    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.lang3.concurrent.locks;
    018
    019import java.util.Objects;
    020import java.util.concurrent.locks.Lock;
    021import java.util.concurrent.locks.ReadWriteLock;
    022import java.util.concurrent.locks.ReentrantReadWriteLock;
    023import java.util.concurrent.locks.StampedLock;
    024import java.util.function.Supplier;
    025
    026import org.apache.commons.lang3.function.Failable;
    027import org.apache.commons.lang3.function.FailableConsumer;
    028import org.apache.commons.lang3.function.FailableFunction;
    029
    030/**
    031 * <p>
    032 * Combines the monitor and visitor pattern to work with {@link java.util.concurrent.locks.Lock locked objects}. Locked
    033 * objects are an alternative to synchronization. This, on Wikipedia, is known as the Visitor pattern
    034 * (https://en.wikipedia.org/wiki/Visitor_pattern), and from the "Gang of Four" "Design Patterns" book's Visitor pattern
    035 * [Gamma, E., Helm, R., &amp; Johnson, R. (1998). Visitor. In Design patterns elements of reusable object oriented software (pp. 331-344). Reading: Addison Wesley.].
    036 * </p>
    037 * <p>
    038 * Locking is preferable, if there is a distinction between read access (multiple threads may have read access
    039 * concurrently), and write access (only one thread may have write access at any given time). In comparison,
    040 * synchronization doesn't support read access, because synchronized access is exclusive.
    041 * </p>
    042 * <p>
    043 * Using this class is fairly straightforward:
    044 * </p>
    045 * <ol>
    046 * <li>While still in single thread mode, create an instance of {@link LockingVisitors.StampedLockVisitor} by calling
    047 * {@link #stampedLockVisitor(Object)}, passing the object which needs to be locked. Discard all references to the
    048 * locked object. Instead, use references to the lock.</li>
    049 * <li>If you want to access the locked object, create a {@link FailableConsumer}. The consumer will receive the locked
    050 * object as a parameter. For convenience, the consumer may be implemented as a Lambda. Then invoke
    051 * {@link LockingVisitors.StampedLockVisitor#acceptReadLocked(FailableConsumer)}, or
    052 * {@link LockingVisitors.StampedLockVisitor#acceptWriteLocked(FailableConsumer)}, passing the consumer.</li>
    053 * <li>As an alternative, if you need to produce a result object, you may use a {@link FailableFunction}. This function
    054 * may also be implemented as a Lambda. To have the function executed, invoke
    055 * {@link LockingVisitors.StampedLockVisitor#applyReadLocked(FailableFunction)}, or
    056 * {@link LockingVisitors.StampedLockVisitor#applyWriteLocked(FailableFunction)}.</li>
    057 * </ol>
    058 * <p>
    059 * Example: A thread safe logger class.
    060 * </p>
    061 *
    062 * <pre>
    063 *   public class SimpleLogger {
    064 *
    065 *     private final StampedLockVisitor&lt;PrintStream&gt; lock;
    066 *
    067 *     public SimpleLogger(OutputStream out) {
    068 *         lock = LockingVisitors.stampedLockVisitor(new PrintStream(out));
    069 *     }
    070 *
    071 *     public void log(String message) {
    072 *         lock.acceptWriteLocked((ps) -&gt; ps.println(message));
    073 *     }
    074 *
    075 *     public void log(byte[] buffer) {
    076 *         lock.acceptWriteLocked((ps) -&gt; { ps.write(buffer); ps.println(); });
    077 *     }
    078 * </pre>
    079 *
    080 * @since 3.11
    081 */
    082public class LockingVisitors {
    083
    084    /**
    085     * Wraps a domain object and a lock for access by lambdas.
    086     *
    087     * @param <O> the wrapped object type.
    088     * @param <L> the wrapped lock type.
    089     */
    090    public static class LockVisitor<O, L> {
    091
    092        /**
    093         * The lock object, untyped, since, for example {@link StampedLock} does not implement a locking interface in
    094         * Java 8.
    095         */
    096        private final L lock;
    097
    098        /**
    099         * The guarded object.
