code: /**
   * This file is part of Eclipse Steady.
   *
   * 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.
   *
   * SPDX-License-Identifier: Apache-2.0
   * SPDX-FileCopyrightText: Copyright (c) 2018-2020 SAP SE or an SAP affiliate company and Eclipse Steady contributors
   */
  package org.eclipse.steady.shared.cache;
  
  import java.util.HashMap;
  import java.util.LinkedList;
  import java.util.Map;
  
  import org.apache.logging.log4j.Logger;
  import org.eclipse.steady.shared.util.StopWatch;
  
  /**
   * Simple cache.
   *
   * @param <S> key type
   * @param <T> value type
   */
  public class Cache<S, T> {
  
    private static Logger log = org.apache.logging.log4j.LogManager.getLogger(Cache.class);
  
    private long refreshMilli = -1;
    private int maxSize = -1;
    private LinkedList<S> keys =
        new LinkedList<S>(); // Used to delete oldest entry if cache size is limited (maxSize!=-1)
    private ObjectFetcher<S, T> reader = null;
    private Map<S, CacheEntry<T>> entries = new HashMap<S, CacheEntry<T>>();
  
    private static int PRINT_STATS_EVERY = 1000;
    private long cacheRequestCount = 0;
    private long cacheHitCount = 0;
    private long cacheMissCount = 0;
    private long cacheDeleteCount = 0;
    private long cacheRefreshCount = 0;
    private long cacheFetchCount = 0;
    private long cacheFetchErrorCount = 0;
    private long cacheFetchDuration = 0;
  
    /** Constant <code>MILLI_IN_MIN=60L * 1000L</code> */
    public static final long MILLI_IN_MIN = 60L * 1000L;
    /** Constant <code>MILLI_IN_HOUR=60L * MILLI_IN_MIN</code> */
    public static final long MILLI_IN_HOUR = 60L * MILLI_IN_MIN;
    /** Constant <code>MILLI_IN_DAY=24L * MILLI_IN_HOUR</code> */
    public static final long MILLI_IN_DAY = 24L * MILLI_IN_HOUR;
  
    /**
     * <p>Constructor for Cache.</p>
     *
     * @param _reader a {@link org.eclipse.steady.shared.cache.ObjectFetcher} object.
     * @param _refresh_min a long.
     */
    public Cache(ObjectFetcher<S, T> _reader, long _refresh_min) {
      this(_reader, _refresh_min, -1);
    }
  
    /**
     * <p>Constructor for Cache.</p>
     *
     * @param _reader a {@link org.eclipse.steady.shared.cache.ObjectFetcher} object.
     * @param _refresh_min a long.
     * @param _max_size a int.
     */
    public Cache(ObjectFetcher<S, T> _reader, long _refresh_min, int _max_size) {
      if (_reader == null) throw new IllegalArgumentException("Object fetcher cannot be [null]");
      if (_refresh_min <= 0)
        throw new IllegalArgumentException("Refresh period must be greater than [0]");
      if (_max_size != -1 && _max_size < 1)
        throw new IllegalArgumentException(
            "Cache size limit must be [-1] (unbounded) or greater than [0]");
  
      this.reader = _reader;
      this.refreshMilli = _refresh_min * MILLI_IN_MIN;
      this.maxSize = _max_size;
    }
  
    /**
     * <p>get.</p>
     *
     * @param _key a S object.
     * @return a T object.
     * @throws org.eclipse.steady.shared.cache.CacheException if any.
     */
    public synchronized T get(S _key) throws CacheException {
      return this.get(_key, false);
    }
  
    /**
     * Reads the cache entry for the given key. In case such an object does not exist, the method
     * calls {@link ObjectFetcher#fetch(Object)} in order to read the object from the remote store.
     *
     * @param _key the key of the entry to be returned
     * @param _force_fetch if true, the entry will be fetched no matter whether it already exists in the cache
     * @return the entry for the given key
     * @throws org.eclipse.steady.shared.cache.CacheException if any.
     */
    public synchronized T get(S _key, boolean _force_fetch) throws CacheException {
      final long current_time = System.currentTimeMillis();
  
      // Print stats
      if (this.cacheRequestCount % PRINT_STATS_EVERY == 0) this.printStats();
  
      this.cacheRequestCount++;
      CacheEntry<T> e = null;
  
      // Check if the key is known and refresh threshold is passed
      if (this.entries.containsKey(_key)) {
        this.cacheHitCount++;
        e = this.entries.get(_key);
        if (_force_fetch || current_time - e.getCreatedAt() > this.refreshMilli) {
          this.cacheRefreshCount++;
          e = null;
        }
      } else {
        this.cacheMissCount++;
      }
  
      // Fetch object (if necessary)
      if (e == null) {
        this.cacheFetchCount++;
        final StopWatch sw = new StopWatch("Fetch cache entry for key [" + _key + "]").start();
        try {
          e = new CacheEntry<T>(this.reader.fetch(_key));
          sw.stop();
          cacheFetchDuration += sw.getRuntimeMillis();
          this.entries.put(_key, e);
          this.keys.add(_key);
        } catch (CacheException e1) {
          sw.stop(e1);
          cacheFetchDuration += sw.getRuntimeMillis();
          cacheFetchErrorCount++;
          throw e1;
        }
  
        // Delete oldest entry if max cache size reached
        if (this.maxSize != -1 && this.entries.size() > this.maxSize) {
          final S to_del = this.keys.poll();
          if (to_del != null) {
            this.cacheDeleteCount++;
            this.entries.remove(to_del);
            log.info(
                "Removed key [ "
                    + to_del.toString()
                    + "] from cache due to cache size limit of ["
                    + this.maxSize
                    + "]");
          }
        }
      }
  
      if (e == null) throw new CacheException(_key);
  
      return e.getObject();
    }
  
    /**
     * <p>getCacheRequest.</p>
     *
     * @return a long.
     */
    public long getCacheRequest() {
      return this.cacheRequestCount;
    }
    /**
     * <p>getCacheHit.</p>
     *
     * @return a long.
     */
    public long getCacheHit() {
      return this.cacheHitCount;
    }
    /**
     * <p>getCacheMiss.</p>
     *
     * @return a long.
     */
    public long getCacheMiss() {
      return this.cacheMissCount;
    }
    /**
     * <p>getCacheFetch.</p>
     *
     * @return a long.
     */
    public long getCacheFetch() {
      return this.cacheFetchCount;
    }
    /**
     * <p>getCacheDelete.</p>
     *
     * @return a long.
     */
    public long getCacheDelete() {
      return this.cacheDeleteCount;
    }
  
    private class CacheEntry<U> {
  
      private U obj = null;
      private long createdAt = System.currentTimeMillis();
  
      private CacheEntry(U _obj) {
        this.obj = _obj;
      }
  
      private long getCreatedAt() {
        return this.createdAt;
      }
  
      private U getObject() {
        return this.obj;
      }
    }
  
    private void printStats() {
      Cache.log.info(
          "Cache requests/hits/misses: ["
              + this.cacheRequestCount
              + "/"
              + this.cacheHitCount
              + "/"
              + this.cacheMissCount
              + "], refreshes ["
              + this.cacheRefreshCount
              + "], fetches, fetch errors and avg. fetch duration ["
              + this.cacheFetchCount
              + "/"
              + this.cacheFetchErrorCount
              + "/"
              + ((float) this.cacheFetchDuration / this.cacheFetchCount)
              + "], deletions ["
              + this.cacheDeleteCount
              + "]");
    }
  }
  