目的
Balking Pattern用于防止对象在不完整或不适当的情况下执行某些代码。
维基百科说
balking模式是一种软件设计模式，仅在对象处于特定状态时才对对象执行操作。例如，如果一个对象读取ZIP文件，而一个调用方法在zip文件未打开时调用该对象的get方法，则该对象将在请求时“停止”。例如，在Java编程语言中，在这些情况下可能会抛出IllegalStateException。
源代码
在这个示例实现中，WashingMachine是一个对象，它有两种状态：启用和清洗。如果机器已启用，则状态将更改为“正在清洗”，任何其他线程都无法调用及执行此操作。另一方面，如果它已经清洗过，则任何其他线程无法再执行wash（）操作。
类图
第1步：创建洗衣机类。
package com.iluwatar.balking;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/<strong>
* Washing machine class
*/
public class WashingMachine {
private static final Logger LOGGER = LoggerFactory.getLogger(WashingMachine.class);
private WashingMachineState washingMachineState;
public WashingMachine() {
washingMachineState = WashingMachineState.ENABLED;
}
public WashingMachineState getWashingMachineState() {
return washingMachineState;
}
/</strong>
* Method responsible for washing
* if the object is in appropriate state
*/
public void wash() {
synchronized (this) {
LOGGER.info("{}: Actual machine state: {}", Thread.currentThread().getName(), getWashingMachineState());
if (washingMachineState == WashingMachineState.WASHING) {
LOGGER.error("ERROR: Cannot wash if the machine has been already washing!");
return;
}
washingMachineState = WashingMachineState.WASHING;
}
LOGGER.info("{}: Doing the washing", Thread.currentThread().getName());
try {
Thread.sleep(50);
} catch (InterruptedException ie) {
ie.printStackTrace();
}
endOfWashing();
}
/<strong>
* Method responsible of ending the washing
* by changing machine state
*/
public synchronized void endOfWashing() {
washingMachineState = WashingMachineState.ENABLED;
LOGGER.info("{}: Washing completed.", Thread.currentThread().getId());
}
}
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第2步： 创建WashingMachineState描述机器的状态，它可以启用并准备好在洗涤期间工作。
package com.iluwatar.balking;
public enum WashingMachineState {
ENABLED, WASHING
}
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第3步：让我们测试一下这种设计模式。
public class BalkingPatternDemo {
private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
/</strong>
* @param args the command line arguments - not used
*/
public static void main(String... args) {
final WashingMachine washingMachine = new WashingMachine();
ExecutorService 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!");
}
}
}
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适用性
使用Balking模式时
- 对象通常只处于一种容易暂时停止的状态，但时间不确定。