There were days when only way to provide mutual exclusion in Java was via synchronized keyword, but it has several shortcomings e.g. you can not extend lock beyond a method or block boundary, you can not give up trying for a lock etc. Java 5 solves this problem by providing more sophisticated control via Lock interface. ReentrantLock is a common implementation of Lock interface and provides re-entrant mutual exclusion Lock with the same basic behaviour and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.
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