为什么 servlet jsp 的 Servlet.service() 抛出异常 java.lang.NullPointerException?

Why Servlet.service() for servlet jsp threw exception java.lang.NullPointerException?

我想通过 servlet 将数据从 javascript 发送到 managedbean 但问题是第一次代码工作正常但是当我返回到包含 javascript 代码的原始 jsp 页面时并再次单击提交按钮以移动到 servlet,值显示在控制台中,但发生错误并且重定向到 jsp 页面失败。

JSP 页面包含 javascript:

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <title>TODO supply a title</title>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <script type="text/javascript" src="js/jquery-2.1.1.js"></script>
        <script src="https://maps.googleapis.com/maps/api/js?v=3.exp&libraries=drawing,geometry"></script>
        <style>
            html, body
            {
                padding: 0;
                margin: 0;
                height: 100%;
            }
            .map{height:40%;}
        </style>
        <script>
       function initialize()
            {
                function getURLParameter(name) {
                    return decodeURI(
                            (RegExp(name + '=' + '(.+?)(&|$)').exec(location.search) || [, null])[1]
                            );
                }
                alert("my function in jason");
                var jsonString = getURLParameter("str");
//                var strr = '{mapBean.str1}';
                alert(jsonString);
                var goo = google.maps,
                        map_in = new goo.Map(document.getElementById('map_in'),
                                {zoom: 12,
                                    center: new goo.LatLng(32.344, 51.048)
                                }),
                        shapes = [],
                        selected_shape = null,
                        drawman = new goo.drawing.DrawingManager({map: map_in}),
                        byId = function(s) {
                            return document.getElementById(s)
                        },
                        clearSelection = function() {
                            if (selected_shape) {
                                selected_shape.set((selected_shape.type
                                        ===
                                        google.maps.drawing.OverlayType.MARKER
                                        ) ? 'draggable' : 'editable', false);
                                selected_shape = null;
                            }
                        },

                goo.event.addListener(drawman, 'overlaycomplete', function(e) {
                    var shape = e.overlay;
                    shape.type = e.type;
                    goo.event.addListener(shape, 'click', function() {
                        setSelection(this);
                    });
                    setSelection(shape);
                    shapes.push(shape);
                });

                goo.event.addListener(map_in, 'click', clearSelection);
                goo.event.addDomListener(byId('clear_shapes'), 'click', clearShapes);            
                goo.event.addDomListener(byId('save_raw'), 'click', function() {
                    var data = IO.IN(shapes, false);
                    byId('data').value = JSON.stringify(data);
                    if (this.shapes) {
                        for (var i = 0; i < this.shapes.length; ++i) {
                            this.shapes[i].setMap(null);
                        }
                    }
                    for (var i = 0; i < shapes.length; ++i) {
                        shapes[i].setMap(null);
                    }
                    shapes = [];
                    alert("clear");

                });
                goo.event.addDomListener(byId('restore'), 'click', function() {

                    if (this.shapes) {
                        for (var i = 0; i < this.shapes.length; ++i) {
                            this.shapes[i].setMap(null);
                        }
                    }
                    //  alert(jsonString);

                    this.shapes = IO.OUT(JSON.parse(byId('data').value), map_in);
                });

            }
 google.maps.event.addDomListener(window, 'load', initialize);
 </script>
    </head>
    <body>
        <br/>
        <div class="map" id="map_in"></div>
        <div style="text-align:center">
            <form method="get" action="GeofencingServlet">
                <input id="clear_shapes" value="clear shapes"    type="button"  />
                <!--<input id="save_encoded" value="save encoded(IO.IN(shapes,true))"    type="button" />-->
                <input id="save_raw"     value="save raw(IO.IN(shapes,false))"        type="button" />
                <input id="data"    name="data"     value=""                style="width:100%" readonly/>
                <input type="submit" id="restore"      value="restore(IO.OUT(array,map))"         type="button" />
                <!--<input type="submit" name="save" value="Create Zone!" id="save" />-->
            </form>
        </div>
    </body>
</html>

GeofencingServlet:

@Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        PrintWriter out = response.getWriter();

        out.println("<html>");
        out.println("<body>");
        out.println("<p>Zone created successfully.</p>");
        out.println("<a href=\"gefencingJason.jsp\">Click here!</a>to go back to Map");
        out.println("</body>");
        out.println("</html>");

        //  String str = request.getParameter("vertices");
        String str = request.getParameter("data");
        System.out.println("str in servlet" + str);
        MapBean mapBean = new MapBean();
        String strret = mapBean.geo(str);
        System.out.println("servlettttt string returned by mapbean" + strret);
        ExternalContext context = FacesContext.getCurrentInstance().getExternalContext();
        context.redirect(context.getRequestContextPath() + "/gefencingJason.jsp?str=" + strret);
    }

地图豆:

 public String geo(String str) throws IOException {
 System.out.println("The data in map bean is str" + str);
        str1 = "my string";
        return str;
    }

我有以下错误:

WARNING:   StandardWrapperValve[GeofencingServlet]: Servlet.service() for servlet GeofencingServlet threw exception
java.lang.NullPointerException
    at servlet.GeofencingServlet.doGet(GeofencingServlet.java:60)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
    at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
    at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
    at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
    at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
    at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:357)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
    at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
    at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
    at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
    at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
    at org.glassfish.grizzly.filterchain.ExecutorResolver.execute(ExecutorResolver.java:119)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
    at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
    at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
    at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
    at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access0(WorkerThreadIOStrategy.java:55)
    at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
    at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
    at java.lang.Thread.run(Thread.java:744)

INFO:   str in servlet[{"type":"RECTANGLE","id":null,"geometry":[[32.33265644167479,50.932273864746094],[32.352090586480486,50.985145568847656]]}]
INFO:   The data in map bean is str[{"type":"RECTANGLE","id":null,"geometry":[[32.33265644167479,50.932273864746094],[32.352090586480486,50.985145568847656]]}]
INFO:   servlettttt string returned by mapbean[{"type":"RECTANGLE","id":null,"geometry":[[32.33265644167479,50.932273864746094],[32.352090586480486,50.985145568847656]]}]

参考this, FacesContext.getCurrentInstance() returns null.

您可以使用以下代码。

//ExternalContext context = FacesContext.getCurrentInstance().getExternalContext();
//context.redirect(context.getRequestContextPath() + "/gefencingJason.jsp?str=" + strret);
response.sendRedirect(request.getContextPath() + "/gefencingJason.jsp?str=" + strret);

或者您可以参考@BalusC 的this文章。

永远记住:在 IDE.

中检查项目的目标运行时(服务器)

对于日食IDE

转到项目的属性并查找 "Targeted Runtimes" 选项。检查所需的 运行时环境