如何关闭一个dao java

Question

在这段代码中，我需要关闭 dao。调用 daoClose() 方法。但我什么时候可以打电话，在哪里？那是最有效的。 我需要确定哪个连接已经挂起，然后关闭它。如何抓住这一刻？

public class UserController extends HttpServlet {
private static final long serialVersionUID = 7589862078860584987L;
private static String INSERT_OR_EDIT = "/user.jsp";
private static String LIST_USER = "/listUser.jsp";
private UserDao dao;

private long begin = System.nanoTime ();



public UserController() {
    super();
    dao = new UserDao();
} 



private void delete(HttpServletRequest request, HttpServletResponse response) {
     int userId = Integer.parseInt(request.getParameter("userId"));
     dao.deleteUser(userId);
     request.setAttribute("users", dao.getAllUsers());
}

private void edit(HttpServletRequest request, HttpServletResponse response) {
     int userId = Integer.parseInt(request.getParameter("userId"));
     User user = dao.getUserById(userId);
     request.setAttribute("user", user);
}

private void listUsers(HttpServletRequest request, HttpServletResponse response) {
     request.setAttribute("users", dao.getAllUsers());
}


private void common (HttpServletRequest request, HttpServletResponse response) {

    String forward="";
    String action = request.getParameter("action");

    if (action.equalsIgnoreCase("delete")){
        delete(request, response);
        forward = LIST_USER;
    } else if (action.equalsIgnoreCase("edit")){
        forward = INSERT_OR_EDIT;
        edit(request, response);
    } else if (action.equalsIgnoreCase("listUser")){
        forward = LIST_USER;
       listUsers(request, response);
    } else {
        forward = INSERT_OR_EDIT;
    }

    RequestDispatcher view = request.getRequestDispatcher(forward);
    try {
        view.forward(request, response);
    } catch (ServletException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }



}

在这段代码中，我需要关闭 dao。调用 daoClose() 方法。但我什么时候可以打电话，在哪里？那是最有效的。 我需要确定哪个连接已经挂起，然后关闭它。如何抓住这一刻？

private void commonSec( HttpServletRequest request, HttpServletResponse response) {

    User user = new User();
    user.setFirstName(request.getParameter("firstName"));
    user.setLastName(request.getParameter("lastName"));
    try {
        Date dob = new SimpleDateFormat("MM/dd/yyyy").parse(request.getParameter("dob"));
        user.setDob(dob);
    } catch (ParseException e) {
        e.printStackTrace();
    }
    user.setEmail(request.getParameter("email"));
    String userid = request.getParameter("userid");
    if(userid == null || userid.isEmpty())
    {
        dao.addUser(user);
    }
    else
    {
        user.setUserid(Integer.parseInt(userid));
        dao.updateUser(user);
    }

    RequestDispatcher views = request.getRequestDispatcher(LIST_USER);
    listUsers(request, response);
    try {
        views.forward(request, response);
    } catch (ServletException e1) {
        e1.printStackTrace();
    } catch (IOException e1) {
        e1.printStackTrace();
    }

}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    common(request, response);
    System.out.println(delta);

}

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    commonSec(request, response);
    System.out.println(delta);

 }

private void daoClose() {

    dao.closeCn();
    System.out.println("run");
}

private long end = System.nanoTime ();
private long delta = end - begin;




 }

Answer 1

为了确保无论成功/失败都关闭 dao，您可以使用 finally 块。

https://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html

例子

try {
    // use your dao
} catch (Exception e) {
    // handle error
} finally {
    daoClose();
}

finally 块中的任何代码都可以保证运行，因此无论是否发生错误，您都可以确保您的 dao 在最后关闭

Answer 2

最好使用try(UserDao dao = new UserDao())，不要把它作为类的成员它不会是线程安全的。我建议您多阅读一些有关交易的内容，并在您开始学习时按照常规的方式进行操作。