java Guice依赖项注入器空指针异常

Question

我在java中使用了以下代码。我不知道我做错了什么

我的主要文件是：查看并检查

package com.sample.test;    
import com.google.inject.Guice;
import com.google.inject.Injector;

public class mymain {    
public static void main(String[] args) {    
Injector injector = Guice.createInjector(new AppInjectory());
ApplicationExample obj =                               injector.getInstance(ApplicationExample.class);
obj.sendMessage();    
}    
}

我的界面是：查看并检查

package com.sample.test;    
public interface MessageService {
boolean sendMessage(String msg, String receipient);
}

我的配置文件是look and check

package com.sample.test;
import com.google.inject.AbstractModule;    
public class AppInjectory extends AbstractModule {    
@Override
protected void configure() {     
bind(MessageService.class).to(EmailService.class);    
}    
}

我的申请文件是：查看并检查

package com.sample.test;
import javax.inject.Inject;    
    public class ApplicationExample {        
    private MessageService service;        
    @Inject
    public void setService(MessageService svc){
    this.service=svc;
    }        
    public void sendMessage() {
    System.out.println(“I am here”);
    service.sendMessage(“welcome”, “java”);        
    }
    }

我的服务级别是：查看和检查

package com.sample.test;
//import com.google.inject.Singleton;
import javax.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;
}    
}

这里我得到了空指针异常。我在这里做错了什么。？请帮助解决此问题。我在这里添加了错误堆栈跟踪。 请看

错误：

Exception in thread “main” I am here java.lang.NullPointerException at com.sample.test.ApplicationExample.sendMessage(ApplicationExample.java:16) at com.sample.test.mymain.main(mymain.java:13)

Answer 1

问题在于这一行：

ApplicationExample obj = injector.getInstance(ApplicationExample.class);

在AppInjectory模块中，您没有将ApplicationExample接口绑定到实现。您是否打算这样做（从您的模块中扣除）：

MessageService obj = injector.getInstance(MessageService.class);