一个线程池 bug 引发的 GC 思考!

2020-03-17 16:06:48来源:博客园 阅读 ()

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一个线程池 bug 引发的 GC 思考!

作者:空无
https://segmentfault.com/a/1190000021109130

问题描述

前几天在帮同事排查生产一个线上偶发的线程池错误,逻辑很简单,线程池执行了一个带结果的异步任务。

但是最近有偶发的报错:

java.util.concurrent.RejectedExecutionException:?Task?java.util.concurrent.FutureTask@a5acd19?rejected?from?java.util.concurrent.ThreadPoolExecutor@30890a38\[Terminated,?pool?size?=?0,?active?threads?=?0,?queued?tasks?=?0,?completed?tasks?=?0\]

本文中的模拟代码已经问题都是在HotSpot java8 (1.8.0_221)版本下模拟&出现的

下面是模拟代码,通过Executors.newSingleThreadExecutor创建一个单线程的线程池,然后在调用方获取Future的结果

public?class?ThreadPoolTest?{????public?static?void?main(String\[\]?args)?{????????final?ThreadPoolTest?threadPoolTest?=?new?ThreadPoolTest();????????for?(int?i?=?0;?i?<?8;?i++)?{????????????new?Thread(new?Runnable()?{????????????????@Override
????????????????public?void?run()?{????????????????????while?(true)?{

????????????????????????Future<String>?future?=?threadPoolTest.submit();????????????????????????try?{
????????????????????????????String?s?=?future.get();
????????????????????????}?catch?(InterruptedException?e)?{
????????????????????????????e.printStackTrace();
????????????????????????}?catch?(ExecutionException?e)?{
????????????????????????????e.printStackTrace();
????????????????????????}?catch?(Error?e)?{
????????????????????????????e.printStackTrace();
????????????????????????}
????????????????????}
????????????????}
????????????}).start();
????????}????????
????????//子线程不停gc,模拟偶发的gc
????????new?Thread(new?Runnable()?{????????????@Override
????????????public?void?run()?{????????????????while?(true)?{
????????????????????System.gc();
????????????????}
????????????}
????????}).start();
????}????/**
?????*?异步执行任务
?????*?@return
?????*/
????public?Future<String>?submit()?{????????//关键点,通过Executors.newSingleThreadExecutor创建一个单线程的线程池
????????ExecutorService?executorService?=?Executors.newSingleThreadExecutor();
????????FutureTask<String>?futureTask?=?new?FutureTask(new?Callable()?{????????????@Override
????????????public?Object?call()?throws?Exception?{
????????????????Thread.sleep(50);????????????????return?System.currentTimeMillis()?+?"";
????????????}
????????});
????????executorService.execute(futureTask);????????return?futureTask;
????}

}

分析&疑问

第一个思考的问题是:线程池为什么关闭了,代码中并没有手动关闭的地方。看一下Executors.newSingleThreadExecotor的源码实现:

public?static?ExecutorService?newSingleThreadExecutor()?{????return?new?FinalizableDelegatedExecutorService
????????????(new?ThreadPoolExecutor(1,?1,????????????????????0L,?TimeUnit.MILLISECONDS,????????????????????new?LinkedBlockingQueue<Runnable>()));
}

这里创建的实际上是一个FinalizableDelegatedExecutorService,这个包装类重写了finalize函数,也就是说这个类会在被GC回收之前,先执行线程池的shutdown方法。

问题来了,GC只会回收不可达(unreachable)的对象,在submit函数的栈帧未执行完出栈之前,executorService应该是可达的才对。

对于此问题,先抛出结论:

当对象仍存在于作用域(stack frame)时,finalize也可能会被执行

oracle jdk文档中有一段关于finalize的介绍:

A reachable object is any object that can be accessed in any potential continuing computation from any live thread.

Optimizing transformations of a program can be designed that reduce the number of objects that are reachable to be less than those which would naively be considered reachable. For example, a Java compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner.

