Merge pull request #397 from gwildor28/master

Lock Pattern #71: Added mutex and semaphore modules to demonstrate locks

mutex/README.md

@@ -0,0 +1,30 @@
+---
+layout: pattern
+title: Mutex
+folder: mutex
+permalink: /patterns/mutex/
+categories: Lock
+tags: 
+ - Java
+ - Difficulty-Beginner
+---
+
+## Also known as
+Mutual Exclusion Lock
+Binary Semaphore
+
+## Intent
+Create a lock which only allows a single thread to access a resource at any one instant.
+
+![alt text](./etc/mutex.png "Mutex")
+
+## Applicability
+Use a Mutex when
+
+* you need to prevent two threads accessing a critical section at the same time
+* concurrent access to a resource could lead to a race condition 
+
+## Credits
+
+* [Lock (computer science)] (http://en.wikipedia.org/wiki/Lock_(computer_science))
+* [Semaphores] (http://tutorials.jenkov.com/java-concurrency/semaphores.html)

mutex/pom.xml

@@ -0,0 +1,42 @@
+<?xml version="1.0"?>
+<!--
+
+    The MIT License
+    Copyright (c) 2014 Ilkka Sepp<70>l<EFBFBD>
+
+    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.
+
+-->
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.12.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>mutex</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency> 
+  </dependencies>
+</project>

mutex/src/main/java/com/iluwatar/mutex/App.java

@@ -0,0 +1,50 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+/**
+ * A Mutex prevents multiple threads from accessing a resource simultaneously.
+ * <p>
+ * In this example we have two thieves who are taking beans from a jar.
+ * Only one thief can take a bean at a time. This is ensured by a Mutex lock
+ * which must be acquired in order to access the jar. Each thief attempts to
+ * acquire the lock, take a bean and then release the lock. If the lock has 
+ * already been acquired, the thief will be prevented from continuing (blocked)
+ * until the lock has been released. The thieves stop taking beans once there
+ * are no beans left to take.
+ */
+public class App {
+
+  /**
+   * main method
+   */
+  public static void main(String[] args) {
+    Mutex mutex = new Mutex();
+    Jar jar = new Jar(1000, mutex);
+    Thief peter = new Thief("Peter", jar);
+    Thief john = new Thief("John", jar);
+    peter.start();
+    john.start();
+  }
+
+}

mutex/src/main/java/com/iluwatar/mutex/Jar.java

@@ -0,0 +1,67 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+/**
+ * A Jar has a resource of beans which can only be accessed by a single Thief
+ * (thread) at any one time. A Mutex lock is used to prevent more than one Thief
+ * taking a bean simultaneously. 
+ */
+public class Jar {
+
+  /**
+   * The lock which must be acquired to access the beans resource.
+   */
+  private final Lock lock;
+
+  /**
+   * The resource within the jar.
+   */
+  private int beans;
+
+  public Jar(int beans, Lock lock) {
+    this.beans = beans;
+    this.lock = lock;
+  }
+
+  /**
+   * Method for a thief to take a bean.
+   */
+  public boolean takeBean() {
+    boolean success = false;
+    try {
+      lock.acquire();
+      success = beans > 0;
+      if (success) {
+        beans = beans - 1;
+      }
+    } catch (Exception e) {
+      e.printStackTrace();
+    } finally {
+      lock.release();
+    }
+
+    return success;
+  }
+
+}

mutex/src/main/java/com/iluwatar/mutex/Lock.java

@@ -0,0 +1,34 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+/**
+ * Lock is an interface for a lock which can be acquired and released.
+ */
+public interface Lock {
+
+  void acquire() throws InterruptedException;
+
+  void release();
+
+}

