1 Introduction
One strength of the Java™ 2 Platform, Standard Edition (J2SE™) is that it performs automatic memory
management, thereby shielding the developer from the complexity of explicit memory management.
This paper provides a broad overview of memory management in the Java HotSpot virtual machine (JVM) in
Sun’s J2SE 5.0 release. It describes the garbage collectors available to perform the memory management, and
gives some advice regarding choosing and configuring a collector and setting sizes for the memory areas on
which the collector operates. It also serves as a resource, listing some of the most commonly-used options that
affect garbage collector behavior and providing numerous links to more detailed documentation.
Section 2 is for readers who are new to the concept of automatic memory management. It has a brief discussion
of the benefits of such management versus requiring programmers to explicitly deallocate space for data.
Section 3 then presents an overview of general garbage collection concepts, design choices, and performance
metrics. It also introduces a commonly-used organization of memory into different areas called
generations
based on the expected lifetimes of objects. This separation into generations has proven effective at reducing
garbage collection pause times and overall costs in a wide range of applications.
The rest of the paper provides information specific to the HotSpot JVM. Section 4 describes the four garbage
collectors that are available, including one that is new in J2SE 5.0 update 6, and documents the generational
memory organization they all utilize. For each collector, Section 4 summarizes the types of collection algorithms
used and specifies when it would be appropriate to choose that collector.
Section 5 describes a technique new in the J2SE 5.0 release that combines (1) automatic selection of garbage
collector, heap sizes, and HotSpot JVM (client or server) based on the platform and operating system on which
the application is running, and (2) dynamic garbage collection tuning based on user-specified desired behavior.
This technique is referred to as
ergonomics.
Section 6 provides recommendations for selecting and configuring a garbage collector. It also provides some
advice as to what to do about
OutOfMemoryErrors. Section 7 briefly describes some of the tools that can be
utilized to evaluate garbage collection performance, and Section 8 lists the most commonly-used command line
options that relate to garbage collector selection and behavior. Finally, Section 9 supplies links to more detailed
documentation for the various topics covered by this paper.
2 Explicit vs. Automatic Memory Management
Memory management is the process of recognizing when allocated objects are no longer needed, deallocating
(freeing) the memory used by such objects, and making it available for subsequent allocations. In some
pr
ogr
amming languages
, memory management is the programmer’s responsibility. The complexity of that task
leads to many common errors that can cause unexpected or erroneous program behavior and crashes. As a
result, a large proportion of developer time is often spent debugging and trying to correct such errors.
One problem that often occurs in programs with explicit memory management is
dangling references. It is
possible to deallocate the space used by an object to which some other object still has a reference. If the object
with tha
t (dangling) r
eference tries to access the original object, but the space has been reallocated to a new
object, the result is unpredictable and not what was intended.
Ano
ther common pr
oblem with explicit memory management is
space l
eaks
. These l
eaks occur when memory is
allocated and no longer referenced but is not released. For example, if you intend to free the space utilized by a
linked list but you make the mistake of just deallocating the first element of the list, the remaining list elements
ar
e no longer r
eferenced but they go out of the program’s reach and can neither be used nor recovered. If
enough leaks occur, they can keep consuming memory until all available memory is exhausted.
An alterna
te approach to memory management that is now commonly utilized, especially by most modern
object-oriented languages, is automatic management by a program called a
garbage collector. Automatic
memory management enables increased abstraction of interfaces and more reliable code.
Sun Microsystems, Inc.
3
Intr
oduction
