code: /*
   * 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 org.apache.skywalking.oap.server.core.analysis.data;
  
  import java.util.List;
  import java.util.concurrent.locks.ReentrantLock;
  
  /**
   * ReadWriteSafeCache provides a read/write isolated cache.
   */
  public class ReadWriteSafeCache<T> {
      /**
       * Pointer of read buffer.
       */
      private volatile BufferedData<T> readBufferPointer;
      /**
       * Pointer of write buffer.
       */
      private volatile BufferedData<T> writeBufferPointer;
      /**
       * Read/Write lock.
       */
      private final ReentrantLock lock;
  
      /**
       * Build the Cache through two given buffer instances.
       *
       * @param buffer1 read/write switchable buffer
       * @param buffer2 read/write switchable buffer. It is the write buffer at the beginning.
       */
      public ReadWriteSafeCache(BufferedData<T> buffer1, BufferedData<T> buffer2) {
          readBufferPointer = buffer1;
          writeBufferPointer = buffer2;
          lock = new ReentrantLock();
      }
  
      /**
       * Write the into the {@link #writeBufferPointer} buffer.
       *
       * @param data to enqueue.
       */
      public void write(T data) {
          lock.lock();
          try {
              writeBufferPointer.accept(data);
          } finally {
              lock.unlock();
          }
      }
  
      /**
       * Write the collection of data into the {@link #writeBufferPointer} buffer.
       *
       * @param data to enqueue.
       */
      public void write(List<T> data) {
          lock.lock();
          try {
              data.forEach(writeBufferPointer::accept);
          } finally {
              lock.unlock();
          }
      }
  
      public List<T> read() {
          lock.lock();
          try {
              // Switch the read and write pointers, when there is no writing.
              BufferedData<T> tempPointer = writeBufferPointer;
              writeBufferPointer = readBufferPointer;
              readBufferPointer = tempPointer;
          } finally {
              lock.unlock();
          }
          // Call read method outside of write lock for concurrency read-write.
          return readBufferPointer.read();
      }
  }
  