Padlock.java

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/*
 * Project: Server for annotations sharing
 * Author: Ing. Jaroslav Dytrych idytrych@fit.vutbr.cz
 * File: Padlock.java
 * Description: Class represents the document protection, and provides methods to lock
 */

/**
 * @file Padlock.java
 *
 * @brief Class represents the document protection, and provides methods to lock
 */

package cz.vutbr.fit.knot.annotations.comet.documentLock;

/**
 * Class represents the document protection, and provides methods to lock.
 *
 * @brief Class represents the document protection, and provides methods to lock.
 * @author Martin Petr (xpetrm05)
 */
public class Padlock {
  /** id of document that lock belongs */
  Integer documentId;
  
  /** paramether indicate locked document */
  boolean isLocked;
  
  /** paramether indicate that some thread waits for change */
  boolean waitForUpdate;
  
  /** count of readers of document */
  int readersCount;

  /**
   * Constructor 
   * 
   * @param documentId id of document that lock belongs
   */
  public Padlock(Integer documentId) {
    this.documentId = documentId;
    isLocked = false;
    waitForUpdate = false;
    readersCount = 0;
  }

  /**
   * Gets id of document that is locked.
   * 
   * @return id of document that is locked
   */
  public Integer getDocumentId() {
    return documentId;
  }

  /**
   * Method try to lock document for read, if there some other thread that locks 
   * document before, current thread will be suspended.
   * 
   * @return returns true if current thread have lock or false if not
   */
  public synchronized boolean lockForRead(){
    if(isLocked || waitForUpdate){
      try{
        wait();
      }catch(InterruptedException ex){
        //handle exception
        return false;
      }
      return false;
    }else{
      readersCount++;
      return true;
    }
  }
  
  /**
   * Method unlocks document for read operations, all suspended threads will be 
   * make runable.
   */
  public synchronized void unlockForRead(){
    readersCount--;
    if(readersCount == 0 && waitForUpdate){
      notifyAll();
    }
  }
  
  /**
   * Method try to lock document for change, if there some other thread that locks 
   * document before, current thread will be suspended.
   * 
   * @return returns true if current thread have lock or false if not
   */
  public synchronized boolean lockForChange(){
    if(isLocked){
      if(!waitForUpdate){
        waitForUpdate = true;
      }
    }else{
      if(readersCount > 0){
        waitForUpdate = true;
      }else{
        // finnaly run
        isLocked = true;
        waitForUpdate = false;
        return true;
      }
    }
    
    try{
      wait();
    }catch(InterruptedException ex){
      //handle exception
    }
    
    return false;
  }
  
  /**
   * Method unlocks document for read operations, all suspended threads will be 
   * make runable.
   */
  public synchronized void unlockForChange(){
    isLocked = false;
    notifyAll();
  }
}  // public class Padlock