如何使自动装配在配置类中工作？

Question

我正在尝试@Service在 @Configuration 类中自动装配一个标记为 a 的属性 (myService)，但我得到了一个 NullPointer。

相反，如果我myService在非配置类中自动装配，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 类

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，这是我的主类：

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，这里有一个不同的类（@RestController），我可以在其中自动装配服务

Answer 1

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）。MyService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。因此，它无法MyService在字段初始化期间访问，因为它尚未注入。您可以通过更改为routingKeys同时在构造函数中使用构造函数注入和初始化来简单地修复它：@Configurationpublic class RabbitConfiguration {&nbsp; &nbsp; private List<String> routingKeys ;&nbsp; &nbsp; private MyService myService;&nbsp; &nbsp; @Autowired&nbsp; &nbsp; public RabbitConfiguration(MyService myService){&nbsp; &nbsp; &nbsp; &nbsp; this.myService = myService&nbsp; &nbsp; &nbsp; &nbsp; this.routingKeys = writeRoutingKeys();&nbsp; &nbsp; }&nbsp; &nbsp; private List<String> writeRoutingKeys() {&nbsp; &nbsp; &nbsp; &nbsp; return myService.getRoutingKeys();&nbsp;&nbsp; &nbsp; }&nbsp;}或者简单地说：@Autowiredpublic RabbitConfiguration(MyService myService){&nbsp; &nbsp; this.myService = myService&nbsp; &nbsp; this.routingKeys = myService.getRoutingKeys();}

Answer 2

@Bean我建议通过任何需要它的创建方法来注入服务：@Bean public&nbsp;MyBean&nbsp;create(MyService&nbsp;myService)然后将服务传递给writeRoutingKeys(MyService myService)方法进行相应的处理。根据文档：@Configuration 类在上下文初始化期间很早就被处理，强制以这种方式注入依赖项可能会导致意外的提前初始化。只要有可能，就如上例那样使用基于参数的注入。