A lock is a tool for controlling access to a shared resource by * multiple threads. Commonly, a lock provides exclusive access to a * shared resource: only one thread at a time can acquire the lock and * all access to the shared resource requires that the lock be * acquired first. However, some locks may allow concurrent access to * a shared resource, such as the read lock of a {@link * ReadWriteLock}. * *
The use of synchronized methods or statements provides * access to the implicit monitor lock associated with every object, but * forces all lock acquisition and release to occur in a block-structured way: * when multiple locks are acquired they must be released in the opposite * order, and all locks must be released in the same lexical scope in which * they were acquired. * *
While the scoping mechanism for synchronized methods * and statements makes it much easier to program with monitor locks, * and helps avoid many common programming errors involving locks, * there are occasions where you need to work with locks in a more * flexible way. For example, some algorithms for traversing * concurrently accessed data structures require the use of * "hand-over-hand" or "chain locking": you * acquire the lock of node A, then node B, then release A and acquire * C, then release B and acquire D and so on. Implementations of the * Lock interface enable the use of such techniques by * allowing a lock to be acquired and released in different scopes, * and allowing multiple locks to be acquired and released in any * order. * *
With this increased flexibility comes additional * responsibility. The absence of block-structured locking removes the * automatic release of locks that occurs with synchronized * methods and statements. In most cases, the following idiom * should be used: * *
Lock l = ...;
* l.lock();
* try {
* // access the resource protected by this lock
* } finally {
* l.unlock();
* }
*
*
* When locking and unlocking occur in different scopes, care must be
* taken to ensure that all code that is executed while the lock is
* held is protected by try-finally or try-catch to ensure that the
* lock is released when necessary.
*
* Lock implementations provide additional functionality * over the use of synchronized methods and statements by * providing a non-blocking attempt to acquire a lock ({@link * #tryLock()}), an attempt to acquire the lock that can be * interrupted ({@link #lockInterruptibly}, and an attempt to acquire * the lock that can timeout ({@link #tryLock(long, TimeUnit)}). * *
A Lock class can also provide behavior and semantics * that is quite different from that of the implicit monitor lock, * such as guaranteed ordering, non-reentrant usage, or deadlock * detection. If an implementation provides such specialized semantics * then the implementation must document those semantics. * *
Note that Lock instances are just normal objects and can * themselves be used as the target in a synchronized statement. * Acquiring the * monitor lock of a Lock instance has no specified relationship * with invoking any of the {@link #lock} methods of that instance. * It is recommended that to avoid confusion you never use Lock * instances in this way, except within their own implementation. * *
Except where noted, passing a null value for any * parameter will result in a {@link NullPointerException} being * thrown. * *
All Lock implementations must enforce the same * memory synchronization semantics as provided by the built-in monitor lock: *
The three forms of lock acquisition (interruptible, * non-interruptible, and timed) may differ in their performance * characteristics, ordering guarantees, or other implementation * qualities. Further, the ability to interrupt the ongoing * acquisition of a lock may not be available in a given Lock * class. Consequently, an implementation is not required to define * exactly the same guarantees or semantics for all three forms of * lock acquisition, nor is it required to support interruption of an * ongoing lock acquisition. An implementation is required to clearly * document the semantics and guarantees provided by each of the * locking methods. It must also obey the interruption semantics as * defined in this interface, to the extent that interruption of lock * acquisition is supported: which is either totally, or only on * method entry. * *
As interruption generally implies cancellation, and checks for * interruption are often infrequent, an implementation can favor responding * to an interrupt over normal method return. This is true even if it can be * shown that the interrupt occurred after another action may have unblocked * the thread. An implementation should document this behavior. * * * @see ReentrantLock * @see Condition * @see ReadWriteLock * * @since 1.5 * @author Doug Lea * **/ public interface Lock { /** * Acquires the lock. *
If the lock is not available then * the current thread becomes disabled for thread scheduling * purposes and lies dormant until the lock has been acquired. *
Implementation Considerations *
