Java – how to improve this singleton?

I have a class that will be single

MySingleton. getInstance(theFile);

Files are required only when building a singleton for the first time After that, that is, once the singleton is built, I don't need to pass the file What should I do? I want to create a mysingleton getInstance(); But this still doesn't work because the caller must call mysingleton getInstance(theFile); Build a valid class for the first time How can I design it better?

Solution

Declare an init () method that uses this file to handle initialization

Simplify getInstance () to return an instance, but throw IllegalStateException if init () has not been called

For example:

public class MySingleton {

    private MySingleton INSTANCE;

    // private constructor is best practice for a singleton 
    private MySingleton(File theFile) {
        // initialize class using "theFile"
    }

    public static void init(File theFile) {
        // if init prevIoUsly called,throw IllegalStateException
        if (INSTANCE != null)
            throw new IllegalStateException();
        // initialize singleton 
        INSTANCE = new MySingleton(theFile);
    }

    public static MySingleton getInstance() {
        // if init hasn't been called yet,throw IllegalStateException
        if (INSTANCE == null)
            throw new IllegalStateException();
        return INSTANCE;
    }

    // rest of class
}

Note that although this is not thread safe, as long as init () is called as early as possible during server startup, there are few race conditions (if any)