Java theory and practice: Building a better HashMap. In July's installment of Java theory and practice ("Concurrent collections classes"), we reviewed scalability bottlenecks and discussed how to achieve higher concurrency and throughput in shared data structures.
Sometimes, the best way to learn is to examine the work of the experts, so this month we're going to look at the implementation of ConcurrentHashMap from Doug Lea's util.concurrent package. A version of ConcurrentHashMap optimized for the new Java Memory Model (JMM), which is being specified by JSR 133, will be included in the java.util.concurrent package in JDK 1.5; the version in util.concurrent has been audited for thread-safety under both the old and new memory models. Optimizing for throughput ConcurrentHashMap uses several tricks to achieve a high level of concurrency and avoid locking, including using multiple write locks for different hash buckets and exploiting the uncertainties of the JMM to minimize the time that locks are held -- or avoid acquiring locks at all.
Java theory and practice: Concurrent collections classes. The first associative collection class to appear in the Java class library was Hashtable, which was part of JDK 1.0.
Hashtable provided an easy-to-use, thread-safe, associative map capability, and it was certainly convenient. However, the thread-safety came at a price -- all methods of Hashtable were synchronized. At that time, uncontended synchronization had a measurable performance cost. The successor to Hashtable, HashMap, which appeared as part of the Collections framework in JDK 1.2, addressed thread-safety by providing an unsynchronized base class and a synchronized wrapper, Collections.synchronizedMap. Separating the base functionality from the thread-safety Collections.synchronizedMap allowed users who needed synchronization to have it, but users who didn't need it didn't have to pay for it. Java theory and practice: To mutate or not to mutate? An immutable object is one whose externally visible state cannot change after it is instantiated.
The String, Integer, and BigDecimal classes in the Java class library are examples of immutable objects -- they represent a single value that cannot change over the lifetime of the object. Benefits of immutability.