Add Query Cursor Datastore sample.

Moved from https://cloud.google.com/appengine/docs/java/datastore/query-cursors#query_cursor_example
GoogleCloudPlatform · Apr 27, 2016 · 902723a · 902723a
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diff --git a/appengine/datastore/src/main/java/com/example/appengine/ListPeopleServlet.java b/appengine/datastore/src/main/java/com/example/appengine/ListPeopleServlet.java
@@ -0,0 +1,91 @@
+/*
+ * Copyright 2016 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.appengine;
+
+// [START cursors]
+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.PreparedQuery;
+import com.google.appengine.api.datastore.Query;
+import com.google.appengine.api.datastore.Query.SortDirection;
+import com.google.appengine.api.datastore.QueryResultList;
+
+import java.io.IOException;
+import java.io.PrintWriter;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+public class ListPeopleServlet extends HttpServlet {
+  static final int PAGE_SIZE = 15;
+  private final DatastoreService datastore;
+
+  public ListPeopleServlet() {
+    datastore = DatastoreServiceFactory.getDatastoreService();
+  }
+
+  @Override
+  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
+      throws ServletException, IOException {
+    FetchOptions fetchOptions = FetchOptions.Builder.withLimit(PAGE_SIZE);
+
+    // If this servlet is passed a cursor parameter, let's use it.
+    String startCursor = req.getParameter("cursor");
+    if (startCursor != null) {
+      fetchOptions.startCursor(Cursor.fromWebSafeString(startCursor));
+    }
+
+    Query q = new Query("Person").addSort("name", SortDirection.ASCENDING);
+    PreparedQuery pq = datastore.prepare(q);
+
+    QueryResultList<Entity> results;
+    try {
+      results = pq.asQueryResultList(fetchOptions);
+    } catch (IllegalArgumentException e) {
+      // IllegalArgumentException happens when an invalid cursor is used.
+      // A user could have manually entered a bad cursor in the URL or there
+      // may have been an internal implementation detail change in App Engine.
+      // Redirect to the page without the cursor parameter to show something
+      // rather than an error.
+      resp.sendRedirect("/people");
+      return;
+    }
+
+    resp.setContentType("text/html");
+    resp.setCharacterEncoding("UTF-8");
+    PrintWriter w = resp.getWriter();
+    w.println("<!DOCTYPE html>");
+    w.println("<meta charset=\"utf-8\">");
+    w.println("<title>Cloud Datastore Cursor Sample</title>");
+    w.println("<ul>");
+    for (Entity entity : results) {
+      w.println("<li>" + entity.getProperty("name") + "</li>");
+    }
+    w.println("</ul>");
+
+    String cursorString = results.getCursor().toWebSafeString();
+
+    // This servlet lives at '/people'.
+    w.println("<a href='/people?cursor=" + cursorString + "'>Next page</a>");
+  }
+}
+// [END cursors]
diff --git a/appengine/datastore/src/main/java/com/example/appengine/StartupServlet.java b/appengine/datastore/src/main/java/com/example/appengine/StartupServlet.java
@@ -0,0 +1,120 @@
+/*
+ * Copyright 2016 Google Inc. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.appengine;
+
+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.EntityNotFoundException;
+import com.google.appengine.api.datastore.Key;
+import com.google.appengine.api.datastore.KeyFactory;
+import com.google.common.collect.ImmutableList;
+
+import java.io.IOException;
+
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+/**
+ * A startup handler to populate the datastore with example entities.
