Spring框架空指针异常

Question

Trying to autowire Spring bean with property, but still getting NPE. Snippets:

INFO: Loading XML bean definitions from class path resource [autoWireByName.xml]
Exception in thread "main" today do push-ups for 30 mins
java.lang.NullPointerException
    at com.springAutoWireByName.KabadiCoach.getFortune(KabadiCoach.java:23)
    at com.springAutoWireByName.AutoWireByName.main(AutoWireByName.java:13)

KabadiCoach.java

package com.springAutoWireByName;

public class KabadiCoach {

    private SadFortune sadFortune;

    /*public KabadiCoach(){

        System.out.println("inside default Constructor");

    }*/
    public String getDailyWorkout()
    {
        return "today do push-ups for 30 mins";
    }
    public void setSadFortune(SadFortune fortune) {

        sadFortune = fortune;
    }

    public String  getFortune() {       

        return sadFortune.getSadFortune();
    }
}

SadFortune.java

package com.springAutoWireByName;

public class SadFortune {

    public  String getSadFortune()
    {
        System.out.println();
        return "your day wont be good enough Sorry!!!";
    }       
 }

<?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.xsd">

<!-- bean definitions here -->

<bean name="Fortune" class="com.springAutoWireByName.SadFortune">
</bean>
<bean id="myCoach" class="com.springAutoWireByName.KabadiCoach" autowire="byName" />


<!-- this is just a prototype to define actual just make use of this file-->

</beans>

main

package com.springAutoWireByName;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class AutoWireByName {

    public static void main(String[] args) {

        ClassPathXmlApplicationContext context =new ClassPathXmlApplicationContext("autoWireByName.xml");

        KabadiCoach co = context.getBean("myCoach",KabadiCoach.class);
        System.out.println(co.getDailyWorkout());
        System.out.println(co.getFortune());
    }
}

after running the above code I am getting the error message as listed and when I change the method to static

public static  String getSadFortune()
{
    System.out.println();
    return "your day wont be good enough Sorry!!!";
}

In class 2 I got the desired output. Why?

Answer 1

Basically the SadFortune member is still null, which is why it works if the getSadFortune() method will be made static.

Until now you only tell Spring to instantiate a Fortune bean and a KabadiCoach, but you have to tell Spring that some members in the KabadiCoach need to be autowired too.

Try this in your Spring configuration file:

<bean id="fortune" class="com.springAutoWireByName.SadFortune"></bean>
<bean id="myCoach" class="com.springAutoWireByName.KabadiCoach" autowire="byName">
    <property name="fortune" ref="fortune" />
</bean>

EDIT: Sorry, overread the autowire="byName" attribute. In this case you probably just have write the name in lower case.

<bean id="fortune" class="com.springAutoWireByName.SadFortune"/>
<bean id="myCoach" class="com.springAutoWireByName.KabadiCoach" autowire="byName"/>