Java setProperty（）在运行时不起作用

Question

I want to implement a Preferences migration from default (Windows HKCU) to my own (filesystem). Everything is working apart but doesn't work together:

public static void main(String[] args){

try {

    for(String s : Preferences.systemNodeForPackage(MyPreferences.class).node(myNode).keys()){
        sysHashMap.put(s, Preferences.systemNodeForPackage(MyPreferences.class).node(myNode).get(s, null));
        logger.info("moving key " + s + " from system");
    }
} catch (BackingStoreException ex) {
    java.util.logging.Logger.getLogger(PreferenceManager.class.getName()).log(Level.SEVERE, null, ex);
}

//then I want to switch to my own implemented preferences 
System.setProperty("java.util.prefs.PreferencesFactory", FilePreferencesFactory.class.getName());
//here if I print System.getProperty - I see that property is set to what I want
AppPreferences myNewPreferences = new Preferences();
myNewPreferences.addEntry("key1", "value1");//sets the value to the registry - i.e. old implementation
}


}

If I coment out the "for" loop - everything works as expected, the key-value is written to the file, specified in FilePreferencesFactory. Why I cannot switch the Preferences Implementation on-the-fly ? Is it possible? How can I fix my code? Thanks in advance. Any question will be answered.

Answer 1

An old question, but for anyone interested:

Your custom preferences factory is only used when you set the system property java.util.prefs.PreferencesFactory before calling systemRoot() or userRoot() (or, in your case executing them indirectly via systemNodeForPackage() ) .

Otherwise the factory class to be used is already set to a default value and it can only be set once.

(See the initialisation code for the factory in the java.util.prefs.Preferences class).