Spring - 自动装配时出现 NullPointerException [重复]

Question

有没有办法直接从 main 方法运行注解配置的 Spring 应用程序？ 我仍然在下面得到 NullPointerException 运行代码。请注意，我没有使用 SpringMVC，并且当我使用在 context.xml 中定义的 bean，通过 context.getBean 方法注入 main 方法时，所有代码都可以正常工作（没有错误）-但我只是想知道这是否是一种方法仅使用注释管理运行它？

//IElem.java
package com.pack.elem;
public interface IElem {
   public abstract String sayHello();
}



//Elem.java
package com.pack.elem;
import org.springframework.stereotype.Component;
@Component
public class Elem implements IElem{
   @Override
   public String sayHello() {
       return ("Hello from Elem class object");
   }
}


//RunElem.java
package com.pack.elem;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class RunElem {
   @Autowired
   private IElem elem;

   public void runHello(){
      System.out.println(elem.sayHello());
   }
}


//Main.java
package com.pack.main;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import com.pack.elem.RunElem;

public class Main {
   @Autowired
   private RunElem runelem;

   public static void main(String[] args) {
       ApplicationContext context= new ClassPathXmlApplicationContext("/META-INF/application-context.xml");
    new Main().runRun();
   }

   private void runRun(){
      runelem.runHello();
   }
 }


<!--/META-INF/application-context.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.2.xsd">

   <context:annotation-config/>
   <context:component-scan base-package="com.pack"/>
</beans>

Answer 1

简单地说，如果 Spring 是实例化 bean 的那个，它就是自动装配的。所以你必须使用getBean()。如果您只想使用注释，那么应该可以这样：

@Component
public class Main {
   @Autowired
   private RunElem runelem;

   public static void main(String[] args) {
       ApplicationContext context= new ClassPathXmlApplicationContext("/META-INF/application-context.xml");
    Main mainBean = context.getBean(Main.class);
    mainBean.runRun();
   }

   private void runRun(){
      runelem.runHello();
   }
 }

或者，您可以自动装配现有的 bean，如下所示，但这不是推荐的 Spring 用法：

public class Main {
   @Autowired
   private RunElem runelem;

   public static void main(String[] args) {
       ApplicationContext context= new ClassPathXmlApplicationContext("/META-INF/application-context.xml");
    Main mainBean = new Main();
    context.getAutowireCapableBeanFactory().autowireBean(mainBean);
    mainBean.runRun();
   }

   private void runRun(){
      runelem.runHello();
   }
 }