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    1   package org.apache.lucene.store;
    2   
    3   /**
    4    * Licensed to the Apache Software Foundation (ASF) under one or more
    5    * contributor license agreements.  See the NOTICE file distributed with
    6    * this work for additional information regarding copyright ownership.
    7    * The ASF licenses this file to You under the Apache License, Version 2.0
    8    * (the "License"); you may not use this file except in compliance with
    9    * the License.  You may obtain a copy of the License at
   10    *
   11    *     http://www.apache.org/licenses/LICENSE-2.0
   12    *
   13    * Unless required by applicable law or agreed to in writing, software
   14    * distributed under the License is distributed on an "AS IS" BASIS,
   15    * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   16    * See the License for the specific language governing permissions and
   17    * limitations under the License.
   18    */
   19   
   20   import java.io.IOException;
   21   
   22   /** An interprocess mutex lock.
   23    * <p>Typical use might look like:<pre>
   24    * new Lock.With(directory.makeLock("my.lock")) {
   25    *     public Object doBody() {
   26    *       <i>... code to execute while locked ...</i>
   27    *     }
   28    *   }.run();
   29    * </pre>
   30    *
   31    *
   32    * @version $Id: Lock.java 595448 2007-11-15 20:42:54Z mikemccand $
   33    * @see Directory#makeLock(String)
   34    */
   35   public abstract class Lock {
   36   
   37     /** How long {@link #obtain(long)} waits, in milliseconds,
   38      *  in between attempts to acquire the lock. */
   39     public static long LOCK_POLL_INTERVAL = 1000;
   40   
   41     /** Pass this value to {@link #obtain(long)} to try
   42      *  forever to obtain the lock. */
   43     public static final long LOCK_OBTAIN_WAIT_FOREVER = -1;
   44   
   45     /** Attempts to obtain exclusive access and immediately return
   46      *  upon success or failure.
   47      * @return true iff exclusive access is obtained
   48      */
   49     public abstract boolean obtain() throws IOException;
   50   
   51     /**
   52      * If a lock obtain called, this failureReason may be set
   53      * with the "root cause" Exception as to why the lock was
   54      * not obtained.
   55      */
   56     protected Throwable failureReason;
   57   
   58     /** Attempts to obtain an exclusive lock within amount of
   59      *  time given. Polls once per {@link #LOCK_POLL_INTERVAL}
   60      *  (currently 1000) milliseconds until lockWaitTimeout is
   61      *  passed.
   62      * @param lockWaitTimeout length of time to wait in
   63      *        milliseconds or {@link
   64      *        #LOCK_OBTAIN_WAIT_FOREVER} to retry forever
   65      * @return true if lock was obtained
   66      * @throws LockObtainFailedException if lock wait times out
   67      * @throws IllegalArgumentException if lockWaitTimeout is
   68      *         out of bounds
   69      * @throws IOException if obtain() throws IOException
   70      */
   71     public boolean obtain(long lockWaitTimeout) throws LockObtainFailedException, IOException {
   72       failureReason = null;
   73       boolean locked = obtain();
   74       if (lockWaitTimeout < 0 && lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER)
   75         throw new IllegalArgumentException("lockWaitTimeout should be LOCK_OBTAIN_WAIT_FOREVER or a non-negative number (got " + lockWaitTimeout + ")");
   76   
   77       long maxSleepCount = lockWaitTimeout / LOCK_POLL_INTERVAL;
   78       long sleepCount = 0;
   79       while (!locked) {
   80         if (lockWaitTimeout != LOCK_OBTAIN_WAIT_FOREVER && sleepCount++ >= maxSleepCount) {
   81           String reason = "Lock obtain timed out: " + this.toString();
   82           if (failureReason != null) {
   83             reason += ": " + failureReason;
   84           }
   85           LockObtainFailedException e = new LockObtainFailedException(reason);
   86           if (failureReason != null) {
   87             e.initCause(failureReason);
   88           }
   89           throw e;
   90         }
   91         try {
   92           Thread.sleep(LOCK_POLL_INTERVAL);
   93         } catch (InterruptedException e) {
   94           throw new IOException(e.toString());
   95         }
   96         locked = obtain();
   97       }
   98       return locked;
   99     }
  100   
  101     /** Releases exclusive access. */
  102     public abstract void release() throws IOException;
  103   
  104     /** Returns true if the resource is currently locked.  Note that one must
  105      * still call {@link #obtain()} before using the resource. */
  106     public abstract boolean isLocked();
  107   
  108   
  109     /** Utility class for executing code with exclusive access. */
  110     public abstract static class With {
  111       private Lock lock;
  112       private long lockWaitTimeout;
  113   
  114   
  115       /** Constructs an executor that will grab the named lock. */
  116       public With(Lock lock, long lockWaitTimeout) {
  117         this.lock = lock;
  118         this.lockWaitTimeout = lockWaitTimeout;
  119       }
  120   
  121       /** Code to execute with exclusive access. */
  122       protected abstract Object doBody() throws IOException;
  123   
  124       /** Calls {@link #doBody} while <i>lock</i> is obtained.  Blocks if lock
  125        * cannot be obtained immediately.  Retries to obtain lock once per second
  126        * until it is obtained, or until it has tried ten times. Lock is released when
  127        * {@link #doBody} exits.
  128        * @throws LockObtainFailedException if lock could not
  129        * be obtained
  130        * @throws IOException if {@link Lock#obtain} throws IOException
  131        */
  132       public Object run() throws LockObtainFailedException, IOException {
  133         boolean locked = false;
  134         try {
  135            locked = lock.obtain(lockWaitTimeout);
  136            return doBody();
  137         } finally {
  138           if (locked)
  139   	      lock.release();
  140         }
  141       }
  142     }
  143   
  144   }

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