Nacos服务发现并发启动scheduleUpdate定时任务的流程分析

private final Map<String, ScheduledFuture<?>> futureMap = new HashMap<String, ScheduledFuture<?>>();

public void scheduleUpdateIfAbsent(String serviceName, String clusters) {
    if (futureMap.get(ServiceInfo.getKey(serviceName, clusters)) != null) {
        return;
    }
    
    synchronized (futureMap) {
        if (futureMap.get(ServiceInfo.getKey(serviceName, clusters)) != null) {
            return;
        }
        
        ScheduledFuture<?> future = addTask(new UpdateTask(serviceName, clusters));
        futureMap.put(ServiceInfo.getKey(serviceName, clusters), future);
    }
}

public class DoubleCheckLazySingleton {

    private volatile static DoubleCheckLazySingleton singleton;

    private DoubleCheckLazySingleton() {
    }

    public DoubleCheckLazySingleton getSingleton() {
        if (singleton == null) {
            synchronized (this) {
                //只有在singleton为null的时候才创建实例
                if (singleton == null) {
                    singleton = new DoubleCheckLazySingleton();
                }
            }
        }
        return singleton;
    }
}