[Chapter 10] 10.2 System Properties

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| Chapter 10 Accessing the Environment |

## 10.2 System Properties

System properties provide a mechanism for getting information about the environment. You can get the value of a system property by passing its name to the `System.getProperty(String)` method. This method returns the value of the named property as a `String`, or it returns `null` if the property is not defined. Since it is common to assume a default value if a property is not specified, there is also a `System.getProperty(String,` `String)` method that takes the name of a property and a default `String` value to return if the property is not defined.

Table 10.1 lists the standard system properties for a Java environment. Many of these properties are guaranteed to be defined in any Java environment. Note, however, that untrusted applets aren't allowed to access many of these properties.

Table 10.1: Standard System Properties
Property Name | Description
--- | ---
`file.encoding` | The character encoding for the default locale ( Java 1.1 only)
`file.encoding.pkg` | The package that contains the converters that handle converting between local encodings and Unicode ( Java 1.1 only)
`file.separator` | The platform-dependent file separator (e.g., "`/`" on UNIX, "`\`" for Windows)
`java.class.path` | The value of the `CLASSPATH` environment variable
`java.class.version` | The version of the Java API
`java.compiler` | The just-in-time compiler to use, if any. The java interpreter provided with the JDK initializes this property from the environment variable `JAVA_COMPILER`
`java.home` | The directory in which Java is installed
`java.version` | The version of the Java interpreter
`java.vendor` | A vendor-specific string
`java.vendor.url` | A vendor URL
`line.separator` | The platform-dependent line separator (e.g., "`\n`" on UNIX, "`\r\n`" for Windows)
`os.name` | The name of the operating system
`os.arch` | The system architecture
`os.version` | The operating system version
`path.separator` | The platform-dependent path separator (e.g., "`:`" on UNIX, "`,`" for Windows)
`user.dir` | The current working directory when the properties were initialized
`user.home` | The home directory of the current user
`user.language` | The two-letter language code of the default locale ( Java 1.1 only)
`user.name` | The username of the current user
`user.region` | The two-letter country code of the default locale ( Java 1.1 only)
`user.timezone` | The default time zone ( Java 1.1 only)

The Java API also provides some convenience methods for getting the value of a system property that should be interpreted as a data type other than `String`:

- ```Boolean.getBoolean()` returns the value of a named system property as a `boolean`.

- `Color.getColor()` returns a `Color` object that represents the color specified by the value of a named system property interpreted as an RGB value. For example, the value `0xFFFFFF` is white, `0xFF000` is red, `0x00FF00` is green, and `0x0000FF` is blue.

- `Font.getFont()` returns a `Font` object that is mapped to a font in the native windowing system. If the string passed to `getFont()` is `"poster"`, the method uses the value of the system property `awt.font.poster` as the name of the native font.

  By default, the font style is plain and the font size is 12 points. If the font name is prefixed with `bold-`, `italic-` or `bolditalic-`, that style is used instead. If the font name is prefixed with a size and a `-`, that size is used instead. If both style and size are specified, style must come first. For example, passing `"italic-14-timesRoman"` to `getFont()` causes it to return a `Font` object that uses the native font identified by the system property `awt.font.timesRoman`. That font should be an italic, 14-point, TimesRoman font.

- `Integer.getInteger()` returns the value of a named system property as an `int`.

- `Long.getLong()` returns the value of a named system property as a `long`.

  There are two built-in mechanisms for setting system properties. Note that you can use these mechanisms to set the standard system properties or to define specific system properties for your own application.

- You can define properties from the command line of the Java virtual machine using the `-D` command line option. For example, to define a property named `time.server` with the value `tm02`, you can invoke the interpreter like this:

      C:\> java -Dtime.server=tm02

- You can define any number of system properties using `-D`, as long as each property is specified with its own `-D` option.

You can programmatically define properties by calling the `System.setProperties()` method. The `Properties` object that you pass to `System.setProperties()` becomes the new source for all system property values.

If a program is running in a browser or other environment that has a `SecurityManager` installed, it may be denied any access to system properties.

I/O |  | Environment Variables