QueryResultIterator

CursorReviewServlet.java

package com.example;

import java.io.IOException;
import java.io.PrintWriter;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.google.appengine.api.datastore.Cursor;
import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.FetchOptions;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.KeyFactory;
import com.google.appengine.api.datastore.Query;
import com.google.appengine.api.datastore.QueryResultIterator;

@SuppressWarnings("serial")
public class CursorReviewServlet extends HttpServlet {

    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws IOException {
        resp.setContentType("text/plain");

        prepare();

        String cursor = null;
        while (true) {
            cursor = exec(resp.getWriter(), cursor);
            if (cursor == null) {
                break;
            }
        }
    }

    private void prepare() {
        List<Entity> entities = new ArrayList<Entity>();
        for (int i = 0; i < 30; i++) {
            Key key = KeyFactory.createKey("CursorReview", String.format("_%04d", i));
            entities.add(new Entity(key));
        }
        DatastoreService ds = DatastoreServiceFactory.getDatastoreService();
        ds.put(entities);
    }

    private String exec(PrintWriter writer, String paging) {
        Query query = new Query("CursorReview");
        DatastoreService ds = DatastoreServiceFactory.getDatastoreService();

        // カーソルを復元
        FetchOptions options = FetchOptions.Builder.withLimit(10);
        if (paging != null) {
            options = options.cursor(Cursor.fromWebSafeString(paging));
        }

        QueryResultIterator<Entity> iter =
            ds.prepare(query).asQueryResultIterator(options);

        writer.println("Serializable?:" + (iter instanceof Serializable));

        // 先頭ひとつだけ
        if (iter.hasNext() == false) {
            writer.println("Result is empty");
            return null;
        }
        else {
            iter.hasNext(); // カーソルを進めるか？
            writer.println("First: " + iter.next().getKey());
        }

        Cursor cursor = iter.getCursor();
        if (cursor == null) {
            writer.println("Cursor is null");
            return null;
        }
        else {
            String next = cursor.toWebSafeString();
            writer.println("continue..");
            return next;
        }
    }
}

result

Serializable?:false
First: CursorReview("_0000")
continue..
Serializable?:false
First: CursorReview("_0001")
continue..
Serializable?:false
First: CursorReview("_0002")
continue..
Serializable?:false
First: CursorReview("_0003")
continue..
Serializable?:false
First: CursorReview("_0004")
continue..
Serializable?:false
First: CursorReview("_0005")
continue..
Serializable?:false
First: CursorReview("_0006")
continue..
Serializable?:false
First: CursorReview("_0007")
continue..
Serializable?:false
First: CursorReview("_0008")
continue..
Serializable?:false
First: CursorReview("_0009")
continue..
Serializable?:false
First: CursorReview("_0010")
continue..
Serializable?:false
First: CursorReview("_0011")
continue..
Serializable?:false
First: CursorReview("_0012")
continue..
Serializable?:false
First: CursorReview("_0013")
continue..
Serializable?:false
First: CursorReview("_0014")
continue..
Serializable?:false
First: CursorReview("_0015")
continue..
Serializable?:false
First: CursorReview("_0016")
continue..
Serializable?:false
First: CursorReview("_0017")
continue..
Serializable?:false
First: CursorReview("_0018")
continue..
Serializable?:false
First: CursorReview("_0019")
continue..
Serializable?:false
First: CursorReview("_0020")
continue..
Serializable?:false
First: CursorReview("_0021")
continue..
Serializable?:false
First: CursorReview("_0022")
continue..
Serializable?:false
First: CursorReview("_0023")
continue..
Serializable?:false
First: CursorReview("_0024")
continue..
Serializable?:false
First: CursorReview("_0025")
continue..
Serializable?:false
First: CursorReview("_0026")
continue..
Serializable?:false
First: CursorReview("_0027")
continue..
Serializable?:false
First: CursorReview("_0028")
continue..
Serializable?:false
First: CursorReview("_0029")
continue..
Serializable?:false
Result is empty