package singleton;

public class AppConfigDCL {

    // volatile is MANDATORY for DCL to work correctly.
    // Without it, the JVM can reorder the writes inside the constructor
    // and return a partially-constructed object to a second thread.
    private static volatile AppConfigDCL instance;

    private AppConfigDCL() { /* load config */ }

    public static AppConfigDCL getInstance() {

        // First check: no lock, instant return for the 99.99% case
        // (instance already created, just return it)
        if (instance == null) {

            // Lock only when first check says we might need to create
            synchronized (AppConfigDCL.class) {

                // Second check: re-verify under the lock.
                // Another thread might have created the instance between
                // our first check and acquiring the lock.
                if (instance == null) {
                    instance = new AppConfigDCL();
                }
            }
        }
        return instance;
    }
}