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Code widgets in this lesson may fail to execute, if Educative.io’s code execution platform doesn’t use Java version 9 or above. This is because the StampedLock class was introduced in Java 9.


The StampedLock was introduced in Java 9 and is a capability-based lock with three modes for controlling read/write access. The StampedLock class is designed for use as an internal utility in the development of other thread-safe components. Its use relies on knowledge of the internal properties of the data, objects, and methods it protects.


The state of a StampedLock is defined by a version and mode. There are three modes the lock can be in:

  • Writing
  • Reading
  • Optimistic Reading

On acquiring a lock, a stamp (long value) is returned that represents and controls access with respect to a lock state. The stamp can be used later on to release the lock or convert the existing acquired lock to a different mode.


The read mode can be acquired using the readLock() method. When reading a thread isn’t expected to make any changes to the shared state and therefore it makes sense to have multiple threads acquire the read lock at the same time. The method returns a stamp that can be used to unlock the read lock. Timed and untimed versions of tryReadLock() also exist.

The code widget below demonstrates multiple threads acquiring the StampedLock in read mode.

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