Java Pocket Guide

Chapter 15. Memory Management

Java has automatic memory management, which is also known as garbage collection (GC). GC’s principal tasks are allocating memory, maintaining referenced objects in memory, and recovering memory from objects that no longer have references to them.

Garbage Collectors

Since the J2SE 5.0 release, the Java HotSpot Virtual Machine performs self-tuning. This process includes the attempted best-fit GC and related settings at startup, based on platform information, as well as ongoing GC tuning.

Although the initial settings and runtime tuning for GC are generally successful, there are times when you may wish to change or tune your GC based on the following goals:

Maximum pause time goal

The maximum pause time goal is the desired time that the GC pauses the application to recover memory.

Throughput goal

The throughput goal is the desired application time, or the time spent outside of GC.

The following is a list of garbage collectors, their main focus, and situations in which they should be used.

Serial Collector

The serial collector is performed via a single thread on a single CPU. When this GC thread is run, the execution of the application will pause until the collection is complete.

This collection is best used when your application has a small data set up to approximately 100 MB and does not have a requirement for low pause times.

Parallel Collector

The parallel collector, also known as the throughput collector, can be performed with multiple threads across several ...