contenthashpastcontent.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.p2p.past;

import java.io.Serializable;

import rice.*;
import rice.p2p.commonapi.*;

/**
 * @(#) ContentHashPastContent.java An abstract class for content-hash objects
 * stored in Past. Provided as a convenience.
 *
 * @version $Id: ContentHashPastContent.java 2688 2005-08-03 01:45:34Z jstewart
 *      $
 * @author Peter Druschel
 * @author Alan Mislove
 */
public abstract class ContentHashPastContent implements PastContent {

  /**
   * DESCRIBE THE FIELD
   */
  protected Id myId;

  private final static long serialVersionUID = 6375789163758367025L;

  // ----- PastCONTENT METHODS -----

  /**
   * Constructor for ContentHashPastContent.
   *
   * @param myId DESCRIBE THE PARAMETER
   */
  public ContentHashPastContent(Id myId) {
    this.myId = myId;
  }

  /**
   * Produces a handle for this content object. The handle is retrieved and
   * returned to the client as a result of the Past.lookupHandles() method.
   *
   * @param local The local past service
   * @return the handle
   */
  public PastContentHandle getHandle(Past local) {
    return new ContentHashPastContentHandle(local.getLocalNodeHandle(), getId());
  }

  /**
   * Returns the Id under which this object is stored in Past.
   *
   * @return the id
   */
  public Id getId() {
    return myId;
  }

  /**
   * States if this content object is mutable. Mutable objects are not subject
   * to dynamic caching in Past.
   *
   * @return true if this object is mutable, else false
   */
  public boolean isMutable() {
    return false;
  }

  /**
   * Checks if a insert operation should be allowed. Invoked when a Past node
   * receives an insert request and it is a replica root for the id; invoked on
   * the object to be inserted. This method determines the effect of an insert
   * operation on an object that already exists: it computes the new value of
   * the stored object, as a function of the new and the existing object.
   *
   * @param id the key identifying the object
   * @param existingContent DESCRIBE THE PARAMETER
   * @return null, if the operation is not allowed; else, the new object to be
   *      stored on the local node.
   * @exception PastException DESCRIBE THE EXCEPTION
   */
  public PastContent checkInsert(Id id, PastContent existingContent) throws PastException {
    // can't overwrite content hash objects
    if (existingContent != null) {
      throw new PastException("ContentHashPastContent: can't insert, object already exists");
    }

    // only allow correct content hash key
    if (!id.equals(getId())) {
      throw new PastException("ContentHashPastContent: can't insert, content hash incorrect");
    }
    return this;
  }
}