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