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One of the major strengths of the Java programming language is its built-in support for multi-threaded programs. An object that is shared between multiple threads can be locked in order to synchronize its access. Java provides primitives to designate critical code regions, which act on a shared object and which may be executed only by one thread at a time. The first thread that enters the region locks the shared object. When a second thread is about to enter the same region, it must wait until the first thread has unlocked the object again.

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In the Java HotSpotHotSpot™ VM, every object is preceded by a class pointer and a header word. The header word, which stores the identity hash code as well as age and marking bits for generational garbage collection, is also used to implement a _thin lock scheme_ \[[Agesen99|#Agesen99], [Bacon98|#Bacon98]\]. The following figure shows the layout of the header word and the representation of different object states.

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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="4ce17c944ccd3627-b04cb7f8-427644c1-9013b257-d3c05e05ea91d55dbfdb7a56"><ac:parameter ac:name="">Agesen99</ac:parameter></ac:structured-macro>\[Agesen99\] O. Agesen, D. Detlefs, A. Garthwaite, R. Knippel, Y. S. Ramakrishna, D. White: _An Efficient Meta-lock for Implementing Ubiquitous Synchronization_. In _Proceedings of the ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications_, pages 207-222. ACM Press, 1999. [doi:10.1145/320384.320402|http://dx.doi.org/10.1145/320384.320402]

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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="c5f147127a1a9f57-b5dcc453-42454a9d-b3cb8e6c-a9b122c490483f1b610c7490"><ac:parameter ac:name="">Bacon98</ac:parameter></ac:structured-macro>\[Bacon98\] D. F. Bacon, R. Konuru, C. Murthy, M. Serrano: _Thin Locks: Featherweight Synchronization for Java_. In _Proceedings of the ACM SIGPLAN Conference on Programming Language Design and Implementation_, pages 258-268. ACM Press, 1998. [doi:10.1145/277650.277734|http://dx.doi.org/10.1145/277650.277734]

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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="ecc792a712a2dba8-ad98cd23-49f846f3-b73e92fd-e705d12840702eb60be94599"><ac:parameter ac:name="">Kawachiya02</ac:parameter></ac:structured-macro>\[Kawachiya02\] K. Kawachiya, A. Koseki, T. Onodera: Lock Reservation: Java Locks can Mostly do without Atomic Operations. In _Proceedings of the ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications_, pages 130-141. ACM Press, 2002. [doi:10.1145/582419.582433|http://dx.doi.org/10.1145/582419.582433]

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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="a3cd45f5affa7663-d894d94f-484f40ca-a69b9726-56eea18360accfd16905a9de"><ac:parameter ac:name="">Russell06</ac:parameter></ac:structured-macro>\[Russel06\] K. Russell, D. Detlefs: Eliminating Synchronization-Related Atomic Operations with Biased Locking and Bulk Rebiasing. In _Proceedings of the ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications_, pages 263-272. ACM Press, 2006. [doi:10.1145/1167473.1167496|http://dx.doi.org/10.1145/1167473.1167496]