Implemented half sync half async pattern #109

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/App.java

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+package com.iluwatar.halfsynchalfasync;
+
+import java.util.concurrent.LinkedBlockingQueue;
+
+/**
+ * This application demonstrates Half-Sync/Half-Async pattern. Key parts of the pattern are
+ * {@link AsyncTask} and {@link AsynchronousService}.
+ * 
+ * <p>
+ * <i>PROBLEM</i>
+ * <br/>
+ * A concurrent system have a mixture of short duration, mid duration and long duration tasks.
+ * Mid or long duration tasks should be performed asynchronously to meet quality of service
+ * requirements.
+ *   
+ * <p><i>INTENT</i>
+ * <br/>
+ * The intent of this pattern is to separate the the synchronous and asynchronous processing
+ * in the concurrent application by introducing two intercommunicating layers - one for sync
+ * and one for async. This simplifies the programming without unduly affecting the performance.
+ *  
+ * <p>
+ * <i>APPLICABILITY</i>
+ * <br/>
+ * <ul>
+ * <li>UNIX network subsystems - In operating systems network operations are carried out
+ * asynchronously with help of hardware level interrupts.</li>
+ * <li>CORBA - At the asynchronous layer one thread is associated with each socket that is
+ * connected to the client. Thread blocks waiting for CORBA requests from the client. On receiving
+ * request it is inserted in the queuing layer which is then picked up by synchronous layer which
+ * processes the request and sends response back to the client.</li>
+ * <li>Android AsyncTask framework - Framework provides a way to execute long running blocking calls,
+ * such as downloading a file, in background threads so that the UI thread remains free to respond
+ * to user inputs.</i>
+ *  </ul>
+ *  
+ * <p>
+ * <i>IMPLEMENTATION</i>
+ * <br/>
+ * The main method creates an asynchronous service which does not block the main thread while
+ * the task is being performed. The main thread continues its work which is similar to Async Method
+ * Invocation pattern. The difference between them is that there is a queuing layer between Asynchronous
+ * layer and synchronous layer, which allows for different communication patterns between both layers.
+ * Such as Priority Queue can be used as queuing layer to prioritize the way tasks are executed.
+ * Our implementation is just one simple way of implementing this pattern, there are many variants possible
+ * as described in its applications.
+ */
+public class App {
+
+	public static void main(String[] args) {
+		AsynchronousService service = new AsynchronousService(new LinkedBlockingQueue<>());
+		/*
+		 *  A new task to calculate sum is received but as this is main thread, it should not block.
+		 *  So it passes it to the asynchronous task layer to compute and proceeds with handling other
+		 *  incoming requests. This is particularly useful when main thread is waiting on Socket to receive
+		 *  new incoming requests and does not wait for particular request to be completed before responding
+		 *  to new request.
+		 */
+		service.execute(new ArithmeticSumTask(1000));
+		
+		/* New task received, lets pass that to async layer for computation. So both requests will be
+		 * executed in parallel.
+		 */
+		service.execute(new ArithmeticSumTask(500));
+		service.execute(new ArithmeticSumTask(2000));
+		service.execute(new ArithmeticSumTask(1));
+	}
+	
+	static class ArithmeticSumTask implements AsyncTask<Long> {
+		private long n;
+
+		public ArithmeticSumTask(long n) {
+			this.n = n;
+		}
+
+		/*
+		 * This is the long running task that is performed in background. In our example 
+		 * the long running task is calculating arithmetic sum with artificial delay.
+		 */
+		@Override
+		public Long call() throws Exception {
+			return ap(n);
+		}
+
+		/*
+		 * This will be called in context of the main thread where some validations can be
+		 * done regarding the inputs. Such as it must be greater than 0. It's a small
+		 * computation which can be performed in main thread. If we did validated the input
+		 * in background thread then we pay the cost of context switching
+		 * which is much more than validating it in main thread. 
+		 */
+		@Override
+		public void onPreCall() {
+			if (n < 0) {
+				throw new IllegalArgumentException("n is less than 0");
+			}
+		}
+
+		@Override
+		public void onPostCall(Long result) {
+			// Handle the result of computation
+			System.out.println(result);
+		}
+
+		@Override
+		public void onError(Throwable throwable) {
+			throw new IllegalStateException("Should not occur");
+		}
+	}
+	
+	private static long ap(long i) {
+		try {
+			Thread.sleep(i);
+		} catch (InterruptedException e) {
+		}
+		return (i) * (i + 1) / 2;
+	}
+}