Add SingletonExtensionFactory (see #232)

pf4j/src/main/java/org/pf4j/SingletonExtensionFactory.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2012-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pf4j;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * An {@link ExtensionFactory} that always returns a specific instance.
+ * Optional, you can specify the extension classes for which you want singletons.
+ *
+ * @author Decebal Suiu
+ */
+public class SingletonExtensionFactory extends DefaultExtensionFactory {
+
+    private final List<String> extensionClassNames;
+
+    private Map<String, Object> cache;
+
+    public SingletonExtensionFactory(String... extensionClassNames) {
+        this.extensionClassNames = Arrays.asList(extensionClassNames);
+
+        cache = new HashMap<>(); // simple cache implementation
+    }
+
+    @Override
+    public Object create(Class<?> extensionClass) {
+        String extensionClassName = extensionClass.getName();
+        if (cache.containsKey(extensionClassName)) {
+            return cache.get(extensionClassName);
+        }
+
+        Object extension = super.create(extensionClass);
+        if (extensionClassNames.isEmpty() || extensionClassNames.contains(extensionClassName)) {
+            cache.put(extensionClassName, extension);
+        }
+
+        return extension;
+    }
+
+}