Question

我将TomCat 8.0服务器，h2数据库和IntelliJ IDEA 14用于我的WebApplication，但是当我运行app（按钮事件）时，我收到了

java.lang.NullPointerException image. 我已成功连接到数据库（已经过测试）：

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;
    }
}

我创建了类RegisterDao，它必须将数据插入到名为“Name”的表中，并且只有一列 - “name”

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;
}}

这是我的类（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;
}}

我使用这些类通过servlet在表“Name”中插入数据 - 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();
    }
}}

但是我得到了错误。我很困惑，因为我在Eclipse中测试了源码并且它在那里工作，但是在IntelliJ中它没有。我是IntelliJ的新手，所以我希望有更多经验的人会给我一个解决问题的想法。

P.S。来自servlet的System.out.println("It Works");可以正常工作，我保证将servlet称为“index.jsp”正确。

对不起这篇长篇文章和最好的问候， D.Balamjiev。

Answer 1

它适用于Eclipse，您确定吗？

DriverManager.getConnection(URL, USER_NAME, PASS);

您尚未将其分配给conn，因此conn仍为null，然后conn.prepareStatement会引发异常，因为conn为null。我认为你需要做的只是：

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

我在IntelliJ IDEA和TomCat中收到了java.lang.NullPointerException

1 个答案: