java - ActionSupport 类上的 Struts2 空指针异常

Question

我目前有两个 ActionSuppport 类，这两个类都是相同的，唯一的区别是它们调用不同的服务方法。

当我使用此代码时(BaseAction.java)

public class BaseAction extends ActionSupport {

    protected Service services;

    protected String redirectUrl;

    public String getRedirectUrl() {
        return redirectUrl;
    }

    public void setRedirectUrl(String redirectUrl) {
        this.redirectUrl = redirectUrl;
    }

    public void setServices(Service services) {
        this.services = services;
    }
}

Listing.java 扩展了 BaseAction.java 一切似乎都工作正常

public class Listing extends BaseAction{
    public String execute(){
        events = services.getEvents();
        return "success";
    }

    List<Event> events;

    public List<Event> getEvents(){ return events; }
}

这是另一个 ActionSupport 类 (EventAction.java)

public class EventAction extends ActionSupport {

    protected EventService services;

    protected String redirectUrl;

    public String getRedirectUrl() {
        return redirectUrl;
    }

    public void setRedirectUrl(String redirectUrl) {
        this.redirectUrl = redirectUrl;
    }

    public void setEventService(EventService services) {
        this.services = services;
    }
}

然后Listing.java扩展到EventAction.java

public class Listing extends EventAction{
    public String execute(){
        events = services.getEvents();
        return "success";
    }

    List<Event> events;

    public List<Event> getEvents(){ return events; }
}

我收到此空指针错误。

java.lang.NullPointerException
    actions.events.Listing.execute(Listing.java:13)
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    java.lang.reflect.Method.invoke(Unknown Source)
    com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:446)
    com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:285)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
    org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
    org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:238)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:238)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:191)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:90)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:252)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:193)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54)
    org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:544)
    org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
    org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
    org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    com.opensymphony.sitemesh.webapp.SiteMeshFilter.obtainContent(SiteMeshFilter.java:129)
    com.opensymphony.sitemesh.webapp.SiteMeshFilter.doFilter(SiteMeshFilter.java:77)
    org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    org.springframework.orm.hibernate4.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:119)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
    org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:225)
    org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
    org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:98)
    org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:927)
    org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:407)
    org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:999)
    org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:565)
    org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:309)
    java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    java.lang.Thread.run(Unknown Source)

在 EventAction 类中分配的服务层是这样的

@Transactional
public class EventService extends Service{
    // So Spring can inject the session factory
    SessionFactory sessionFactory;
    public void setSessionFactory(SessionFactory value) {
        sessionFactory = value;
    }

    // Shortcut for sessionFactory.getCurrentSession()
    public Session sess() {
        return sessionFactory.getCurrentSession();
    }

    @SuppressWarnings("unchecked")
    public List<Event> getEvents() {
        return sess().createQuery("from Event").list();
    }
}

我不知道这是否有帮助，但我的 applicationContext.xml 中有这个

<bean id="services" class ="services.Service" scope ="singleton">
    <property name = "sessionFactory" ref = "sessionFactory"/>
</bean>


<bean  class ="services.EventService" scope ="singleton">
    <property name = "sessionFactory" ref = "sessionFactory"/>
</bean>

为什么我会收到此错误？两个 Action 支持类是相同的吗？

Answer 1

根据评论将其发布为答案:

<bean  class ="services.EventService" scope ="singleton">
    <property name = "sessionFactory" ref = "sessionFactory"/>
</bean>

创建 Bean 时没有任何 ID，因为在这种情况下，ID 将由 Spring 内部创建，但与操作类中使用的定义不匹配。

向 bean 提供 ID 解决了问题。