2016-03-28 03:32 UTC

Freeing MappedByteBuffer

While investigating a production bug that indicated that the operating system of a server running only a JVM continually ran out of memory under a specific set of circumstances, I discovered a very interesting idiosyncrasy of the JVM. Specifically, memory allocated with mmap through the MappedByteBuffer (the super class of ... read more

2015-03-02 02:45 UTC

Dropwizard IT Port Randomisation

Dropwizard's testing package includes a @ClassRule, DropwizardAppRule, that may be used to create end-to-end integration tests. However, you may rapidly discover that parallel test execution is severely impaired, as each test attempts to use the same application and admin ports as another. You can get around this by ... read more

2014-01-11 15:48 UTC

Perceiving The Effects Of Cache Coherency In Spin Locks

The "test-and-test-and-set" algorithm is a fairly well-understood solution to reducing latency in spin locks. In particular, the algorithm is often employed to reduce the latency exhibited in the similar "test-and-set" algorithm. Understanding how and why a seeming-superfluous read results in lower latency is rather interesting. ... read more

2013-07-22 05:01 UTC

Downcasting Longs To Ints On x86

Last week, my esteemed colleague and close friend asked a remarkably straight-forward question about downcasting a long to an int in Java. I'll admit the question caught me off guard. While the JLS offered the correct answer, I couldn't help but ponder what's actually happening in the machine. ... read more

2013-07-22 04:59 UTC

Finally, Rollback Your Transaction

Lately, I've been working with code that exhibits a fairly poor practice: programmatic transactions (namely Spring) don't always get committed or rolled-back, even though the code appears correct. Consider the following contrived scenario: ... read more

2013-05-02 03:40 UTC

Enum Hash Codes Are Non-Deterministic

An annoying discovery: Java Enum objects have non-deterministic hash codes. Specifically, a hash code is the Enum's identity (or memory address). ... read more

2013-03-28 00:51 UTC

No ConcurrentHashSet? No Problem

The JDK provides several concurrent Set classes, each with their own application, but it does not provide a ConcurrentHashSet. Using the Collections utility class, however, such a structure may be created. ... read more

2013-03-18 00:53 UTC

The Concurrency Of ConcurrentHashMap

Java's ConcurrentHashMap is an excellent point of study when learning about Java's Memory Model. It employs numerous techniques to provide lock-free reads while still guaranteeing safe publication of objects. This article represents my attempt to take the magic out of the ConcurrentHashMap implementation. ... read more

2013-01-22 02:54 UTC

Escape Analysis & Stack Allocated Objects

In 1999, a paper, Escape Analysis For Java was to appear in the ACM SIGPLAN Conference on Object-Oriented Programming Systems, Languages, and Applications. This paper outlined an algorithm to detect whether an object, O, escaped from the currently executed method, M, or current thread, T. This paper ... read more

2013-01-02 05:20 UTC

Impact Of Local Variable Declaration Location

Does the location of a local variable's declaration result in performance changes of a method? For example, will a String variable, s, declared outside of a loop exhibit "better" performance than when declared inside a loop? In Java, looking at a class' bytecode explains, irrefutably, why the answer is ... read more