    100         */
    101        private final O object;
    102
    103        /**
    104         * Supplies the read lock, usually from the lock object.
    105         */
    106        private final Supplier<Lock> readLockSupplier;
    107
    108        /**
    109         * Supplies the write lock, usually from the lock object.
    110         */
    111        private final Supplier<Lock> writeLockSupplier;
    112
    113        /**
    114         * Constructs an instance.
    115         *
    116         * @param object The object to guard.
    117         * @param lock The locking object.
    118         * @param readLockSupplier Supplies the read lock, usually from the lock object.
    119         * @param writeLockSupplier Supplies the write lock, usually from the lock object.
    120         */
    121        protected LockVisitor(final O object, final L lock, final Supplier<Lock> readLockSupplier, final Supplier<Lock> writeLockSupplier) {
    122            this.object = Objects.requireNonNull(object, "object");
    123            this.lock = Objects.requireNonNull(lock, "lock");
    124            this.readLockSupplier = Objects.requireNonNull(readLockSupplier, "readLockSupplier");
    125            this.writeLockSupplier = Objects.requireNonNull(writeLockSupplier, "writeLockSupplier");
    126        }
    127
    128        /**
    129         * <p>
    130         * Provides read (shared, non-exclusive) access to the locked (hidden) object. More precisely, what the method
    131         * will do (in the given order):
    132         * </p>
    133         * <ol>
    134         * <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
    135         * lock is granted.</li>
    136         * <li>Invokes the given {@link FailableConsumer consumer}, passing the locked object as the parameter.</li>
    137         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    138         * lock will be released anyways.</li>
    139         * </ol>
    140         *
    141         * @param consumer The consumer, which is being invoked to use the hidden object, which will be passed as the
    142         *        consumers parameter.
    143         * @see #acceptWriteLocked(FailableConsumer)
    144         * @see #applyReadLocked(FailableFunction)
    145         */
    146        public void acceptReadLocked(final FailableConsumer<O, ?> consumer) {
    147            lockAcceptUnlock(readLockSupplier, consumer);
    148        }
    149
    150        /**
    151         * <p>
    152         * Provides write (exclusive) access to the locked (hidden) object. More precisely, what the method will do (in
    153         * the given order):
    154         * </p>
    155         * <ol>
    156         * <li>Obtain a write (shared) lock on the locked (hidden) object. The current thread may block, until such a
    157         * lock is granted.</li>
    158         * <li>Invokes the given {@link FailableConsumer consumer}, passing the locked object as the parameter.</li>
    159         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    160         * lock will be released anyways.</li>
    161         * </ol>
    162         *
    163         * @param consumer The consumer, which is being invoked to use the hidden object, which will be passed as the
    164         *        consumers parameter.
    165         * @see #acceptReadLocked(FailableConsumer)
    166         * @see #applyWriteLocked(FailableFunction)
    167         */
    168        public void acceptWriteLocked(final FailableConsumer<O, ?> consumer) {
    169            lockAcceptUnlock(writeLockSupplier, consumer);
    170        }
    171
    172        /**
    173         * <p>
    174         * Provides read (shared, non-exclusive) access to the locked (hidden) object for the purpose of computing a
    175         * result object. More precisely, what the method will do (in the given order):
    176         * </p>
    177         * <ol>
    178         * <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
    179         * lock is granted.</li>
    180         * <li>Invokes the given {@link FailableFunction function}, passing the locked object as the parameter,
    181         * receiving the functions result.</li>
    182         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    183         * lock will be released anyways.</li>
    184         * <li>Return the result object, that has been received from the functions invocation.</li>
    185         * </ol>
    186         * <p>
    187         * <em>Example:</em> Consider that the hidden object is a list, and we wish to know the current size of the
    188         * list. This might be achieved with the following:
    189         * </p>
    190         * <pre>
    191         * private Lock&lt;List&lt;Object&gt;&gt; listLock;
    192         *
    193         * public int getCurrentListSize() {
    194         *     final Integer sizeInteger = listLock.applyReadLocked((list) -&gt; Integer.valueOf(list.size));
    195         *     return sizeInteger.intValue();
    196         * }
    197         * </pre>
    198         *
    199         * @param <T> The result type (both the functions, and this method's.)
