爲什麼我得到NullPointerException錯誤？

我試圖找出什麼是我的錯誤，但我找不到任何與我的代碼被稱爲空指針。爲什麼我得到NullPointerException錯誤？

這裏是堆棧跟蹤（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)

這裏是堆棧跟蹤日誌（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)

以下是錯誤頁面：

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

以下是我的代碼：

數據庫類：

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類：

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()); 
     } 
    } 
}

的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>

來源

2012-07-31 malhobayyeb

如果你得到一個NullPointerException，你也得到一個堆棧跟蹤。堆棧跟蹤的第一行顯示出錯的地方。 – 2012-07-31 12:32:31

我很確定'dbConnection'爲空。 – Paranaix 2012-07-31 12:33:11

我在哪裏可以看到堆棧跟蹤？ – malhobayyeb 2012-07-31 12:35:16

的問題是在Database.createStatement()，線：

return dbConnection.createStatement();

引發錯誤，這意味着dbConnection爲空。

在你connect()方法你有這些行：

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

所以，即使連接失敗，你不會知道它，因爲你沒有在catch塊。

我建議你將其更改爲：

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

，我打賭你會發現，你有一個問題存在。

來源

2012-07-31 12:41:24 Tomer

如果我確實在哪裏打印日誌？ – malhobayyeb 2012-07-31 12:47:27

沒有打印日誌，它會把它扔到屏幕上。 – Tomer 2012-07-31 12:48:13

@ MIH1406控制檯。 – 2012-07-31 12:48:19

正是這些檢查

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

你輸入就可以派上作爲空值，並且你已經解釋了這一點，但是如果確實如此，它將嘗試首先執行此聲明adminUsername.equals("")並拋出NPE。簡單地顛倒他們的順序，它應該可以解決你的問題。

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

@ ftom2也是正確的，也可能是dbConnection.createStatement();線，但同出一堆棧跟蹤，我們真的不能告訴。您應該進行這些更改以適應null的可能性，以便稍後不會出現問題。

來源

2012-07-31 12:33:48

我做到了，但現在使用。 – malhobayyeb 2012-07-31 12:38:13

或「」.equals（admin用戶名） – Woot4Moo 2012-07-31 12:39:57

爲什麼我得到NullPointerException錯誤？

回答

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