ExecutorService中的所有线程都在忙于等待等待执行器服务队列中阻塞的任务的任务。

示例代码：

ExecutorService es=Executors.newFixedThreadPool(8);
        Set<Future<Set<String>>> outerSet=new HashSet<>();
        for(int i=0;i<8;i++){
            outerSet.add(es.submit(new Callable<Set<String>>() {

                @Override
                public Set<String> call() throws Exception {
                    Thread.sleep(10000); //to simulate work
                    Set<Future<String>> innerSet=new HashSet<>();
                    for(int j=0;j<8;j++) {
                        int k=j;
                        innerSet.add(es.submit(new Callable<String>() {
                            @Override
                            public String call() throws Exception {
                                return "number "+k+" in inner loop";
                            }

                        }));
                    }
                    Set<String> out=new HashSet<>();
                    while(!innerSet.isEmpty()) {            //we are stuck at this loop because all the
                        for(Future<String> f:innerSet) {    //callable in innerSet are stuckin the queue
                            if(f.isDone()) {                //of es and can't start since all the threads
                                out.add(f.get());           //in es are busy waiting for them to finish
                            }
                        }
                    }
                    return out;
                }
            }));
        }

除了为每一层创建更多线程池或通过设置大小不固定的线程池之外，还有什么方法可以避免这种情况？

一个实际的例子是，如果将一些可调用对象提交给ForkJoinPool.commonPool（），然后这些任务使用的对象也通过其方法之一提交给commonPool。