为什么我收到NullPointerException错误？

Question

我试图弄清楚我的错误是什么，但我找不到任何名为空指针的代码。

这是StackTrace（1）：

java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1678)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1523)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:186)
    at daftoor.Database.connect(Database.java:92)
    at daftoor.DoSignIn.checkSignin(DoSignIn.java:86)
    at daftoor.DoSignIn.processRequest(DoSignIn.java:40)
    at daftoor.DoSignIn.doGet(DoSignIn.java:122)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:964)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:722)

以下是StackTrace日志（2）：

Jul 31, 2012 4:07:41 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [DoSignIn] in context with path [/Daftoor] threw exception
java.lang.NullPointerException
    at daftoor.Database.createStatement(Database.java:105)
    at daftoor.DoSignIn.checkSignin(DoSignIn.java:91)
    at daftoor.DoSignIn.processRequest(DoSignIn.java:40)
    at daftoor.DoSignIn.doGet(DoSignIn.java:122)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:224)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:964)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
    at java.lang.Thread.run(Thread.java:722)

以下是错误页面：

HTTP Status 500 -

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
    myproject.Database.createStatement(Database.java:101)
    myproject.DoSignIn.checkSignin(DoSignIn.java:91)
    myproject.DoSignIn.processRequest(DoSignIn.java:40)
    myproject.DoSignIn.doGet(DoSignIn.java:127)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)

note The full stack trace of the root cause is available in the Apache Tomcat/7.0.22 logs.
Apache Tomcat/7.0.22

以下是我的代码：

数据库类：

package somePackage;

import java.io.*;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.HashMap;
import java.util.Properties;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


/**
 *
 * @author mih1406
 */
public class Database {
    private HashMap dbConfig;
    private Connection dbConnection;

    // debugging
    HttpServletRequest request;
    HttpServletResponse response;



    public Database(HttpServletRequest request, HttpServletResponse respnse) {
        this.request = request;
        this.response = response;
        setDatabaseConfig();

    }

    public void setDatabaseConfig() {
        Properties config = new Properties();
        InputStream configFile;

        try {
            configFile = this.getClass().getResourceAsStream("config.properties");
            config.load(configFile);
        } catch(FileNotFoundException fnfe) {
            // TODO properties file not found error
        } catch(IOException ioe) {
            // TODO i/o Exception
        }

        String dbHost = config.getProperty("dbHost");
        String dbPort = config.getProperty("dbPort");
        String dbUsername = config.getProperty("dbUsername");
        String dbPassword = config.getProperty("dbPassword");
        String dbName = config.getProperty("dbName");
        String dbPrefix = config.getProperty("dbPrefix");

        setDatabaseConfig(dbHost, dbPort, dbUsername, dbPassword,
                dbName, dbPrefix);
    }

    public void setDatabaseConfig(String dbHost, String dbPort, String dbUsername,
            String dbPassword, String dbName, String dbPrefix) {
        dbConfig = new HashMap();

        dbConfig.put("dbHost", dbHost);
        dbConfig.put("dbPort", dbPort);
        dbConfig.put("dbUsername", dbUsername);
        dbConfig.put("dbPassword", dbPassword);
        dbConfig.put("dbName", dbName);
        dbConfig.put("dbPrefix", dbPrefix);
    }

    public void connect() {
        String dbHost = (String)dbConfig.get("dbHost") + "/";
        String dbPort = (String)dbConfig.get("dbPort");
        String dbUsername = (String)dbConfig.get("dbUsername");
        String dbPassword = (String)dbConfig.get("dbPassword");
        String dbName = (String)dbConfig.get("dbName");
        String dbPrefix = (String)dbConfig.get("dbPrefix");

        String dbDriver = "com.mysql.jdbc.Driver";
        String dbURL = "jdbc:mysql://" + dbHost + ":" + dbPort + "/";

        try {
            Class.forName(dbDriver);
            dbConnection = DriverManager.getConnection(dbURL, dbURL, dbPassword);
        } catch (ClassNotFoundException cnfe) {
            // TODO Class Exception
        } catch (SQLException sqle) {
            // TODO SQL Exception
        }
    }

    public Statement createStatement() {
        try {
            return dbConnection.createStatement();
        } catch(SQLException sqle) {
            // TODO SQL Exception
        }

        return null;
    }

    public PreparedStatement prepareStatement(String sql) {
        PreparedStatement pstmt = null;
        try {
            pstmt = dbConnection.prepareStatement(sql);
        } catch(SQLException sqle) {
            // TODO SQL Exception
        }

        return pstmt;
    }

    public void close() {
        try {
            if(dbConnection.isClosed()) {
                return;
            }

            dbConnection.close();
        } catch(SQLException sqle) {
            // TODO SQL Exception
        }
    }

    public String getPrefix() {
        return (String)dbConfig.get("dbPrefix");
    }

    public String getName() {
        return (String)dbConfig.get("dbName");
    }

    public String getHost() {
        return (String)dbConfig.get("dbHost");
    }

    public String getPort() {
        return (String)dbConfig.get("dbPort");
    }

    public String getUsername() {
        return (String)dbConfig.get("dbUsername");
    }

    public String getPassword() {
        return (String)dbConfig.get("dbPassword");
    }
}

