Interfaces and classes providing a framework for locking and waiting for conditions that is distinct from built-in synchronization and monitors. The framework permits much greater flexibility in the use of locks and conditions, at the expense of more awkward syntax.

The {@link java.util.concurrent.locks.Lock} interface supports locking disciplines that differ in semantics (reentrant, fair, etc), and that can be used in non-block-structured contexts including hand-over-hand and lock reordering algorithms. The main implementation is {@link java.util.concurrent.locks.ReentrantLock}.

The {@link java.util.concurrent.locks.ReadWriteLock} interface similarly defines locks that may be shared among readers but are exclusive to writers. Only a single implementation, {@link java.util.concurrent.locks.ReentrantReadWriteLock}, is provided, since it covers most standard usage contexts. But programmers may create their own implementations to cover nonstandard requirements.

The {@link java.util.concurrent.locks.Condition} interface describes condition variables that may be associated with Locks. These are similar in usage to the implicit monitors accessed using Object.wait, but offer extended capabilities. In particular, multiple Condition objects may be associated with a single Lock. To avoid compatibility issues, the names of Condition methods are different than the corresponding Object versions.

The {@link java.util.concurrent.locks.AbstractQueuedSynchronizer} class serves as a useful superclass for defining locks and other synchronizers that rely on queuing blocked threads. The {@link java.util.concurrent.locks.LockSupport} class provides lower-level blocking and unblocking support that is useful for those developers implementing their own customized lock classes. @since 1.5