/**

* Copyright 2016 Google Inc.

*

* 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.memcache;

import com.google.appengine.api.memcache.MemcacheService;

import com.google.appengine.api.memcache.MemcacheService.IdentifiableValue;

import com.google.appengine.api.memcache.MemcacheServiceFactory;

import java.io.IOException;

import java.math.BigInteger;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

// [START example]

@SuppressWarnings("serial")

// With @WebServlet annotation the webapp/WEB-INF/web.xml is no longer required.

@WebServlet(name = "MemcacheConcurrent", description = "Memcache: Concurrent",

urlPatterns = "/memcache/concurrent")

public class MemcacheConcurrentServlet extends HttpServlet {

@Override

public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException,

ServletException {

String path = req.getRequestURI();

if (path.startsWith("/favicon.ico")) {

return; // ignore the request for favicon.ico

}

String key = "count-concurrent";

// Using the synchronous cache.

MemcacheService syncCache = MemcacheServiceFactory.getMemcacheService();

// Write this value to cache using getIdentifiable and putIfUntouched.

for (long delayMs = 1; delayMs < 1000; delayMs *= 2) {

IdentifiableValue oldValue = syncCache.getIdentifiable(key);

byte[] newValue = oldValue == null

? BigInteger.valueOf(0).toByteArray()

: increment((byte[]) oldValue.getValue()); // newValue depends on old value

resp.setContentType("text/plain");

resp.getWriter().print("Value is " + new BigInteger(newValue).intValue() + "\n");

if (oldValue == null) {

// Key doesn't exist. We can safely put it in cache.

syncCache.put(key, newValue);

break;

} else if (syncCache.putIfUntouched(key, oldValue, newValue)) {

// newValue has been successfully put into cache.

break;

} else {

// Some other client changed the value since oldValue was retrieved.

// Wait a while before trying again, waiting longer on successive loops.

try {

Thread.sleep(delayMs);

} catch (InterruptedException e) {

throw new ServletException("Error when sleeping", e);

}

}

}

}

/**

* Increments an integer stored as a byte array by one.

* @param oldValue a byte array with the old value

* @return a byte array as the old value increased by one

*/

private byte[] increment(byte[] oldValue) {

long val = new BigInteger(oldValue).intValue();

val++;

return BigInteger.valueOf(val).toByteArray();

}

}

// [END example]