    200         * @param function The function, which is being invoked to compute the result. The function will receive the
    201         *        hidden object.
    202         * @return The result object, which has been returned by the functions invocation.
    203         * @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
    204         *         access to the hidden object beyond this methods lifetime and will therefore be prevented.
    205         * @see #acceptReadLocked(FailableConsumer)
    206         * @see #applyWriteLocked(FailableFunction)
    207         */
    208        public <T> T applyReadLocked(final FailableFunction<O, T, ?> function) {
    209            return lockApplyUnlock(readLockSupplier, function);
    210        }
    211
    212        /**
    213         * <p>
    214         * Provides write (exclusive) access to the locked (hidden) object for the purpose of computing a result object.
    215         * More precisely, what the method will do (in the given order):
    216         * </p>
    217         * <ol>
    218         * <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
    219         * lock is granted.</li>
    220         * <li>Invokes the given {@link FailableFunction function}, passing the locked object as the parameter,
    221         * receiving the functions result.</li>
    222         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    223         * lock will be released anyways.</li>
    224         * <li>Return the result object, that has been received from the functions invocation.</li>
    225         * </ol>
    226         *
    227         * @param <T> The result type (both the functions, and this method's.)
    228         * @param function The function, which is being invoked to compute the result. The function will receive the
    229         *        hidden object.
    230         * @return The result object, which has been returned by the functions invocation.
    231         * @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
    232         *         access to the hidden object beyond this methods lifetime and will therefore be prevented.
    233         * @see #acceptReadLocked(FailableConsumer)
    234         * @see #applyWriteLocked(FailableFunction)
    235         */
    236        public <T> T applyWriteLocked(final FailableFunction<O, T, ?> function) {
    237            return lockApplyUnlock(writeLockSupplier, function);
    238        }
    239
    240        /**
    241         * Gets the lock.
    242         *
    243         * @return the lock.
    244         */
    245        public L getLock() {
    246            return lock;
    247        }
    248
    249        /**
    250         * Gets the guarded object.
    251         *
    252         * @return the object.
    253         */
    254        public O getObject() {
    255            return object;
    256        }
    257
    258        /**
    259         * This method provides the default implementation for {@link #acceptReadLocked(FailableConsumer)}, and
    260         * {@link #acceptWriteLocked(FailableConsumer)}.
    261         *
    262         * @param lockSupplier A supplier for the lock. (This provides, in fact, a long, because a {@link StampedLock} is used
    263         *        internally.)
    264         * @param consumer The consumer, which is to be given access to the locked (hidden) object, which will be passed
    265         *        as a parameter.
    266         * @see #acceptReadLocked(FailableConsumer)
    267         * @see #acceptWriteLocked(FailableConsumer)
    268         */
    269        protected void lockAcceptUnlock(final Supplier<Lock> lockSupplier, final FailableConsumer<O, ?> consumer) {
    270            final Lock lock = lockSupplier.get();
    271            lock.lock();
    272            try {
    273                consumer.accept(object);
    274            } catch (final Throwable t) {
    275                throw Failable.rethrow(t);
    276            } finally {
    277                lock.unlock();
    278            }
    279        }
    280
    281        /**
    282         * This method provides the actual implementation for {@link #applyReadLocked(FailableFunction)}, and
    283         * {@link #applyWriteLocked(FailableFunction)}.
    284         *
    285         * @param <T> The result type (both the functions, and this method's.)
    286         * @param lockSupplier A supplier for the lock. (This provides, in fact, a long, because a {@link StampedLock} is used
    287         *        internally.)
    288         * @param function The function, which is being invoked to compute the result object. This function will receive
    289         *        the locked (hidden) object as a parameter.
    290         * @return The result object, which has been returned by the functions invocation.
    291         * @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
    292         *         access to the hidden object beyond this methods lifetime and will therefore be prevented.
