我收到顯示java.lang.NullPointerException在IntelliJ IDEA的和Tomcat

我使用Tomcat 8.0服務器，數據庫，H2和IntelliJ IDEA 14我WebApplication的，但是當我運行的應用程序（按鈕事件）我收到我收到顯示java.lang.NullPointerException在IntelliJ IDEA的和Tomcat

的Java。 lang.NullPointerException image. 我有（它已經測試）這樣的成功連接到數據庫：

public class DbConnection { 
private static final String DRIVER = "org.h2.Driver"; 
private static final String URL = "jdbc:h2:tcp://localhost/~/Students"; 
private static final String USER_NAME = "doncho"; 
private static final String PASS = ""; 
private static DbConnection instance; 
private static Connection conn; 

private DbConnection(){ 

} 

public static DbConnection getInstance(){ 
    if(instance == null){ 
     instance = new DbConnection(); 
    } 
    return instance; 
} 

public Connection getConnect() throws SQLException{ 
    Connect(); 
    return conn; 
} 

private void Connect() throws SQLException { 
    if (conn == null) { 
     try { 
      Class.forName(DRIVER); 
     } catch (ClassNotFoundException e) { 
      System.out.println("Driver isn't found"); 
      e.printStackTrace(); 
     } 
     DriverManager.getConnection(URL, USER_NAME, PASS); 
    } 
} 

public void Disconnect() throws SQLException{ 
    if(conn != null){ 
     conn.close(); 
     conn=null; 
    } 
}

我已創建類RegisterDao必須名稱爲「名」將數據插入表剛一列 - 「名」

public class RegisterDAO { 

Connection conn = null; 
PreparedStatement state = null; 

public StudentBean registerStudent(String userName) throws SQLException{ 

    StudentBean result = null; 
    String sql = "INSERT INTO Name VALUES(?)"; 

    try { 
     conn = DbConnection.getInstance().getConnect(); 
     state = conn.prepareStatement(sql); 
     state.setString(1, userName); 
     state.execute(); 
    } catch (SQLException e) { 
     e.printStackTrace(); 
    }finally { 
     if(state != null){ 
      state.close(); 
     } 
    } 
    DbConnection.getInstance().Disconnect(); 
    return result; 
}}

這是我的課（的Java bean）StudentBean

@javax.ejb.Stateless(name = "StudentEJB") 
public class StudentBean { 

private String name; 


public StudentBean() { 
} 

public String getName() { 
    return name; 
} 
public void setName(String name) { 
    this.name = name; 
}}

我用thеsе類通過servlet來插入表「名稱」數據 - RegisterServlet

public class RegisterServlet extends HttpServlet { 

protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { 
    try { 
     Connection conn = DbConnection.getInstance().getConnect(); 
     System.out.println("It Works"); 
    } catch (SQLException e) { 
     e.printStackTrace(); 
    } 

    String username = request.getParameter("usernameReg"); 
    StudentBean student; 
    try { 
     student = new RegisterDAO().registerStudent(username); 
     request.getServletContext().getRequestDispatcher("/index.jsp").forward(request, response); 
    } catch (SQLException e) { 
     e.printStackTrace(); 
    } 

    try { 
     DbConnection.getInstance().Disconnect(); 
    } catch (SQLException e) { 
     e.printStackTrace(); 
    } 
}}

但我得到上面的錯誤。我很困惑，因爲我在Eclipse中測試了源代碼，並且它在那裏工作，但是在IntelliJ中沒有。我是IntelliJ新手，所以我希望有更多經驗的人會給我一個解決問題的想法。

P.S.來自servlet的System.out.println("It Works");工作，我保證我調用servlet到「index.jsp」正確。

對不起，對這篇長文章和最好的問候， D.Balamjiev。

來源

2016-11-16 Doncho Balamjiev

它在Eclipse中有效，你確定嗎？

DriverManager.getConnection(URL, USER_NAME, PASS);

您沒有將其分配給conn所以conn仍然null，然後conn.prepareStatement拋出異常，因爲conn是null。我認爲你所需要做的只是：

conn = DriverManager.getConnection(URL, USER_NAME, PASS);

來源

2016-11-16 12:58:55 Shadov

錯誤是我的。我忘了設置連接初始化。感謝您的幫助，現在工作正常。 –

我收到顯示java.lang.NullPointerException在IntelliJ IDEA的和Tomcat

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