jsp中的java.lang.NullPointerException

时间：2015-04-21 06:28:20

标签： java json twitter-bootstrap jsp

大家好我是java和jsp的新手，说实话，我不懂json。但我有一些要求，我必须从数据库中提取数据并在jsp页面上显示。但标准是我必须使用Bootstrap数据表为什么我使用它，因为它提供了很多灵活性，如分页过滤排序多列排序单个列过滤器。 我的java.lang.NullPointerException收到jsp 谁能告诉我为什么我会收到这个错误。 这是我的jsp页面

dataTable.jsp

<%@page import="org.codehaus.jettison.json.JSONObject"%>
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
   <%@ page import ="java.util.*" %>
  <%--  <%@ page import="com.varun.DataBase"%> --%>
   <%@ page import="java.sql.*" %>
<%--    <%@ page import="org.json.*"%> --%>
   <%@ page import ="net.sf.json.JSONArray" %>


 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>DataTable</title>
</head>
<body>
<h1>Lets display the data from database using dataTabel</h1>

<%

String[] cols = { "id","engine", "browser", "platform", "version", "grade" };
String table = "ajax";

JSONObject result = new JSONObject();
JSONArray arr = new JSONArray();


int amount = 10;
int start = 0;
int echo = 0;
int col = 0;

int  id=0;
String engine = "";
String browser = "";
String platform = "";
String version = "";
String grade = "";



String dir = "asc";
String sStart = request.getParameter("iDisplayStart");
String sAmount = request.getParameter("iDisplayLength");
String sEcho = request.getParameter("sEcho");
String sCol = request.getParameter("iSortCol_0");
String sdir = request.getParameter("sSortDir_0");

engine = request.getParameter("sSearch_0");
browser = request.getParameter("sSearch_1");
platform = request.getParameter("sSearch_2");
version = request.getParameter("sSearch_3");
grade = request.getParameter("sSearch_4");


List<String> sArray = new ArrayList<String>();
if (!engine.equals("")) {
    String sEngine = " engine like '%" + engine + "%'";
    sArray.add(sEngine);
    //or combine the above two steps as:
    //sArray.add(" engine like '%" + engine + "%'");
    //the same as followings
}

if (!browser.equals("")) {
    String sBrowser = " browser like '%" + browser + "%'";
    sArray.add(sBrowser);
}
if (!platform.equals("")) {
    String sPlatform = " platform like '%" + platform + "%'";
    sArray.add(sPlatform);
}
if (!version.equals("")) {
    String sVersion = " version like '%" + version + "%'";
    sArray.add(sVersion);
}
if (!grade.equals("")) {
    String sGrade = " grade like '%" + grade + "%'";
    sArray.add(sGrade);
}

String individualSearch = "";
if(sArray.size()==1){
    individualSearch = sArray.get(0);
}else if(sArray.size()>1){
    for(int i=0;i<sArray.size()-1;i++){
        individualSearch += sArray.get(i)+ " and ";
    }
    individualSearch += sArray.get(sArray.size()-1);
}


if (sStart != null) {
    start = Integer.parseInt(sStart);
    if (start < 0)
        start = 0;
}
if (sAmount != null) {
    amount = Integer.parseInt(sAmount);
    if (amount < 10 || amount > 100)
        amount = 10;
}
if (sEcho != null) {
    echo = Integer.parseInt(sEcho);
}
if (sCol != null) {
    col = Integer.parseInt(sCol);
    if (col < 0 || col > 5)
        col = 0;
}
if (sdir != null) {
    if (!sdir.equals("asc"))
        dir = "desc";
}


String colName = cols[col];
int total = 0;
Connection conn = DriverManager.getConnection("jdbc:oracle:thin:@localhost:1521:orcl","system","admin");
try {
    String sql = "SELECT count(*) FROM "+table;
    PreparedStatement ps = conn.prepareStatement(sql);
    ResultSet rs = ps.executeQuery();
    if(rs.next()){
        total = rs.getInt("count(*)");
    }
}catch(Exception e){

}
int totalAfterFilter = total;
//result.put("sEcho",echo);

try {
    String searchSQL = "";
    String sql = "SELECT * FROM "+table;
    String searchTerm = request.getParameter("sSearch");
    String globeSearch =  " where (engine like '%"+searchTerm+"%'"
                            + " or browser like '%"+searchTerm+"%'"
                            + " or platform like '%"+searchTerm+"%'"
                            + " or version like '%"+searchTerm+"%'"
                            + " or grade like '%"+searchTerm+"%')";
    if(searchTerm!=""&&individualSearch!=""){
        searchSQL = globeSearch + " and " + individualSearch;
    }
    else if(individualSearch!=""){
        searchSQL = " where " + individualSearch;
    }else if(searchTerm!=""){
        searchSQL=globeSearch;
    }
    sql += searchSQL;
    sql += " order by " + colName + " " + dir;
    sql += " limit " + start + ", " + amount;

