379 lines
18 KiB
Java
379 lines
18 KiB
Java
/*
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership.
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* The ASF licenses this file to You under the Apache License, Version 2.0
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* (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package org.apache.commons.lang3.concurrent.locks;
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import java.util.Objects;
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import java.util.concurrent.locks.Lock;
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import java.util.concurrent.locks.ReadWriteLock;
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import java.util.concurrent.locks.ReentrantReadWriteLock;
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import java.util.concurrent.locks.StampedLock;
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import java.util.function.Supplier;
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import org.apache.commons.lang3.function.Failable;
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import org.apache.commons.lang3.function.FailableConsumer;
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import org.apache.commons.lang3.function.FailableFunction;
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/**
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* <p>
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* Combines the monitor and visitor pattern to work with {@link java.util.concurrent.locks.Lock locked objects}. Locked
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* objects are an alternative to synchronization. This, on Wikipedia, is known as the Visitor pattern
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* (https://en.wikipedia.org/wiki/Visitor_pattern), and from the "Gang of Four" "Design Patterns" book's Visitor pattern
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* [Gamma, E., Helm, R., & Johnson, R. (1998). Visitor. In Design patterns elements of reusable object oriented software (pp. 331-344). Reading: Addison Wesley.].
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* </p>
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* <p>
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* Locking is preferable, if there is a distinction between read access (multiple threads may have read access
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* concurrently), and write access (only one thread may have write access at any given time). In comparison,
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* synchronization doesn't support read access, because synchronized access is exclusive.
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* </p>
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* <p>
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* Using this class is fairly straightforward:
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* </p>
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* <ol>
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* <li>While still in single thread mode, create an instance of {@link LockingVisitors.StampedLockVisitor} by calling
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* {@link #stampedLockVisitor(Object)}, passing the object which needs to be locked. Discard all references to the
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* locked object. Instead, use references to the lock.</li>
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* <li>If you want to access the locked object, create a {@link FailableConsumer}. The consumer will receive the locked
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* object as a parameter. For convenience, the consumer may be implemented as a Lambda. Then invoke
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* {@link LockingVisitors.StampedLockVisitor#acceptReadLocked(FailableConsumer)}, or
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* {@link LockingVisitors.StampedLockVisitor#acceptWriteLocked(FailableConsumer)}, passing the consumer.</li>
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* <li>As an alternative, if you need to produce a result object, you may use a {@link FailableFunction}. This function
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* may also be implemented as a Lambda. To have the function executed, invoke
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* {@link LockingVisitors.StampedLockVisitor#applyReadLocked(FailableFunction)}, or
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* {@link LockingVisitors.StampedLockVisitor#applyWriteLocked(FailableFunction)}.</li>
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* </ol>
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* <p>
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* Example: A thread safe logger class.
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* </p>
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*
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* <pre>
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* public class SimpleLogger {
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*
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* private final StampedLockVisitor<PrintStream> lock;
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*
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* public SimpleLogger(OutputStream out) {
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* lock = LockingVisitors.stampedLockVisitor(new PrintStream(out));
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* }
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*
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* public void log(String message) {
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* lock.acceptWriteLocked((ps) -> ps.println(message));
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* }
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*
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* public void log(byte[] buffer) {
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* lock.acceptWriteLocked((ps) -> { ps.write(buffer); ps.println(); });
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* }
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* </pre>
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*
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* @since 3.11
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*/
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public class LockingVisitors {
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/**
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* Wraps a domain object and a lock for access by lambdas.
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*
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* @param <O> the wrapped object type.
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* @param <L> the wrapped lock type.
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*/
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public static class LockVisitor<O, L> {
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/**
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* The lock object, untyped, since, for example {@link StampedLock} does not implement a locking interface in
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* Java 8.
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*/
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private final L lock;
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/**
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* The guarded object.
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*/
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private final O object;
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/**
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* Supplies the read lock, usually from the lock object.
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*/
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private final Supplier<Lock> readLockSupplier;
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/**
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* Supplies the write lock, usually from the lock object.
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*/
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private final Supplier<Lock> writeLockSupplier;
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/**
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* Constructs an instance.
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*
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* @param object The object to guard.
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* @param lock The locking object.
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* @param readLockSupplier Supplies the read lock, usually from the lock object.
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* @param writeLockSupplier Supplies the write lock, usually from the lock object.
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*/
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protected LockVisitor(final O object, final L lock, final Supplier<Lock> readLockSupplier, final Supplier<Lock> writeLockSupplier) {
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this.object = Objects.requireNonNull(object, "object");
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this.lock = Objects.requireNonNull(lock, "lock");
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this.readLockSupplier = Objects.requireNonNull(readLockSupplier, "readLockSupplier");
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this.writeLockSupplier = Objects.requireNonNull(writeLockSupplier, "writeLockSupplier");
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}
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/**
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* <p>
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* Provides read (shared, non-exclusive) access to the locked (hidden) object. More precisely, what the method
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* will do (in the given order):
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* </p>
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* <ol>
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* <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
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* lock is granted.</li>
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* <li>Invokes the given {@link FailableConsumer consumer}, passing the locked object as the parameter.</li>
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* <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
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* lock will be released anyways.</li>
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* </ol>
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*
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* @param consumer The consumer, which is being invoked to use the hidden object, which will be passed as the
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* consumers parameter.
