如何使自动装配在配置 class 中工作？

Question

我正在尝试在 @Configuration class 中自动装配一个标记为 @Service 的属性 (myService)，但我得到一个 NullPointer。
相反，如果我在非配置 classes 中自动装配 myService，我没有问题。

这是我在自动装配时遇到问题的@Service：

package com.myapp.resources;

@Service
class MyService {
    public List<String> getRoutingKeys() {
        List<String> routingKeys;

        //Do stuff

        return routingKeys;
    }

    public String aMethod() {

        return "hello";
    }
}

这是我无法自动装配服务的 @Configuration class

package com.myapp.messaging;

import com.myapp.resources;
import org.springframework.amqp.core.Binding;
import org.springframework.amqp.core.BindingBuilder;
import org.springframework.amqp.core.Queue;
import org.springframework.amqp.core.TopicExchange;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import java.util.List;

@Configuration
public class RabbitConfiguration {
    private List<String> routingKeys = writeRoutingKeys();

    @Autowired
    private MyService myService;

    private List<String> writeRoutingKeys() {
        boolean test = myService == null;
        System.out.println("is the service null? " + test); //output: true!!!

        return myService.getRoutingKeys(); //here I get a NullPointer
    }

    //Methods with bean declarations for RabbitMQ
 }

如果有帮助，这是我的主要class:

package com.myapp;

import com.myapp.resources;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

import java.util.List;

@SpringBootApplication
public class Application {

    public static void main(String[] args) {
        ConfigurableApplicationContext appContext = SpringApplication.run(Application.class, args);

        MyService myService = (MyService) appContext.getBean(MyService.class);

        boolean test = myService == null;
        System.out.println("is the service null? " + test); //output: false

        //Do stuff
    }
}

如果有帮助，这里有一个不同的 class（@RestController），我可以在其中自动连接服务

package com.myapp.resources;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class MyController {

    @Autowired
    private MyService myService;

    @GetMapping("/endpoint")
    public String myRestMethod() {
        boolean test = myService == null;
        System.out.println("is the service null? " + test); //output: false

        return myService.aMethod();
    }
}

我也试过在配置中添加 @ComponentScan class，但我仍然得到一个 NullPointer

package com.myapp.messaging;

//list of imports...

@Configuration
@ComponentScan("com.myapp.demo")
public class RabbitConfiguration {
   @Autowired
   private MyService myService;

   //...
}

Answer 1

我建议通过任何需要它的 @Bean 创建方法来注入服务：

@Bean
public MyBean create(MyService myService)

然后将服务传入writeRoutingKeys(MyService myService)方法进行相应的处理。

根据文档：

@Configuration classes are processed quite early during the initialization of the context and forcing a dependency to be injected this way may lead to unexpected early initialization. Whenever possible, resort to parameter-based injection as in the example above.

Answer 2

Spring 只会在实例化 bean 之后或实例化时注入依赖项（取决于是否使用构造函数注入）。但是，您现在在初始化 bean 之前发生的字段初始化期间访问依赖项 MyService。因此，它无法在字段初始化期间访问 MyService，因为它尚未注入。

您可以通过更改为使用构造函数注入并同时在构造函数中初始化 routingKeys 来简单地修复它：

@Configuration
public class RabbitConfiguration {

    private List<String> routingKeys ;
    private MyService myService;

    @Autowired
    public RabbitConfiguration(MyService myService){
        this.myService = myService
        this.routingKeys = writeRoutingKeys();
    }

    private List<String> writeRoutingKeys() {
        return myService.getRoutingKeys(); 
    }
 }

或者简单地说：

@Autowired
public RabbitConfiguration(MyService myService){
    this.myService = myService
    this.routingKeys = myService.getRoutingKeys();
}

如何使自动装配在配置 class 中工作？

How do I make Autowired work inside a Configuration class?

java

spring

autowired

spring-boot