mutex/src/main/java/com/iluwatar/mutex/Mutex.java

@@ -0,0 +1,67 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+/**
+ * Mutex is an implementation of a mutual exclusion lock.
+ */
+public class Mutex implements Lock {
+
+  /**
+   * The current owner of the lock.
+   */
+  private Object owner;
+
+  /**
+   * Returns the current owner of the Mutex, or null if available
+   */
+  public Object getOwner() {
+    return owner;
+  }
+  
+  /**
+   * Method called by a thread to acquire the lock. If the lock has already
+   * been acquired this will wait until the lock has been released to 
+   * re-attempt the acquire.
+   */
+  @Override
+  public synchronized void acquire() throws InterruptedException {
+    while (owner != null) {
+      wait();
+    }
+
+    owner = Thread.currentThread();
+  }
+
+  /**
+   * Method called by a thread to release the lock.
+   */
+  @Override
+  public synchronized void release() {
+    if (Thread.currentThread() == owner) {
+      owner = null;
+      notify();
+    }
+  }
+
+}

mutex/src/main/java/com/iluwatar/mutex/Thief.java

@@ -0,0 +1,62 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+/**
+ * Thief is a class which continually tries to acquire a jar and take a bean
+ * from it. When the jar is empty the thief stops.
+ */
+public class Thief extends Thread {
+
+  /**
+   * The name of the thief. 
+   */
+  private final String name;
+  
+  /**
+   * The jar
+   */
+  private final Jar jar;
+
+  public Thief(String name, Jar jar) {
+    this.name = name;
+    this.jar = jar;
+  }
+
+  /**
+   * In the run method the thief repeatedly tries to take a bean until none
+   * are left.
+   */
+  @Override
+  public void run() {
+    int beans = 0;
+
+    while (jar.takeBean()) {
+      beans = beans + 1;
+      System.out.println(name + " took a bean.");
+    }
+
+    System.out.println(name + " took " + beans + " beans.");
+  }
+
+}

mutex/src/test/java/com/iluwatar/mutex/AppTest.java

@@ -0,0 +1,34 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+import org.junit.Test;
+import java.io.IOException;
+
+public class AppTest{
+  @Test
+  public void test() throws IOException {
+    String[] args = {};
+    App.main(args);
+  }
+}

mutex/src/test/java/com/iluwatar/mutex/JarTest.java

@@ -0,0 +1,43 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ * Test case for taking beans from a Jar
+ */
+public class JarTest {
+
+  @Test
+  public void testTakeBeans() {
+    Mutex mutex = new Mutex();
+    Jar jar = new Jar(10, mutex);
+    for (int i = 0; i < 10; i++) {
+      assertTrue(jar.takeBean());
+    }
+    assertFalse(jar.takeBean());
+  }
+
+}

mutex/src/test/java/com/iluwatar/mutex/MutexTest.java

@@ -0,0 +1,47 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.mutex;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ * Test case for acquiring and releasing a Mutex
+ */
+public class MutexTest {
+
+  @Test
+  public void acquireReleaseTest() {
+    Mutex mutex = new Mutex();
+    assertNull(mutex.getOwner());
+    try {
+      mutex.acquire();
+      assertEquals(mutex.getOwner(), Thread.currentThread());
+    } catch (InterruptedException e) {
+      fail(e.toString());
+    }
+    mutex.release();
+    assertNull(mutex.getOwner());
+  }
+
+}

pom.xml

@@ -1,19 +1,15 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!--
-
     The MIT License
     Copyright (c) 2014 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
@@ -21,7 +17,6 @@
     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.
-
 -->
 <project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
 		 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
@@ -125,6 +120,8 @@
 		<module>value-object</module>
 		<module>monad</module>
 		<module>mute-idiom</module>
+		<module>mutex</module>
+		<module>semaphore</module>
 	</modules>
 
 	<dependencyManagement>
@@ -402,4 +399,4 @@
     </plugins>
   </reporting>
 
-</project>
+</project>

semaphore/README.md

@@ -0,0 +1,33 @@
+---
+layout: pattern
+title: Semaphore
+folder: semaphore
+permalink: /patterns/semaphore/
+categories: Lock
+tags: 
+ - Java
+ - Difficulty-Beginner
+---
+
+## Also known as
+Counting Semaphore
+
+## Intent
+Create a lock which mediates access to a pool of resources. 
+Only a limited number of threads, specified at the creation 
+of the semaphore, can access the resources at any given time.
+A semaphore which only allows one concurrent access to a resource
+is called a binary semaphore.
+
+![alt text](./etc/semaphore.png "Semaphore")
+
+## Applicability
+Use a Semaphore when 
+
+* you have a pool of resources to allocate to different threads
+* concurrent access to a resource could lead to a race condition 
+
+## Credits
+
+* [Semaphore(programming)] (http://en.wikipedia.org/wiki/Semaphore_(programming))
+* [Semaphores] (http://tutorials.jenkov.com/java-concurrency/semaphores.html)

