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Java - how to make a library I made use config values

Long story short, I'm making a java library that requires several 'config values'. Currently, I'm just using a config.properties file in the root directory and reading from it, but I don't see how that can work with a distributable jar file.
I've thought about making these config values parameters to a constructor of a class in the library, but there are too many values- it just doesn't seem like the correct way of doing things.
Essentially, I just need some way that a user of my library can just use the jar file, but also have the ability to change several configuration values that affect the function of the library.
If it makes any difference, I'm using maven to build my project.
Thanks in advance.

(Assuming you are just working in JavaSE, as Java EE has other configuration mechanisms.)
A pattern is to create a singleton class in your jar that provides configuration to the other classes. Which is reads default values from the property file in the jar. Allow the caller of the jar to override properties by setting them as system properties.
In the java doc for Property class there is a constructor to provide defaults and overrides and get the 'net' properties.
Giving the caller the option specify a property file by giving a file path as a system property.
Log4j and java.util.logging work of like this. Reading through there config documentation will help explain.

I will split your question into two parts and then answer each part separately.
I've thought about making these config values parameters to a constructor of a class in the library, but there are too many values- it just doesn't seem like the correct way of doing things.
For the part of your question I have quoted above, see the accepted answer (written by me) to the following StackOverflow question: How to handle large number of configuration parameters across a program conceptually?
Essentially, I just need some way that a user of my library can just use the jar file, but also have the ability to change several configuration values that affect the function of the library.
It sounds like your configuration file will contain N variables, and a user might want to customize the values of, say, just 2 or 3 of those variables. If N is relatively small, then it will probably be okay to use an either-or approach to configuration:
either the user provides a configuration file containing values for all N variables (the location of this file might be specified via, say, a system property, an environment variable, or a parameter to the constructor of your library);
or your library uses a "built-in" set of default configuration values, which I suggest you place in a properties file that you bundle into your library's jar file and then access by calling ClassLoader.getSystemResourceAsStream().
However, if N is large, then you might want your library to provide and semantics for configuration variables:
your library uses a "built-in" set of default configuration values, which I suggest you place in a properties file that you bundle into your library's jar file and then access by calling ClassLoader.getSystemResourceAsStream(). This provides default values for all N variables.
and your library loads an (optional) user-specified configuration file that might contain anywhere between 0 and N variables. The values of the user-specified configuration variables are used to override the built-in default values.
Obviously, you will need to implement the and logic within your library, but that is really just a SMOP (Simple Matter of Programming). Perhaps the simplest way is to iterate over the entries in the user-provided Properties object and copy them into the Properties object that contains the default values. In this way, the user-specified value will override default values.

A "Resource bundle" for persisting other then localization data

I need system similar to ResourceBundles except that I will not use it for localization. Basically, I need to store several versions of settings. One version of that settings is String-to-String map, that can be represented by Properties file. These settings versions must be easily persisted to file system inside application .jar (alike PropertyResourceBundle).
The idea is to have different versions of application settings (settings profiles), represented by key-value pairs of type string, that can be chosen from at application start up based of user decision. Again these are not language versions so ResourceBundle (according to its javadoc) is not the right way to implement it.
Any easy way how to do that without implementing the whole think myself? Please do not suggest third-party it should only use Java SE classes.
Edit: I forgot to mention one important detail. It would be hard for me to get stream like this: Thread.currentThread().getContextClassLoader().getResourceAsStream("/your/resource/here");. That is because the project that would contain properties file is compiled by Ant and used as a dynamically loaded library in different GUI projects that actually run. I might have all properties files in fixed project folder but since Thread.currentThread().getContextClassLoader() returns context of GUI project and I do not know where in that project was the .jar with property file placed by Ant I do not know what to use as "/your/resource/here".

I might have misunderstood the question however seems to me you can easily do something like this:
InputStream inputStream = Thread.currentThread().getContextClassLoader().getResourceAsStream("/your/resource/here");
Properties prop = new Properties();
prop.load(inputStream);
This will be safe in a Java EE environment as well and you can call you anytime you need if you want settings to change.
Update: as long as the resource is on the classpath you should be able to find it without knowing the full path of the resource as well.

Where to place a writable properties file in a java webapp?

I'm using a properties file to store configuration information for a webapp. I'd like to allow the user to set certain values via the webapp. I'm just wondering where I should place it?

I see this is tagged with Spring. Have you looked at the PropertyPlaceHolderConfigurer ?
This can be used to specify multiple property files used to expand properties in the Spring configs. You can use this to (say) specify a properties file on the classpath (in your app bundle) and an optional overriding properties file in (say) the user's home dir.
That way you can provide defaults in the app, and override using optional properties files in the user's home directory, the /etc/ filesystem etc. This is what I typically do, providing locations for different prperties files that are user-editable (in /home) and admin editable (in /etc).
I appreciate that this doesn't quite answer your question, but gives you new options for what you really want to achieve.

If it is project setting conf file, I would put it to ${user.home}/app/app-conf.properties
platform independent & uniform and easily accessible place

List all available ResourceBundle Files

i am using ResourceBundle and I want to give the user an option to select a language for the GUI.
i want to get a list of all resource files that are under a specific package.
i don't know what resources I will have since this application is based on plug-ins.
Is there an option to ask from java to search all available resources under a package?
(if no i guess the plug-in should give all available local for it)
thank you all

The files may reside on a web server. There is no standard way of listing files (pages) on a web server. So, in general, what you need to do is remember which locales you have resources for (perhaps create a list as part of your build process).

Possible ways to abstract away hard-coded path-names for properties file in java app

I'm trying to bind in a third party app to our project but have discovered that the unix paths of their property files have been hard-coded into several classes. It is probably possible to make minimal changes to their setup, so my question is: what are the possible (and quickest) ways to achieve this? The third party app uses neither ant nor spring, but just a build script that compiles the code into a .jar that is called from the command line.

You could read the properties files as resources. Then you only have to put the directories containing these files into your classpath.

You haven't actually said what you want to achieve. Would your application determine some paths and pass them to third party app via an API? Would something in your environment (for example a command line argument) specify the location of the files?
I would first refactor their code so that I know for certain that any hard coded strings are held in one defined place. So if for example they have
readProperties("/usr/thing/propertyFileXxx.txt");
or
static final String PROPERTY_XXX = "/usr/thing/propertyFileXxx.txt";
readProperties(PROPERTY_XXX);
I would first consolidate to a single accessor for the properties
readProperties(PROPERTY_XXX_ENUM);
So now we have a well-defined single piece of code that determines where to obtain the properties of each type and a specific list of the types of properties.
Then we need some controllable way to define the set of property files to be used at run-time. I have used the idea suggested by #tangens of loading the properties as resourcees from a specific directory added to the classpath.