Add simple Registry class to manage singletons

Typically, singleton classes have lots of static methods, or use
a getInstance() method to get the singleton.  This provides an alternative
mechanism, similar to the JDNI and the common-paramater property registry,
where singletons can be registered.

Clean up registry after junit test.
Modified a few comments. (I prefer 90 characters for comments.)

Added a method to register or replace an existing key without throwing
an exception.

Change-Id: I3b62719013d3b5f71adb5e9299d3c1257fb55c80
Issue-ID: POLICY-1542
Signed-off-by: Jim Hahn <jrh3@att.com>

1 files changed, 151 insertions, 0 deletions

diff --git a/utils/src/main/java/org/onap/policy/common/utils/services/Registry.java b/utils/src/main/java/org/onap/policy/common/utils/services/Registry.java
new file mode 100644
index 00000000..c209379f
--- /dev/null
+++ b/utils/src/main/java/org/onap/policy/common/utils/services/Registry.java
@@ -0,0 +1,151 @@
+/*
+ * ============LICENSE_START=======================================================
+ * ONAP
+ * ================================================================================
+ * Copyright (C) 2019 AT&T Intellectual Property. All rights reserved.
+ * ================================================================================
+ * 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.
+ * ============LICENSE_END=========================================================
+ */
+
+package org.onap.policy.common.utils.services;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * This is a simple object registry, similar in spirit to JNDI, but suitable for use in a
+ * stand-alone JVM.
+ */
+public class Registry {
+    private static final Logger logger = LoggerFactory.getLogger(Registry.class);
+
+    private static volatile Registry instance = new Registry();
+
+    /**
+     * Registry map.
+     */
+    private Map<String, Object> name2object = new ConcurrentHashMap<>();
+
+    /**
+     * Constructs the object.
+     */
+    private Registry() {
+        super();
+    }
+
+    /**
+     * Registers an object.
+     *
+     * @param name name by which the object is known
+     * @param object object to be registered
+     * @throws IllegalStateException if an object is already registered for that name
+     * @throws IllegalArgumentException if either argument is null
+     */
+    public static void register(String name, Object object) {
+        if (name == null) {
+            throw new IllegalArgumentException("attempt to register: " + name);
+        }
+
+        if (object == null) {
+            throw new IllegalArgumentException("attempt to register null object for " + name);
+        }
+
+        instance.name2object.compute(name, (key, oldval) -> {
+
+            if (oldval != null) {
+                throw new IllegalStateException("already registered: " + name);
+            }
+
+            return object;
+        });
+    }
+
+    /**
+     * Registers an object, replacing any previously existing binding.
+     *
+     * @param name name by which the object is known
+     * @param object object to be registered
+     * @throws IllegalArgumentException if either argument is null
+     */
+    public static void registerOrReplace(String name, Object object) {
+        if (name == null) {
+            throw new IllegalArgumentException("attempt to register: " + name);
+        }
+
+        if (object == null) {
+            throw new IllegalArgumentException("attempt to register null object for " + name);
+        }
+
+        instance.name2object.compute(name, (key, oldval) -> {
+
+            if (oldval != null) {
+                logger.warn("replacing previously registered: {}", name);
+            }
+
+            return object;
+        });
+    }
+
+    /**
+     * Unregisters an object.
+     *
+     * @param name name by which the object is known
+     * @return {@code true} if the object was unregistered, {@code false} if it did not
+     *         exist
+     */
+    public static boolean unregister(String name) {
+        return (instance.name2object.remove(name) != null);
+    }
+
+    /**
+     * Gets the object by the given name.
+     *
+     * @param name name of the object to get
+     * @param clazz object's class
+     * @return the object
+     * @throws IllegalArgumentException if no object is registered by the given name
+     */
+    public static <T> T get(String name, Class<T> clazz) {
+        Object obj = instance.name2object.get(name);
+        if (obj == null) {
+            throw new IllegalArgumentException("not registered: " + name);
+        }
+
+        return clazz.cast(obj);
+    }
+
+    /**
+     * Gets the object by the given name, providing a default value if the name is not
+     * registered.
+     *
+     * @param name name of the object to get
+     * @param clazz object's class
+     * @param defaultVal the default value to return, if the object does not exist
+     * @return the object, if it exists, the default value, otherwise
+     */
+    public static <T> T getOrDefault(String name, Class<T> clazz, T defaultVal) {
+        Object obj = instance.name2object.get(name);
+        return (obj != null ? clazz.cast(obj) : defaultVal);
+    }
+
+    /**
+     * Creates a new registry instance. This is typically only used by junit tests, as it
+     * discards any previous registry entries.
+     */
+    public static void newRegistry() {
+        instance = new Registry();
+    }
+}