEmailService {

    private final int i;

    public FancyEmailService(int i){
        this.i = i;
    }

    public void doSomething(){
        System.out.println("i is injected with " + i);
    }

}

@Singleton
public class EmailService implements MessageService {

    private FancyEmailService fancyEmailService;

    @Inject
    public EmailService(@Named("fancy") int someValue){
       fancyEmailService = new FancyEmailService(someValue);
    }

    public boolean sendMessage(String msg, String receipient) {
        //some fancy code to send email
        System.out.println("Email Message sent to "+receipient+" with message="+msg);
        fancyEmailService.doSomething();
        return true;
    }
}

让我们测试

public class GuiceTest {

    public static void main(String[] args) {


        Injector injector = Guice.createInjector(new GuiceTestModule());

        MyApplication appn = injector.getInstance(MyApplication.class);
        appn.sendMessage("msg", "guru");
    }

}

输出：

Email Message sent to guru with message=msg
i is injected with 10

EmailService 也可以使用@Provides 以这种方式编写

@Singleton
public class EmailService implements MessageService {

    private FancyEmailService fancyEmailService;

    @Inject
    public EmailService(FancyEmailService fes){
       fancyEmailService = fes;
    }

    public boolean sendMessage(String msg, String receipient) {
        //some fancy code to send email
        System.out.println("Email Message sent to "+receipient+" with message="+msg);
        fancyEmailService.doSomething();
        return true;
    }
}

【讨论】：