我用以下代码创建了一个简单的 xhtml 页面(如 Primefaces showcase)：
<body>
<div>My test</div>
<h:form>
<p:messages showDetail="true"/>
<p:captcha label="Captcha" />
</h:form>
</body>
加载此页面时，我收到以下异常：
exception
javax.servlet.ServletException: Expression cannot be null
root cause
javax.el.ELException: Expression cannot be null
我的范围是在表单中使用验证码组件，并检查用户是否通过按钮(提交)选中了它。
这是全栈跟踪：
严重：渲染视图时出错[/test.xhtml] javax.el.ELException：表达式不能为空 at com.sun.el.lang.ExpressionBuilder.createNodeInternal(ExpressionBuilder.java：172) at com.sun.el.lang.ExpressionBuilder.build(ExpressionBuilder.java：237) at com.sun.el.lang.ExpressionBuilder.createValueExpression(ExpressionBuilder.java：295) at com.sun.el.ExpressionFactoryImpl.createValueExpression(ExpressionFactoryImpl.java：112) at org.jboss.weld.util.el.ForwardingExpressionFactory.createValueExpression(ForwardingExpressionFactory.java：53) at org.jboss.weld.el.WeldExpressionFactory.createValueExpression(WeldExpressionFactory.java：48) at com.sun.faces.application.ApplicationImpl.evaluateExpressionGet(ApplicationImpl.java：446) at javax.faces.application.ApplicationWrapper.evaluateExpressionGet(ApplicationWrapper.java：684) at org.primefaces.component.captcha.CaptchaRenderer.getPublicKey(CaptchaRenderer.java：87) at org.primefaces.component.captcha.CaptchaRenderer.encodeEnd(CaptchaRenderer.java：50) at javax.faces.component.UIComponentBase.encodeEnd(UIComponentBase.java：919) at javax.faces.component.UIComponent.encodeAll(UIComponent.java：1863) at javax.faces.render.Renderer.encodeChildren(Renderer.java：176) at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java：889) at javax.faces.component.UIComponent.encodeAll(UIComponent.java：1856) 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 javax.faces.application.ViewHandlerWrapper.renderView(ViewHandlerWrapper.java：337) 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.StandardWrapper.service(StandardWrapper.java：1682) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java：318) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java：160) at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java：734) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java：673) at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java：99) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java：174) at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java：415) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java：282) at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java：459) at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java：167) at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java：201) at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java：175) at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java：235) at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java：119) at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java：284) at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java：201) at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java：133) at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java：112) at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java：77) at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java：561) at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java：112) at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java：117) at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java：56) at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java：137) at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java：565) at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java：545) at java.lang.Thread.run(Thread.java：748)信息：发现异常排队事件：真 Severe： javax.el.ELException： 表达式不能为空 at com.sun.el.lang.ExpressionBuilder.createNodeInternal(ExpressionBuilder.java：172) at com.sun.el.lang.ExpressionBuilder.build(ExpressionBuilder.java：237) at com.sun.el.lang.ExpressionBuilder.createValueExpression(ExpressionBuilder.java：295) at com.sun.el.ExpressionFactoryImpl.createValueExpression(ExpressionFactoryImpl.java：112) at org.jboss.weld.util.el.ForwardingExpressionFactory.createValueExpression(ForwardingExpressionFactory.java：53) at org.jboss.weld.el.WeldExpressionFactory.createValueExpression(WeldExpressionFactory.java：48) at com.sun.faces.application.ApplicationImpl.evaluateExpressionGet(ApplicationImpl.java：446) at javax.faces.application.ApplicationWrapper.evaluateExpressionGet(ApplicationWrapper.java：684) at org.primefaces.component.captcha.CaptchaRenderer.getPublicKey(CaptchaRenderer.java：87) at org.primefaces.component.captcha.CaptchaRenderer.encodeEnd(CaptchaRenderer.java：50) at javax.faces.component.UIComponentBase.encodeEnd(UIComponentBase.java：919) at javax.faces.component.UIComponent.encodeAll(UIComponent.java：1863) at javax.faces.render.Renderer.encodeChildren(Renderer.java：176) at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java：889) at javax.faces.component.UIComponent.encodeAll(UIComponent.java：1856) 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 javax.faces.application.ViewHandlerWrapper.renderView(ViewHandlerWrapper.java：337) 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.StandardWrapper.service(StandardWrapper.java：1682) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java：318) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java：160) at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java：734) at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java：673) at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java：99) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java：174) at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java：415) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java：282) at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java：459) at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java：167) at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java：201) at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java：175) at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java：235) at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java：119) at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java：284) at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java：201) at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java：133) at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java：112) at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java：77) at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java：561) at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java：112) at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java：117) at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java：56) at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java：137) at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java：565) at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java：545) at java.lang.Thread.run(Thread.java：748) 警告：StandardWrapperValve[Faces Servlet]：Servlet.service() for servlet Faces Servlet 抛出异常
你必须去 https://www.google.com/recaptcha/admin
获取私钥和公钥，然后在 Web 中注册.xml
<context-param>
<param-name>primefaces.PUBLIC_CAPTCHA_KEY</param-name>
<param-value>PUBLIC_KEY</param-value>
</context-param>
<context-param>
<param-name>primefaces.PRIVATE_CAPTCHA_KEY</param-name>
<param-value>PRIVATE_KEY</param-value>
</context-param>
考虑到 @carlos-ob，当使用 spring-boot-joinfaces-starter 时，它需要在 application.yml 中：
joinfaces:
jsf:
project-stage: Development
state-saving-method: server
#interpret-empty-string-submitted-values-as-null: true
#datetimeconverter-default-timezone-is-system-timezone: true
primefaces:
theme: ultima
font-awesome: true
PUBLIC_CAPTCHA_KEY: PUBLIC_K
PRIVATE_CAPTCHA_KEY: PRIVATE_K