使用@Autowired属性时为NPE（但使用ApplicationContext设置属性时不为NPE）

Question

I'm trying to use the @Autowired annotation to set a property on a jax-rs restful service but I get a null pointer exception when the property is referenced. This is the first time I've tried using this annotation.

package com.pallelli.mvcpract.rest;

import javax.servlet.ServletContext;
import javax.ws.rs.Consumes;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.Context;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;
import org.springframework.web.context.support.WebApplicationContextUtils;

import com.pallelli.hibpract.policymodel.PolicyDao;
import com.pallelli.hibpract.policymodel.beans.Risk;

@Service("riskService")
@Path("risk")
@Component
public class RiskService {

    //@Context
    //private ServletContext context; 

    @Autowired
    private PolicyDao policyDao;

    @POST
    @Consumes(MediaType.APPLICATION_JSON)
    @Produces(MediaType.APPLICATION_JSON)
    public Response storeRisk(Risk risk) {

        //ApplicationContext ctx = 
        //WebApplicationContextUtils.getWebApplicationContext(context);
        //policyDao = ctx.getBean(PolicyDao.class);

        policyDao.addRisk(risk);
        risk.setName(risk.getName()+" : processed");
        return Response.ok(risk).status(200).build();
    }
}

Everything works if I remove the comments so that policyDao is created using an application context, hence I think that spring is being made aware of the bean.

I'm using the following in mvc-dispatcher-servlet.xml to get spring to look for the beans.

<context:component-scan base-package="com.pallelli.mvcpract.rest" />
<context:component-scan base-package="com.pallelli.hibpract.policymodel" />

This is the PolicyDao class (and I know that it is 'wrong')

package com.pallelli.hibpract.policymodel;

import org.hibernate.Session;
import org.springframework.stereotype.Component;

import com.pallelli.hibpract.policymodel.beans.Risk;

@Component
public class PolicyDao {
    public void addRisk(Risk risk) {
        Session session = null;
        try {
            session = Main.getSessionFactory().openSession();
            session.beginTransaction();
            session.persist(risk);
            session.getTransaction().commit();
        }
        finally {
            if(session != null) session.close();
        }
    }
}

The debug log seems to suggest that the autowiring worked

20:10:41 DEBUG DefaultListableBeanFactory:220 - Creating shared instance of singleton bean 'riskService'
20:10:41 DEBUG DefaultListableBeanFactory:449 - Creating instance of bean 'riskService'
20:10:41 DEBUG InjectionMetadata:71 - Registered injected element on class [com.pallelli.mvcpract.rest.RiskService]: AutowiredFieldElement for private com.pallelli.hibpract.policymodel.PolicyDao com.pallelli.mvcpract.rest.RiskService.policyDao
20:10:41 DEBUG DefaultListableBeanFactory:523 - Eagerly caching bean 'riskService' to allow for resolving potential circular references
20:10:41 DEBUG InjectionMetadata:85 - Processing injected method of bean 'riskService': AutowiredFieldElement for private com.pallelli.hibpract.policymodel.PolicyDao com.pallelli.mvcpract.rest.RiskService.policyDao
20:10:41 DEBUG DefaultListableBeanFactory:220 - Creating shared instance of singleton bean 'policyDao'
20:10:41 DEBUG DefaultListableBeanFactory:449 - Creating instance of bean 'policyDao'
20:10:41 DEBUG DefaultListableBeanFactory:523 - Eagerly caching bean 'policyDao' to allow for resolving potential circular references
20:10:41 DEBUG DefaultListableBeanFactory:477 - Finished creating instance of bean 'policyDao'
20:10:41 DEBUG AutowiredAnnotationBeanPostProcessor:427 - Autowiring by type from bean name 'riskService' to bean named 'policyDao'
20:10:41 DEBUG DefaultListableBeanFactory:477 - Finished creating instance of bean 'riskService'
...
20:18:45 DEBUG DefaultListableBeanFactory:247 - Returning cached instance of singleton bean 'policyDao'

Any ideas as to why the autowired property on RiskService is null?

Answer 1

It seems like jax-rs is not able to process Spring annotations.

Some additional setup is required to make jax-rs objects aware of Spring Beans. Without proper initialization Autowired, Transactional or any other Spring annotation is not processed.

Assuming that you are using Jersey and Spring 3 you need to include library which provides a "bridge" between Jersey and Spring:

<dependency>
    <groupId>org.glassfish.jersey.ext</groupId>
    <artifactId>jersey-spring3</artifactId>
    <version>2.22.1</version>
</dependency>

There is a separate library which provides support for Spring 4. For details see Jersey documentation and this example .

You managed to get ApplicationContext as it is stored as an attribute in ServletContext so it can be retrieved from any place in the application using static method call. As you noticed beans are properly initialized but both frameworks just do not talk to each other.