    293         * @see #applyReadLocked(FailableFunction)
    294         * @see #applyWriteLocked(FailableFunction)
    295         */
    296        protected <T> T lockApplyUnlock(final Supplier<Lock> lockSupplier, final FailableFunction<O, T, ?> function) {
    297            final Lock lock = lockSupplier.get();
    298            lock.lock();
    299            try {
    300                return function.apply(object);
    301            } catch (final Throwable t) {
    302                throw Failable.rethrow(t);
    303            } finally {
    304                lock.unlock();
    305            }
    306        }
    307
    308    }
    309
    310    /**
    311     * This class implements a wrapper for a locked (hidden) object, and provides the means to access it. The basic
    312     * idea, is that the user code forsakes all references to the locked object, using only the wrapper object, and the
    313     * accessor methods {@link #acceptReadLocked(FailableConsumer)}, {@link #acceptWriteLocked(FailableConsumer)},
    314     * {@link #applyReadLocked(FailableFunction)}, and {@link #applyWriteLocked(FailableFunction)}. By doing so, the
    315     * necessary protections are guaranteed.
    316     *
    317     * @param <O> The locked (hidden) objects type.
    318     */
    319    public static class ReadWriteLockVisitor<O> extends LockVisitor<O, ReadWriteLock> {
    320
    321        /**
    322         * Creates a new instance with the given locked object. This constructor is supposed to be used for subclassing
    323         * only. In general, it is suggested to use {@link LockingVisitors#stampedLockVisitor(Object)} instead.
    324         *
    325         * @param object The locked (hidden) object. The caller is supposed to drop all references to the locked object.
    326         * @param readWriteLock the lock to use.
    327         */
    328        protected ReadWriteLockVisitor(final O object, final ReadWriteLock readWriteLock) {
    329            super(object, readWriteLock, readWriteLock::readLock, readWriteLock::writeLock);
    330        }
    331    }
    332
    333    /**
    334     * This class implements a wrapper for a locked (hidden) object, and provides the means to access it. The basic
    335     * idea is that the user code forsakes all references to the locked object, using only the wrapper object, and the
    336     * accessor methods {@link #acceptReadLocked(FailableConsumer)}, {@link #acceptWriteLocked(FailableConsumer)},
    337     * {@link #applyReadLocked(FailableFunction)}, and {@link #applyWriteLocked(FailableFunction)}. By doing so, the
    338     * necessary protections are guaranteed.
    339     *
    340     * @param <O> The locked (hidden) objects type.
    341     */
    342    public static class StampedLockVisitor<O> extends LockVisitor<O, StampedLock> {
    343
    344        /**
    345         * Creates a new instance with the given locked object. This constructor is supposed to be used for subclassing
    346         * only. In general, it is suggested to use {@link LockingVisitors#stampedLockVisitor(Object)} instead.
    347         *
    348         * @param object The locked (hidden) object. The caller is supposed to drop all references to the locked object.
    349         * @param stampedLock the lock to use.
    350         */
    351        protected StampedLockVisitor(final O object, final StampedLock stampedLock) {
    352            super(object, stampedLock, stampedLock::asReadLock, stampedLock::asWriteLock);
    353        }
    354    }
    355
    356    /**
    357     * Creates a new instance of {@link ReadWriteLockVisitor} with the given (hidden) object.
    358     *
    359     * @param <O> The locked objects type.
    360     * @param object The locked (hidden) object.
    361     * @return The created instance, a {@link StampedLockVisitor lock} for the given object.
    362     */
    363    public static <O> ReadWriteLockVisitor<O> reentrantReadWriteLockVisitor(final O object) {
    364        return new LockingVisitors.ReadWriteLockVisitor<>(object, new ReentrantReadWriteLock());
    365    }
    366
    367    /**
    368     * Creates a new instance of {@link StampedLockVisitor} with the given (hidden) object.
    369     *
    370     * @param <O> The locked objects type.
    371     * @param object The locked (hidden) object.
    372     * @return The created instance, a {@link StampedLockVisitor lock} for the given object.
    373     */
    374    public static <O> StampedLockVisitor<O> stampedLockVisitor(final O object) {
    375        return new LockingVisitors.StampedLockVisitor<>(object, new StampedLock());
    376    }
    377
    378}