lrucache.java

pastry的java实现的2.0b版

/*************************************************************************

"FreePastry" Peer-to-Peer Application Development Substrate 

Copyright 2002, Rice University. All rights reserved.

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********************************************************************************/

package rice.persistence;

/*
 *  @(#) LRUCache.java
 *
 *  @author Ansley Post
 *  @author Alan Mislove
 *
 *  @version $Id: LRUCache.java 2808 2005-11-22 14:38:49Z jeffh $
 */
import java.io.*;
import java.util.*;
import java.util.zip.*;

import rice.*;
import rice.Continuation.*;
import rice.environment.Environment;
import rice.environment.logging.Logger;
import rice.p2p.commonapi.*;
import rice.p2p.util.*;

/**
 * This class is an encapsulation of a least-recently-used (LRU) cache. It uses
 * the provided storage service in order to store the cached data. If the
 * Storage provides non-corruption services, these services will also be
 * provided by this cache.
 *
 * @version $Id: pretty.settings 2305 2005-03-11 20:22:33Z jeffh $
 * @author jeffh
 */
public class LRUCache implements Cache {

  // the maximum size of the cache
  private int maximumSize;

  // the back-end storage used by this cache
  private Storage storage;

  // the list of keys, in MRU -> LRU order
  private LinkedList order;

  /**
   * DESCRIBE THE FIELD
   */
  protected Environment environment;

  /**
   * Builds a LRU cache given a storage object to store the cached data in and a
   * maximum cache size.
   *
   * @param storage The storage service to use as a back-end storage
   * @param maximumSize The maximum size, in bytes, of storage to use
   * @param env DESCRIBE THE PARAMETER
   */
  public LRUCache(Storage storage, int maximumSize, Environment env) {
    this.environment = env;
    this.storage = storage;
    this.maximumSize = maximumSize;

    this.order = new LinkedList();
    Iterator i = storage.scan().getIterator();

    while (i.hasNext()) {
      order.addLast(i.next());
    }
  }

  /**
   * Returns the backing storage object
   *
   * @return The storage backing this cache
   */
  public Storage getStorage() {
    return storage;
  }

  /**
   * Returns the object identified by the given id.
   *
   * @param id The id of the object in question.
   * @param c The command to run once the operation is complete
   */
  public void getObject(Id id, Continuation c) {
    if (!exists(id)) {
      c.receiveResult(null);
      return;
    }

    synchronized (this) {
      order.remove(id);
      order.addFirst(id);
    }

    storage.getObject(id, c);
  }

  /**
   * Returns the metadata associated with the provided object, or null if no
   * metadata exists. The metadata must be stored in memory, so this operation
   * is guaranteed to be fast and non-blocking.
   *
   * @param id The id for which the metadata is needed
   * @return The metadata, or null of non exists
   */
  public Serializable getMetadata(Id id) {
    return storage.getMetadata(id);
  }

  /**
   * Returns the maximum size of the cache, in bytes. The result is returned via
   * the receiveResult method on the provided Continuation with an Integer
   * representing the size.
   *
   * @return The MaximumSize value
   */
  public long getMaximumSize() {
    return maximumSize;
  }

  /**
   * Returns the number of Ids currently stored in the catalog
   *
   * @return The number of ids in the catalog
   */
  public int getSize() {
    return storage.getSize();
  }

  /**
   * Returns the total size of the stored data in bytes. The result is returned
   * via the receiveResult method on the provided Continuation with an Integer
   * representing the size.
   *
   * @return The total size, in bytes, of data stored.
   */
  public long getTotalSize() {
    return storage.getTotalSize();
  }

  /**
   * Returns the size of the given object, in bytes.
   *
   * @param obj The object to determine the size of
   * @return The size, in bytes
   */
  private int getSize(Object obj) {
    try {
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      ObjectOutputStream oos = new XMLObjectOutputStream(new BufferedOutputStream(new GZIPOutputStream(baos)));

      oos.writeObject(obj);
      oos.flush();

      return baos.toByteArray().length;
    } catch (IOException e) {
      Logger logger = environment.getLogManager().getLogger(LRUCache.class, null);
      if (logger.level <= Logger.WARNING) {
        logger.logException("", e);
      }
      // returns maximum value here, so it won't be cached
      return Integer.MAX_VALUE;
    }
  }

  /**
   * Updates the metadata stored under the given key to be the provided value.
   * As this may require a disk access, the requestor must also provide a
   * continuation to return the result to.
   *
   * @param id The id for the metadata
   * @param metadata The metadata to store
   * @param command The new Metadata value
   */
  public void setMetadata(Id id, Serializable metadata, Continuation command) {
    storage.setMetadata(id, metadata, command);
  }

  /**
   * Sets the maximum size of the cache, in bytes. Setting this value to a
   * smaller value than the current value may result in object being evicted
   * from the cache.
   *
   * @param size The new maximum size, in bytes, of the cache.
   * @param c The command to run once the operation is complete
   */
  public void setMaximumSize(final int size, final Continuation c) {
    Continuation local =
      new Continuation() {
        public void receiveResult(Object o) {
          maximumSize = size;

          c.receiveResult(new Boolean(true));
        }

        public void receiveException(Exception e) {
          c.receiveException(e);
        }
      };

    if (size < maximumSize) {
      resize(size, local);
    } else {
      local.receiveResult(new Boolean(true));
    }
  }