大概意思是:可达对象(reachable object)是可以从任何活动线程的任何潜在的持续访问中的任何对象;java编译器或代码生成器可能会对不再访问的对象提前置为null,使得对象可以被提前回收

也就是说,在jvm的优化下,可能会出现对象不可达之后被提前置空并回收的情况,关注微信公众号:Java技术栈,在后台回复:java,可以获取我整理的 N 篇最新 Java 教程,都是干货。

举个例子来验证一下(摘自https://stackoverflow.com/questions/24376768/can-java-finalize-an-object-when-it-is-still-in-scope):

class?A?{
????@Override?protected?void?finalize()?{
????????System.out.println(this?+?"?was?finalized!");
????}????public?static?void?main(String\[\]?args)?throws?InterruptedException?{
????????A?a?=?new?A();
????????System.out.println("Created?"?+?a);????????for?(int?i?=?0;?i?<?1\_000\_000\_000;?i++)?{????????????if?(i?%?1\_000_00?==?0)
????????????????System.gc();
????????}
????????System.out.println("done.");
????}
}//打印结果Created?A@1be6f5c3
A@1be6f5c3?was?finalized!//finalize方法输出done.

从例子中可以看到,如果a在循环完成后已经不再使用了,则会出现先执行finalize的情况;虽然从对象作用域来说,方法没有执行完,栈帧并没有出栈,但是还是会被提前执行。

现在来增加一行代码,在最后一行打印对象a,让编译器/代码生成器认为后面有对象a的引用

...System.out.println(a);//打印结果Created?A@1be6f5c3
done.
A@1be6f5c3

从结果上看,finalize方法都没有执行(因为main方法执行完成后进程直接结束了),更不会出现提前finalize的问题了

基于上面的测试结果,再测试一种情况,在循环之前先将对象a置为null,并且在最后打印保持对象a的引用

A?a?=?new?A();System.out.println("Created?"?+?a);
a?=?null;//手动置nullfor?(int?i?=?0;?i?<?1\_000\_000\_000;?i++)?{????if?(i?%?1\_000_00?==?0)????????System.gc();
}System.out.println("done.");System.out.println(a);//打印结果Created?A@1be6f5c3
A@1be6f5c3?was?finalized!
done.null

从结果上看,手动置null的话也会导致对象被提前回收,虽然在最后还有引用,但此时引用的也是null了


现在再回到上面的线程池问题,根据上面介绍的机制,在分析没有引用之后,对象会被提前finalize

可在上述代码中,return之前明明是有引用的executorService.execute(futureTask),为什么也会提前finalize呢?

猜测可能是由于在execute方法中,会调用threadPoolExecutor,会创建并启动一个新线程,这时会发生一次主动的线程切换,导致在活动线程中对象不可达

结合上面Oracle Jdk文档中的描述“可达对象(reachable object)是可以从任何活动线程的任何潜在的持续访问中的任何对象”,可以认为可能是因为一次显示的线程切换,对象被认为不可达了,导致线程池被提前finalize了

下面来验证一下猜想:

//入口函数public?class?FinalizedTest?{????public?static?void?main(String\[\]?args)?{????????final?FinalizedTest?finalizedTest?=?new?FinalizedTest();????????for?(int?i?=?0;?i?<?8;?i++)?{????????????new?Thread(new?Runnable()?{????????????????@Override
????????????????public?void?run()?{????????????????????while?(true)?{
????????????????????????TFutureTask?future?=?finalizedTest.submit();
????????????????????}
????????????????}
????????????}).start();
????????}????????new?Thread(new?Runnable()?{????????????@Override
????????????public?void?run()?{????????????????while?(true)?{
????????????????????System.gc();
????????????????}
????????????}
????????}).start();
????}????public?TFutureTask?submit(){
????????TExecutorService?TExecutorService?=?Executors.create();
????????TExecutorService.execute();????????return?null;
????}
}//Executors.java,模拟juc的Executorspublic?class?Executors?{????/**
?????*?模拟Executors.createSingleExecutor
?????*?@return
?????*/
????public?static?TExecutorService?create(){????????return?new?FinalizableDelegatedTExecutorService(new?TThreadPoolExecutor());
????}????static?class?FinalizableDelegatedTExecutorService?extends?DelegatedTExecutorService?{

