What are singletons used for in Java applications?

Singletons are classes which can have no more than one object. They're most useful for storing global state across a system.

Some situations where one might use a singleton include:

1. A system-wide "global value," that many parts of the system may need to access—e.g. the software's license number. Some software requires a valid "license" in order to run. Such software might want to make the current license available to different parts of the software system while it's running. A singleton is a good place to store that information, since there's only ever one correct answer to the question "what license are we using?"
2. Logging. You might want different loggers with different configurations. For example, you might want a "loud" logger that emails exceptions back to the software maintainer, to alert her of crucial issues, as well as a "quiet" logger that simply logs errors to a file on the user's system. Your software might have several components (read: several classes) that want to use the loud logger (e.g. payment-related stuff) and several components that want to use the quiet logger (e.g. caching systems—if the cache fails the system might still run correctly, just more slowly). java.util.logging.LogManager manages a set of individual loggers which are singletons. You can access them by name with getLogger(), and you can add new ones with addLogger().

Singletons are contentious these days. Many people believe they should be avoided, or at least be used less often than they generally are.

Even so, implementing a singleton is an interesting coding challenge.

Suppose we wanted a singleton called InstallationDetails that stored some information, including the licenseNumber. How would we implement this?

We have several options. The first is lazy: have the class get or create its instance just in time, as it's requested:

public final class InstallationDetails {
    private static InstallationDetails INSTANCE = null;

    private long licenseNumber;

    public long getLicenseNumber() {
        return licenseNumber;
    }

    // by making the constructor private, we prevent instantiation
    private InstallationDetails() {
        this.licenseNumber = ... ;
    }

    public static InstallationDetails getInstance() {
        if(INSTANCE == null) {
            INSTANCE = new InstallationDetails();
        }
        return INSTANCE;
    }
}

Java

To make this thread-safe:

public final class InstallationDetails {
    private static volatile InstallationDetails INSTANCE = null;

    private long licenseNumber;

    public long getLicenseNumber() {
        return licenseNumber;
    }

    // by making the constructor private, we prevent instantiation
    private InstallationDetails() {
        this.licenseNumber = ... ;
    }

    public static InstallationDetails getInstance() {
        if(INSTANCE == null) {
            synchronized (InstallationDetails.class) {
                if(INSTANCE == null) {
                    INSTANCE = new InstallationDetails();
                }
            }
        }
        return INSTANCE;
    }
}

Java

Another is to eagerly have the class instantiate its singleton object even before one is requested:

public final class InstallationDetails {
    private static final InstallationDetails INSTANCE = new InstallationDetails();

    private long licenseNumber;

    public long getLicenseNumber() {
        return licenseNumber;
    }

    // by making the constructor private, we prevent instantiation
    private InstallationDetails() {
        this.licenseNumber = ... ;
    }

    public static InstallationDetails getInstance() {
        return INSTANCE;
    }
}

Java

There's also the initialization-on-demand way:

public final class InstallationDetails {

    private long licenseNumber;

    public long getLicenseNumber() {
        return licenseNumber;
    }

    // by making the constructor private, we prevent instantiation
    private InstallationDetails() {
        this.licenseNumber = ... ;
    }

    private static class InstallationDetailsHolder {
        private static final InstallationDetails INSTANCE = new InstallationDetails();
    }

    public static InstallationDetails getInstance() {
        return InstallationDetailsHolder.INSTANCE;
    }
}

Java