Questions about JSP & SQL. Java web application

问题

I am recently working on making a Java web application, which must focus on CRUD(You know). But I am stuck with the read & update & delete operations(Three operations). Only Create Operation works fine. In detailed, the Java web application I am working on is not completed so far. Inside my code, the function called "findUsers" is used to implement read operation. By the way, I already checked code for a long period of time. I guess the problem may be in findUsers function(not sure, just an assumption). Every time, I try to type R to invoke that function, Netbeans returns "User does not exist". I don't know why. Moreover, the database is connected successfully. enter image description here This photo is my database structure.

DBManager.java

//read operation
   public User findUsers(String email, String password) throws SQLException {
    String sqll = "SELECT * FROM XWB.USERS WHERE EMAIL = ' " + email + " ' AND PASSWORD = ' " + password + " ' ";
    // " SELECT * FROM XWB.USERS WHERE EMAIL = ' " + email + " ' AND PASSWORD = ' " + password + " ' "; 
    //select * from XWB.Users where EMAIL = 'TargetEmail' and PASSWORD = 'TargetPassword';
    ResultSet rs = st.executeQuery(sqll);

    while (rs.next()) {
        String UserEmail = rs.getString("EMAIL");
        String UserPassword = rs.getString("PASSWORD");
        if (UserEmail.equals(email) && UserPassword.equals(password)) {
            String UserName = rs.getString("NAME"); 
            String UserGender = rs.getString("GENDER");
            String UserColor = rs.getString("FAVOURITECOLOR");
            return new User(UserEmail, UserName, UserPassword, UserGender, UserColor);
        }
    }
    return null;
}

TestDB.java(I use this class to test the DBManager)

 // findUsers()
    private void testRead() throws SQLException {
        System.out.print("User email: ");
        String email = in.nextLine();
        System.out.print("User password: ");
        String password = in.nextLine();
        User user = db.findUsers(email, password); // returns nothing
        //System.out.println(user);
        if( user != null) {
            System.out.println("User " + user.getName() + " exists in the database.");
        }else { //user == null
            System.out.println("User does not exit.");
        }
    }

This is the result I got from Netbeans. It always tells me "User does not exist. " enter image description here

回答1:

I think you should remove space after and before ['] character if your data value are correct with email and password. Your code:

 String sqll = "SELECT * FROM XWB.USERS WHERE EMAIL = ' " + email + " ' AND PASSWORD = ' " + password + " ' ";

Replace:

 String sqll = "SELECT * FROM XWB.USERS WHERE EMAIL = '" + email + "' AND PASSWORD = '" + password + "' ";

回答2:

In your code you have return null so when this get check at your calling class if( user != null) will return false and the else part of your if-statement will get executed.So to overcome do below changes :

Your method will look like below:

User user; //create class object
String sqll = "SELECT * FROM XWB.USERS WHERE EMAIL = ? AND PASSWORD = ? ";
PreparedStatement ps = con.prepareStatement(
 sqll);
//setting value for "?" 
ps.setString(1, email);
ps.setString(2, password);
//execute query
ResultSet rs = ps.executeQuery();
//if found
if (rs.next()) {
 //fetch
 String UserEmail = rs.getString("EMAIL");
 String UserPassword = rs.getString("PASSWORD");
 String UserName = rs.getString("NAME");
 String UserGender = rs.getString("GENDER");
 String UserColor = rs.getString("FAVOURITECOLOR");
 //pass in constructor
 user = new User(UserEmail, UserName, UserPassword, UserGender, UserColor);
} else {
 //set the object to null (no match)
 user = null;
}
//send object back
return user;

来源：https://stackoverflow.com/questions/61999571/questions-about-jsp-sql-java-web-application

标签

java

sql

jsp

model-view-controller

crud