+++ /dev/null
-/*
- * Copyright 2005 the original author or authors.
- *
- * 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.
- */
-package org.wamblee.concurrency;
-
-import java.util.HashSet;
-
-
-/**
- * Read-write lock for allowing multiple concurrent readers or at most one
- * writer. This implementation does not aim for high performance but for
- * robustness and simplicity. Users of this class should not synchronize on
- * objects of this class.
- */
-public class ReadWriteLock {
- /**
- * Sets containing the references to the threads that are currently
- * reading. This administration is useful to check that the lock has
- * already been acquired before it is release. This check adds robustness
- * to the application.
- */
- private HashSet<Thread> _readers;
-
- /**
- * The thread that has acquired the lock for writing or null if no such
- * thread exists currently.
- */
- private Thread _writer;
-
- /**
- * Constructs read-write lock.
- */
- public ReadWriteLock() {
- _readers = new HashSet<Thread>();
- _writer = null;
- }
-
- /**
- * Acquires the lock for reading. This call will block until the lock can
- * be acquired.
- *
- * @throws IllegalStateException Thrown if the read or write lock is
- * already acquired.
- */
- public synchronized void acquireRead() {
- if (_readers.contains(Thread.currentThread())) {
- throw new IllegalStateException(
- "Read lock already acquired by current thread: "
- + Thread.currentThread());
- }
-
- if (_writer == Thread.currentThread()) {
- throw new IllegalStateException(
- "Trying to acquire the read lock while already holding a write lock: "
- + Thread.currentThread());
- }
-
- while (_writer != null) {
- try {
- wait();
- } catch (InterruptedException e) {
- notifyAll();
- }
- }
-
- _readers.add(Thread.currentThread());
- }
-
- /**
- * Releases the lock for reading. Note: This implementation assumes that
- * the lock has already been acquired for reading previously.
- *
- * @throws IllegalStateException Thrown when the lock was not acquired by
- * this thread.
- */
- public synchronized void releaseRead() {
- if (!_readers.remove(Thread.currentThread())) {
- throw new IllegalStateException(
- "Cannot release read lock because current thread has not acquired it.");
- }
-
- if (_readers.size() == 0) {
- notifyAll();
- }
- }
-
- /**
- * Acquires the lock for writing. This call will block until the lock has
- * been acquired.
- *
- * @throws IllegalStateException Thrown if the read or write lock is
- * already acquired.
- */
- public synchronized void acquireWrite() {
- if (_writer == Thread.currentThread()) {
- throw new IllegalStateException(
- "Trying to acquire a write lock while already holding the write lock: "
- + Thread.currentThread());
- }
-
- if (_readers.contains(Thread.currentThread())) {
- throw new IllegalStateException(
- "Trying to acquire a write lock while already holding the read lock: "
- + Thread.currentThread());
- }
-
- // wait until there are no more writers and no more
- // readers
- while ((_writer != null) || (_readers.size() > 0)) {
- try {
- wait();
- } catch (InterruptedException e) {
- notifyAll();
- }
- }
-
- _writer = Thread.currentThread();
-
- // notification not necessary since all writers and
- // readers are now blocked by this thread.
- }
-
- /**
- * Releases the lock for writing.
- *
- * @throws IllegalStateException Thrown when the lock was not acquired.
- */
- public synchronized void releaseWrite() {
- if (_writer != Thread.currentThread()) {
- throw new IllegalStateException(
- "Cannot release write lock because it was not acquired. ");
- }
-
- _writer = null;
- notifyAll();
- }
-}
git://wamblee.org / utils / blobdiff