add named Singleton manager

* most useful for sharing caches on Spark Executors

Author: axtimwalde

src/main/java/net/imglib2/algorithm/util/Singleton.java

@@ -0,0 +1,90 @@
+/**
+ *
+ */
+package net.imglib2.algorithm.util;
+
+import java.util.HashMap;
+
+/**
+ * Manage named singleton instances.
+ *
+ * Useful for sharing instances of objects between threads.
+ *
+ * Our most relevant use case for this is to share cached ImgLib2 cell images
+ * such as N5 datasets and lazily generated cell images between tasks of a Spark
+ * cluster running on the same executor.
+ *
+ * Example:
+ *
+ * <pre>
+ * final String url = "https://janelia-cosem.s3.amazonaws.com/jrc_hela-2/jrc_hela-2.n5";
+ * final String dataset = "/em/fibsem-uint16/s4";
+ *
+ * final N5Reader n5 = Singleton.get(
+ * 		url,
+ * 		() -> new N5Factory().openReader( url ) );
+ *
+ * final RandomAccessibleInterval< T > img = Singleton.get(
+ * 		url + ":" + dataset,
+ * 		() -> N5Utils.open( n5, dataset ) );
+ * </pre>
+ *
+ * @author Stephan Saalfeld
+ *
+ */
+public class Singleton
+{
+	@FunctionalInterface
+	public static interface ThrowingSupplier< T, E extends Exception >
+	{
+		public T get() throws E;
+	}
+
+	private Singleton()
+	{}
+
+	static private HashMap< String, Object > singletons = new HashMap<>();
+
+	/**
+	 * Remove and retrieve a named singleton instance.
+	 *
+	 * @param key
+	 * @return
+	 */
+	public static synchronized Object remove( final String key )
+	{
+		return singletons.remove( key );
+	}
+
+	/**
+	 * Get or create the named singleton instance of T.
+	 *
+	 * @param <T>
+	 * @param <E>
+	 * @param key
+	 * @param supplier
+	 * @return
+	 * @throws E
+	 */
+	public static synchronized < T, E extends Exception > T get( final String key, final ThrowingSupplier< T, E > supplier ) throws E
+	{
+		@SuppressWarnings( "unchecked" )
+		final T t = ( T ) singletons.get( key );
+		if ( t == null )
+		{
+			final T s = supplier.get();
+			singletons.put( key, s );
+			return s;
+		}
+		else
+			return t;
+	}
+
+	/**
+	 * Clear all named singletons.
+	 */
+	public static synchronized void clear()
+	{
+		singletons.clear();
+	}
+}