Java 排序并行流时遇到顺序错误
我有一个Java 排序并行流时遇到顺序错误,java,parallel-processing,java-8,java-stream,Java,Parallel Processing,Java 8,Java Stream,我有一个记录课程: public class Record implements Comparable<Record> { private String myCategory1; private int myCategory2; private String myCategory3; private String myCategory4; private int myValue1; private double myValue2;
记录
课程:
public class Record implements Comparable<Record>
{
private String myCategory1;
private int myCategory2;
private String myCategory3;
private String myCategory4;
private int myValue1;
private double myValue2;
public Record(String category1, int category2, String category3, String category4,
int value1, double value2)
{
myCategory1 = category1;
myCategory2 = category2;
myCategory3 = category3;
myCategory4 = category4;
myValue1 = value1;
myValue2 = value2;
}
// Getters here
}
请注意,前10000条记录的类别2
为0
,然后下10000条记录的1
,以此类推,而value1
值依次为0-114999
我创建了一个流
,它既并行
又排序
Stream<Record> stream = list.stream()
.parallel()
.sorted(
//(r1, r2) -> Integer.compare(r1.getCategory2(), r2.getCategory2())
)
//.parallel()
;
Stream<Record> stream = list.stream().sorted().parallel();
我用这个把戏
压缩输出:
0
1
2
3
...
69996
69997
69998
69999
71875 // discontinuity!
71876
71877
71878
...
79058
79059
79060
79061
70000 // discontinuity!
70001
70002
70003
...
71871
71872
71873
71874
79062 // discontinuity!
79063
79064
79065
79066
...
114996
114997
114998
114999
output
的size()
是115000
,所有元素似乎都在那里,只是顺序略有不同
所以我写了一些检查代码,看看排序是否稳定。如果它是稳定的,那么所有的value1
值都应该保持顺序。此代码验证订单,打印任何差异
int prev = -1;
boolean verified = true;
for (Record record : output)
{
int curr = record.getValue1();
if (prev != -1)
{
if (prev + 1 != curr)
{
System.out.println("Warning: " + prev + " followed by " + curr + "!");
verified = false;
}
}
prev = curr;
}
System.out.println("Verified: " + verified);
输出:
Warning: 69999 followed by 71875!
Warning: 79061 followed by 70000!
Warning: 71874 followed by 79062!
Warning: 99999 followed by 100625!
Warning: 107811 followed by 100000!
Warning: 100624 followed by 107812!
Verified: false
如果执行以下任一操作,此情况将持续存在:
true
true
false
- 将
替换为ForkJoinPool
ThreadPoolExecutor
ThreadPoolExecutor pool = new ThreadPoolExecutor(8, 8, 0, TimeUnit.SECONDS, new ArrayBlockingQueue<>(10));
- 调用
在我调用parallel()
之后sorted
Stream<Record> stream = list.stream() .parallel() .sorted( //(r1, r2) -> Integer.compare(r1.getCategory2(), r2.getCategory2()) ) //.parallel() ;
Stream<Record> stream = list.stream().sorted().parallel();
- 使用
。请注意,此排序标准不同于我为比较器进行排序
接口定义的“自然”顺序,尽管从一开始就以顺序排列的结果开始,但结果应该仍然相同可比
我的Java版本是1.8.0_05。这种异常现象也存在,它似乎正在运行Java8U25 更新 在撰写本文时,我已经将JDK升级到最新版本1.8.0_45,问题没有改变 问题 结果Stream<Record> stream = list.stream().parallel().sorted( (r1, r2) -> Integer.compare(r1.getCategory2(), r2.getCategory2()) );
列表中的记录顺序(
)是否由于排序不稳定、遭遇顺序未保留或其他原因而出现顺序错误输出
当我创建并行流并对其排序时,如何确保保留遭遇顺序?它看起来像数组。在某些情况下,并行排序不稳定。好的。流并行排序是根据
,因此它也会影响流。下面是一个简化的示例:数组实现的。parallelSort
当然,它应该打印三次public class StableSortBug { static final int SIZE = 50_000; static class Record implements Comparable<Record> { final int sortVal; final int seqNum; Record(int i1, int i2) { sortVal = i1; seqNum = i2; } @Override public int compareTo(Record other) { return Integer.compare(this.sortVal, other.sortVal); } } static Record[] genArray() { Record[] array = new Record[SIZE]; Arrays.setAll(array, i -> new Record(i / 10_000, i)); return array; } static boolean verify(Record[] array) { return IntStream.range(1, array.length) .allMatch(i -> array[i-1].seqNum + 1 == array[i].seqNum); } public static void main(String[] args) { Record[] array = genArray(); System.out.println(verify(array)); Arrays.sort(array); System.out.println(verify(array)); Arrays.parallelSort(array); System.out.println(verify(array)); } }
。这是在当前的JDK 9开发版本上。考虑到您已经尝试过的情况,如果它出现在迄今为止所有的JDK 8版本中,我不会感到惊讶。奇怪的是,减小大小或除数将改变行为。20000的大小和10000的除数是稳定的,50000的大小和1000的除数也是稳定的。问题似乎与比较相等与平行分割大小的足够大的值运行有关true
OpenJDK的问题涉及到这个bug。我会尝试制作最简单的程序来重现这个问题,在最新的JDK版本上运行它,如果它被重现,我会提交一个bug:排序应该是稳定的:它是这样记录的。(真、真、假)同样在Windows7(64),8u40上。@StefanZobel哦,是的,谢谢,我已将新错误作为旧错误的副本关闭。@StuartMarks感谢您确定此错误的根本原因。我们知道什么时候能修好吗?修复程序将应用于Java 8还是只应用于Java 9未来的开发?@rgetman抱歉,对此没有估计。就我个人而言,我建议将修复程序后移植到8-update发布行,但我不能保证是否会发生这种情况。Stream<Record> stream = list.parallelStream().sorted();
Stream<Record> stream = list.stream().parallel().sorted( (r1, r2) -> Integer.compare(r1.getCategory2(), r2.getCategory2()) );
Verified: true
public class StableSortBug { static final int SIZE = 50_000; static class Record implements Comparable<Record> { final int sortVal; final int seqNum; Record(int i1, int i2) { sortVal = i1; seqNum = i2; } @Override public int compareTo(Record other) { return Integer.compare(this.sortVal, other.sortVal); } } static Record[] genArray() { Record[] array = new Record[SIZE]; Arrays.setAll(array, i -> new Record(i / 10_000, i)); return array; } static boolean verify(Record[] array) { return IntStream.range(1, array.length) .allMatch(i -> array[i-1].seqNum + 1 == array[i].seqNum); } public static void main(String[] args) { Record[] array = genArray(); System.out.println(verify(array)); Arrays.sort(array); System.out.println(verify(array)); Arrays.parallelSort(array); System.out.println(verify(array)); } }
true true false