????????FinalizableDelegatedTExecutorService(TExecutorService?executor)?{????????????super(executor);
????????}????????
????????/**
?????????*?析构函数中执行shutdown,修改线程池状态
?????????*?@throws?Throwable
?????????*/
????????@Override
????????protected?void?finalize()?throws?Throwable?{????????????super.shutdown();
????????}
????}????static?class?DelegatedTExecutorService?extends?TExecutorService?{????????protected?TExecutorService?e;????????public?DelegatedTExecutorService(TExecutorService?executor)?{????????????this.e?=?executor;
????????}????????@Override
????????public?void?execute()?{
????????????e.execute();
????????}????????@Override
????????public?void?shutdown()?{
????????????e.shutdown();
????????}
????}
}//TThreadPoolExecutor.java,模拟juc的ThreadPoolExecutorpublic?class?TThreadPoolExecutor?extends?TExecutorService?{????/**
?????*?线程池状态,false:未关闭,true已关闭
?????*/
????private?AtomicBoolean?ctl?=?new?AtomicBoolean();????@Override
????public?void?execute()?{????????//启动一个新线程,模拟ThreadPoolExecutor.execute
????????new?Thread(new?Runnable()?{????????????@Override
????????????public?void?run()?{

????????????}
????????}).start();????????//模拟ThreadPoolExecutor,启动新建线程后,循环检查线程池状态,验证是否会在finalize中shutdown
????????//如果线程池被提前shutdown,则抛出异常
????????for?(int?i?=?0;?i?<?1\_000\_000;?i++)?{????????????if(ctl.get()){????????????????throw?new?RuntimeException("reject!!!\["+ctl.get()+"\]");
????????????}
????????}
????}????@Override
????public?void?shutdown()?{
????????ctl.compareAndSet(false,true);
????}
}

执行若干时间后报错:

Exception?in?thread?"Thread-1"?java.lang.RuntimeException:?reject!!!\[true\]

从错误上来看,“线程池”同样被提前shutdown了,那么一定是由于新建线程导致的吗?

下面将新建线程修改为Thread.sleep测试一下:

//TThreadPoolExecutor.java,修改后的execute方法public?void?execute()?{????try?{????????//显式的sleep?1?ns,主动切换线程
????????TimeUnit.NANOSECONDS.sleep(1);
????}?catch?(InterruptedException?e)?{
????????e.printStackTrace();
????}????//模拟ThreadPoolExecutor,启动新建线程后,循环检查线程池状态,验证是否会在finalize中shutdown
????//如果线程池被提前shutdown,则抛出异常
????for?(int?i?=?0;?i?<?1\_000\_000;?i++)?{????????if(ctl.get()){????????????throw?new?RuntimeException("reject!!!\["+ctl.get()+"\]");
????????}
????}
}

执行结果一样是报错

Exception?in?thread?"Thread-3"?java.lang.RuntimeException:?reject!!!\[true\]

由此可得,如果在执行的过程中,发生一次显式的线程切换,则会让编译器/代码生成器认为外层包装对象不可达

总结

虽然GC只会回收不可达GC ROOT的对象,但是在编译器(没有明确指出,也可能是JIT)/代码生成器的优化下,可能会出现对象提前置null,或者线程切换导致的“提前对象不可达”的情况。

所以如果想在finalize方法里做些事情的话,一定在最后显示的引用一下对象(toString/hashcode都可以),保持对象的可达性(reachable)

上面关于线程切换导致的对象不可达,没有官方文献的支持,只是个人一个测试结果,如有问题欢迎指出

综上所述,这种回收机制并不是JDK的bug,而算是一个优化策略,提前回收而已;但Executors.newSingleThreadExecutor的实现里通过finalize来自动关闭线程池的做法是有Bug的,在经过优化后可能会导致线程池的提前shutdown,从而导致异常。

线程池的这个问题,在JDK的论坛里也是一个公开但未解决状态的问题https://bugs.openjdk.java.net/browse/JDK-8145304。

不过在JDK11下,该问题已经被修复:

JUC??Executors.FinalizableDelegatedExecutorServicepublic?void?execute(Runnable?command)?{????try?{
????????e.execute(command);
????}?finally?{?reachabilityFence(this);?}
}

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原文链接:https://www.cnblogs.com/javastack/p/12511606.html
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