Map<Entity, Long> 上的 <h:selectOneRadio> 导致 java.lang.String 上的 ClassCastException

Question

  private Map<Linha, Long> mapLinhas = new HashMap<Linha, Long>();
  for (final Linha linha : this.mapLinhas.keySet())
        {
            final Long idLinha = this.mapLinhas.get(linha);
            final Unidade b = Cache.getInstance().getUnidade(a);
            linha.setUnidade(b);         
        }

映射值是Long 类型，但是当我将它分配给另一个Long 变量时，它会引发以下异常：

02/02/2015 11:50:50 com.sun.faces.application.ActionListenerImpl processAction
GRAVE: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
javax.faces.el.EvaluationException: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
    at javax.faces.component.UICommand.broadcast(UICommand.java:315)
    at javax.faces.component.UIData.broadcast(UIData.java:912)
    at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:775)
    at javax.faces.component.UIViewRoot.processDecodes(UIViewRoot.java:943)
    at com.sun.faces.lifecycle.ApplyRequestValuesPhase.execute(ApplyRequestValuesPhase.java:78)
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:312)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.primefaces.webapp.filter.FileUploadFilter.doFilter(FileUploadFilter.java:79)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2466)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2455)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at com.wplexeo.controller.UnidadeController.salvaUnidadeLinha(UnidadeController.java:101)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.el.parser.AstValue.invoke(AstValue.java:278)
    at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:273)
    at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:98)
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:88)
    ... 30 more
02/02/2015 11:50:50 com.sun.faces.lifecycle.ApplyRequestValuesPhase execute
AVISO: #{unidadeController.salvaUnidadeLinha}: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
javax.faces.FacesException: #{unidadeController.salvaUnidadeLinha}: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:118)
    at javax.faces.component.UICommand.broadcast(UICommand.java:315)
    at javax.faces.component.UIData.broadcast(UIData.java:912)
    at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:775)
    at javax.faces.component.UIViewRoot.processDecodes(UIViewRoot.java:943)
    at com.sun.faces.lifecycle.ApplyRequestValuesPhase.execute(ApplyRequestValuesPhase.java:78)
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:312)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.primefaces.webapp.filter.FileUploadFilter.doFilter(FileUploadFilter.java:79)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2466)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2455)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)
Caused by: javax.faces.el.EvaluationException: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
    ... 29 more
Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at com.wplexeo.controller.UnidadeController.salvaUnidadeLinha(UnidadeController.java:101)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.el.parser.AstValue.invoke(AstValue.java:278)
    at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:273)
    at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:98)
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:88)
    ... 30 more
javax.faces.FacesException: #{unidadeController.salvaUnidadeLinha}: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at com.sun.faces.lifecycle.ApplyRequestValuesPhase.execute(ApplyRequestValuesPhase.java:86)
    at com.sun.faces.lifecycle.Phase.doPhase(Phase.java:101)
    at com.sun.faces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java:118)
    at javax.faces.webapp.FacesServlet.service(FacesServlet.java:312)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.primefaces.webapp.filter.FileUploadFilter.doFilter(FileUploadFilter.java:79)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2466)
    at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2455)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)
Caused by: javax.faces.FacesException: #{unidadeController.salvaUnidadeLinha}: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:118)
    at javax.faces.component.UICommand.broadcast(UICommand.java:315)
    at javax.faces.component.UIData.broadcast(UIData.java:912)
    at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:775)
    at javax.faces.component.UIViewRoot.processDecodes(UIViewRoot.java:943)
    at com.sun.faces.lifecycle.ApplyRequestValuesPhase.execute(ApplyRequestValuesPhase.java:78)
    ... 24 more
Caused by: javax.faces.el.EvaluationException: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:102)
    at com.sun.faces.application.ActionListenerImpl.processAction(ActionListenerImpl.java:102)
    ... 29 more
Caused by: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Long
    at com.wplexeo.controller.UnidadeController.salvaUnidadeLinha(UnidadeController.java:101)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.apache.el.parser.AstValue.invoke(AstValue.java:278)
    at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:273)
    at com.sun.faces.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:98)
    at javax.faces.component.MethodBindingMethodExpressionAdapter.invoke(MethodBindingMethodExpressionAdapter.java:88)
    ... 30 more

Obs: UnidadeController.java:101 是 idLinha 线

如果我执行 Long.parseLong(this.mapLinha.get(linha) 之类的操作，它会说方法 parseLong 不接受类型 Long。

地图由 xhtml 页面中的 selectOneRadio 填充，因为如果有帮助，我正在使用 JSF，尽管我认为这并不重要。

谁能解释一下？

Answer 1

您的问题是由以下事实引起的：

1. Java 泛型是编译时语法糖，在运行时完全不存在。
2. EL 表达式在运行时而不是在编译时运行。
3. HTTP 请求参数值为obtained 和String。

换句话说，EL 看不到Map<K, Long>，而只看到Map。因此，当您没有为模型值显式指定转换器时，EL 将在获得String 提交的值后将其原封不动地放在Map 中。当您尝试在运行时之后将其转换为 Long 时，您确实会遇到 ClassCastException。

解决方案很简单：明确指定String 到Long 的转换器。您可以为此使用 JSF 内置 LongConverter，它的转换器 ID 为 javax.faces.Long。

<h:selectOneRadio ... converter="javax.faces.Long">

Answer 2

mapLinhas 真的是类属性吗？你为什么叫this.mapLinhas 而不是mapLinhas？真的有Long 对象插入到地图中吗？

第二件事是，出于性能原因，您永远不应该以您的方式迭代 HashMap。您正在为每个地图条目进行密钥查找。更好用：

for (Map.Entry<Linha, Long> entry : map.entrySet()) {
  Linha key = entry.getKey();
  Long value = entry.getValue();
}