A high performance version of java.util.LinkedHashMap for use as a software cache.

• Design
• Features
• Status
• Future
• Maven
• Performance

Design

• A linked list runs through a ConcurrentHashMap to provide eviction ordering.
• Avoids lock contention by amortizing the penalty under lock.

See the design document and the StrangeLoop conference slides (Concurrent Caching at Google).

Features

• LRU page replacement policy (currently being upgraded to LIRS).
• Equivalent performance to ConcurrentHashMap under load.
• Can bound by the size of the values (e.g. Multimap cache).
• Can notify a listener when an entry is evicted.

See the tutorial for examples of using this library.

Status

• Released v1.4 with Least-Recently-Used page replacement policy.
• Integrated into Google Guava (MapMaker, CacheBuilder)

See the Changelog for version history.

Future

• v2.x: Implement Low Inter-reference Recency Set page replacement policy.

See the Changelog for more details and current progress.

Maven

Maven users should choose one of the dependencies based on their JDK version.

<!-- JDK 6 -->
<dependency>
  <groupId>com.googlecode.concurrentlinkedhashmap</groupId>
  <artifactId>concurrentlinkedhashmap-lru</artifactId>
  <version>1.4</version>
</dependency>

<!-- JDK 5 -->
<dependency>
  <groupId>com.googlecode.concurrentlinkedhashmap</groupId>
  <artifactId>concurrentlinkedhashmap-lru</artifactId>
  <version>1.2_jdk5</version>
</dependency>

Performance

In this benchmark an unbounded ConcurrentHashMap is compared to a ConcurrentLinkedHashMap v1.0 with a maximum size of 5,000 entries under an artificially high load (250 threads, 4-cores).