Advantages of Preferences

• Your app does not need to locate disk space to store them. Java does that for you.
• Your app does not need a mechanism to find the data again on disk. Java does that for you.
• You can store complex hierarchical configuration data with it.
• It is designed so your names won’t collide with anyone else’s.

Disadvantages of Preferences

• The preferences mechanism cannot be counted on to preserve the order of your data.
• Though you can embed dots in the key names to create a tree structure, there is no support for arrays or keys with multiple values. If you have unique values in your array, you can use them as keys. You can, however, get a list of all keys under a node with keys(), You might use key/value pairs like this
  The problem with that scheme is insert and delete would be clumsy since you would have to renumber all the keys and resave them, after removing the old keys. The lines are limited to 8192, so you are limited to the number of elements with the composite technique.

  Another technique would be to serialize with a ByteArrayOutputStream ⇒ ObjectOutputStream and use putByteArray. The problem with that method is the registry would be unintelligible (it is encoded in Base64) and you could not do manual patching. The other catch is keys are limited to 80 bytes and values to a mere 8K.

• The registry is fragile. It is hard to back up and restore. You can’t restore at all if it is badly damaged. There is good chance you will eventually lose everything. the Preferences class has methods to import and export the preferences to *.xml files.

Use

systemNodeForPackage lets you specify Preferences for the whole machine for each package. userNodeForPackage lets you specify Preferences for a particular user for that package. If the package is used in more than one context, e.g. the same class used in two different programs or the same class is used to manage two separate sets of files, it is up to you to create another level of name to keep track of the context. Note though you specify a class name, the Preferences are on a per package, not a per class basis.

The registry will be more legible with regedit if you stick to lower case keys and values since upper case values are encoded in a bizarre fluffy way.

Location

• In W98/Me/NT/W2K/XP/W2K3/Vista this information is stored in the fragile, hard-to-back-up registry in HKEY_LOCAL_MACHINE\JavaSoft\Prefs for system Preferences and HKEY_CURRENT_USER\JavaSoft\Prefs for user Preferences in a very fluffy format. Every capital letter is preceded with a / and any fields containing accented letters are encoded in Base64.
• In Windows, user Preferences show up at HKEY_CURRENT_USER\Software\JavaSoft\Prefs\com\mindprod\replicator and HKEY_USERS\usernamexxx\Software\JavaSoft\Prefs\com\mindprod\replicator where the package name is com.mindprod.replicator.
• In Windows, system Preferences show up at HKEY_LOCAL_MACHINE\Software\JavaSoft\Prefs\com\mindprod\replicator, where the package name is com.mindprod.replicator
• In Linux, preferences are stored in ordinary xml files. System Preferences are stored in etc/.java.
• In Linux, user preferences are stored in ~/.java. The file for user preferences may have a goofy base64-encoded name something like this:

  /home/username/.java/.userPrefs/com/mindprod/replicator/_!':!bw "t!#4!b@"p!'4!~!"w!()!bw"k!#4!cg"l!(!!b!"p!'}@"0!'8!cg==

  The contents of the file will look something like this:

  <?xml version="1.0" encoding="UTF-8" standalone="no"?>
  <!DOCTYPE map SYSTEM "http://java.sun.com/dtd/preferences.dtd">
  <map MAP_XML_VERSION="1.0">
    <entry key="LAN_ZIP_URL" value="http://mindprod.com/replicator"/>
    <entry key="PASSWORD" value=""/>
    <entry key="PREFS_EXIST" value="true"/>
    <entry key="RECEIVER_BASE_DIR" value="/home/Mindprod"/>
    <entry key="RECEIVER_ZIP_STAGING_DIR" value="/home/MPstaging"/>
    <entry key="USERID" value=""/>
  </map>

• In theory, a JVM could hide them in an SQL database or any other scheme it concocted.

Tips

• Remember to call flush when you have finished changing your Preferences to ensure they get written to backing store.

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