Trail:
Essential Java Classes
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Accessing System Resources
Sometimes, a program requires access to system resources such as system
properties, standard input and output, or the current time. Your
program could make system calls directly to the window or operating
system, but then your program would be able to run only in that
particular environment. Each time you want to run the program in a new
environment, you'd have to port your program by rewriting the
system-dependent sections of code.
The Java platform lets your program access system resources through a
(relatively) system-independent API implemented by the
System class and
through a system-dependent API implemented by the
Runtime class.
Purity Tip:
Some of the system resources available through the System
and Runtime classes cannot be used in 100% Pure Java programs.
These resources are noted throughout this lesson.
Most system programming needs are met through the System class.
However, in rare cases, a program might have to access the system
through the Runtime object that represents the current runtime
environment. The last section of this lesson, The
Runtime Object
explains how to do this and talks about the trade-offs of
accessing the system directly via the Runtime object.
The following diagram shows that the System
class allows your Java programs to use system resources
but insulates them from system-specific details.
If you've experimented with other lessons in this tutorial, you've no
doubt already seen the System class's
standard output stream used in
several examples to display text. This and other resources available
through System are briefly described here and
covered in the sections indicated.
All of System's methods and variables
are class methods and class variables.
You don't instantiate the System class to use it;
you use the System class's
methods and variables directly from a reference
to the System class.
Probably the most frequently used items from the System
class are the streams used for reading and writing text. The
System class provides
one stream for reading text--the standard input stream--and two streams
for writing text--the standard output and standard error streams.
Properties are key/value pairs that your Java programs can use to set
up various attributes or parameters between invocations. The Java
platform itself maintains a set of system properties that contain
information about the current platform. You can access the system
properties through the System class.
In Java, you don't have to free an object when you're done with it-the
garbage collector runs periodically in the background and cleans up
unreferenced objects. Or you can force the garbage collector to run
using System's gc method. Also, you can
request that the runtime system
perform object finalization using System's
runFinalization method.
The security manager is an application-wide object that determines
whether potentially threatening operations should be allowed. You use
the System class to set and get the security manager for an
application. Subclasses of java.lang.SecurityManager
implement a specific management policy.
The System class includes several miscellaneous methods
that let you get the current time in milliseconds, exit the
interpreter, and copy arrays.
Most system programming needs are met through the programming interface
provided by the System class.
However, in rare cases, a program must
bypass the system-independent interface
of the System class and use system
resources directly from the runtime environment.
The Java environment provides a
Runtimeobject, which represents the current runtime environment.
You can use a Runtime object
to access system resources directly.
Note:
Messaging the Runtime object directly compromises
your ability to run your program on different systems.
You should do this only in special situations.
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