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// © 2016 and later: Unicode, Inc. and others.
// License & terms of use: http://www.unicode.org/copyright.html
/**
*******************************************************************************
* Copyright (C) 2001-2013, International Business Machines Corporation and *
* others. All Rights Reserved. *
*******************************************************************************
*/
package com.ibm.icu.impl;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* A Reader/Writer lock originally written for ICU service
* implementation. The internal implementation was replaced
* with the JDK's stock read write lock (ReentrantReadWriteLock)
* for ICU 52.
*
* This assumes that there will be little writing contention.
* It also doesn't allow active readers to acquire and release
* a write lock, or deal with priority inversion issues.
*
* Access to the lock should be enclosed in a try/finally block
* in order to ensure that the lock is always released in case of
* exceptions:
* try {
* lock.acquireRead();
* // use service protected by the lock
* }
* finally {
* lock.releaseRead();
* }
*
*
* The lock provides utility methods getStats and clearStats
* to return statistics on the use of the lock.
*/
public class ICURWLock {
private ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
private Stats stats = null;
/**
* Internal class used to gather statistics on the RWLock.
*/
public final static class Stats {
/**
* Number of times read access granted (read count).
*/
public int _rc;
/**
* Number of times concurrent read access granted (multiple read count).
*/
public int _mrc;
/**
* Number of times blocked for read (waiting reader count).
*/
public int _wrc; // wait for read
/**
* Number of times write access granted (writer count).
*/
public int _wc;
/**
* Number of times blocked for write (waiting writer count).
*/
public int _wwc;
private Stats() {
}
private Stats(int rc, int mrc, int wrc, int wc, int wwc) {
this._rc = rc;
this._mrc = mrc;
this._wrc = wrc;
this._wc = wc;
this._wwc = wwc;
}
private Stats(Stats rhs) {
this(rhs._rc, rhs._mrc, rhs._wrc, rhs._wc, rhs._wwc);
}
/**
* Return a string listing all the stats.
*/
@Override
public String toString() {
return " rc: " + _rc +
" mrc: " + _mrc +
" wrc: " + _wrc +
" wc: " + _wc +
" wwc: " + _wwc;
}
}
/**
* Reset the stats. Returns existing stats, if any.
*/
public synchronized Stats resetStats() {
Stats result = stats;
stats = new Stats();
return result;
}
/**
* Clear the stats (stop collecting stats). Returns existing stats, if any.
*/
public synchronized Stats clearStats() {
Stats result = stats;
stats = null;
return result;
}
/**
* Return a snapshot of the current stats. This does not reset the stats.
*/
public synchronized Stats getStats() {
return stats == null ? null : new Stats(stats);
}
/**
* Acquire a read lock, blocking until a read lock is
* available. Multiple readers can concurrently hold the read
* lock.
*
* If there's a writer, or a waiting writer, increment the
* waiting reader count and block on this. Otherwise
* increment the active reader count and return. Caller must call
* releaseRead when done (for example, in a finally block).
*/
public void acquireRead() {
if (stats != null) { // stats is null by default
synchronized (this) {
stats._rc++;
if (rwl.getReadLockCount() > 0) {
stats._mrc++;
}
if (rwl.isWriteLocked()) {
stats._wrc++;
}
}
}
rwl.readLock().lock();
}
/**
* Release a read lock and return. An error will be thrown
* if a read lock is not currently held.
*
* If this is the last active reader, notify the oldest
* waiting writer. Call when finished with work
* controlled by acquireRead.
*/
public void releaseRead() {
rwl.readLock().unlock();
}
/**
* Acquire the write lock, blocking until the write lock is
* available. Only one writer can acquire the write lock, and
* when held, no readers can acquire the read lock.
*
* If there are no readers and no waiting writers, mark as
* having an active writer and return. Otherwise, add a lock to the
* end of the waiting writer list, and block on it. Caller
* must call releaseWrite when done (for example, in a finally
* block).
*/
public void acquireWrite() {
if (stats != null) { // stats is null by default
synchronized (this) {
stats._wc++;
if (rwl.getReadLockCount() > 0 || rwl.isWriteLocked()) {
stats._wwc++;
}
}
}
rwl.writeLock().lock();
}
/**
*
Release the write lock and return. An error will be thrown
* if the write lock is not currently held.
*
* If there are waiting readers, make them all active and
* notify all of them. Otherwise, notify the oldest waiting
* writer, if any. Call when finished with work controlled by
* acquireWrite.
*/
public void releaseWrite() {
rwl.writeLock().unlock();
}
}