Add demo for CompletableFuture.get()

Author: mincong-h

concurrency/src/main/java/io/mincong/concurrency/completablefuture/BlockingThreadDemo.java

@@ -0,0 +1,77 @@
+package io.mincong.concurrency.completablefuture;
+
+import java.time.LocalTime;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Demo for comment 5130660255 in my blog "Why Do We Need Completable Future?":
+ * https://mincong.io/2020/06/26/completable-future/#comment-5130660255
+ */
+public class BlockingThreadDemo {
+
+  /**
+   * Possible output of the program:
+   *
+   * <pre>
+   * [20:40:38.195492][main] Started
+   * [20:40:38.189831][Thread-0] Started
+   * [20:40:38.310267][main] 0
+   * [20:40:38.312907][main] 1
+   * [20:40:38.313041][main] 2
+   * [20:40:38.313145][main] 3
+   * [20:40:38.313252][main] 4
+   * [20:40:38.313344][main] 5
+   * [20:40:38.313429][main] 6
+   * [20:40:38.313518][main] 7
+   * [20:40:38.313604][main] 8
+   * [20:40:38.313700][main] 9
+   * [20:40:38.313802][main] Finished
+   * [20:40:40.178991][Thread-0] Hello!
+   * [20:40:40.179161][Thread-0] Finished
+   * </pre>
+   *
+   * The "main" thread finished around 20:40:38.3 while the "Thread-0" finished around 20:40:40.1.
+   * The two seconds delay was caused by the scheduler, which schedule the completion of the future
+   * 2 seconds in the future. From this demo, we can see that where the main thread was not blocked
+   * by the completable future. Only the worker thread "Thread-0" was blocked.
+   */
+  public static void main(String[] args) {
+    var workerFuture = new CompletableFuture<String>();
+
+    var thread =
+        new Thread(
+            () -> {
+              print("Started");
+              final String workerAnswer;
+              try {
+                workerAnswer = workerFuture.get();
+              } catch (Exception e) {
+                e.printStackTrace();
+                return;
+              }
+              print(workerAnswer);
+              print("Finished");
+            });
+    thread.start();
+
+    var executor = Executors.newSingleThreadScheduledExecutor();
+    try {
+      executor.schedule(() -> workerFuture.complete("Hello!"), 2, TimeUnit.SECONDS);
+
+      print("Started");
+      for (int i = 0; i < 10; i++) {
+        print(i);
+      }
+      print("Finished");
+    } finally {
+      executor.shutdown();
+    }
+  }
+
+  private static void print(Object message) {
+    var thread = Thread.currentThread().getName();
+    System.out.println("[" + LocalTime.now() + "][" + thread + "] " + message);
+  }
+}