+ */
+public class StartupServlet extends HttpServlet {
+  static final String IS_POPULATED_ENTITY = "IsPopulated";
+  static final String IS_POPULATED_KEY_NAME = "is-populated";
+
+  private static final String PERSON_ENTITY = "Person";
+  private static final String NAME_PROPERTY = "name";
+  private static final ImmutableList<String> US_PRESIDENTS =
+      ImmutableList.<String>builder()
+          .add("George Washington")
+          .add("John Adams")
+          .add("Thomas Jefferson")
+          .add("James Madison")
+          .add("James Monroe")
+          .add("John Quincy Adams")
+          .add("Andrew Jackson")
+          .add("Martin Van Buren")
+          .add("William Henry Harrison")
+          .add("John Tyler")
+          .add("James K. Polk")
+          .add("Zachary Taylor")
+          .add("Millard Fillmore")
+          .add("Franklin Pierce")
+          .add("James Buchanan")
+          .add("Abraham Lincoln")
+          .add("Andrew Johnson")
+          .add("Ulysses S. Grant")
+          .add("Rutherford B. Hayes")
+          .add("James A. Garfield")
+          .add("Chester A. Arthur")
+          .add("Grover Cleveland")
+          .add("Benjamin Harrison")
+          .add("Grover Cleveland")
+          .add("William McKinley")
+          .add("Theodore Roosevelt")
+          .add("William Howard Taft")
+          .add("Woodrow Wilson")
+          .add("Warren G. Harding")
+          .add("Calvin Coolidge")
+          .add("Herbert Hoover")
+          .add("Franklin D. Roosevelt")
+          .add("Harry S. Truman")
+          .add("Dwight D. Eisenhower")
+          .add("John F. Kennedy")
+          .add("Lyndon B. Johnson")
+          .add("Richard Nixon")
+          .add("Gerald Ford")
+          .add("Jimmy Carter")
+          .add("Ronald Reagan")
+          .add("George H. W. Bush")
+          .add("Bill Clinton")
+          .add("George W. Bush")
+          .add("Barack Obama")
+          .build();
+
+  @Override
+  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
+      throws ServletException, IOException {
+    resp.setContentType("text/plain");
+    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
+
+    Key isPopulatedKey = KeyFactory.createKey(IS_POPULATED_ENTITY, IS_POPULATED_KEY_NAME);
+    boolean isAlreadyPopulated;
+    try {
+      datastore.get(isPopulatedKey);
+      isAlreadyPopulated = true;
+    } catch (EntityNotFoundException expected) {
+      isAlreadyPopulated = false;
+    }
+    if (isAlreadyPopulated) {
+      resp.getWriter().println("ok");
+      return;
+    }
+
+    ImmutableList.Builder<Entity> people = ImmutableList.builder();
+    for (String name : US_PRESIDENTS) {
+      Entity person = new Entity(PERSON_ENTITY);
+      person.setProperty(NAME_PROPERTY, name);
+      people.add(person);
+    }
+    datastore.put(people.build());
+    datastore.put(new Entity(isPopulatedKey));
+    resp.getWriter().println("ok");
+  }
+}
diff --git a/appengine/datastore/src/main/webapp/WEB-INF/web.xml b/appengine/datastore/src/main/webapp/WEB-INF/web.xml
@@ -24,7 +24,7 @@
   </servlet>
   <servlet-mapping>
     <servlet-name>guestbook-strong</servlet-name>
-    <url-pattern>/guestbook-strong</url-pattern>
+    <url-pattern>/</url-pattern>
   </servlet-mapping>
   <servlet>
     <servlet-name>guestbook</servlet-name>
@@ -34,6 +34,14 @@
     <servlet-name>guestbook</servlet-name>
     <url-pattern>/guestbook</url-pattern>
   </servlet-mapping>
+  <servlet>
+    <servlet-name>people</servlet-name>
+    <servlet-class>com.example.appengine.ListPeopleServlet</servlet-class>
+  </servlet>
+  <servlet-mapping>
+    <servlet-name>people</servlet-name>
+    <url-pattern>/people</url-pattern>
+  </servlet-mapping>
   <servlet>
     <servlet-name>projection</servlet-name>
     <servlet-class>com.example.appengine.ProjectionServlet</servlet-class>
@@ -43,6 +51,16 @@
     <url-pattern>/projection</url-pattern>
   </servlet-mapping>
 
+  <!-- Special handler to populate Datastore with default values. -->
+  <servlet>
+    <servlet-name>startup</servlet-name>
+    <servlet-class>com.example.appengine.StartupServlet</servlet-class>
+  </servlet>
+  <servlet-mapping>
+    <servlet-name>startup</servlet-name>
+    <url-pattern>/_ah/start</url-pattern>
+  </servlet-mapping>
+
   <security-constraint>
     <web-resource-collection>
       <web-resource-name>profile</web-resource-name>