semaphore/pom.xml

@@ -0,0 +1,42 @@
+<?xml version="1.0"?>
+<!--
+
+    The MIT License
+    Copyright (c) 2014 Ilkka Sepp<70>l<EFBFBD>
+
+    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.
+
+-->
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.12.0-SNAPSHOT</version>
+  </parent>
+  <artifactId>semaphore</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

semaphore/src/main/java/com/iluwatar/semaphore/App.java

@@ -0,0 +1,50 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+/**
+ * A Semaphore mediates access by a group of threads to a pool of resources.
+ * <p>
+ * In this example a group of customers are taking fruit from a fruit shop.
+ * There is a bowl each of apples, oranges and lemons. Only one customer can 
+ * access a bowl simultaneously. A Semaphore is used to indicate how many 
+ * resources are currently available and must be acquired in order for a bowl 
+ * to be given to a customer. Customers continually try to take fruit until 
+ * there is no fruit left in the shop. 
+ */
+public class App {
+    
+  /**
+   * main method
+   */
+  public static void main(String[] args) {
+    FruitShop shop = new FruitShop();
+    new Customer("Peter", shop).start();
+    new Customer("Paul", shop).start();
+    new Customer("Mary", shop).start();
+    new Customer("John", shop).start();
+    new Customer("Ringo", shop).start();
+    new Customer("George", shop).start();
+  }
+  
+}

semaphore/src/main/java/com/iluwatar/semaphore/Customer.java

@@ -0,0 +1,76 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+/**
+ * A Customer attempts to repeatedly take Fruit from the FruitShop by
+ * taking Fruit from FruitBowl instances.
+ */
+public class Customer extends Thread {
+
+  /**
+   * Name of the Customer.
+   */
+  private final String name;
+  
+  /**
+   * The FruitShop he is using.
+   */
+  private final FruitShop fruitShop;
+  
+  /**
+   * Their bowl of Fruit.
+   */
+  private final FruitBowl fruitBowl;
+  
+  /**
+   * Customer constructor
+   */
+  public Customer(String name, FruitShop fruitShop) {
+    this.name = name;
+    this.fruitShop = fruitShop;
+    this.fruitBowl = new FruitBowl();
+  }
+  
+  /**
+   * The Customer repeatedly takes Fruit from the FruitShop until no Fruit
+   * remains.
+   */   
+  public void run() {
+        
+    while (fruitShop.countFruit() > 0) {
+      FruitBowl bowl = fruitShop.takeBowl();
+      Fruit fruit;
+            
+      if (bowl != null && (fruit = bowl.take()) != null) {
+        System.out.println(name + " took an " + fruit);
+        fruitBowl.put(fruit);
+        fruitShop.returnBowl(bowl);
+      }
+    }
+        
+    System.out.println(name + " took " + fruitBowl);
+        
+  }
+    
+}

semaphore/src/main/java/com/iluwatar/semaphore/Fruit.java

@@ -0,0 +1,60 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+/**
+ * Fruit is a resource stored in a FruitBowl. 
+ */
+public class Fruit {
+
+  public static enum FruitType {
+    ORANGE, APPLE, LEMON
+  }
+    
+  private FruitType type;
+    
+  public Fruit(FruitType type) {
+    this.type = type;
+  }
+    
+  public FruitType getType() {
+    return type;
+  }
+  
+  /**
+   * toString method
+   */ 
+  public String toString() {
+    switch (type) {
+      case ORANGE:
+        return "Orange";
+      case APPLE: 
+        return "Apple";
+      case LEMON: 
+        return "Lemon";
+      default: 
+        return "";
+    }
+  }
+
+}

