    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;
    029import org.apache.commons.lang3.function.Suppliers;
    030
    031/**
    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 *
    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         * Provides read (shared, non-exclusive) access to the locked (hidden) object. More precisely, what the method
    130         * will do (in the given order):
    131         *
    132         * <ol>
    133         * <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
    134         * lock is granted.</li>
    135         * <li>Invokes the given {@link FailableConsumer consumer}, passing the locked object as the parameter.</li>
    136         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    137         * lock will be released anyways.</li>
    138         * </ol>
    139         *
    140         * @param consumer The consumer, which is being invoked to use the hidden object, which will be passed as the
    141         *        consumers parameter.
    142         * @see #acceptWriteLocked(FailableConsumer)
    143         * @see #applyReadLocked(FailableFunction)
    144         */
    145        public void acceptReadLocked(final FailableConsumer<O, ?> consumer) {
    146            lockAcceptUnlock(readLockSupplier, consumer);
    147        }
    148
    149        /**
    150         * Provides write (exclusive) access to the locked (hidden) object. More precisely, what the method will do (in
    151         * the given order):
    152         *
    153         * <ol>
    154         * <li>Obtain a write (shared) lock on the locked (hidden) object. The current thread may block, until such a
    155         * lock is granted.</li>
    156         * <li>Invokes the given {@link FailableConsumer consumer}, passing the locked object as the parameter.</li>
    157         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    158         * lock will be released anyways.</li>
    159         * </ol>
    160         *
    161         * @param consumer The consumer, which is being invoked to use the hidden object, which will be passed as the
    162         *        consumers parameter.
    163         * @see #acceptReadLocked(FailableConsumer)
    164         * @see #applyWriteLocked(FailableFunction)
    165         */
    166        public void acceptWriteLocked(final FailableConsumer<O, ?> consumer) {
    167            lockAcceptUnlock(writeLockSupplier, consumer);
    168        }
    169
    170        /**
    171         * Provides read (shared, non-exclusive) access to the locked (hidden) object for the purpose of computing a
    172         * result object. More precisely, what the method will do (in the given order):
    173         *
    174         * <ol>
    175         * <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
    176         * lock is granted.</li>
    177         * <li>Invokes the given {@link FailableFunction function}, passing the locked object as the parameter,
    178         * receiving the functions result.</li>
    179         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    180         * lock will be released anyways.</li>
    181         * <li>Return the result object, that has been received from the functions invocation.</li>
    182         * </ol>
    183         * <p>
    184         * <em>Example:</em> Consider that the hidden object is a list, and we wish to know the current size of the
    185         * list. This might be achieved with the following:
    186         * </p>
    187         * <pre>
    188         * private Lock&lt;List&lt;Object&gt;&gt; listLock;
    189         *
    190         * public int getCurrentListSize() {
    191         *     final Integer sizeInteger = listLock.applyReadLocked((list) -&gt; Integer.valueOf(list.size));
    192         *     return sizeInteger.intValue();
    193         * }
    194         * </pre>
    195         *
    196         * @param <T> The result type (both the functions, and this method's.)
    197         * @param function The function, which is being invoked to compute the result. The function will receive the
    198         *        hidden object.
    199         * @return The result object, which has been returned by the functions invocation.
    200         * @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
    201         *         access to the hidden object beyond this methods lifetime and will therefore be prevented.
    202         * @see #acceptReadLocked(FailableConsumer)
    203         * @see #applyWriteLocked(FailableFunction)
    204         */
    205        public <T> T applyReadLocked(final FailableFunction<O, T, ?> function) {
    206            return lockApplyUnlock(readLockSupplier, function);
    207        }
    208
    209        /**
    210         * Provides write (exclusive) access to the locked (hidden) object for the purpose of computing a result object.