  /**
   * Renames the given object to the new id. This method is potentially faster
   * than store/cache and unstore/uncache.
   *
   * @param oldId The id of the object in question.
   * @param newId The new id of the object in question.
   * @param c The command to run once the operation is complete
   */
  public void rename(final Id oldId, final Id newId, Continuation c) {
    storage.rename(oldId, newId,
      new StandardContinuation(c) {
        public void receiveResult(Object o) {
          synchronized (order) {
            order.remove(oldId);
            order.addFirst(newId);
          }
        }
      });
  }

  /**
   * Caches an object in this storage. This method is non-blocking. If the
   * object has already been stored at the location id, this method has the
   * effect of calling <code>uncachr(id)</code> followed by <code>cache(id, obj)</code>
   * . This method finishes by calling receiveResult() on the provided
   * continuation with whether or not the object was cached. Note that the
   * object may not actually be cached due to the cache replacement policy.
   * Returns <code>True</code> if the cache actaully stores the object, else
   * <code>False</code> (through receiveResult on c).
   *
   * @param id The object's id.
   * @param metadata The object's metadata
   * @param obj The object to cache.
   * @param c The command to run once the operation is complete
   */
  public void cache(final Id id, final Serializable metadata, final Serializable obj, final Continuation c) {
    final int size = getSize(obj);

    if (exists(id)) {
      synchronized (order) {
        order.remove(id);
        order.addFirst(id);
      }

      c.receiveResult(new Boolean(true));
      return;
    }

    if (size > maximumSize) {
      c.receiveResult(new Boolean(false));
      return;
    }

    //System.out.println("\nCaching object of size " + size + " with ID " + id + " num " + order.size());

    final Continuation store =
      new Continuation() {
        public void receiveResult(Object o) {
          synchronized (order) {
            order.addFirst(id);
          }

          storage.store(id, metadata, obj, c);
        }

        public void receiveException(Exception e) {
          c.receiveException(e);
        }
      };

    long totalSize = storage.getTotalSize();

    if (maximumSize - size < totalSize) {
      resize(maximumSize - size, store);
    } else {
      store.receiveResult(Boolean.TRUE);
    }
  }

  /**
   * Removes the object from the list of cached objects. This method is
   * non-blocking. If the object was not in the cached list in the first place,
   * nothing happens and <code>False</code> is returned. Returns <code>True</code>
   * if the action succeeds, else <code>False</code> (through receiveResult on
   * c).
   *
   * @param c The command to run once the operation is complete
   * @param id DESCRIBE THE PARAMETER
   */
  public void uncache(Id id, Continuation c) {
    synchronized (order) {
      order.remove(id);
    }

    storage.unstore(id, c);
  }

  /**
   * Returns whether or not an object is cached in the location <code>id</code>.
   *
   * @param id The id of the object in question.
   * @return Whether or not an object is present at id.
   */
  public boolean exists(Id id) {
    return storage.exists(id);
  }

  /**
   * Return the objects identified by the given range of ids. The IdSet returned
   * contains the Ids of the stored objects. The range is partially inclusive,
   * the lower range is inclusive, and the upper exclusive. NOTE: This method
   * blocks so if the behavior of this method changes and uses the disk, this
   * method may be deprecated.
   *
   * @param range The range to query
   * @return The idset containg the keys
   */
  public synchronized IdSet scan(IdRange range) {
    return storage.scan(range);
  }

  /**
   * Return all objects currently stored by this catalog NOTE: This method
   * blocks so if the behavior of this method changes and no longer stored in
   * memory, this method may be deprecated.
   *
   * @return The idset containg the keys
   */
  public synchronized IdSet scan() {
    return storage.scan();
  }

  /**
   * Returns a map which contains keys mapping ids to the associated metadata.
   *
   * @param range The range to query
   * @return The map containg the keys
   */
  public SortedMap scanMetadata(IdRange range) {
    return storage.scanMetadata(range);
  }

  /**
   * Returns a map which contains keys mapping ids to the associated metadata.
   *
   * @return The treemap mapping ids to metadata
   */
  public SortedMap scanMetadata() {
    return storage.scanMetadata();
  }

  /**
   * Returns the submapping of ids which have metadata less than the provided
   * value.
   *
   * @param value The maximal metadata value
   * @return The submapping
   */
  public SortedMap scanMetadataValuesHead(Object value) {
    return storage.scanMetadataValuesHead(value);
  }

  /**
   * Returns the submapping of ids which have metadata null
   *
   * @return The submapping
   */
  public SortedMap scanMetadataValuesNull() {
    return storage.scanMetadataValuesNull();
  }

  /**
   * Internal method which removes objects from the cache until the cache is
   * smaller than the specified size
   *
   * @param size The maximum number of bytes to make the cache
   * @param c The command to run once the operation is complete
   */
  private void resize(final int size, Continuation c) {
    final Continuation remove =
      new StandardContinuation(c) {
        private boolean waitingForSize = true;

        public void receiveResult(Object o) {
          if (storage.getTotalSize() > size) {
            uncache((Id) order.getLast(), this);
          } else {
            parent.receiveResult(new Boolean(true));
          }
        }
      };

    remove.receiveResult(Boolean.TRUE);
  }

  /**
   * Method which is used to erase all data stored in the Cache. Use this method
   * with care!
   *
   * @param c The command to run once done
   */
  public void flush(Continuation c) {
    storage.flush(
      new StandardContinuation(c) {
        public void receiveResult(Object o) {
          order.clear();
          parent.receiveResult(Boolean.TRUE);
        }
      });
  }
}