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* @see #acceptWriteLocked(FailableConsumer)
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* @see #applyReadLocked(FailableFunction)
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*/
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public void acceptReadLocked(final FailableConsumer<O, ?> consumer) {
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lockAcceptUnlock(readLockSupplier, consumer);
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}
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/**
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* <p>
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* Provides write (exclusive) access to the locked (hidden) object. More precisely, what the method will do (in
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* the given order):
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* </p>
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* <ol>
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* <li>Obtain a write (shared) lock on the locked (hidden) object. The current thread may block, until such a
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* lock is granted.</li>
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* <li>Invokes the given {@link FailableConsumer consumer}, passing the locked object as the parameter.</li>
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* <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
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* lock will be released anyways.</li>
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* </ol>
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*
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* @param consumer The consumer, which is being invoked to use the hidden object, which will be passed as the
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* consumers parameter.
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* @see #acceptReadLocked(FailableConsumer)
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* @see #applyWriteLocked(FailableFunction)
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*/
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public void acceptWriteLocked(final FailableConsumer<O, ?> consumer) {
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lockAcceptUnlock(writeLockSupplier, consumer);
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}
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/**
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* <p>
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* Provides read (shared, non-exclusive) access to the locked (hidden) object for the purpose of computing a
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* result object. More precisely, what the method will do (in the given order):
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* </p>
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* <ol>
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* <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
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* lock is granted.</li>
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* <li>Invokes the given {@link FailableFunction function}, passing the locked object as the parameter,
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* receiving the functions result.</li>
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* <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
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* lock will be released anyways.</li>
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* <li>Return the result object, that has been received from the functions invocation.</li>
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* </ol>
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* <p>
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* <em>Example:</em> Consider that the hidden object is a list, and we wish to know the current size of the
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* list. This might be achieved with the following:
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* </p>
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* <pre>
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* private Lock<List<Object>> listLock;
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*
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* public int getCurrentListSize() {
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* final Integer sizeInteger = listLock.applyReadLocked((list) -> Integer.valueOf(list.size));
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* return sizeInteger.intValue();
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* }
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* </pre>
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*
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* @param <T> The result type (both the functions, and this method's.)
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* @param function The function, which is being invoked to compute the result. The function will receive the
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* hidden object.
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* @return The result object, which has been returned by the functions invocation.
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* @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
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* access to the hidden object beyond this methods lifetime and will therefore be prevented.
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* @see #acceptReadLocked(FailableConsumer)
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* @see #applyWriteLocked(FailableFunction)
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*/
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public <T> T applyReadLocked(final FailableFunction<O, T, ?> function) {
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return lockApplyUnlock(readLockSupplier, function);
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}
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/**
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* <p>
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* Provides write (exclusive) access to the locked (hidden) object for the purpose of computing a result object.
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* More precisely, what the method will do (in the given order):
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* </p>
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* <ol>
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* <li>Obtain a read (shared) lock on the locked (hidden) object. The current thread may block, until such a
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* lock is granted.</li>
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* <li>Invokes the given {@link FailableFunction function}, passing the locked object as the parameter,
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* receiving the functions result.</li>
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* <li>Release the lock, as soon as the consumers invocation is done. If the invocation results in an error, the
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* lock will be released anyways.</li>
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* <li>Return the result object, that has been received from the functions invocation.</li>
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* </ol>
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*
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* @param <T> The result type (both the functions, and this method's.)
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* @param function The function, which is being invoked to compute the result. The function will receive the
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* hidden object.
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* @return The result object, which has been returned by the functions invocation.
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* @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
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* access to the hidden object beyond this methods lifetime and will therefore be prevented.
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* @see #acceptReadLocked(FailableConsumer)
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* @see #applyWriteLocked(FailableFunction)
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*/
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public <T> T applyWriteLocked(final FailableFunction<O, T, ?> function) {
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return lockApplyUnlock(writeLockSupplier, function);
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}
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/**
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* Gets the lock.
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*
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* @return the lock.
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*/
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public L getLock() {
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return lock;
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}
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/**
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* Gets the guarded object.
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*
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* @return the object.
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*/
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public O getObject() {
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return object;
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}
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/**
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* This method provides the default implementation for {@link #acceptReadLocked(FailableConsumer)}, and
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* {@link #acceptWriteLocked(FailableConsumer)}.
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*
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* @param lockSupplier A supplier for the lock. (This provides, in fact, a long, because a {@link StampedLock} is used
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* internally.)
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* @param consumer The consumer, which is to be given access to the locked (hidden) object, which will be passed
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* as a parameter.