    211         * More precisely, what the method will do (in the given order):
    212         *
    213         * <ol>
    214         * <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
    215         * lock is granted.</li>
    216         * <li>Invokes the given {@link FailableFunction function}, passing the locked object as the parameter,
    217         * receiving the functions result.</li>
    218         * <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
    219         * lock will be released anyways.</li>
    220         * <li>Return the result object, that has been received from the functions invocation.</li>
    221         * </ol>
    222         *
    223         * @param <T> The result type (both the functions, and this method's.)
    224         * @param function The function, which is being invoked to compute the result. The function will receive the
    225         *        hidden object.
    226         * @return The result object, which has been returned by the functions invocation.
    227         * @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
    228         *         access to the hidden object beyond this methods lifetime and will therefore be prevented.
    229         * @see #acceptReadLocked(FailableConsumer)
    230         * @see #applyWriteLocked(FailableFunction)
    231         */
    232        public <T> T applyWriteLocked(final FailableFunction<O, T, ?> function) {
    233            return lockApplyUnlock(writeLockSupplier, function);
    234        }
    235
    236        /**
    237         * Gets the lock.
    238         *
    239         * @return the lock.
    240         */
    241        public L getLock() {
    242            return lock;
    243        }
    244
    245        /**
    246         * Gets the guarded object.
    247         *
    248         * @return the object.
    249         */
    250        public O getObject() {
    251            return object;
    252        }
    253
    254        /**
    255         * This method provides the default implementation for {@link #acceptReadLocked(FailableConsumer)}, and
    256         * {@link #acceptWriteLocked(FailableConsumer)}.
    257         *
    258         * @param lockSupplier A supplier for the lock. (This provides, in fact, a long, because a {@link StampedLock} is used
    259         *        internally.)
    260         * @param consumer The consumer, which is to be given access to the locked (hidden) object, which will be passed
    261         *        as a parameter.
    262         * @see #acceptReadLocked(FailableConsumer)
    263         * @see #acceptWriteLocked(FailableConsumer)
    264         */
    265        protected void lockAcceptUnlock(final Supplier<Lock> lockSupplier, final FailableConsumer<O, ?> consumer) {
    266            final Lock lock = Objects.requireNonNull(Suppliers.get(lockSupplier), "lock");
    267            lock.lock();
    268            try {
    269                if (consumer != null) {
    270                    consumer.accept(object);
    271                }
    272            } catch (final Throwable t) {
    273                throw Failable.rethrow(t);
    274            } finally {
    275                lock.unlock();
    276            }
    277        }
    278
    279        /**
    280         * This method provides the actual implementation for {@link #applyReadLocked(FailableFunction)}, and
    281         * {@link #applyWriteLocked(FailableFunction)}.
    282         *
    283         * @param <T> The result type (both the functions, and this method's.)
    284         * @param lockSupplier A supplier for the lock. (This provides, in fact, a long, because a {@link StampedLock} is used
    285         *        internally.)
    286         * @param function The function, which is being invoked to compute the result object. This function will receive
    287         *        the locked (hidden) object as a parameter.
    288         * @return The result object, which has been returned by the functions invocation.
    289         * @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
    290         *         access to the hidden object beyond this methods lifetime and will therefore be prevented.
    291         * @see #applyReadLocked(FailableFunction)
    292         * @see #applyWriteLocked(FailableFunction)
    293         */
    294        protected <T> T lockApplyUnlock(final Supplier<Lock> lockSupplier, final FailableFunction<O, T, ?> function) {
    295            final Lock lock = Objects.requireNonNull(Suppliers.get(lockSupplier), "lock");
    296            lock.lock();
    297            try {
    298                return function.apply(object);
    299            } catch (final Throwable t) {
    300                throw Failable.rethrow(t);
    301            } finally {
    302                lock.unlock();
    303            }
    304        }
    305
    306    }
    307
    308    /**
    309     * This class implements a wrapper for a locked (hidden) object, and provides the means to access it. The basic
    310     * idea, is that the user code forsakes all references to the locked object, using only the wrapper object, and the
    311     * accessor methods {@link #acceptReadLocked(FailableConsumer)}, {@link #acceptWriteLocked(FailableConsumer)},
    312     * {@link #applyReadLocked(FailableFunction)}, and {@link #applyWriteLocked(FailableFunction)}. By doing so, the
    313     * necessary protections are guaranteed.
