使用java ConcurrentSkipListSet添加方法时线程卡住了(Thread stuck when use java ConcurrentSkipListSet add method)

 我正在使用ConcurrentSkipListSet集合来处理并发运算符。 我发现它有时会卡住，用这段代码重现： 
 
import java.util.Comparator
import java.util.concurrent._

object SetDeadLock extends App {
  private val tasks = new ConcurrentSkipListSet[Task](new Comparator[Task](){
    override def compare(o1: Task, o2: Task): Int = {
      val compare = (o1.systemTime - o2.systemTime).toInt
      if (compare == 0) 1 else compare  //distinct same time task
    }
  })

  for(i <- 1 to 20) {
    tasks.add(Task())
    println(s"added - $i")
  }

  case class Task() {
    val systemTime = System.currentTimeMillis()
  }
}
 
 产量 
 
added - 1
added - 2
 
 它或许卡在别人身上，另外，比较器自定义排序数据方法尤其是它们相同（因为Set不支持相同的元素）而且所有Task都是新实例，它不应该与其他实例冲突。 
 
 使用jstack cmd，主线程卡住了 
 
at java.util.concurrent.ConcurrentSkipListMap.findPredecessor(ConcurrentSkipListMap.java:685)
 
 这是一个错误还是我误导了一些原则？ 
 
 感谢任何帮助或建议。 
 
 UPDATE 
 我现在尝试修改if (compare == 0) 1 else compare if (compare == 0) -1 else compare ，令人惊讶，我运作良好！ 
 
 有没有人能澄清它是如何工作的？ 源代码对我来说很难（我认为很多人都同意我的观点），毕竟，jdk为机器执行速度做了很多工作而不是代码阅读器。 
 
 最后 
 为了避免尴尬的情况，只需对comparator Set的语义进行一些不同的因素，例如附加一个随机值。但我认为它会更好地找到另一个真正合适的集合。 
 
 几天前，我发现在systemTime等于时使用hashCode作为第二次验证是个好主意。 希望有用〜 

I'm using ConcurrentSkipListSet collection to deal with concurrent operator. I find it will stuck sometimes, reproduct with this code:
 
import java.util.Comparator
import java.util.concurrent._

object SetDeadLock extends App {
  private val tasks = new ConcurrentSkipListSet[Task](new Comparator[Task](){
    override def compare(o1: Task, o2: Task): Int = {
      val compare = (o1.systemTime - o2.systemTime).toInt
      if (compare == 0) 1 else compare  //distinct same time task
    }
  })

  for(i <- 1 to 20) {
    tasks.add(Task())
    println(s"added - $i")
  }

  case class Task() {
    val systemTime = System.currentTimeMillis()
  }
}
 
Output 
 
added - 1
added - 2
 
It perhaps stuck at others, besides, the comparator custom sort data method especially they are same (because Set not support same element) and all Task is new instance, it shouldn't be conflict with others.
 
with jstack cmd, the main thread stuck 
 
at java.util.concurrent.ConcurrentSkipListMap.findPredecessor(ConcurrentSkipListMap.java:685)
 
does it was a bug or I just misleading some principle?
 
Thanks any help or advise.
 
UPDATE
 I just now try modify if (compare == 0) 1 else compare to if (compare == 0) -1 else compare, surprising, I works well! 
 
Does anyone could clarify how it work? Source code its difficult for me(and I think many people agree with me), after all, jdk do many works for machine execute speed not for code reader. 
 
Finally
 To avoid the awkward situation just make some different factor to comparator which match Set's semantics, such as appends a random value.But I think its will be better find another real suitable collection.
 
With a few days ago, I find a good idea to use hashCode as second verify when systemTime equals. hope helpful~

原文：https://stackoverflow.com/questions/37558703