Facade

About 3 min

Intent

Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

Explanation

Real-world example

How does a goldmine work? "Well, the miners go down there and dig gold!" you say. That is what you believe because you are using a simple interface that goldmine provides on the outside, internally it has to do a lot of stuff to make it happen. This simple interface to the complex subsystem is a facade.

In plain words

Facade pattern provides a simplified interface to a complex subsystem.

Wikipedia says

A facade is an object that provides a simplified interface to a larger body of code, such as a class library.

Programmatic Example

Let's take our goldmine example from above. Here we have the dwarven mine worker hierarchy. First, there's a base class DwarvenMineWorker:


@Slf4j
public abstract class DwarvenMineWorker {

    public void goToSleep() {
        LOGGER.info("{} goes to sleep.", name());
    }

    public void wakeUp() {
        LOGGER.info("{} wakes up.", name());
    }

    public void goHome() {
        LOGGER.info("{} goes home.", name());
    }

    public void goToMine() {
        LOGGER.info("{} goes to the mine.", name());
    }

    private void action(Action action) {
        switch (action) {
            case GO_TO_SLEEP -> goToSleep();
            case WAKE_UP -> wakeUp();
            case GO_HOME -> goHome();
            case GO_TO_MINE -> goToMine();
            case WORK -> work();
            default -> LOGGER.info("Undefined action");
        }
    }

    public void action(Action... actions) {
        Arrays.stream(actions).forEach(this::action);
    }

    public abstract void work();

    public abstract String name();

    enum Action {
        GO_TO_SLEEP, WAKE_UP, GO_HOME, GO_TO_MINE, WORK
    }
}

Then we have the concrete dwarf classes DwarvenTunnelDigger, DwarvenGoldDigger and DwarvenCartOperator:


@Slf4j
public class DwarvenTunnelDigger extends DwarvenMineWorker {

    @Override
    public void work() {
        LOGGER.info("{} creates another promising tunnel.", name());
    }

    @Override
    public String name() {
        return "Dwarven tunnel digger";
    }
}

@Slf4j
public class DwarvenGoldDigger extends DwarvenMineWorker {

    @Override
    public void work() {
        LOGGER.info("{} digs for gold.", name());
    }

    @Override
    public String name() {
        return "Dwarf gold digger";
    }
}

@Slf4j
public class DwarvenCartOperator extends DwarvenMineWorker {

    @Override
    public void work() {
        LOGGER.info("{} moves gold chunks out of the mine.", name());
    }

    @Override
    public String name() {
        return "Dwarf cart operator";
    }
}

To operate all these goldmine workers we have the DwarvenGoldmineFacade:

public class DwarvenGoldmineFacade {

    private final List<DwarvenMineWorker> workers;

    public DwarvenGoldmineFacade() {
        workers = List.of(
                new DwarvenGoldDigger(),
                new DwarvenCartOperator(),
                new DwarvenTunnelDigger());
    }

    public void startNewDay() {
        makeActions(workers, DwarvenMineWorker.Action.WAKE_UP, DwarvenMineWorker.Action.GO_TO_MINE);
    }

    public void digOutGold() {
        makeActions(workers, DwarvenMineWorker.Action.WORK);
    }

    public void endDay() {
        makeActions(workers, DwarvenMineWorker.Action.GO_HOME, DwarvenMineWorker.Action.GO_TO_SLEEP);
    }

    private static void makeActions(Collection<DwarvenMineWorker> workers,
                                    DwarvenMineWorker.Action... actions) {
        workers.forEach(worker -> worker.action(actions));
    }
}

Now let's use the facade:

var facade = new DwarvenGoldmineFacade();
facade.startNewDay();
facade.digOutGold();
facade.endDay();

Program output:

// Dwarf gold digger wakes up.
// Dwarf gold digger goes to the mine.
// Dwarf cart operator wakes up.
// Dwarf cart operator goes to the mine.
// Dwarven tunnel digger wakes up.
// Dwarven tunnel digger goes to the mine.
// Dwarf gold digger digs for gold.
// Dwarf cart operator moves gold chunks out of the mine.
// Dwarven tunnel digger creates another promising tunnel.
// Dwarf gold digger goes home.
// Dwarf gold digger goes to sleep.
// Dwarf cart operator goes home.
// Dwarf cart operator goes to sleep.
// Dwarven tunnel digger goes home.
// Dwarven tunnel digger goes to sleep.

Class diagram

Applicability

Use the Facade pattern when

You want to provide a simple interface to a complex subsystem.
Subsystems are getting more complex and depend on multiple classes, but most clients only need a part of the functionality.
There is a need to layer your subsystems. Use a facade to define an entry point to each subsystem level.

Tutorials

Known Uses

Java libraries such as java.net.URL and javax.faces.context.FacesContext use Facade to simplify complex underlying classes.
In many Java frameworks, facades are used to simplify the usage of APIs by providing a simpler interface to more complex underlying code structures.

Consequences

Benefits:

Isolates clients from subsystem components, making it easier to use and reducing dependencies.
Promotes weak coupling between the subsystem and its clients.
Often simplifies the API of complex systems.

Trade-offs:

A facade can become a god object coupled to all classes of an app if not implemented correctly.

Often used with other design patterns like Singleton and Abstract Factory.
Command pattern can use Facade to define an interface that simplifies methods invocation.