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Add Query Cursor Datastore sample. (#207)
Moved from https://cloud.google.com/appengine/docs/java/datastore/query-cursors#query_cursor_example
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‎appengine/datastore/src/main/java/com/example/appengine/ListPeopleServlet.java

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/*
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* Copyright 2016 Google Inc. All Rights Reserved.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package com.example.appengine;
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// [START cursors]
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import com.google.appengine.api.datastore.Cursor;
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import com.google.appengine.api.datastore.DatastoreService;
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import com.google.appengine.api.datastore.DatastoreServiceFactory;
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import com.google.appengine.api.datastore.Entity;
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import com.google.appengine.api.datastore.FetchOptions;
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import com.google.appengine.api.datastore.PreparedQuery;
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import com.google.appengine.api.datastore.Query;
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import com.google.appengine.api.datastore.Query.SortDirection;
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import com.google.appengine.api.datastore.QueryResultList;
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import java.io.IOException;
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import java.io.PrintWriter;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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public class ListPeopleServlet extends HttpServlet {
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static final int PAGE_SIZE = 15;
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private final DatastoreService datastore;
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public ListPeopleServlet() {
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datastore = DatastoreServiceFactory.getDatastoreService();
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}
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@Override
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protected void doGet(HttpServletRequest req, HttpServletResponse resp)
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throws ServletException, IOException {
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FetchOptions fetchOptions = FetchOptions.Builder.withLimit(PAGE_SIZE);
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// If this servlet is passed a cursor parameter, let's use it.
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String startCursor = req.getParameter("cursor");
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if (startCursor != null) {
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fetchOptions.startCursor(Cursor.fromWebSafeString(startCursor));
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}
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Query q = new Query("Person").addSort("name", SortDirection.ASCENDING);
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PreparedQuery pq = datastore.prepare(q);
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QueryResultList<Entity> results;
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try {
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results = pq.asQueryResultList(fetchOptions);
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} catch (IllegalArgumentException e) {
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// IllegalArgumentException happens when an invalid cursor is used.
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// A user could have manually entered a bad cursor in the URL or there
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// may have been an internal implementation detail change in App Engine.
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// Redirect to the page without the cursor parameter to show something
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// rather than an error.
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resp.sendRedirect("/people");
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return;
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}
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resp.setContentType("text/html");
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resp.setCharacterEncoding("UTF-8");
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PrintWriter w = resp.getWriter();
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w.println("<!DOCTYPE html>");
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w.println("<meta charset=\"utf-8\">");
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w.println("<title>Cloud Datastore Cursor Sample</title>");
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w.println("<ul>");
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for (Entity entity : results) {
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w.println("<li>" + entity.getProperty("name") + "</li>");
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}
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w.println("</ul>");
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String cursorString = results.getCursor().toWebSafeString();
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// This servlet lives at '/people'.
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w.println("<a href='/people?cursor=" + cursorString + "'>Next page</a>");
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}
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}
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// [END cursors]

