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Java performance programming, Part 1: Smart object-management saves the day

Learn how to reduce program overhead and improve performance by controlling object creation and garbage collection

By Dennis M. Sosnoski, JavaWorld.com, 08/20/01

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Objects are a powerful software engineering construct, and Java uses them extensively. In fact, it encourages the use of objects so much that developers sometimes forget the costs behind the construct. The result can be object churn, a program state in which most of your processor time is soaked up by repeatedly creating and then garbage collecting objects.

Java performance programming: Read the whole series!

This is the first in a series of articles focused on performance issues in Java. In this series, we'll examine a number of areas in which Java performance can be less than ideal, and provide techniques for bypassing many of these performance roadblocks. Actual timing measurements will be used throughout to demonstrate the performance improvements possible with the right coding techniques.

This month, we'll take a look at the issue of object management in Java. Since Java is often in competition with C/C++ as a language choice for implementing applications, we'll start out by reviewing the differences in how these languages manage the allocation and deallocation of objects, and examine what impact these differences have on performance. In the remainder of the article, we'll look at three ways to reduce the amount of object churn in your Java programs.

Java memory management

Java's simplified memory management is one of the key features that appeals to developers with backgrounds in languages such as C/C++. In contrast with the explicit deallocation required by C/C++, Java lets you allocate objects as necessary and trust that they'll be reclaimed and recycled by the JVM when they're no longer needed. The work required to make this happen goes on behind the scenes, in the garbage collection process.

Garbage collection has been used for memory management in programming languages dating back to the dawn of the computer era in the 1960s. The basic principle of garbage collection is the same in all cases: identify objects that are no longer in use by the program, and recycle the memory used by these objects to create new ones.

JVMs generally use a reachability analysis to identify the objects that are in use, then recycle all the other objects. This starts with a base set of variables that the program uses directly, including object references in local or argument variables on the method call stack of every active thread, and in static variables of loaded classes. Each object that one of these variables references is added to the set of reachable objects. Next, each object that a member variable references in one of these objects is also added to the reachable set. This process continues until closure is obtained; in the end, every object referenced by any object in the reachable set is also in the reachable set. Any objects that are not in the reachable set are by definition not in use by the program, so they can safely be recycled.

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  • Recent JavaWorld articles covering object pools for resource management