Spring IoC - 依赖注入 -> NullPointerException答案

【问题标题】：Spring IoC - Dependency Injection -> NullPointerExceptionSpring IoC - 依赖注入 -> NullPointerException
【发布时间】：2010-12-09 22:19:16
【问题描述】：

我有这个结构：

IProjectService 接口，由 ProjectServiceImpl 实现。

现在我有一个控制器，我想在其中使用依赖注入：

@Controller
@RequestMapping(value="/testFolder")
public class MyDIController {
    ...
    private IProjectService projectServiceImpl;

    public void setProjectServiceImpl(IProjectService projectServiceImpl) {
        this.projectServiceImpl= projectServiceImpl;
    }
    ...
    @RequestMapping(value="/project", method=RequestMethod.GET)
    public @ResponseBody Project getProject() {
        Project project = this.projectServiceImpl.getProject(12); // line 37! (id 12 is a test case, it does exist, here is he error: this.projectServiceImpl = null)
        return project;
    }
}

在我的 applicationContext.xml 我有这些代码行：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
    <bean id="projectServiceImpl" class="com.mydomain.project.service.impl.ProjectServiceImpl" />
    <bean id="projectService" class="com.mydomain.project.service.IProjectService">
        <property name="projectServiceImpl">
            <ref local="projectServiceImpl"/>
        </property>
    </bean>
</beans>

但是，如果我运行我的应用程序，我的 MyDIController 在这行代码中会收到一个错误 (NullPointerException)：

... = this.projectServiceImpl.getProject(12);

问题是，

this.projectServiceImpl

为空，因此依赖注入似乎不起作用。

这里怎么了，有谁知道吗？

最好的问候蒂姆。

更新：

完整的错误跟踪：

09.12.2010 11:16:53 org.apache.catalina.core.ApplicationDispatcher invoke
SCHWERWIEGEND: Servlet.service() for servlet project threw exception
java.lang.NullPointerException
    at com.mydomain.project.controller.MyDIController.getAllProjects(MyDIController.java:37)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:176)
    at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:426)
    at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:414)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:790)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:719)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:644)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:549)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
    at org.tuckey.web.filters.urlrewrite.NormalRewrittenUrl.doRewrite(NormalRewrittenUrl.java:195)
    at org.tuckey.web.filters.urlrewrite.RuleChain.handleRewrite(RuleChain.java:159)
    at org.tuckey.web.filters.urlrewrite.RuleChain.doRules(RuleChain.java:141)
    at org.tuckey.web.filters.urlrewrite.UrlRewriter.processRequest(UrlRewriter.java:90)
    at org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:417)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Thread.java:619)
09.12.2010 11:16:53 org.apache.catalina.core.StandardWrapperValve invoke
SCHWERWIEGEND: Servlet.service() for servlet default threw exception
java.lang.NullPointerException
    at com.mydomain.project.controller.MyDIController.getProject(MyDIController.java:37)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:176)
    at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:426)
    at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:414)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:790)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:719)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:644)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:549)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
    at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
    at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
    at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
    at org.tuckey.web.filters.urlrewrite.NormalRewrittenUrl.doRewrite(NormalRewrittenUrl.java:195)
    at org.tuckey.web.filters.urlrewrite.RuleChain.handleRewrite(RuleChain.java:159)
    at org.tuckey.web.filters.urlrewrite.RuleChain.doRules(RuleChain.java:141)
    at org.tuckey.web.filters.urlrewrite.UrlRewriter.processRequest(UrlRewriter.java:90)
    at org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:417)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Thread.java:619)

applicationContext.xml：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
    <bean id="projectService" class="com.mydomain.project.service.impl.ProjectServiceImpl" />

    <bean id="myDIController" class="com.mydomain.project.controller.MyDIController">
        <property name="projectServiceImpl">
            <ref local="projectService"/>
        </property>
    </bean>
</beans>

web.xml：

    <servlet>
    <servlet-name>project</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/spring/app-config.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>

app-config.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-3.0.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <!-- Scans the classpath of this application for @Components to deploy as beans -->
    <context:component-scan base-package="com.mydomain.project" />

    <!-- Application Message Bundle -->
    <bean id="messageSource" class="org.springframework.context.support.ReloadableResourceBundleMessageSource">
        <property name="basename" value="/WEB-INF/messages/messages" />
        <property name="cacheSeconds" value="0" />
    </bean>

    <!-- Configures Spring MVC -->
    <import resource="mvc-config.xml" />
</beans>

【问题讨论】：

如何获取你的控制器的实例？
小变量命名提示：不要用 *Impl 命名具有接口类型的变量...这是一个理所当然的事实，您使用实现
@Liviu T.: 好的，这个问题解决后我会改这个的。

标签： java spring dependency-injection

【解决方案1】：

你还没有为MyDIController注入属性projectServiceImpl

会议。应该是这样的

<bean id="projectService" class="com.mydomain.project.service.impl.ProjectServiceImpl" />

<bean id="myDIController " class="com.mydomain.project.service.MyDIController">
        <property name="projectServiceImpl">
            <ref local="projectService"/>
        </property>
</bean>

在 web.xml 中

<web-app>
     <context-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>
                /WEB-INF/application-context.xml                     
            </param-value>
        </context-param>
.
.
.
.
<servlet>
.
.
.

【讨论】：

@Bozho 他没有在控制器中注入服务，服务也应该在上下文中引用它的实现
啊，是的.. 他甚至没有映射控制器。
我仍然得到相同的错误：Servlet 项目的 Servlet.service() 在 = this.projectServiceImpl.getProject(12); 的行中抛出异常 java.lang.NullPointerException；
@Tim 你如何获得 myDIController 的实例？
@Tim 它将以这种方式工作，因为现在您的容器正在从 app-context 读取 bean，但更好的是这样做，就像服务 bean 应该在另一个上下文文件中并且控制器在不同的和您应该使用 <import resource="file name.xml"> 在控制器的 xml 文件中导入服务 bean，干杯 :)

【解决方案2】：

你有几个选择。

为您的控制器声明一个 XML Bean，并在其中为您的服务设置属性。
根据您的 Spring 版本，使用 Spring 的注释/自动装配支持。

如果您选择数字 2，请务必在您的 xml 配置中包含 <context:component-scan base-package="com.your.package"/>，并为您的 projectService 属性使用 @Controller 和 @Autowired 标记您的控制器类。

【讨论】：