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* @see #acceptReadLocked(FailableConsumer)
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* @see #acceptWriteLocked(FailableConsumer)
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*/
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protected void lockAcceptUnlock(final Supplier<Lock> lockSupplier, final FailableConsumer<O, ?> consumer) {
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final Lock lock = lockSupplier.get();
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lock.lock();
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try {
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consumer.accept(object);
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} catch (final Throwable t) {
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throw Failable.rethrow(t);
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} finally {
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lock.unlock();
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}
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}
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/**
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* This method provides the actual implementation for {@link #applyReadLocked(FailableFunction)}, and
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* {@link #applyWriteLocked(FailableFunction)}.
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*
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* @param <T> The result type (both the functions, and this method's.)
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* @param lockSupplier A supplier for the lock. (This provides, in fact, a long, because a {@link StampedLock} is used
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* internally.)
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* @param function The function, which is being invoked to compute the result object. This function will receive
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* the locked (hidden) object as a parameter.
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* @return The result object, which has been returned by the functions invocation.
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* @throws IllegalStateException The result object would be, in fact, the hidden object. This would extend
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* access to the hidden object beyond this methods lifetime and will therefore be prevented.
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* @see #applyReadLocked(FailableFunction)
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* @see #applyWriteLocked(FailableFunction)
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*/
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protected <T> T lockApplyUnlock(final Supplier<Lock> lockSupplier, final FailableFunction<O, T, ?> function) {
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final Lock lock = lockSupplier.get();
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lock.lock();
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try {
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return function.apply(object);
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} catch (final Throwable t) {
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throw Failable.rethrow(t);
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} finally {
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lock.unlock();
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}
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}
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}
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/**
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* This class implements a wrapper for a locked (hidden) object, and provides the means to access it. The basic
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* idea, is that the user code forsakes all references to the locked object, using only the wrapper object, and the
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* accessor methods {@link #acceptReadLocked(FailableConsumer)}, {@link #acceptWriteLocked(FailableConsumer)},
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* {@link #applyReadLocked(FailableFunction)}, and {@link #applyWriteLocked(FailableFunction)}. By doing so, the
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* necessary protections are guaranteed.
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*
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* @param <O> The locked (hidden) objects type.
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*/
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public static class ReadWriteLockVisitor<O> extends LockVisitor<O, ReadWriteLock> {
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/**
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* Creates a new instance with the given locked object. This constructor is supposed to be used for subclassing
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* only. In general, it is suggested to use {@link LockingVisitors#stampedLockVisitor(Object)} instead.
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*
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* @param object The locked (hidden) object. The caller is supposed to drop all references to the locked object.
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* @param readWriteLock the lock to use.
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*/
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protected ReadWriteLockVisitor(final O object, final ReadWriteLock readWriteLock) {
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super(object, readWriteLock, readWriteLock::readLock, readWriteLock::writeLock);
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}
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}
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/**
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* This class implements a wrapper for a locked (hidden) object, and provides the means to access it. The basic
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* idea is that the user code forsakes all references to the locked object, using only the wrapper object, and the
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* accessor methods {@link #acceptReadLocked(FailableConsumer)}, {@link #acceptWriteLocked(FailableConsumer)},
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* {@link #applyReadLocked(FailableFunction)}, and {@link #applyWriteLocked(FailableFunction)}. By doing so, the
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* necessary protections are guaranteed.
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*
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* @param <O> The locked (hidden) objects type.
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*/
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public static class StampedLockVisitor<O> extends LockVisitor<O, StampedLock> {
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/**
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* Creates a new instance with the given locked object. This constructor is supposed to be used for subclassing
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* only. In general, it is suggested to use {@link LockingVisitors#stampedLockVisitor(Object)} instead.
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*
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* @param object The locked (hidden) object. The caller is supposed to drop all references to the locked object.
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* @param stampedLock the lock to use.
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*/
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protected StampedLockVisitor(final O object, final StampedLock stampedLock) {
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super(object, stampedLock, stampedLock::asReadLock, stampedLock::asWriteLock);
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}
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}
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/**
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* Creates a new instance of {@link ReadWriteLockVisitor} with the given (hidden) object.
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*
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* @param <O> The locked objects type.
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* @param object The locked (hidden) object.
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* @return The created instance, a {@link StampedLockVisitor lock} for the given object.
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*/
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public static <O> ReadWriteLockVisitor<O> reentrantReadWriteLockVisitor(final O object) {
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return new LockingVisitors.ReadWriteLockVisitor<>(object, new ReentrantReadWriteLock());
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}
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/**
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* Creates a new instance of {@link StampedLockVisitor} with the given (hidden) object.
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*
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* @param <O> The locked objects type.
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* @param object The locked (hidden) object.
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* @return The created instance, a {@link StampedLockVisitor lock} for the given object.
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*/
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public static <O> StampedLockVisitor<O> stampedLockVisitor(final O object) {
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return new LockingVisitors.StampedLockVisitor<>(object, new StampedLock());
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}
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}
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