Allow shutdown of executors, to clean up resources.

src/main/java/net/jodah/expiringmap/ExpiringMap.java

@@ -1388,4 +1388,19 @@ private void initListenerService() {
       }
     }
   }
+
+  /**
+   * Free up {@link Executors} used by <code>ExpiringMap</code> to manage expiration and listeners.
+   * This will stop the expiration process for all running instances of this class, but it is useful before unloading this class from the classloader in order not to leave classes running (e.g. in a {@link javax.servlet.ServletContextListener#contextDestroyed(ServletContextEvent)}).
+   */
+  public synchronized static void shutdown() {
+    if (EXPIRER != null) {
+      EXPIRER.shutdownNow();
+      EXPIRER = null;
+    }
+    if (LISTENER_SERVICE != null) {
+      LISTENER_SERVICE.shutdownNow();
+      LISTENER_SERVICE = null;
+    }
+  }
 }

src/test/java/net/jodah/expiringmap/ShutdownTest.java

@@ -0,0 +1,49 @@
+package net.jodah.expiringmap;
+
+import net.jodah.expiringmap.internal.NamedThreadFactory;
+import org.testng.annotations.Test;
+
+import java.util.concurrent.TimeUnit;
+
+import static org.testng.Assert.assertEquals;
+
+/**
+ * Tests ExpiringMap shutdown procedure.
+ */
+@Test
+public class ShutdownTest {
+  private static final String BASE_NAME = "TestThread-";
+
+  private static int expirationThreads() {
+    int n = 0;
+    for (Thread t : Thread.getAllStackTraces().keySet())
+      if (t.getName().startsWith(BASE_NAME))
+        ++n;
+    return n;
+  }
+
+  /**
+   * Ensures that the service thread are properly shutdown.
+   */
+  public void shouldCloseThreads() throws Exception {
+    // Initial cleanup
+    ExpiringMap.setThreadFactory(new NamedThreadFactory(BASE_NAME + "%s"));
+    ExpiringMap.shutdown();
+
+    // Given
+    ExpiringMap<String, String> map = ExpiringMap.builder().expiration(100, TimeUnit.MILLISECONDS).build();
+
+    // When
+    for (int i = 0; i < 100; i++)
+      map.put("John" + i, "Joe");
+
+    // Then
+    assertEquals(expirationThreads(), 1);
+    ExpiringMap.shutdown();
+    Thread.sleep(100);
+    assertEquals(expirationThreads(), 0);
+
+    // Final cleanup
+    ExpiringMap.THREAD_FACTORY = null;
+  }
+}