JSP中的NullPointerException

Question

當我運行該程序時，我不斷收到NullPointerException。

java.lang.NullPointerException 
     at org.apache.jsp.student_jsp._jspService(student_jsp.java from :65) 
     at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:109) 
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) 
     at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:406) 
     at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:483) 
     at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:373) 
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) 
     at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1523) 
     at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:279) 
     at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:188) 
     at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:641) 
     at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:97) 
     at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:85) 
     at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:185) 
     at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:325) 
     at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:226) 
     at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:165) 
     at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:791) 
     at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:693) 
     at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:954) 
     at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:170) 
     at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:135) 
     at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:102) 
     at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:88) 
     at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:76) 
     at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:53) 
     at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:57) 
     at com.sun.grizzly.ContextTask.run(ContextTask.java:69) 
     at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:330) 
     at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:309) 
     at java.lang.Thread.run(Thread.java:619) 

---

SEVERE: org.hibernate.hql.ast.QuerySyntaxException: student is not mapped [from student] 
     at org.hibernate.hql.ast.util.SessionFactoryHelper.requireClassPersister(SessionFactoryHelper.java:158) 
     at org.hibernate.hql.ast.tree.FromElementFactory.addFromElement(FromElementFactory.java:87) 
     at org.hibernate.hql.ast.tree.FromClause.addFromElement(FromClause.java:70) 
     at org.hibernate.hql.ast.HqlSqlWalker.createFromElement(HqlSqlWalker.java:255) 
     at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElement(HqlSqlBaseWalker.java:3056) 
     at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElementList(HqlSqlBaseWalker.java:2945) 
     at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromClause(HqlSqlBaseWalker.java:688) 
     at org.hibernate.hql.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:544) 
     at org.hibernate.hql.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:281) 
     at org.hibernate.hql.antlr.HqlSqlBaseWalker.statement(HqlSqlBaseWalker.java:229) 
     at org.hibernate.hql.ast.QueryTranslatorImpl.analyze(QueryTranslatorImpl.java:228) 
     at org.hibernate.hql.ast.QueryTranslatorImpl.doCompile(QueryTranslatorImpl.java:160) 
     at org.hibernate.hql.ast.QueryTranslatorImpl.compile(QueryTranslatorImpl.java:111) 
     at org.hibernate.engine.query.HQLQueryPlan.<init>(HQLQueryPlan.java:77) 
     at org.hibernate.engine.query.HQLQueryPlan.<init>(HQLQueryPlan.java:56) 
     at org.hibernate.engine.query.QueryPlanCache.getHQLQueryPlan(QueryPlanCache.java:72) 
     at org.hibernate.impl.AbstractSessionImpl.getHQLQueryPlan(AbstractSessionImpl.java:133) 
     at org.hibernate.impl.AbstractSessionImpl.createQuery(AbstractSessionImpl.java:112) 
     at org.hibernate.impl.SessionImpl.createQuery(SessionImpl.java:1623) 
     at servlet.Servlet.processRequest(Servlet.java:52) 
     at servlet.Servlet.doGet(Servlet.java:128) 
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:734) 
     at javax.servlet.http.HttpServlet.service(HttpServlet.java:847) 
     at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1523) 
     at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:279) 
     at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:188) 
     at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:641) 
     at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:97) 
     at com.sun.enterprise.web.PESessionLockingStandardPipeline.invoke(PESessionLockingStandardPipeline.java:85) 
     at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:185) 
     at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:325) 
     at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:226) 
     at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:165) 
     at com.sun.grizzly.http.ProcessorTask.invokeAdapter(ProcessorTask.java:791) 
     at com.sun.grizzly.http.ProcessorTask.doProcess(ProcessorTask.java:693) 
     at com.sun.grizzly.http.ProcessorTask.process(ProcessorTask.java:954) 
     at com.sun.grizzly.http.DefaultProtocolFilter.execute(DefaultProtocolFilter.java:170) 
     at com.sun.grizzly.DefaultProtocolChain.executeProtocolFilter(DefaultProtocolChain.java:135) 
     at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:102) 
     at com.sun.grizzly.DefaultProtocolChain.execute(DefaultProtocolChain.java:88) 
     at com.sun.grizzly.http.HttpProtocolChain.execute(HttpProtocolChain.java:76) 
     at com.sun.grizzly.ProtocolChainContextTask.doCall(ProtocolChainContextTask.java:53) 
     at com.sun.grizzly.SelectionKeyContextTask.call(SelectionKeyContextTask.java:57) 
     at com.sun.grizzly.ContextTask.run(ContextTask.java:69) 
     at com.sun.grizzly.util.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:330) 
     at com.sun.grizzly.util.AbstractThreadPool$Worker.run(AbstractThreadPool.java:309) 
     at java.lang.Thread.run(Thread.java:619) 

任何想法，爲什麼我得到這個？

Answer 1

反轉您的字符串比較。在常數值上調用equals（）。這將有助於避免NullPointerExceptions。
下面是一個例子： if ("student".equalsIgnoreCase(view))

如果hibernateSession.createQuery（）返回null，你會得到一個NullPointerException

Answer 2

如果它變成是你的字符串比較的一個，那麼你可能想採取一個更安全的戰略

 if ("student".equalsIgnoreCase(view)) { ... } 
else if ("course".equalsIgnoreCase(view)) { ... } 
// etc 

你得到避免空檢查，而你保護的NPE

Answer 3

這裏的相關位

java.lang.NullPointerException 
    at org.apache.jsp.student_jsp._jspService(student_jsp.java from :65) 

（你的問題的殘餘其實是無關緊要，我非常善良的從你的原始問題中刪除怪物servlet代碼和所有INFO消息，以儘量減少噪音）。

此異常和跟蹤表明你在student.jsp文件，而不是標籤庫和EL使用老式小腳本（那些<% %>的東西）（這是空安全）。你的罪魁禍首是在JSP文件的已編譯類文件的第65行。你可以在servletcontainer的工作文件夾中找到這個類文件。您需要將其跟蹤到JSP文件本身的代碼。它會是這個樣子：

<%= someBean.getSomeProperty(); %> 

的NullPointerException表明someBean是null。修理它。理想的解決方案將全部用於avoid腳本。例如。

${someBean.someProperty} 

這不會拋出NPE每當${someBean}是null。

---

按照其它堆棧跟蹤：

org.hibernate.hql.ast.QuerySyntaxException: student is not mapped [from student] 

這意味着實體student是行不通映射。這通常應該是實體的簡短類名稱。由於正常的Java約定規定類名應以大寫開頭，我敢打賭，它實際上被稱爲Student而不是student。您需要在您的HQL查詢中相應地修復它。

如果情況並非如此，那麼您只需將其添加到您的Hibernate映射。