Eclipse运行JSF动态Web项目报Faces Servlet空指针异常如何解决？

问题描述 #

我在Eclipse中运行动态Web项目时遇到报错，具体场景如下：我在NewUserBean类中添加了@ManagedBean(name = "newUserBean")注解，在xhtml文件中调用该newUserBean，未在face-config.xml中添加任何配置。

控制台错误日志 #

Kas 30, 2021 11:11:06 AM com.sun.faces.application.view.FaceletViewHandlingStrategy handleRenderException
SEVERE: Error Rendering View[/index.xhtml]
javax.el.ELException: /index.xhtml: java.lang.NullPointerException
    at com.sun.faces.facelets.compiler.AttributeInstruction.write(AttributeInstruction.java:94)
    at com.sun.faces.facelets.compiler.UIInstructions.encodeBegin(UIInstructions.java:82)
    at com.sun.faces.facelets.compiler.UILeaf.encodeAll(UILeaf.java:183)
    at javax.faces.component.UIComponent.encodeAll(UIComponent.java:1859)
    at com.sun.faces.application.view.FaceletViewHandlingStrategy.renderView(FaceletViewHandlingStrategy.java:456)
    at com.sun.faces.application.view.MultiViewHandler.renderView(MultiViewHandler.java:133)
    at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:120)
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
    at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:219)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:647)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:543)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:688)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:609)
    at com.sun.faces.facelets.compiler.AttributeInstruction.write(AttributeInstruction.java:89)
    ... 31 more

Kas 30, 2021 11:11:06 AM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [Faces Servlet] in context with path [/Aident] threw exception [null] with root cause
java.lang.NullPointerException
    at createBeans.NewUserBean.createUser(NewUserBean.java:159)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:748)

补充说明 #

• 原先网页可以正常加载，在xhtml文件的注册按钮上添加actionListener="#{newUserBean.createUser()}"属性后，网页无法正常显示，仅出现白屏。
• 附加报错信息：

Kas 30, 2021 12:03:09 PM com.sun.faces.application.view.FaceletViewHandlingStrategy handleRenderException
SEVERE: Error Rendering View[/index.xhtml]
Kas 30, 2021 12:03:09 PM com.sun.faces.context.ExceptionHandlerImpl throwIt
INFO: Exception when handling error trying to reset the response.
Kas 30, 2021 12:03:09 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [Faces Servlet] in context with path [/Aident] threw exception [null] with root cause

解决方案 #

从错误日志可以定位到根因是*NewUserBean类的createUser方法第159行出现空指针异常，可按照以下步骤排查修复：

• 首先查看NewUserBean.java第159行代码，检查该行所有调用属性/方法的对象是否完成初始化，常见空值场景包括：
  • 注入的依赖对象（@EJB/@Inject/@ManagedProperty注解修饰的属性）未成功注入
  • 方法内部定义的对象未实例化直接调用方法/属性
  • 前端传递的参数为空，后端未做空值校验直接调用
• 检查actionListener写法是否规范：actionListener对应的方法需要接收ActionEvent参数，如果你不需要事件参数，应该用action属性而非actionListener，修改方式二选一：
  1. 保留无参createUser方法，将按钮属性修改为action="#{newUserBean.createUser()}"
  2. 保留actionListener属性，修改createUser方法定义为public void createUser(ActionEvent event)，同时导入javax.faces.event.ActionEvent包
• 检查ManagedBean的作用域注解是否配置：仅添加@ManagedBean没有加作用域注解时默认是@RequestScoped，页面渲染、提交请求过程中可能出现对象重新实例化导致属性为空，根据业务场景补充@ViewScoped/@SessionScoped等对应作用域注解即可
• 完成修改后清空浏览器缓存、清理Tomcat工作目录重启项目验证

内容的提问来源于stack exchange，提问作者Furkan Basoglu