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An attribute has two parts: a name and a value. For example, "os.name
" is the name for one of the Java platform's system attributes; its value contains the name of the current operating system, such as "Solaris
".The
Properties
class in the
java.util
package manages a set of key/value pairs. A key/value pair is like a dictionary entry: The key is the word, and the value is the definition. This is a perfect match for managing the names and values of attributes. EachProperties
key contains the name of a system attribute, and its correspondingProperties
value is the current value of that attribute.The
System
class uses aProperties
object for managing system properties. Any Java program can use aProperties
object to manage its program attributes. TheProperties
class itself provides methods for the following:
- Loading key/value pairs into a
Properties
object from a stream- Retrieving a value from its key
- Listing the keys and their values
- Enumerating over the keys
- Saving the properties to a stream
Properties
extends theHashtableProperties
class and inherits methods from it for doing the following:
- Testing to see if a particular key or value is in the
Properties
object- Getting the current number of key/value pairs
- Removing a key and its value
- Adding a key/value pair to the
Properties
list- Enumerating over the values or the keys
- Retrieving a value by its key
- Finding out if the
Properties
object is empty
Security Considerations: Access to properties is subject to approval by the current security manager. The example code segments in this section are assumed to be in standalone applications, which, by default, have no security manager. If you attempt to use this code in an applet, it may not work, depending on the browser or viewer in which it is running. See Security Restrictionsfor information about security restrictions on applets.
Properties
The following figure illustrates how a typical program might manage its attributes with aProperties
object over the course of its execution.
- Starting Up
- The actions given in the first three boxes occur when the program is starting up. First, the program loads the default properties from a well-known location into a
Properties
object. Normally, the default properties are stored in a file on disk along with the.class
and other resource files for the program.Next, the program creates another
Properties
object and loads the properties that were saved from the last time the program was run. Many applications store properties on a per-user basis, so the properties loaded in this step are usually in a specific file in a particular directory maintained by this application in the user's home directory. Finally, the program uses the default and remembered properties to initialize itself.The key here is consistency. The application must always load and save properties to the same location so that it can find them the next time it's executed.
- Running
- During the execution of the program, the user may change some settings, perhaps in a Preferences window, and the
Properties
object is updated to reflect these changes. For them to have a permanent effect, they must be saved.- Exiting
- Upon exiting, the program saves the properties to its well-known location, to be loaded again when the program is next started up.
Properties
ObjectThe following Java code performs the first two steps described in the previous section: loading the default properties and loading the remembered properties:First, the application sets up a default. . . // create and load default properties Properties defaultProps = new Properties(); FileInputStream in = new FileInputStream("defaultProperties"); defaultProps.load(in); in.close(); // create program properties with default Properties applicationProps = new Properties(defaultProps); // now load properties from last invocation in = new FileInputStream("appProperties"); applicationProps.load(in); in.close(); . . .Properties
object. This object contains the set of properties to use if values are not explicitly set elsewhere. Then the load method reads the default values from a file on disk nameddefaultProperties
.Next, the application uses a different constructor to create a second
Properties
object,applicationProps
, whose default values are contained indefaultProps
. The defaults come into play when a property is being retrieved. If the property can't be found inapplicationProps
, then its default list is searched.Finally, the code loads a set of properties into
applicationProps
from a file namedappProperties
. The properties in this file are those that were saved from the program the last time it was invoked (the next section shows you how this was done).
Properties
The following example writes out the application properties from the previous example usingProperties
'ssave
method. The default properties don't need to be saved each time because they never change.TheFileOutputStream out = new FileOutputStream("appProperties"); applicationProps.save(out, "---No Comment---"); out.close();save
method needs a stream to write to, as well as a string that it uses as a comment at the top of the output.
Once you've set up yourProperties
object, you can query it for information about various keys/values that it contains. An application gets information from aProperties
object after start up so that it can initialize itself based on choices made by the user. TheProperties
class has several methods for getting property information:
- contains(Object value)
containsKey(Object key)- Returns
true
if the value or the key is in theProperties
object.Properties
inherits these methods fromHashtable
. Thus they acceptObject
arguments. You should pass inString
s.- getProperty(String key)
getProperty(String key, String default)- Returns the value for the specified property. The second version allows you to provide a default value. If the key is not found, the default is returned.
- list(PrintStream s)
list(PrintWriter w)- Writes all of the properties to the specified stream or writer. This is useful for debugging.
- elements()
keys()
propertyNames()- Returns an
Enumeration
containing the keys or values (as indicated by the method name) contained in theProperties
object.- size()
- Returns the current number of key/value pairs.
Properties
A user's interaction with a program during its execution may impact property settings. These changes should be reflected in theProperties
object so that they are saved when the program exits (and calls the save method). You can use the following methods to change the properties in aProperties
object:Both put and remove come from
- put(Object key, Object value)
- Puts the key/value pair in the
Properties
object.- remove(Object key)
- Removes the key/value pair associated with key.
Hashtable
and thus takeObjects
. You should pass inStrings
.
BINGO!, shows and documents a complete client/server application that implements the game of BINGO. Both the client and the server application in that example use
Properties
to maintain program attributes.
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