System Properties
In Properties, we examined the way an application can use
The following table describes some of the most important system properties
Security consideration: Access to system properties can be restricted by the Security Manager. This is most often an issue in applets, which are prevented from reading some system properties, and from writing any system properties. For more on accessing system properties in applets, refer to System Properties in the Doing More With Java Rich Internet Applications lesson.
The
The
The other version of
The last method provided by the
Warning: Changing system properties is potentially dangerous and should be done with discretion. Many system properties are not reread after start-up and are there for informational purposes. Changing some properties may have unexpected side-effects.
The next example,
Note how
This statement initializes the new properties object,
Also note that the value of system properties can be overwritten! For example, if
In general, be careful not to overwrite system properties.
The
Properties
objects to maintain its configuration. The Java platform itself uses a Properties
object to maintain its own configuration. The System
class maintains a Properties
object that describes the configuration of the current working environment. System properties include information about the current user, the current version of the Java runtime, and the character used to separate components of a file path name.The following table describes some of the most important system properties
Key | Meaning |
---|---|
"file.separator" | Character that separates components of a file path. This is "/ " on UNIX and "\ " on Windows. |
"java.class.path" | Path used to find directories and JAR archives containing class files. Elements of the class path are separated by a platform-specific character specified in the path.separator property. |
"java.home" | Installation directory for Java Runtime Environment (JRE) |
"java.vendor" | JRE vendor name |
"java.vendor.url" | JRE vendor URL |
"java.version" | JRE version number |
"line.separator" | Sequence used by operating system to separate lines in text files |
"os.arch" | Operating system architecture |
"os.name" | Operating system name |
"os.version" | Operating system version |
"path.separator" | Path separator character used in java.class.path |
"user.dir" | User working directory |
"user.home" | User home directory |
"user.name" | User account name |
Security consideration: Access to system properties can be restricted by the Security Manager. This is most often an issue in applets, which are prevented from reading some system properties, and from writing any system properties. For more on accessing system properties in applets, refer to System Properties in the Doing More With Java Rich Internet Applications lesson.
Reading System Properties
TheSystem
class has two methods used to read system properties: getProperty
and getProperties
.The
System
class has two different versions of getProperty
. Both retrieve the value of the property named in the argument list. The simpler of the two getProperty
methods takes a single argument, a property key For example, to get the value of path.separator
, use the following statement:System.getProperty("path.separator");
getProperty
method returns a string containing the value of the property. If the property does not exist, this version of getProperty
returns null.The other version of
getProperty
requires two String
arguments: the first argument is the key to look up and the second argument is a default value to return if the key cannot be found or if it has no value. For example, the following invocation of getProperty
looks up the System
property called subliminal.message
. This is not a valid system property, so instead of returning null, this method returns the default value provided as a second argument: "Buy StayPuft Marshmallows!
"System.getProperty("subliminal.message",
"Buy StayPuft Marshmallows!");
System
class to access property values is the getProperties
method, which returns a Properties
object. This object contains a complete set of system property definitions.Writing System Properties
To modify the existing set of system properties, useSystem.setProperties
. This method takes a Properties
object that has been initialized to contain the properties to be set. This method replaces the entire set of system properties with the new set represented by the Properties
object.Warning: Changing system properties is potentially dangerous and should be done with discretion. Many system properties are not reread after start-up and are there for informational purposes. Changing some properties may have unexpected side-effects.
PropertiesTest
, creates a Properties
object and initializes it from myProperties.txt
.subliminal.message=Buy StayPuft Marshmallows!
PropertiesTest
then uses System.setProperties
to install the new Properties
objects as the current set of system properties.import java.io.FileInputStream;
import java.util.Properties;
public class PropertiesTest {
public static void main(String[] args)
throws Exception {
// set up new properties object
// from file "myProperties.txt"
FileInputStream propFile =
new FileInputStream( "myProperties.txt");
Properties p =
new Properties(System.getProperties());
p.load(propFile);
// set the system properties
System.setProperties(p);
// display new properties
System.getProperties().list(System.out);
}
}
PropertiesTest
creates the Properties
object, p
, which is used as the argument to setProperties
:Properties p =
new Properties(System.getProperties());
p
, with the current set of system properties, which in the case of this small application, is the set of properties initialized by the runtime system. Then the application loads additional properties into p
from the file myProperties.txt
and sets the system properties to p
. This has the effect of adding the properties listed in myProperties.txt
to the set of properties created by the runtime system at startup. Note that an application can create p
without any default Properties
object, like this:Properties p = new Properties();
myProperties.txt
contains the following line, the java.vendor
system property will be overwritten:java.vendor=Acme Software Company
The
setProperties
method changes the set of system properties for the current running application. These changes are not persistent. That is, changing the system properties within an application will not affect future invocations of the Java interpreter for this or any other application. The runtime system re-initializes the system properties each time its starts up. If changes to system properties are to be persistent, then the application must write the values to some file before exiting and read them in again upon startup.