我最近开始将activejdbc用于我必须为学校做的小型Java EE项目。我试图在Servlet中的JDBC管理的数据库的Object表中获取外键字段的值。手头的关系是两张桌子，一张桌子和＃34;艺术家＆＃34;和一个表＆＃34;记录＆＃34;，后者持有艺术家PK中的外国钥匙＆＃34;艺术家＆＃34; table，因为我的模型中的Artist：Records = 1：n。
我想实现以下目标：我有一套＆＃34;记录＆＃34;并希望为我的JSP中的每条记录获取相应的Artist。在我的Java代码中，这样做很好：
//Query the db:
List<Record> mostRecentRecords = Record.findAll().limit(10).orderBy("id asc");
for (Iterator i = mostRecentRecords.iterator(); i.hasNext(); ) {
Record r = (Record) i.next();
Artist a = r.parent(Artist.class);
System.out.println("artist of record: " + a.get("artist_name"));
}
在我的控制台输出中，我可以看到艺术家打印到stdout。
然后我继续将此列表传递给requestDispatcher并呈现jsp模板。
因此，通过类比，我认为我应该能够在我的jsp中做类似的事情：
<c:forEach items="${mostRecentRecords}" var="record">
<c:out value="${record.parent(Artist.class).get('title')}" />
</c:forEach>
如果我打印＆＃34;记录＆＃34;我的模板循环中的变量表示＆＃34; class model.Record＆＃34;这是正确的。
在渲染servlet时，我得到一个org.apache.jasper.JasperException：javax.el.ELException：java.lang.NullPointerException。
我在下面列出了完整的堆栈跟踪。
[2018-03-12T21:06:40.489+0100] [glassfish 5.0] [WARNING] [] [javax.enterprise.web.core] [tid: _ThreadID=29 _ThreadName=http-listener-1(2)] [timeMillis: 1520885200489] [levelValue: 900] [[
Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at org.javalite.activejdbc.Model.parent(Model.java:1178)
at org.javalite.activejdbc.Model.parent(Model.java:1167)
at sun.reflect.GeneratedMethodAccessor65.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at javax.el.ELUtil.invokeMethod(ELUtil.java:304)
at javax.el.BeanELResolver.invoke(BeanELResolver.java:535)
at javax.el.CompositeELResolver.invoke(CompositeELResolver.java:256)
at com.sun.el.parser.AstValue.getValue(AstValue.java:135)
at com.sun.el.parser.AstValue.getValue(AstValue.java:203)
at com.sun.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:226)
at org.apache.jasper.runtime.PageContextImpl.evaluateExpression(PageContextImpl.java:1016)
at org.apache.jsp.index_jsp._jspx_meth_c_out_3(index_jsp.java:235)
at org.apache.jsp.index_jsp._jspx_meth_c_forEach_0(index_jsp.java:147)
at org.apache.jsp.index_jsp._jspService(index_jsp.java:83)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:791)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:791)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1580)
at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:819)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:683)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:519)
at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:490)
at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:372)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:322)
at servlet.Welcome.doGet(Welcome.java:38)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:686)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:791)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1580)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:258)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:652)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:591)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:371)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:238)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:463)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:168)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:242)
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:539)
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$Worker.doWork(AbstractThreadPool.java:593)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:573)
at java.lang.Thread.run(Thread.java:748)
]]
[2018-03-12T21:06:40.490+0100] [glassfish 5.0] [WARNING] [] [javax.enterprise.web] [tid: _ThreadID=29 _ThreadName=http-listener-1(2)] [timeMillis: 1520885200490] [levelValue: 900] [[
StandardWrapperValve[welcome]: Servlet.service() for servlet welcome threw exception
java.lang.NullPointerException
at org.javalite.activejdbc.Model.parent(Model.java:1178)
at org.javalite.activejdbc.Model.parent(Model.java:1167)
at sun.reflect.GeneratedMethodAccessor65.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at javax.el.ELUtil.invokeMethod(ELUtil.java:304)
at javax.el.BeanELResolver.invoke(BeanELResolver.java:535)
at javax.el.CompositeELResolver.invoke(CompositeELResolver.java:256)
at com.sun.el.parser.AstValue.getValue(AstValue.java:135)
at com.sun.el.parser.AstValue.getValue(AstValue.java:203)
at com.sun.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:226)
at org.apache.jasper.runtime.PageContextImpl.evaluateExpression(PageContextImpl.java:1016)
at org.apache.jsp.index_jsp._jspx_meth_c_out_3(index_jsp.java:235)
at org.apache.jsp.index_jsp._jspx_meth_c_forEach_0(index_jsp.java:147)
at org.apache.jsp.index_jsp._jspService(index_jsp.java:83)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:111)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:791)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:411)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:473)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:377)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:791)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1580)
at org.apache.catalina.core.ApplicationDispatcher.doInvoke(ApplicationDispatcher.java:819)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:683)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:519)
at org.apache.catalina.core.ApplicationDispatcher.doDispatch(ApplicationDispatcher.java:490)
at org.apache.catalina.core.ApplicationDispatcher.dispatch(ApplicationDispatcher.java:372)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:322)
at servlet.Welcome.doGet(Welcome.java:38)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:686)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:791)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1580)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:258)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:652)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:591)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:371)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:238)
at com.sun.enterprise.v3.services.impl.ContainerMapper$HttpHandlerCallable.call(ContainerMapper.java:463)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:168)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:206)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:180)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:242)
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:539)
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$Worker.doWork(AbstractThreadPool.java:593)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:573)
at java.lang.Thread.run(Thread.java:748)
]]