当一个系统的功能越来越强，子系统会越来越多，客户对系统的访问也变得越来越复杂。这时如果系统内部发生改变，客户端也要跟着改变，这违背了“开闭原则”，也违背了“迪米特法则”，所以有必要为多个子系统提供一个统一的接口，从而降低系统的耦合度，这就是外观模式的目标。
外观（Facade）模式的定义：是一种通过为多个复杂的子系统提供一个一致的接口，而使这些子系统更加容易被访问的模式。该模式对外有一个统一接口，外部应用程序不用关心内部子系统的具体的细节，这样会大大降低应用程序的复杂度，提高了程序的可维护性。
这种模式隐藏了更大系统的复杂性，并提供了一个更简单的系统。
在本例中，Facade是（{@link dwarvengoldminefaccade}），它为金矿子系统提供了一个更简单的接口。
public class App { /** * Program entry point. * * @param args command line args */ public static void main(String[] args) { var facade = new DwarvenGoldmineFacade(); facade.startNewDay(); facade.digOutGold(); facade.endDay(); } } public class DwarvenGoldmineFacade { private final List<DwarvenMineWorker> workers; /** * Constructor. */ 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)); } } /** * DwarvenMineWorker is one of the goldmine subsystems. */ public abstract class DwarvenMineWorker { private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenMineWorker.class); public void goToSleep() { LOGGER.info("{} goes to sleep.(去睡觉)", name()); } public void wakeUp() { LOGGER.info("{} wakes up.(醒来)", name()); } public void goHome() { LOGGER.info("{} goes home.（回家）", name()); } public void goToMine() { LOGGER.info("{} goes to the mine.（去矿井）", name()); } private void action(Action action) { switch (action) { case GO_TO_SLEEP: goToSleep(); break; case WAKE_UP: wakeUp(); break; case GO_HOME: goHome(); break; case GO_TO_MINE: goToMine(); break; case WORK: work(); break; default: LOGGER.info("Undefined action（未定义动作）"); break; } } /** * Perform actions. */ 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 } } /** * DwarvenCartOperator is one of the goldmine subsystems. */ public class DwarvenCartOperator extends DwarvenMineWorker { private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenCartOperator.class); @Override public void work() { LOGGER.info("{} moves gold chunks out of the mine.（把黄金块从矿里搬出来）", name()); } @Override public String name() { return "Dwarf cart operator（矮车操作员）"; } } /** * DwarvenGoldDigger is one of the goldmine subsystems. */ public class DwarvenGoldDigger extends DwarvenMineWorker { private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenGoldDigger.class); @Override public void work() { LOGGER.info("{} digs for gold.(掘金)", name()); } @Override public String name() { return "Dwarf gold digger(矮人掘金者)"; } } /** * DwarvenTunnelDigger is one of the goldmine subsystems. */ public class DwarvenTunnelDigger extends DwarvenMineWorker { private static final Logger LOGGER = LoggerFactory.getLogger(DwarvenTunnelDigger.class); @Override public void work() { LOGGER.info("{} creates another promising tunnel.（创造另一条有希望的隧道）", name()); } @Override public String name() { return "Dwarven tunnel digger（矮人隧道挖掘机）"; } }