    314     *
    315     * @param <O> The locked (hidden) objects type.
    316     */
    317    public static class ReadWriteLockVisitor<O> extends LockVisitor<O, ReadWriteLock> {
    318
    319        /**
    320         * Creates a new instance with the given locked object. This constructor is supposed to be used for subclassing
    321         * only. In general, it is suggested to use {@link LockingVisitors#stampedLockVisitor(Object)} instead.
    322         *
    323         * @param object The locked (hidden) object. The caller is supposed to drop all references to the locked object.
    324         * @param readWriteLock the lock to use.
    325         */
    326        protected ReadWriteLockVisitor(final O object, final ReadWriteLock readWriteLock) {
    327            super(object, readWriteLock, readWriteLock::readLock, readWriteLock::writeLock);
    328        }
    329    }
    330
    331    /**
    332     * This class implements a wrapper for a locked (hidden) object, and provides the means to access it. The basic
    333     * idea is that the user code forsakes all references to the locked object, using only the wrapper object, and the
    334     * accessor methods {@link #acceptReadLocked(FailableConsumer)}, {@link #acceptWriteLocked(FailableConsumer)},
    335     * {@link #applyReadLocked(FailableFunction)}, and {@link #applyWriteLocked(FailableFunction)}. By doing so, the
    336     * necessary protections are guaranteed.
    337     *
    338     * @param <O> The locked (hidden) objects type.
    339     */
    340    public static class StampedLockVisitor<O> extends LockVisitor<O, StampedLock> {
    341
    342        /**
    343         * Creates a new instance with the given locked object. This constructor is supposed to be used for subclassing
    344         * only. In general, it is suggested to use {@link LockingVisitors#stampedLockVisitor(Object)} instead.
    345         *
    346         * @param object The locked (hidden) object. The caller is supposed to drop all references to the locked object.
    347         * @param stampedLock the lock to use.
    348         */
    349        protected StampedLockVisitor(final O object, final StampedLock stampedLock) {
    350            super(object, stampedLock, stampedLock::asReadLock, stampedLock::asWriteLock);
    351        }
    352    }
    353
    354    /**
    355     * Creates a new instance of {@link ReadWriteLockVisitor} with the given (hidden) object and lock.
    356     *
    357     * @param <O> The locked objects type.
    358     * @param object The locked (hidden) object.
    359     * @param readWriteLock The lock to use.
    360     * @return The created instance, a {@link StampedLockVisitor lock} for the given object.
    361     * @since 3.13.0
    362     */
    363    public static <O> ReadWriteLockVisitor<O> create(final O object, final ReadWriteLock readWriteLock) {
    364        return new LockingVisitors.ReadWriteLockVisitor<>(object, readWriteLock);
    365    }
    366
    367    /**
    368     * Creates a new instance of {@link ReadWriteLockVisitor} 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> ReadWriteLockVisitor<O> reentrantReadWriteLockVisitor(final O object) {
    375        return create(object, new ReentrantReadWriteLock());
    376    }
    377
    378    /**
    379     * Creates a new instance of {@link StampedLockVisitor} with the given (hidden) object.
    380     *
    381     * @param <O> The locked objects type.
    382     * @param object The locked (hidden) object.
    383     * @return The created instance, a {@link StampedLockVisitor lock} for the given object.
    384     */
    385    public static <O> StampedLockVisitor<O> stampedLockVisitor(final O object) {
    386        return new LockingVisitors.StampedLockVisitor<>(object, new StampedLock());
    387    }
    388
    389    /**
    390     * Make private in 4.0.
    391     *
    392     * @deprecated TODO Make private in 4.0.
    393     */
    394    @Deprecated
    395    public LockingVisitors() {
    396        // empty
    397    }
    398}