small semaphore demo program

fishercoder1534 · fishercoder1534 · commit e7add79ce8a4 · 2016-08-06T10:58:22.000-07:00
diff --git a/RandomProblems/src/sporadic/semaphore/ConnectionLimiter.java b/RandomProblems/src/sporadic/semaphore/ConnectionLimiter.java
@@ -0,0 +1,71 @@
+package sporadic.semaphore;
+
+import java.util.concurrent.Semaphore;
+
+/**
+ * This is a small program to demo how semaphore can create trouble for us.
+ * One rule of thumb is:
+ * 	Always release what you acquire!
+ * Copied from online.
+ *
+ */
+//this is a bad public class name, it doesn't apply to 
+//what this class really do, it's a name from a different class that I copied earlier.
+public class ConnectionLimiter {
+
+	private static class DoubleResourceGrabber implements Runnable{
+		
+		private Semaphore first;
+		private Semaphore second;
+		
+		public DoubleResourceGrabber(Semaphore s1, Semaphore s2){
+			first = s1;
+			second = s2;
+		}
+
+		@Override
+		public void run() {
+			Thread t = Thread.currentThread();
+			
+			try {
+				first.acquire();
+				System.out.println(t.getName() + " acquired " + first);
+				
+//				Thread.sleep(20);//to demo a deadlock
+				
+//				second.acquire();
+//				System.out.println(t.getName() + " acquired " + second);
+				
+//				second.release();
+//				System.out.println(t.getName() + " released " + second);
+				
+				first.release();
+				System.out.println(t.getName() + " released " + first);
+				
+			} catch (InterruptedException e) {
+				e.printStackTrace();
+			}
+		}
+	}
+
+	/**
+	 * @param args
+	 * @throws InterruptedException 
+	 */
+	public static void main(String[] args) throws InterruptedException {
+		Semaphore s1 = new Semaphore(1);//give it only 1 permit
+		Semaphore s2 = new Semaphore(1);//give it only 1 permit as well
+		Thread t1 = new Thread(new DoubleResourceGrabber(s1, s2));
+		//now reverse them, here comes the trouble
+		Thread t2 = new Thread(new DoubleResourceGrabber(s2, s1));
+		
+		t1.start();
+		t2.start();
+		
+		t1.join();
+		t2.join();
+		System.out.println("We got lucky!");
+		
+	}
+
+}
