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The source code

package org.apache.lucene.store;

/**
 * Copyright 2004 The Apache Software Foundation
 *
 * Licensed 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.
 */

import org.apache.lucene.index.IndexWriter;

import java.io.IOException;

/** An interprocess mutex lock.
 * 

Typical use might look like:

 * new Lock.With(directory.makeLock("my.lock")) {
 *     public Object doBody() {
 *       ... code to execute while locked ...
 *     }
 *   }.run();
 * 
* * @author Doug Cutting * @version $Id: Lock.java,v 1.12 2004/05/11 17:43:28 cutting Exp $ * @see Directory#makeLock(String) */ public abstract class Lock { public static long LOCK_POLL_INTERVAL = 1000; /** Attempts to obtain exclusive access and immediately return * upon success or failure. * @return true iff exclusive access is obtained */ public abstract boolean obtain() throws IOException; /** Attempts to obtain an exclusive lock within amount * of time given. Currently polls once per second until * lockWaitTimeout is passed. * @param lockWaitTimeout length of time to wait in ms * @return true if lock was obtained * @throws IOException if lock wait times out or obtain() throws an IOException */ public boolean obtain(long lockWaitTimeout) throws IOException { boolean locked = obtain(); int maxSleepCount = (int)(lockWaitTimeout / LOCK_POLL_INTERVAL); int sleepCount = 0; while (!locked) { if (++sleepCount == maxSleepCount) { throw new IOException("Lock obtain timed out: " + this.toString()); } try { Thread.sleep(LOCK_POLL_INTERVAL); } catch (InterruptedException e) { throw new IOException(e.toString()); } locked = obtain(); } return locked; } /** Releases exclusive access. */ public abstract void release(); /** Returns true if the resource is currently locked. Note that one must * still call {@link #obtain()} before using the resource. */ public abstract boolean isLocked(); /** Utility class for executing code with exclusive access. */ public abstract static class With { private Lock lock; private long lockWaitTimeout; /** Constructs an executor that will grab the named lock. * Defaults lockWaitTimeout to Lock.COMMIT_LOCK_TIMEOUT. * @deprecated Kept only to avoid breaking existing code. */ public With(Lock lock) { this(lock, IndexWriter.COMMIT_LOCK_TIMEOUT); } /** Constructs an executor that will grab the named lock. */ public With(Lock lock, long lockWaitTimeout) { this.lock = lock; this.lockWaitTimeout = lockWaitTimeout; } /** Code to execute with exclusive access. */ protected abstract Object doBody() throws IOException; /** Calls {@link #doBody} while lock is obtained. Blocks if lock * cannot be obtained immediately. Retries to obtain lock once per second * until it is obtained, or until it has tried ten times. Lock is released when * {@link #doBody} exits. */ public Object run() throws IOException { boolean locked = false; try { locked = lock.obtain(lockWaitTimeout); return doBody(); } finally { if (locked) lock.release(); } } } }
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