Added tests for service-locator pattern

Fix NPE when requested service is unknown

service-locator/src/main/java/com/iluwatar/servicelocator/ServiceLocator.java

@@ -32,7 +32,9 @@ public class ServiceLocator {
        */
       InitContext ctx = new InitContext();
       serviceObj = (Service) ctx.lookup(serviceJndiName);
+      if (serviceObj != null) { // Only cache a service if it actually exists
         serviceCache.addService(serviceObj);
+      }
       return serviceObj;
     }
   }

service-locator/src/test/java/com/iluwatar/servicelocator/ServiceLocatorTest.java

@@ -0,0 +1,42 @@
+package com.iluwatar.servicelocator;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+/**
+ * Date: 12/29/15 - 19:07 PM
+ *
+ * @author Jeroen Meulemeester
+ */
+public class ServiceLocatorTest {
+
+  /**
+   * Verify if we just receive 'null' when requesting a non-existing service
+   */
+  @Test
+  public void testGetNonExistentService() {
+    assertNull(ServiceLocator.getService("fantastic/unicorn/service"));
+    assertNull(ServiceLocator.getService("another/fantastic/unicorn/service"));
+  }
+
+  /**
+   * Verify if we get the same cached instance when requesting the same service twice
+   */
+  @Test
+  public void testServiceCache() {
+    final String[] serviceNames = new String[]{
+            "jndi/serviceA", "jndi/serviceB"
+    };
+
+    for (final String serviceName : serviceNames) {
+      final Service service = ServiceLocator.getService(serviceName);
+      assertNotNull(service);
+      assertEquals(serviceName, service.getName());
+      assertTrue(service.getId() > 0); // The id is generated randomly, but the minimum value is '1'
+      assertSame(service, ServiceLocator.getService(serviceName));
+    }
+
+  }
+
+}