target is null for setProperty(null,）错误的引发原因及解决办法

使用SSH框架开发一个网站项目，登录时报出这个错误。

错误信息：

警告: Error setting expression 'user.upw' with value '[Ljava.lang.String;@6581a654'ognl.OgnlException: target is null for setProperty(null, "upw", [Ljava.lang.String;@6581a654)at ognl.OgnlRuntime.setProperty(OgnlRuntime.java:2309)at ognl.ASTProperty.setValueBody(ASTProperty.java:127)at ognl.SimpleNode.evaluateSetValueBody(SimpleNode.java:220)at ognl.SimpleNode.setValue(SimpleNode.java:301)at ognl.ASTChain.setValueBody(ASTChain.java:227)at ognl.SimpleNode.evaluateSetValueBody(SimpleNode.java:220)at ognl.SimpleNode.setValue(SimpleNode.java:301)at ognl.Ognl.setValue(Ognl.java:737)at com.opensymphony.xwork2.ognl.OgnlUtil.setValue(OgnlUtil.java:217)at com.opensymphony.xwork2.ognl.OgnlValueStack.trySetValue(OgnlValueStack.java:186)at com.opensymphony.xwork2.ognl.OgnlValueStack.setValue(OgnlValueStack.java:173)at com.opensymphony.xwork2.ognl.OgnlValueStack.setParameter(OgnlValueStack.java:151)at com.opensymphony.xwork2.interceptor.ParametersInterceptor.setParameters(ParametersInterceptor.java:317)at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:228)at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:236)at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:90)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:192)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54)at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:511)at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)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$AbstractConnectionHandler.process(AbstractProtocol.java:611)at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)at java.lang.Thread.run(Thread.java:744)

这个错误的意思是调用setProperty()的对象为空。
造成这类错误的原因就在JSP页面中使用user.upw传值到action，而指定的action中user对象是为空的，所以无法接收该值。

出现这种错误的时候首先检查一下struts.xml配置文件，看指定的action是否有错，是否有action的name重名。