Add a singleton storage container

hippo4j-common/src/main/java/cn/hippo4j/common/toolkit/Singleton.java

@@ -0,0 +1,82 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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 cn.hippo4j.common.toolkit;
+
+import lombok.AccessLevel;
+import lombok.NoArgsConstructor;
+
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Supplier;
+
+/**
+ * Singleton object container.
+ */
+@NoArgsConstructor(access = AccessLevel.PRIVATE)
+public final class Singleton {
+    
+    private static final ConcurrentHashMap<String, Object> SINGLE_OBJECT_POOL = new ConcurrentHashMap();
+    
+    /**
+     * Get a singleton object by key.
+     *
+     * @param key
+     * @param <T>
+     * @return
+     */
+    public static <T> T get(String key) {
+        Object result = SINGLE_OBJECT_POOL.get(key);
+        return result == null ? null : (T) result;
+    }
+    
+    /**
+     * Get a singleton object by key.
+     *
+     * <p> When empty, build a singleton object through supplier and put it into the container.
+     *
+     * @param key
+     * @param supplier
+     * @param <T>
+     * @return
+     */
+    public static <T> T get(String key, Supplier<T> supplier) {
+        Object result = SINGLE_OBJECT_POOL.get(key);
+        if (result == null && (result = supplier.get()) != null) {
+            SINGLE_OBJECT_POOL.put(key, result);
+        }
+        return result != null ? (T) result : null;
+    }
+    
+    /**
+     * Object into container.
+     *
+     * @param value
+     */
+    public static void put(Object value) {
+        put(value.getClass().getName(), value);
+    }
+    
+    /**
+     * Object into container.
+     *
+     * @param key
+     * @param value
+     */
+    public static void put(String key, Object value) {
+        SINGLE_OBJECT_POOL.put(key, value);
+    }
+}