🔍目的
止步模式用于防止对象在不完整或不合适的状态下执行某些代码。
🔍解释
真实世界例子
洗衣机中有一个开始按钮，用于启动衣物洗涤。当洗衣机处于非活动状态时，按钮将按预期工作，但是如果已经在洗涤，则按钮将不起任何作用。
通俗描述
使用止步模式，仅当对象处于特定状态时才执行特定代码。
维基百科
禁止模式是一种软件设计模式，仅当对象处于特定状态时才对对象执行操作。例如，一个对象读取zip压缩文件并在压缩文件没打开的时候调用get方法，对象将在请求的时候”止步“。
程序示例
洗衣机是一个具有两个状态的对象，可以处于两种状态：ENABLED和WASHING。 如果机器已启用，则使用线程安全方法将状态更改为WASHING。 另一方面，如果已经进行了清洗并且任何其他线程执行wash（），则它将不执行该操作，而是不执行任何操作而返回。
创建洗衣机类相关部分
@Slf4j public class WashingMachine { private final DelayProvider delayProvider; private WashingMachineState washingMachineState; public WashingMachine(DelayProvider delayProvider) { this.delayProvider = delayProvider; this.washingMachineState = WashingMachineState.ENABLED; } public WashingMachineState getWashingMachineState() { return washingMachineState; } public void wash() { synchronized (this) { var machineState = getWashingMachineState(); LOGGER.info("{}: Actual machine state: {}", Thread.currentThread().getName(), machineState); if (this.washingMachineState == WashingMachineState.WASHING) { LOGGER.error("Cannot wash if the machine has been already washing!"); return; } this.washingMachineState = WashingMachineState.WASHING; } LOGGER.info("{}: Doing the washing", Thread.currentThread().getName()); this.delayProvider.executeAfterDelay(50, TimeUnit.MILLISECONDS, this::endOfWashing); } public synchronized void endOfWashing() { washingMachineState = WashingMachineState.ENABLED; LOGGER.info("{}: Washing completed.", Thread.currentThread().getId()); } }
这里是一个WashingMachine所使用的DelayProvider简单接口。
public interface DelayProvider { void executeAfterDelay(long interval, TimeUnit timeUnit, Runnable task); }
使用WashingMachine
public static void main(String... args) { final var washingMachine = new WashingMachine(); var executorService = Executors.newFixedThreadPool(3); for (int i = 0; i < 3; i++) { executorService.execute(washingMachine::wash); } executorService.shutdown(); try { executorService.awaitTermination(10, TimeUnit.SECONDS); } catch (InterruptedException ie) { LOGGER.error("ERROR: Waiting on executor service shutdown!"); Thread.currentThread().interrupt(); } }
程序的输出。
14:02:52.268 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-2: Actual machine state: ENABLED 14:02:52.272 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-2: Doing the washing 14:02:52.272 [pool-1-thread-3] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-3: Actual machine state: WASHING 14:02:52.273 [pool-1-thread-3] ERROR com.iluwatar.balking.WashingMachine - Cannot wash if the machine has been already washing! 14:02:52.273 [pool-1-thread-1] INFO com.iluwatar.balking.WashingMachine - pool-1-thread-1: Actual machine state: WASHING 14:02:52.273 [pool-1-thread-1] ERROR com.iluwatar.balking.WashingMachine - Cannot wash if the machine has been already washing! 14:02:52.324 [pool-1-thread-2] INFO com.iluwatar.balking.WashingMachine - 14: Washing completed.
🔍类图
🔍适用场景
使用止步模式场景
- 您只想在对象处于特定状态时才对其调用操作
- 对象通常仅处于容易暂时停止但状态未知的状态
