赌你半天都想不出的Bug

赌你半天都想不出的Bug 背景： 之前实现一个办公系统，对查询进行一个更改，添加一个“QueryHelper”类，但是代码改完之后，结果一直都没有出来，宝宝心急了，这是怎么回事？ 问题一： Struts Problem Report Struts has detected an unhandled exception: Messages: · node to traverse cannot be null! File: org/hibernate/hql/ast/util/NodeTraverser.Java Line number: 63 Stacktraces java.lang.IllegalArgumentException: node to traversecannot be null! org.hibernate.hql.ast.util.NodeTraverser.traverseDepthFirst(NodeTraverser.java:63) org.hibernate.hql.ast.QueryTranslatorImpl.parse(QueryTranslatorImpl.java:280) org.hibernate.hql.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:182) org.hibernate.hql.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:136) org.hibernate.engine.query.HQLQueryPlan.(HQLQueryPlan.java:101) org.hibernate.engine.query.HQLQueryPlan.(HQLQueryPlan.java:80) org.hibernate.engine.query.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:124) org.hibernate.impl.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:156) org.hibernate.impl.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:135) org.hibernate.impl.SessionImpl.createQuery(SessionImpl.java:1770) cn.itcast.oa.base.DaoSupportImpl.getPageBean(DaoSupportImpl.java:128) sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) java.lang.reflect.Method.invoke(Method.java:601) org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:307) org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182) org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149) org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:106) org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204) com.sun.proxy.$Proxy20.getPageBean(Unknown Source) cn.itcast.oa.view.action.ForumAction.show(ForumAction.java:153) sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) java.lang.reflect.Method.invoke(Method.java:601) com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:441) com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:280) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:243) com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:165) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:252) org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:179) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:235) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:130) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:165) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:179) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) cn.itcast.oa.util.CheckPrivilegeInterceptor.intercept(CheckPrivilegeInterceptor.java:43) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237) org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52) org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:488) 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:241) org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:198) org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76) org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241) org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208) org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218) org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110) org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:506) org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169) org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103) org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:962) org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116) org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:445) org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1115) org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637) org.apache.tomcat.util.NET.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2549) org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2538) java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) java.lang.Thread.run(Thread.java:722) You are seeing this page because development mode is enabled. Developmentmode, or devMode, enables extra debugging behaviors and reports to assistdevelopers. To disable this mode, set: struts.devMode=false in your WEB-INF/classes/struts.properties file. 问题分析： 出现这种问题，经过主要的语句：“node to traverse cannot be null!” 和相关的语句提示，我们可以分析出，主要是HQL语句出问题了，引起内部的查询对象为空，无法处理控制引起的。解决方案很简单：检查HQL语句（尤其是关键字呗） ``` 

问题二： Struts Problem Report Struts has detected an unhandled exception: Messages: · Topict is not mapped [FROM Topict WHERE t.forum=? ORDER BY (CASE t.type WHEN 2 THEN 2 ELSE 0 END) DESC, t.lastUpdateTime DESC] File: org/hibernate/hql/ast/util/SessionFactoryHelper.java Line number: 180 Stacktraces org.hibernate.hql.ast.QuerySyntaxException:Topict is not mapped [FROM Topict WHERE t.forum=? ORDER BY (CASE t.type WHEN 2THEN 2 ELSE 0 END) DESC, t.lastUpdateTime DESC] org.hibernate.hql.ast.util.SessionFactoryHelper.requireClassPersister(SessionFactoryHelper.java:180) org.hibernate.hql.ast.tree.FromElementFactory.addFromElement(FromElementFactory.java:111) org.hibernate.hql.ast.tree.FromClause.addFromElement(FromClause.java:93) org.hibernate.hql.ast.HqlSqlWalker.createFromElement(HqlSqlWalker.java:322) org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElement(HqlSqlBaseWalker.java:3441) org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElementList(HqlSqlBaseWalker.java:3325) org.hibernate.hql.antlr.HqlSqlBaseWalker.fromClause(HqlSqlBaseWalker.java:733) org.hibernate.hql.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:584) org.hibernate.hql.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:301) org.hibernate.hql.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:244) org.hibernate.hql.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:254) org.hibernate.hql.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:185) org.hibernate.hql.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:136) org.hibernate.engine.query.HQLQueryPlan.(HQLQueryPlan.java:101) org.hibernate.engine.query.HQLQueryPlan.(HQLQueryPlan.java:80) org.hibernate.engine.query.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:124) org.hibernate.impl.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:156) org.hibernate.impl.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:135) org.hibernate.impl.SessionImpl.createQuery(SessionImpl.java:1770) cn.itcast.oa.base.DaoSupportImpl.getPageBean(DaoSupportImpl.java:128) sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) java.lang.reflect.Method.invoke(Method.java:601) org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:307) org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:182) org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:149) org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:106) org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171) org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204) com.sun.proxy. Proxy20.getPageBean(UnknownSource)cn.itcast.oa.view.action.ForumAction.show(ForumAction.java:153)sun.reflect.NativeMethodAccessorImpl.invoke0(NativeMethod)sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)java.lang.reflect.Method.invoke(Method.java:601)com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:441)com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:280)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:243)com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:165)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:252)org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:179)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:235)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:130)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138)com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:165)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:179)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)cn.itcast.oa.util.CheckPrivilegeInterceptor.intercept(CheckPrivilegeInterceptor.java:43)com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:237)org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:488)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:241)org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:198)org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:506)org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:962)org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:445)org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1115)org.apache.coyote.AbstractProtocol AbstractConnectionHandler.process(AbstractProtocol.java:637) org.apache.tomcat.util.net.AprEndpoint SocketProcessor.doRun(AprEndpoint.java:2549)org.apache.tomcat.util.net.AprEndpoint SocketProcessor.run(AprEndpoint.java:2538) java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) java.util.concurrent.ThreadPoolExecutor Worker.run(ThreadPoolExecutor.java:615)org.apache.tomcat.util.threads.TaskThread WrappingRunnable.run(TaskThread.java:61) java.lang.Thread.run(Thread.java:722)

You are seeing this page because development mode isenabled. Development mode, or devMode, enables extra debugging behaviors andreports to assist developers. To disable this mode, set: struts.devMode=false in your WEB-INF/classes/struts.properties file.

解决方案： 寻宝寻宝，看着寻宝图往前走就可以了，这不：“Topict is not mapped [FROM Topict WHERE t.forum=? ORDER BY (CASEt.type WHEN 2 THEN 2 ELSE 0 END) DESC, t.lastUpdateTime DESC]” 和相关的语句提示，我们可以分析出，主要是HQL语句出问题了，引起内部的查询对象为空，无法处理控制引起的。解决方案很简单：检查HQL语句（尤其是关键字） “`

总结： 其实这两个问题都是如出一辙，都是HQL语句，其实HQL语句是没有错的，是写的格式不正确。 在这里是要有空格，否则怎么查怎么查都没有查出原因来，

添加空格之后，宝藏马上浮出水面，那成就感，油然而生！所以，写HQL语句，SQL语句和引号之间有没有空格，这是极其重要的；继续延伸每一行的代码，拼写这样看似很小的细节，也是非常容易让我们找很久很久的，细节从平时做起，少甚至不出现这种低级的bug。Have a nice day, thanks for your time!

参考资料：

转载于：http://blog.csdn.net/u013067756/article/details/73211180