[提问]关于主线程创建 N 多的线程，抛出'OOME: unable to create new native thread'error 后， catch 住 error，然后清理线程，此时主线程的表现问题。

运行环境:

java version "1.8.0_191"
Java(TM) SE Runtime Environment (build 1.8.0_191-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.191-b12, mixed mode)

macOS 10.13.6

一个朋友问，线程可不可以从 OOM 中恢复，然后给了一篇网上看到的文章，大意是如果一个线程发生了 OOM,是不影响其他线程的正常运行的。

然后我就想，如果主线程疯狂创建线程，也会抛出 OOM，那么如果我 catch 住异常，然后把创建的线程 stop 掉，是不是可行，于是写了下面一段代码测试。

JVM 参数:-Xms256m -Xmx256m -Xss1m

public class TestThread {

    public static void main(String[] args) throws InterruptedException {
        List<Thread> list = new ArrayList<>();
        try {
            while (true) {
                Thread t = new Thread(() -> {
                    System.out.println(Thread.currentThread().getName());
                    try {
                        Thread.sleep(Integer.MAX_VALUE);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                });
                list.add(t);
                t.start();
            }

        } catch (Throwable e) {
            e.printStackTrace();
            System.out.println("list size =================" + list.size());
            System.out.println("thread active =================" + Thread.activeCount());
            list.forEach(testThread -> {
                testThread.stop();
            });
            System.out.println("stop thread over");
        }
        Thread.sleep(2000L);
        System.out.println("thread active =================" + Thread.activeCount());
        Thread.currentThread().getThreadGroup().list();
    }
}

部分日志如下:

Thread-2024
Thread-2025
Thread-2026
Thread-2027
java.lang.OutOfMemoryError: unable to create new native thread
	at java.lang.Thread.start0(Native Method)
	at java.lang.Thread.start(Thread.java:717)
	at com.xzy.test.TestThread.main(TestThread.java:25)
list size =================2029
thread active =================2030
stop thread over
thread active =================2
java.lang.ThreadGroup[name=main,maxpri=10]
    Thread[main,5,main]
    Thread[Monitor Ctrl-Break,5,main]

Process finished with exit code 0

可以看到，在 catch 住异常后，主线程正常执行了剩下的内容，正常退出了(exit code 0)。然后我就想，如果这里的是线程池呢？这个时候测试的出来的结果就有点不理解了。

JVM 参数依然是:-Xms256m -Xmx256m -Xss1m

public class TestThreadPool {

    public static void main(String[] args) throws InterruptedException {
        List<ExecutorService> list = new ArrayList<>();
        try {
            while (true) {
                ExecutorService executorService = Executors.newFixedThreadPool(10);
                int i = 0;
                while (i++ <= 10) {
                    executorService.submit(() -> {System.out.println(Thread.currentThread().getName());});
                }
                list.add(executorService);
            }
        } catch (Throwable e) {
            System.out.println("catch throwable");
            e.printStackTrace();
            System.out.println("============" + Thread.currentThread().getName() + " collect thread start!");
            System.out.println("list size ============" + list.size());
            list.forEach(ExecutorService::shutdownNow);
            System.out.println("============" + Thread.currentThread().getName() + " shutdown success!");
        }
        list.forEach(executorService -> {
            if (!executorService.isShutdown()) {
                System.out.println("============ alive executorservice");
            }
        });
        Thread.sleep(2000L);
        System.out.println("thread active count ============" + Thread.activeCount());
        Thread.currentThread().getThreadGroup().list();
    }
}

部分日志输出如下:

pool-203-thread-4
pool-203-thread-5
pool-203-thread-6
pool-203-thread-7
catch throwable
java.lang.OutOfMemoryError: unable to create new native thread
	at java.lang.Thread.start0(Native Method)
	at java.lang.Thread.start(Thread.java:717)
	at java.util.concurrent.ThreadPoolExecutor.addWorker(ThreadPoolExecutor.java:957)
	at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1367)
	at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:112)
	at com.xzy.test.TestThreadPool.main(TestThreadPool.java:20)
============main collect thread start!
list size ============202
============main shutdown success!
thread active count ============9
java.lang.ThreadGroup[name=main,maxpri=10]
    Thread[main,5,main]
    Thread[Monitor Ctrl-Break,5,main]
    Thread[pool-203-thread-1,5,main]
    Thread[pool-203-thread-2,5,main]
    Thread[pool-203-thread-3,5,main]
    Thread[pool-203-thread-4,5,main]
    Thread[pool-203-thread-5,5,main]
    Thread[pool-203-thread-6,5,main]
    Thread[pool-203-thread-7,5,main]

主线程没有退出，没理解错的话，因为pool-203线程池还在运行，但是为什么pool-203这个线程池会卡住呢，还是 catch 住异常之前的样子(都是新建了 7 个线程)，我用 VisualVM 看了下线程的状态，发现pool-203-thread-这一批线程因为调用了java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#await()这个方法，都处于 waiting 状态。 看了眼 ThreadPoolExecutor，应该是java.util.concurrent.ThreadPoolExecutor#awaitTermination这里调用的，这个方法是有 shutdwon 请求时，阻塞等待所有任务完成执行。

不知道我前面的分析有没有哪个地方有错。如果前面分析的没错的话，我的疑问是，为什么会卡在java.util.concurrent.ThreadPoolExecutor#awaitTermination这里呢，是因为创建线程过多发生了 oom，但是最后一组线程池没有正确创建完毕，导致主线程恢复过来后，无法彻底回收最后那组线程池吗？