Balking
大约 2 分钟
意图
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。
通俗地说
使用止步模式，仅当对象处于特定状态时才执行特定代码。
维基百科说
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
在此示例中， WashingMachine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。
这里是WashingMachine 类相关的部分。
@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);
}现在，我们使用WashingMachine介绍该应用程序。
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();
}
}下面是程序的输出。
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
适用性
使用止步模式当
- 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