mono subscribe源码分析

        Mono.just("hello")
         .filter(t->t.startsWith("h"))
         .map(String::toUpperCase)
         .subscribe(System.out::println);

    /***
    **该方法会根据声明部分创建完整发布、订阅关系链
    *本例子中涉及到下面几个订阅者类：LambdaMonoSubscriber、MapFuseableSubscriber以及 
    *FilterFuseableSubscriber
    ***/
	public final void subscribe(Subscriber<? super T> actual) {
		CorePublisher publisher = Operators.onLastAssembly(this);
		CoreSubscriber subscriber = Operators.toCoreSubscriber(actual);

        //本例中的filter、map节点都是实现了OptimizableOperator
		if (publisher instanceof OptimizableOperator) {
			OptimizableOperator operator = (OptimizableOperator) publisher;
            //下面创建发布、订阅链，创建过程跟声明阶段相比，即map-->filter
			while (true) {
				subscriber = operator.subscribeOrReturn(subscriber);
				if (subscriber == null) {
					// null means "I will subscribe myself", returning...
					return;
				}
                
                //map上一节点为filter，filter不是数据源，filter节点的上一节点
                //为MonoJust，为数据源，所以filter节点nextOptimizableSource返回null
				OptimizableOperator newSource = operator.nextOptimizableSource();
				if (newSource == null) {
                    //最后在这里更新发布者为最终的发布者，例如该例子中的MonoJust
					publisher = operator.source();
					break;
				}
				operator = newSource;
			}
		}
        //对于本文来说，publisher为MonoJust对象，subscriber为FilterFuseableSubscriber
        //MonoJust对象调用subscribe方法会调用其subscriber（即FilterFuseableSubscriber）的onSubscribe方法，然后进入onSubscribe阶段，onSubscribe调用顺序跟声明阶段相同，当onSubscribe传到subscribe方法的订阅者时将进入request阶段，request阶段执行顺序跟声明阶段相反，当request阶段执行到数据源端又会触发调用阶段的执行，常见的为subscriber.onNext方法
		publisher.subscribe(subscriber);
	}