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7.5 Properties

The java.util.Properties class is a specialized hashtable for strings. Java uses the Properties object to replace the environment variables used in other programming environments. You can use a Properties table to hold arbitrary configuration information for an application in an easily accessible format. The Properties object can also load and store information using streams (see Chapter 8, Input/Output Facilities for information on streams).

Any string values can be stored as key/value pairs in a Properties table. However, the convention is to use a dot-separated naming hierarchy to group property names into logical structures, as is done with X resources on UNIX systems.[4] The java.lang.System class provides system-environment information in this way, through a system Properties table I'll describe shortly.

[4] Unfortunately, this is just a naming convention right now, so you can't access logical groups of properties as you can with X resources.

Create an empty Properties table and add String key/value pairs just as with any Hashtable:

Properties props = new Properties(); 
props.put("myApp.xsize", "52"); 
props.put("myApp.ysize", "79"); 

Thereafter, you can retrieve values with the getProperty()method:

String xsize = props.getProperty( "myApp.xsize" ); 

If the named property doesn't exist, getProperty() returns null. You can get an Enumeration of the property names with the propertyNames() method:

for ( Enumeration e = props.propertyNames(); e.hasMoreElements; ) { 
    String name = e.nextElement(); 
    ... 
} 

Default Values

When you create a Properties table, you can specify a second table for default property values:

Properties defaults; 
... 
Properties props = new Properties( defaults ); 

Now when you call getProperty(), the method searches the default table if it doesn't find the named property in the current table. An alternative version of getProperty() also accepts a default value; this value is returned if the property is not found in the current list or in the default list:

String xsize = props.getProperty( "myApp.xsize", "50" ); 

Loading and Storing

You can save a Properties table to an OutputStream using the save() method. The property information is output in flat ASCII format. Continuing with the above example, output the property information to System.out as follows:

props.save( System.out, "Application Parameters" ); 

As we'll discuss in Chapter 8, Input/Output Facilities, System.out is a standard output stream similar to C's stdout. We could also save the information to a file by using a FileOutputStream as the first argument to save(). The second argument to save() is a String that is used as a header for the data. The above code outputs something like the following to System.out:

#Application Parameters 
#Mon Feb 12 09:24:23 CST 1997 
myApp.ysize=79 
myApp.xsize=52 

The load() method reads the previously saved contents of a Properties object from an InputStream:

FileInputStream fin; 
... 
Properties props = new Properties() 
props.load( fin ); 

The list() method is useful for debugging. It prints the contents to an OutputStream in a format that is more human-readable but not retrievable by load().

System Properties

The java.lang.System class provides access to basic system environment information through the static System.getProperty() method. This method returns a Properties table that contains system properties. System properties take the place of environment variables in other programming environments.

Table 7.7 summarizes system properties that are guaranteed to be defined in any Java environment.

Table 7.7: System Properties
System Property Meaning
java.vendor Vendor-specific string
java.vendor.url URL of vendor
java.version Java version
java.home Java installation directory
java.class.version Java class version
java.class.path The class path
os.name Operating-system name
os.arch Operating-system architecture
os.version Operating-system version
file.separator File separator (such as "/" or " \")
path.separator Path separator (such as ":" or ";")
line.separator Line separator (such as "\n" or "\r\n")
user.name User account name
user.home User's home directory
user.dir Current working directory

Applets are, by current Web browser conventions, prevented from reading the following properties: java.home, java.class.path, user.name, user.home, and user.dir. As you'll see in the next section, these restrictions are implemented by a SecurityManager object.


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