com.epam.deltix.util.concurrent.VolatileCopyMap Maven / Gradle / Ivy

Go to download
/*
 * Copyright 2021 EPAM Systems, Inc
 *
 * See the NOTICE file distributed with this work for additional information
 * regarding copyright ownership. 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 com.epam.deltix.util.concurrent;

import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/**
 * Thread-safe version of Map. Suitable for scenarios when number of reads vastly outnumbers number of writes. Reads are lock-free.
 * Writes can synchronize on instance of this class to implement atomic operations. For example to escalate frequent locks.
 */
public final class VolatileCopyMap {
    private final Map writableMap = new HashMap();
    private volatile Map readOnlySnapshot = Collections.emptyMap();
    
    public V get (K key) {
        return readOnlySnapshot.get(key);
    }
    
    /**
     * Method does not allow overriding already associated value (first put() wins).
     * @return value associated with given key at the end of this method
     */
    public synchronized V put (K key, V value) {
        V collision = writableMap.put(key, value); // optimistic assumption: collisions will be rare
        if (collision != null) {
            writableMap.put(key, collision); // restore original value
            value = collision;
        }
        readOnlySnapshot = new HashMap(writableMap);  // writes are super rare compared to number of reads
        return value;
    }

    /** Caller is not allowed to modify result */
    public Collection keySet() {
        return readOnlySnapshot.keySet();
    }

    /** Caller is not allowed to modify result */
    public Collection values() {
        return readOnlySnapshot.values();
    }
    
}