semaphore/src/main/java/com/iluwatar/semaphore/FruitBowl.java

@@ -0,0 +1,88 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+import java.util.ArrayList;
+
+/**
+ * A FruitBowl contains Fruit. 
+ */
+public class FruitBowl {
+    
+  private ArrayList<Fruit> fruit = new ArrayList<>();
+
+  /**
+   * 
+   * @return The amount of Fruit left in the bowl. 
+   */
+  public int countFruit() {
+    return fruit.size();
+  }
+
+  /**
+   * Put an item of Fruit into the bowl.
+   * 
+   * @param f fruit
+   */
+  public void put(Fruit f) {
+    fruit.add(f);
+  }
+  
+  /**
+   * Take an item of Fruit out of the bowl.
+   * @return The Fruit taken out of the bowl, or null if empty.
+   */
+  public Fruit take() {
+    if (fruit.isEmpty()) {
+      return null;
+    } else {
+      return fruit.remove(0);
+    }
+  }
+  
+  /**
+   * toString method
+   */   
+  public String toString() {
+    int apples = 0;
+    int oranges = 0;
+    int lemons = 0;
+        
+    for (Fruit f : fruit) {
+      switch (f.getType()) {
+        case APPLE:
+          apples++;
+          break;
+        case ORANGE:
+          oranges++;
+          break;
+        case LEMON:
+          lemons++;
+          break;
+        default:
+      }
+    }
+        
+    return apples + " Apples, " + oranges + " Oranges, and " + lemons + " Lemons";
+  }
+}

semaphore/src/main/java/com/iluwatar/semaphore/FruitShop.java

@@ -0,0 +1,120 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+/**
+ * A FruitShop contains three FruitBowl instances and controls access to them.
+ */
+public class FruitShop {
+   
+  /**
+   * The FruitBowl instances stored in the class.
+   */
+  private FruitBowl[] bowls = {
+    new FruitBowl(),
+    new FruitBowl(),
+    new FruitBowl()
+  };
+
+  /**
+   * Access flags for each of the FruitBowl instances.
+   */
+  private boolean[] available = {
+    true,
+    true,
+    true
+  };
+
+  /**
+   * The Semaphore that controls access to the class resources.
+   */
+  private Semaphore semaphore;
+  
+  /**
+   * FruitShop constructor
+   */
+  public FruitShop() {
+    for (int i = 0; i < 100; i++) {
+      bowls[0].put(new Fruit(Fruit.FruitType.APPLE));
+      bowls[1].put(new Fruit(Fruit.FruitType.ORANGE));
+      bowls[2].put(new Fruit(Fruit.FruitType.LEMON));
+    }
+        
+    semaphore = new Semaphore(3);
+  }
+
+  /**
+   * 
+   * @return The amount of Fruit left in the shop.
+   */
+  public synchronized int countFruit() {
+    return bowls[0].countFruit() + bowls[1].countFruit() + bowls[2].countFruit();
+  }
+  
+  /**
+   * Method called by Customer to get a FruitBowl from the shop. This method
+   * will try to acquire the Semaphore before returning the first available 
+   * FruitBowl.
+   */   
+  public synchronized FruitBowl takeBowl() {
+        
+    FruitBowl bowl = null;
+        
+    try {
+      semaphore.acquire();
+            
+      if (available[0]) {
+        bowl = bowls[0];
+        available[0] = false;
+      } else if (available[1]) {
+        bowl = bowls[1];
+        available[1] = false;
+      } else if (available[2]) {
+        bowl = bowls[2];
+        available[2] = false;
+      }
+            
+    } catch (InterruptedException e) {
+      e.printStackTrace();
+    } finally {
+      semaphore.release();
+    }
+    return bowl;
+  }
+  
+  /**
+   * Method called by a Customer instance to return a FruitBowl to the shop. 
+   * This method releases the Semaphore, making the FruitBowl available to 
+   * another Customer.
+   */   
+  public synchronized void returnBowl(FruitBowl bowl) {
+    if (bowl == bowls[0]) {
+      available[0] = true;
+    } else if (bowl == bowls[1]) {
+      available[1] = true;
+    } else if (bowl == bowls[2]) {
+      available [2] = true;
+    }
+  }
+    
+}

