GAE数据存储放置和获取

Question

Hi I'm just playing around with the datastore and hoping someone can tell my why my simple code doesn't work? I have just been playing around with how to put and get strings from the datastore... unfortunately my GAE ability is weak alongside my weak java ability...

@SuppressWarnings("serial")
public class LoopingProgramServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws IOException {


    Key key = KeyFactory.createKey("a","b");        
    String Teststring = "pleasework";
    Entity greeting = new Entity("meh", key);
    greeting.setProperty("teststring", Teststring);     
    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    datastore.put(greeting);



    Entity greetings = null;
    resp.setContentType("text/plain");
    resp.getWriter().println("doing something");


    try {
        greetings = datastore.get(key);
        String string = (String) greetings.getProperty("teststring");


        if (string == null) {
            resp.setContentType("text/plain");
            resp.getWriter().println("null");   
            System.out.println("null");
        } else {
            resp.setContentType("text/plain");
            resp.getWriter().println("contains something"); 
            System.out.println("contains something");
        }

        resp.setContentType("text/plain");
        resp.getWriter().println(string);   

    } catch (EntityNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }




}

}

Answer 1

The Entity construct that you're using is interpreting the Key argument as the key to a parent entity. Since no id or keyName are given (as would happen if you instead used one of the other Entity constructors described here ), an id will be generated for the greeting at put time.

datastore.get(key) is returning null because key refers to an entity of kind "a" with a keyName of "b", which (I'm guessing) doesn't exist.