A Lock implementation may be able to detect * erroneous use of the lock, such as an invocation that would cause * deadlock, and may throw an (unchecked) exception in such circumstances. * The circumstances and the exception type must be documented by that * Lock implementation. * **/ void lock(); /** * Acquires the lock unless the current thread is * {@link Thread#interrupt interrupted}. *
Acquires the lock if it is available and returns immediately. *
If the lock is not available then * the current thread becomes disabled for thread scheduling * purposes and lies dormant until one of two things happens: *
If the current thread: *
Implementation Considerations * *
The ability to interrupt a lock acquisition in some * implementations may not be possible, and if possible may be an * expensive operation. The programmer should be aware that this * may be the case. An implementation should document when this is * the case. * *
An implementation can favor responding to an interrupt over * normal method return. * *
A Lock implementation may be able to detect * erroneous use of the lock, such as an invocation that would * cause deadlock, and may throw an (unchecked) exception in such * circumstances. The circumstances and the exception type must * be documented by that Lock implementation. * * @throws InterruptedException if the current thread is interrupted * while acquiring the lock (and interruption of lock acquisition is * supported). * * @see Thread#interrupt * **/ void lockInterruptibly() throws InterruptedException; /** * Acquires the lock only if it is free at the time of invocation. *
Acquires the lock if it is available and returns immediately * with the value true. * If the lock is not available then this method will return * immediately with the value false. *
A typical usage idiom for this method would be: *
* Lock lock = ...;
* if (lock.tryLock()) {
* try {
* // manipulate protected state
* } finally {
* lock.unlock();
* }
* } else {
* // perform alternative actions
* }
*
* This usage ensures that the lock is unlocked if it was acquired, and
* doesn't try to unlock if the lock was not acquired.
*
* @return true if the lock was acquired and false
* otherwise.
**/
boolean tryLock();
/**
* Acquires the lock if it is free within the given waiting time and the
* current thread has not been {@link Thread#interrupt interrupted}.
*
* If the lock is available this method returns immediately * with the value true. * If the lock is not available then * the current thread becomes disabled for thread scheduling * purposes and lies dormant until one of three things happens: *
If the lock is acquired then the value true is returned. *
If the current thread: *
If the specified waiting time elapses then the value false * is returned. * If the time is * less than or equal to zero, the method will not wait at all. * *
Implementation Considerations *
The ability to interrupt a lock acquisition in some implementations * may not be possible, and if possible may * be an expensive operation. * The programmer should be aware that this may be the case. An * implementation should document when this is the case. *
An implementation can favor responding to an interrupt over normal * method return, or reporting a timeout. *
A Lock implementation may be able to detect * erroneous use of the lock, such as an invocation that would cause * deadlock, and may throw an (unchecked) exception in such circumstances. * The circumstances and the exception type must be documented by that * Lock implementation. * * @param time the maximum time to wait for the lock * @param unit the time unit of the time argument. * @return true if the lock was acquired and false * if the waiting time elapsed before the lock was acquired. * * @throws InterruptedException if the current thread is interrupted * while acquiring the lock (and interruption of lock acquisition is * supported). * * @see Thread#interrupt * **/ boolean tryLock(long time, TimeUnit unit) throws InterruptedException; /** * Releases the lock. *
Implementation Considerations *
A Lock implementation will usually impose * restrictions on which thread can release a lock (typically only the * holder of the lock can release it) and may throw * an (unchecked) exception if the restriction is violated. * Any restrictions and the exception * type must be documented by that Lock implementation. **/ void unlock(); /** * Returns a new {@link Condition} instance that is bound to this * Lock instance. *
Before waiting on the condition the lock must be held by the * current thread. * A call to {@link Condition#await()} will atomically release the lock * before waiting and re-acquire the lock before the wait returns. *
Implementation Considerations *
The exact operation of the {@link Condition} instance depends on the * Lock implementation and must be documented by that * implementation. * * @return A new {@link Condition} instance for this Lock * instance. * @throws UnsupportedOperationException if this Lock * implementation does not support conditions. **/ Condition newCondition(); }