‎appengine/datastore/src/main/java/com/example/appengine/StartupServlet.java

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/*
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* Copyright 2016 Google Inc. All Rights Reserved.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package com.example.appengine;
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import com.google.appengine.api.datastore.DatastoreService;
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import com.google.appengine.api.datastore.DatastoreServiceFactory;
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import com.google.appengine.api.datastore.Entity;
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import com.google.appengine.api.datastore.EntityNotFoundException;
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import com.google.appengine.api.datastore.Key;
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import com.google.appengine.api.datastore.KeyFactory;
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import com.google.common.collect.ImmutableList;
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import java.io.IOException;
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import javax.servlet.ServletException;
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import javax.servlet.http.HttpServlet;
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import javax.servlet.http.HttpServletRequest;
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import javax.servlet.http.HttpServletResponse;
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/**
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* A startup handler to populate the datastore with example entities.
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*/
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public class StartupServlet extends HttpServlet {
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static final String IS_POPULATED_ENTITY = "IsPopulated";
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static final String IS_POPULATED_KEY_NAME = "is-populated";
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private static final String PERSON_ENTITY = "Person";
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private static final String NAME_PROPERTY = "name";
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private static final ImmutableList<String> US_PRESIDENTS =
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ImmutableList.<String>builder()
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.add("George Washington")
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.add("John Adams")
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.add("Thomas Jefferson")
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.add("James Madison")
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.add("James Monroe")
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.add("John Quincy Adams")
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.add("Andrew Jackson")
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.add("Martin Van Buren")
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.add("William Henry Harrison")
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.add("John Tyler")
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.add("James K. Polk")
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.add("Zachary Taylor")
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.add("Millard Fillmore")
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.add("Franklin Pierce")
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.add("James Buchanan")
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.add("Abraham Lincoln")
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.add("Andrew Johnson")
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.add("Ulysses S. Grant")
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.add("Rutherford B. Hayes")
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.add("James A. Garfield")
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.add("Chester A. Arthur")
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.add("Grover Cleveland")
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.add("Benjamin Harrison")
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.add("Grover Cleveland")
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.add("William McKinley")
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.add("Theodore Roosevelt")
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.add("William Howard Taft")
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.add("Woodrow Wilson")
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.add("Warren G. Harding")
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.add("Calvin Coolidge")
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.add("Herbert Hoover")
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.add("Franklin D. Roosevelt")
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.add("Harry S. Truman")
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.add("Dwight D. Eisenhower")
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.add("John F. Kennedy")
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.add("Lyndon B. Johnson")
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.add("Richard Nixon")
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.add("Gerald Ford")
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.add("Jimmy Carter")
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.add("Ronald Reagan")
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.add("George H. W. Bush")
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.add("Bill Clinton")
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.add("George W. Bush")
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.add("Barack Obama")
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.build();
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@Override
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protected void doGet(HttpServletRequest req, HttpServletResponse resp)
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throws ServletException, IOException {
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resp.setContentType("text/plain");
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DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
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Key isPopulatedKey = KeyFactory.createKey(IS_POPULATED_ENTITY, IS_POPULATED_KEY_NAME);
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boolean isAlreadyPopulated;
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try {
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datastore.get(isPopulatedKey);
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isAlreadyPopulated = true;
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} catch (EntityNotFoundException expected) {
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isAlreadyPopulated = false;
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}
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if (isAlreadyPopulated) {
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resp.getWriter().println("ok");
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return;
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}
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ImmutableList.Builder<Entity> people = ImmutableList.builder();
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for (String name : US_PRESIDENTS) {
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Entity person = new Entity(PERSON_ENTITY);
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person.setProperty(NAME_PROPERTY, name);
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people.add(person);
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}
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datastore.put(people.build());
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datastore.put(new Entity(isPopulatedKey));
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resp.getWriter().println("ok");
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}
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}

‎appengine/datastore/src/main/webapp/WEB-INF/web.xml

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</servlet>
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<servlet-mapping>
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<servlet-name>guestbook-strong</servlet-name>
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<url-pattern>/guestbook-strong</url-pattern>
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<url-pattern>/</url-pattern>
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</servlet-mapping>
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<servlet>
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<servlet-name>guestbook</servlet-name>
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<servlet-name>guestbook</servlet-name>
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<url-pattern>/guestbook</url-pattern>
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</servlet-mapping>
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<servlet>
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<servlet-name>people</servlet-name>
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<servlet-class>com.example.appengine.ListPeopleServlet</servlet-class>
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</servlet>
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<servlet-mapping>
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<servlet-name>people</servlet-name>
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<url-pattern>/people</url-pattern>
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</servlet-mapping>
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<servlet>
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<servlet-name>projection</servlet-name>
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<servlet-class>com.example.appengine.ProjectionServlet</servlet-class>
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<url-pattern>/projection</url-pattern>
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</servlet-mapping>
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<!-- Special handler to populate Datastore with default values. -->
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<servlet>
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<servlet-name>startup</servlet-name>
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<servlet-class>com.example.appengine.StartupServlet</servlet-class>
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</servlet>
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<servlet-mapping>
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<servlet-name>startup</servlet-name>
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<url-pattern>/_ah/start</url-pattern>
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</servlet-mapping>
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<security-constraint>
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<web-resource-collection>
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<web-resource-name>profile</web-resource-name>

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