    PreparedStatement ps = conn.prepareStatement(sql);
    ResultSet rs = ps.executeQuery();
    while (rs.next()) {
       // JSONArray ja = new JSONArray();
        net.sf.json.JSONArray ja = new net.sf.json.JSONArray(); 

        ja.add(rs.getInt("id"));
        ja.add(rs.getString("engine"));
        ja.add(rs.getString("browser"));
        ja.add(rs.getString("platform"));
        ja.add(rs.getString("version"));
        ja.add(rs.getString("grade"));
        arr.add(ja); 
    } 
    String sql2 = "SELECT count(*) FROM "+table;
    if (searchTerm != "") {
        sql2 += searchSQL;
        PreparedStatement ps2 = conn.prepareStatement(sql2);
        ResultSet rs2 = ps2.executeQuery();
        if (rs2.next()) {
            totalAfterFilter = rs2.getInt("count(*)");
        }
    }
    result.put("iTotalRecords", total);
    result.put("iTotalDisplayRecords", totalAfterFilter);
    result.put("aaData", arr);
    response.setContentType("application/json");
    response.setHeader("Cache-Control", "no-store");
    out.print(result);
    conn.close();
} catch (Exception e) {

}

%>

</body>
</html>

堆栈跟踪

11:26:50,224 ERROR [[jsp]] Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
    at org.apache.jsp.dataTable_jsp._jspService(dataTable_jsp.java:114)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:322)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:249)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:235)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:190)
    at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:92)
    at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.process(SecurityContextEstablishmentValve.java:126)
    at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:70)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:158)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:330)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:829)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:598)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
    at java.lang.Thread.run(Unknown Source)

3 个答案:

答案 0 :(得分：2)

如果Ajil Mohan的答案阻止出现错误，则表示您的ParametersMap中没有值。 

engine = request.getParameter("sSearch_0");
browser = request.getParameter("sSearch_1");
platform = request.getParameter("sSearch_2");
version = request.getParameter("sSearch_3");
grade = request.getParameter("sSearch_4");

这些行只是获取请求的参数映射中的数据。如果给定的不在地图中，则返回null（https://docs.oracle.com/javaee/6/api/javax/servlet/ServletRequest.html#getParameter%28java.lang.String%29）。 当您在上一页上发送包含表单中包含的数据的POST请求时，将创建此映射。您的输入字段必须使用您为getParameter方法指定的名称进行设置。

如果您没有包含表单的上一页，那么您可能希望将数据从服务器发送到JSP。如果您使用Servlet，那么您应该使用请求AttributeMap。 

public class BasicTotoServlet extends HttpServlet {
 /**
  * Manage GET HTTP Request
  */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) {
        String data = getData();
        request.setAttibute("sSearch_0", data)
    }
}

然后在你的JSP中：

engine = request.getAttribute("sSearch_0");

我希望我足够清楚。 如果需要，要求精确度

编辑： 这是我的理解。

您到达JSP并从上一页的表单中获得了几个参数。此表格可能不会向您提供所有信息，这就是您在此

上获得NPE的原因 
engine = request.getParameter("sSearch_0");
engine.equals("")

要解决这个问题，就像Ajil Mohan告诉你的那样，只需将其他方向相反转动，这样如果引擎为NULL则没有问题

"".equals(engine);

As＆＃34;＆＃34;从来没有你永远不会得到NPE。

现在，您表中没有任何数据。为什么？如果我更正，您可以通过连接多个数据来创建SQL查询。 searchSQL = globeSearch + " and " + individualSearch;

individualSearch是连接参数的结果，但如果它们为空则会怎样？

if (!"".equals(browser)) {
    String sBrowser = " browser like '%" + browser + "%'";
    sArray.add(sBrowser);
}

在此代码中，如果浏览器为null（正如我们之前所见），您将使用和关键字将字符串" browser like '%null%'"添加到查询中。我不确定会有任何浏览器＆＃34; null＆＃34;以它的名字。

你最好把这个测试：

if (!"".equals(browser) && browser != null) {
    String sBrowser = " browser like '%" + browser + "%'";
    sArray.add(sBrowser);
}

答案 1 :(得分：1)

由于它是一个jsp文件，因此很难调试。在这里你得到一个NullPointerExceotion。从您的代码中我们可以看到一些会发生NullPointerException的地方。

例如： - 您正在从“sSearch_0”获取参数。 engine = request.getParameter（“sSearch_0”）;

想象一下，如果它是一个空值。

然后下面的代码将产生一个nullpointer异常。

if（！engine.equals（“”））

所以检查如下是安全的， if（！“”。equals（engine））...

这样即使引擎具有空值，也不会出现空指针异常。

这可能不是问题，但我想提一下。

答案 2 :(得分：0)

您是否考虑在控制器中使用类似Google gson的内容从模型类中自动生成json？它可以减少jsp中的字符串操作。

更常见的模式是从前端的ajax调用所消耗的restful api中公开此信息。

Java Jersey与gson或Jackson相结合可以为你实现很多这方面的功能

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