DoSingin课程：

package somePackage;

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 *
 * @author mih1406
 */
@WebServlet(name = "DoSignIn", urlPatterns = {"/DoSignIn"})
public class DoSignIn extends HttpServlet {

    /**
     * Processes requests for both HTTP
     * <code>GET</code> and
     * <code>POST</code> methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        checkValues(request, response);
        checkSignin(request, response);

    }

    private void checkValues(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String adminUsername = request.getParameter("adminUsername");
        String adminPassword = request.getParameter("adminPassword");

        boolean someEmpty = false;

        if((adminUsername.equals("") || adminUsername == null)) {
            someEmpty = true;
            request.setAttribute("emptyEmail", "Required");
        } else {
            request.setAttribute("adminUsername",
                    request.getParameter("adminUsername"));
        }

        if((adminPassword.equals("") || adminPassword == null)) {
            someEmpty = true;
            request.setAttribute("emptyPassword", "Required");
        }

        if(someEmpty) {
            String url = "signin.jsp";
            forwardToPage(request, response, url);
        }
    }

    private void forwardToPage(HttpServletRequest request,
            HttpServletResponse response, String url)
            throws ServletException, IOException {
        RequestDispatcher dispatcher = request.getRequestDispatcher(url);
        dispatcher.forward(request, response);
    }

    private void checkSignin(HttpServletRequest request, HttpServletResponse response)
        throws ServletException,IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        String adminUsername = request.getParameter("adminUsername");
        String adminPassword = request.getParameter("adminPassword");

        out.println("<br />HereDBCreation!!");
        Database db = new Database(request, response);
        db.connect();
        out.println("<br />HereDBConnectionDone!!");
        String query = "SELECT adminUsername, adminPassword FROM " +
                db.getPrefix() + db.getName() + " WHERE adminUsername = '" +
                adminUsername + "' AND adminPassword = '" + adminPassword + "'";
        Statement stmt = db.createStatement();
        ResultSet result = null;
if(stmt == null)         out.println("<br />HereDBStmtDone!!");

        try {
            result = stmt.executeQuery(query);
            result.beforeFirst();
            out.println("Here!!");
            if(result.next()){
                    HttpSession session = request.getSession(true);
                    session.setMaxInactiveInterval(21*60); // 21 minutes
                    session.setAttribute("JSESSIONID", adminUsername);

                    out.println("Correct!!");
            } else {
                out.println("Invalid!!");
            }
        } catch(SQLException sqle) {
            // TODO SQL Exception
            System.out.println(sqle.getMessage());
        }
    }
}

signin.jsp JSP文件：

<%--
    Document   : login
    Created on : Jul 19, 2012, 3:36:48 PM
    Author     : mih1406
--%>

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>Administration</title>
    </head>
    <body>
        <form action="DoSignIn" method="get" name="adminSignInForm">
            <table>
                <tr>
                    <td colspan="2">Administration<td>
                        <br />
                </tr>
                <tr>
                    <td><label for="adminUsername">Email:</label></td>
                    <td><input type="text" value="${adminUsername}" name="adminUsername" />${emptyEmail}</td>
                </tr>

                <tr>
                    <td><label for="adminPassword">Password:</label></td>
                    <td><input type="password" value="" name="adminPassword" />${emptyPassword}</td>
                </tr>
                <tr>
                    <td colspan="2">
                        <input type="checkbox" value="yes" checked="checked" name="persistentCookie" />
                        <label for="persistentCookie">Stay signed in</label>
                    </td>
                </tr>
                <tr>
                    <td colspan="2"><input type="submit" value="Sign in" /></td>
                </tr>
            </table>
        </form>
    </body>
</html>

Answer 1

这是这些检查

(adminUsername.equals("") || adminUsername == null)

您的输入可以为null，并且您已经考虑到了这一点，但如果确实如此，它将尝试首先执行此语句adminUsername.equals("")并抛出NPE。只需反转他们的订单，它就可以解决您的问题。

( adminUsername == null || adminUsername.equals(""))

@ ftom2也是正确的，它可能是dbConnection.createStatement();行，但是如果没有堆栈跟踪，我们无法确定。您应该进行这些更改以适应null的可能性，以便以后不会出现问题。

Answer 2

问题出在Database.createStatement()，行：

return dbConnection.createStatement();

抛出错误，这意味着dbConnection为空。

在connect()方法中，您有以下几行：

try {
            Class.forName(dbDriver);
            dbConnection = DriverManager.getConnection(dbURL, dbURL, dbPassword);
        } catch (ClassNotFoundException cnfe) {
            // TODO Class Exception
        } catch (SQLException sqle) {
            // TODO SQL Exception
        }

因此，即使连接失败，您也不会知道它，因为您在catch块中什么都不做。

我建议您将其更改为：

try {
            Class.forName(dbDriver);
            dbConnection = DriverManager.getConnection(dbURL, dbURL, dbPassword);
        } catch (ClassNotFoundException cnfe) {
           cnfe.printStacktrace();
        } catch (SQLException sqle) {
            sqle.printStacktrace();
        }

我打赌你会发现那里有问题。