使用GAE数据存储进行身份验证时，实体用户返回null

Question

I'm trying to create my own authentication. Everytime I try to login, and the username is not found in the GAE datastore, I get to have INTERNAL_SERVER_ERROR.

It says that:

java.lang.NullPointerException
at com.pawnsoftware.User.authenticate(User.java:16)
at com.pawnsoftware.UserLoginServlet.doPost(UserLoginServlet.java:24)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)

and so on...

How do you avoid having this error?

The error is on the line that says:

if (user==null) {
    message = "Username not found.";
}

Authenticate User:

public static String authenticate(String username, String password) {
    String message;
    Entity user = UserUtil.findUserEntity(username);
    String pass = user.getProperty("password").toString();
    if (user==null) {
        message = "Username not found.";
    } else if (user!=null && !password.equals(pass)) {
        message = "Password is incorrect.";
    } else if (user!=null && password.equals(pass)) {
        message = "Successfully logged in!";
    } else {
        message = "Sorry, cannot find the username and password.";
    } 
    return message;
}

Find User Entity:

public static Entity findUserEntity (String username) {
    Key userKey = KeyFactory.createKey("User", username);
    try {
      return datastore.get(userKey);
    } catch (EntityNotFoundException e) {
      return null;
    }
}

Update on authentication:

public static String authenticate(String username, String password) {
    String message;
    Entity user = UserUtil.findUserEntity(username);
    password = encrypt(password);
    String pass = "";   
        try {
            pass = user.getProperty("password").toString();
        } catch (NullPointerException e) {
            message = "Username not found.";
        }
    if (user==null) {
        message = "Username not found.";
    } else if (user!=null && !password.equals(pass)) {
        message = "Password is incorrect.";
    } else if (user!=null && password.equals(pass)) {
        message = "Successfully logged in!";
    } else {
        message = "Sorry, cannot find the username and password.";
    } 
    return message;
}

Answer 1

如果use为null，则此行将失败：

String pass = user.getProperty("password").toString();

Answer 2

You don't display if you get get a EntityNotFoundException and return null . This, in turn, can cause a NullPointerException for the variable user :

user.getProperty("password").toString();

You could add a guard statement here:

if (user != null) {
   pass = user.getProperty("password").toString();
}