semaphore/src/main/java/com/iluwatar/semaphore/Lock.java

@@ -0,0 +1,34 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+/**
+ * Lock is an interface for a lock which can be acquired and released.
+ */
+public interface Lock {
+    
+  void acquire() throws InterruptedException;
+  
+  void release();
+    
+}

semaphore/src/main/java/com/iluwatar/semaphore/Semaphore.java

@@ -0,0 +1,77 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+/**
+ * Semaphore is an implementation of a semaphore lock.
+ */
+public class Semaphore implements Lock {
+
+  private final int licenses;
+  /**
+   * The number of concurrent resource accesses which are allowed.
+   */
+  private int counter;
+    
+  public Semaphore(int licenses) {
+    this.licenses = licenses;
+    this.counter = licenses;    
+  }
+  
+  /**
+   * Returns the number of licenses managed by the Semaphore
+   */
+  public int getNumLicenses() {
+    return licenses;
+  }
+  
+  /**
+   * Returns the number of available licenses
+   */
+  public int getAvailableLicenses() {
+    return counter; 
+  }
+  
+  /**
+   * Method called by a thread to acquire the lock. If there are no resources
+   * available this will wait until the lock has been released to re-attempt
+   * the acquire.
+   */
+  public synchronized void acquire() throws InterruptedException {
+    while (counter == 0) {
+      wait();
+    }
+    counter = counter - 1;
+  }
+  
+  /**
+   * Method called by a thread to release the lock.
+   */
+  public synchronized void release() {
+    if (counter < licenses) {
+      counter = counter + 1;
+      notify();
+    }
+  }
+
+}

semaphore/src/test/java/com/iluwatar/semaphore/AppTest.java

@@ -0,0 +1,34 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+import org.junit.Test;
+import java.io.IOException;
+
+public class AppTest{
+  @Test
+  public void test() throws IOException {
+    String[] args = {};
+    App.main(args);
+  }
+}

semaphore/src/test/java/com/iluwatar/semaphore/FruitBowlTest.java

@@ -0,0 +1,51 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ * Test taking from and putting Fruit into a FruitBowl
+ */
+public class FruitBowlTest {
+
+  @Test
+  public void fruitBowlTest() {
+    FruitBowl fbowl = new FruitBowl();
+    
+    assertEquals(fbowl.countFruit(), 0);
+    
+    for (int i = 1; i <= 10; i++) {
+      fbowl.put(new Fruit(Fruit.FruitType.LEMON));
+      assertEquals(fbowl.countFruit(), i);
+    }
+
+    for (int i = 9; i >= 0; i--) {
+      assertNotNull(fbowl.take());
+      assertEquals(fbowl.countFruit(), i);
+    }
+
+    assertNull(fbowl.take());
+  }
+}

semaphore/src/test/java/com/iluwatar/semaphore/SemaphoreTest.java

@@ -0,0 +1,56 @@
+/**
+ * The MIT License
+ * Copyright (c) 2014 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.semaphore;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+/**
+ * Test case for acquiring and releasing a Semaphore
+ */
+public class SemaphoreTest {
+
+  @Test
+  public void acquireReleaseTest() {
+    Semaphore sphore = new Semaphore(3);
+
+    assertEquals(sphore.getAvailableLicenses(), 3);
+
+    for (int i = 2; i >= 0; i--) {
+      try {
+        sphore.acquire();
+        assertEquals(sphore.getAvailableLicenses(), i);
+      } catch (InterruptedException e) {
+        fail(e.toString());
+      }
+    }
+  
+    for (int i = 1; i <= 3; i++) {
+      sphore.release();
+      assertEquals(sphore.getAvailableLicenses(), i);
+    }
+
+    sphore.release();
+    assertEquals(sphore.getAvailableLicenses(), 3);
+  }
+}