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我将 TomCat 8.0 服务器、h2 数据库和 IntelliJ IDEA 14 用于我的 Web 应用程序，但是当我运行应用程序(按钮事件)时，我收到了
java.lang.NullPointerException image. I have successful connection to Database like this(it's already tested):
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”且只有一列 - “name”的表中
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 - 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();
}
}}
但是我得到了上层错误。我很困惑，因为我在 Eclipse 中测试了源代码并且它在那里工作，但在 IntelliJ 中它没有。 我是 IntelliJ 的新手，所以我希望有更多经验的人能给我一个解决问题的想法。
附言System.out.println("It Works"); 来自 servlet 的作品，我保证我将 servlet 调用到“index.jsp”中是正确的。
很抱歉发了这么长的帖子，并致以最诚挚的问候， D.Balamjiev。