spring autowire is not working returning null

问题

i'm learning spring now. here's my sample code. i'm using jersey, spring, hibernate, and mysql for my REST service.

CustomerServiceImpl.java which is REST endpoint (partial code)

package com.samples.service.impl;

@Path("customers")
public class CustomerServiceImpl implements CustomerService {

    private static Logger logger = Logger.getLogger(CustomerServiceImpl.class);

    @Autowired
    CustomerBO customerBO;

here is CustomerBOImpl.java (partial code)

package com.samples.BO.impl;

@Component
public class CustomerBOImpl implements CustomerBO {

    @Autowired
    CustomerDAO customerDAO;

    @Autowired
    CustomerAdapter customerAdapter;

CustomerDAOImpl.class package com.samples.DAO.impl;

@Repository
public class CustomerDAOImpl implements CustomerDAO {

this is applicationContext.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"
    xmlns:tx="http://www.springframework.org/schema/tx"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
              http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
              http://www.springframework.org/schema/context 
              http://www.springframework.org/schema/context/spring-context-3.0.xsd
              http://www.springframework.org/schema/tx 
              http://www.springframework.org/schema/tx/spring-tx-3.1.xsd">

<context:property-placeholder location="classpath*:database.properties"/>
<context:component-scan base-package="com.samples"/>
<context:annotation-config />

</beans>

this is first few lines of exception i'm getting.

http-bio-8090-exec-1] [class: CustomerServiceImpl] INFO  - list all customers
[http-bio-8090-exec-1] [class: CustomerServiceImpl] INFO  - customerBO is null
May 08, 2014 10:55:29 AM com.sun.jersey.spi.container.ContainerResponse mapMappableContainerException
SEVERE: The RuntimeException could not be mapped to a response, re-throwing to the HTTP container
java.lang.NullPointerException
    at com.samples.service.impl.CustomerServiceImpl.getAllCustomers(CustomerServiceImpl.java:40)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.sun.jersey.spi.container.JavaMethodInvokerFactory$1.invoke(JavaMethodInvokerFactory.java:60)
    at com.sun.jersey.server.impl.model.method.dispatch.AbstractResourceMethodDispatchProvider$TypeOutInvo

this is web.xml

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
version="2.5">
    <display-name>Employee Service</display-name>
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>

        classpath*:applicationContext.xml
        </param-value>
    </context-param>

    <context-param>
        <param-name>initializeContextOnStartup</param-name> 
        <param-value>true</param-value> 
    </context-param>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> 
    </listener>

    <servlet>
        <servlet-name>jersey-serlvet</servlet-name>
        <servlet-class>com.sun.jersey.spi.container.servlet.ServletContainer</servlet-class>
            <init-param>
            <param-name>com.sun.jersey.config.property.packages</param-name>
            <param-value>com.samples.service</param-value>
        </init-param>
        <init-param>
            <param-name>com.sun.jersey.api.json.POJOMappingFeature</param-name>
            <param-value>true</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>jersey-serlvet</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>


</web-app>

so if i'm understanding how this works correctly, i'm configuring xml to have my components autoscanned by providing the package under which i want to run autoscan. and for objects that i want to autowire. in CustomerServiceImpl class, i use @autowired for customerBO object which should have been scanned by @Component annotation on CustomerBOImpl.class definition. can you please help why my auto-scan is not picking up autowired customerBO object? thanks.

回答1:

I suspect the problem is your CustomerServiceImpl class is being instantiated outside of Spring, either by the servlet container or explicitly in your code. You need to either have Spring instantiate it (by making it a bean, etc) or use <context:spring-configured/>. I'm not sure if the latter will work if the servlet container instantiates the object as it will likely depend on the order in which things occur...

回答2:

Spring only know to autowire fields that have been marked as beans. From your code CustomerDAO is not a bean. For example to define it as a bean in the xml configuration file you need to add <bean id="customerDao" class="com.packagesNames.CustomerDaoImpl"/> in the xml file. or to have the class annotated with @Component or to have it defined with @Bean in a @Configuration class.

回答3:

Jersey 2 and Spring integration is now supported.

Jersey Docs: Chapter 22. Spring DI

Blog: Implementing Jersey 2 Spring Integration

Sample App: Jersey's Hello World Spring WebApp

However, as of this writing, Spring beans can't be injected directly into JAX-RS classes by using Spring XML configuration. Annotations (@Autowired, etc.) must be used.

回答4:

I had the same problem. My injected bean in the resource class was always coming as NULL.

I spent 2 days trying to figure it out why I can't inject the Spring Bean into the JAX-RS resource class. Then I found the below in the Jersey documentation:

"Limitations:

Spring beans can't be injected directly into JAX-RS classes by using Spring XML configuration"

Note: I was using Jersey 2 and Spring 3.x in my project.

来源：https://stackoverflow.com/questions/23549322/spring-autowire-is-not-working-returning-null

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autowired