App.java

/**
 * The MIT License
 * Copyright (c) 2014-2016 Ilkka Seppälä
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package com.iluwatar.async.method.invocation;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.Callable;

/**
 * This application demonstrates the async method invocation pattern. Key parts of the pattern are
 * <code>AsyncResult</code> which is an intermediate container for an asynchronously evaluated value,
 * <code>AsyncCallback</code> which can be provided to be executed on task completion and <code>AsyncExecutor</code>
 * that manages the execution of the async tasks.
 * <p>
 * The main method shows example flow of async invocations. The main thread starts multiple tasks with variable
 * durations and then continues its own work. When the main thread has done it's job it collects the results of the
 * async tasks. Two of the tasks are handled with callbacks, meaning the callbacks are executed immediately when the
 * tasks complete.
 * <p>
 * Noteworthy difference of thread usage between the async results and callbacks is that the async results are collected
 * in the main thread but the callbacks are executed within the worker threads. This should be noted when working with
 * thread pools.
 * <p>
 * Java provides its own implementations of async method invocation pattern. FutureTask, CompletableFuture and
 * ExecutorService are the real world implementations of this pattern. But due to the nature of parallel programming,
 * the implementations are not trivial. This example does not take all possible scenarios into account but rather
 * provides a simple version that helps to understand the pattern.
 *
 * @see AsyncResult
 * @see AsyncCallback
 * @see AsyncExecutor
 *
 * @see java.util.concurrent.FutureTask
 * @see java.util.concurrent.CompletableFuture
 * @see java.util.concurrent.ExecutorService
 */
public class App {

  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);

  /**
   * Program entry point
   */
  public static void main(String[] args) throws Exception {
    // construct a new executor that will run async tasks
    AsyncExecutor executor = new ThreadAsyncExecutor();

    // start few async tasks with varying processing times, two last with callback handlers
    AsyncResult<Integer> asyncResult1 = executor.startProcess(lazyval(10, 500));
    AsyncResult<String> asyncResult2 = executor.startProcess(lazyval("test", 300));
    AsyncResult<Long> asyncResult3 = executor.startProcess(lazyval(50L, 700));
    AsyncResult<Integer> asyncResult4 = executor.startProcess(lazyval(20, 400), callback("Callback result 4"));
    AsyncResult<String> asyncResult5 = executor.startProcess(lazyval("callback", 600), callback("Callback result 5"));

    // emulate processing in the current thread while async tasks are running in their own threads
    Thread.sleep(350); // Oh boy I'm working hard here
    log("Some hard work done");

    // wait for completion of the tasks
    Integer result1 = executor.endProcess(asyncResult1);
    String result2 = executor.endProcess(asyncResult2);
    Long result3 = executor.endProcess(asyncResult3);
    asyncResult4.await();
    asyncResult5.await();

    // log the results of the tasks, callbacks log immediately when complete
    log("Result 1: " + result1);
    log("Result 2: " + result2);
    log("Result 3: " + result3);
  }

  /**
   * Creates a callable that lazily evaluates to given value with artificial delay.
   *
   * @param value
   *          value to evaluate
   * @param delayMillis
   *          artificial delay in milliseconds
   * @return new callable for lazy evaluation
   */
  private static <T> Callable<T> lazyval(T value, long delayMillis) {
    return () -> {
      Thread.sleep(delayMillis);
      log("Task completed with: " + value);
      return value;
    };
  }

  /**
   * Creates a simple callback that logs the complete status of the async result.
   *
   * @param name
   *          callback name
   * @return new async callback
   */
  private static <T> AsyncCallback<T> callback(String name) {
    return (value, ex) -> {
      if (ex.isPresent()) {
        log(name + " failed: " + ex.map(Exception::getMessage).orElse(""));
      } else {
        log(name + ": " + value);
      }
    };
  }

  private static void log(String msg) {
    LOGGER.info(msg);
  }
}