java - Stream.toList() 会比 Collectors.toList() 表现得更好吗

Question

JDK 引入了一个 API Stream.toList() 和 JDK-8180352 。这是我尝试将其性能与现有 Collectors.toList 进行比较的基准测试代码:

@BenchmarkMode(Mode.All)
@Fork(1)
@State(Scope.Thread)
@Warmup(iterations = 20, time = 1, batchSize = 10000)
@Measurement(iterations = 20, time = 1, batchSize = 10000)
public class CollectorsVsStreamToList {

    @Benchmark
    public List<Integer> viaCollectors() {
        return IntStream.range(1, 1000).boxed().collect(Collectors.toList());
    }

    @Benchmark
    public List<Integer> viaStream() {
        return IntStream.range(1, 1000).boxed().toList();
    }
}

结果总结如下:

Benchmark                                                       Mode  Cnt   Score    Error  Units
CollectorsVsStreamToList.viaCollectors                         thrpt   20  17.321 ±  0.583  ops/s
CollectorsVsStreamToList.viaStream                             thrpt   20  23.879 ±  1.682  ops/s
CollectorsVsStreamToList.viaCollectors                          avgt   20   0.057 ±  0.002   s/op
CollectorsVsStreamToList.viaStream                              avgt   20   0.040 ±  0.001   s/op
CollectorsVsStreamToList.viaCollectors                        sample  380   0.054 ±  0.001   s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p0.00    sample        0.051            s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p0.50    sample        0.054            s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p0.90    sample        0.058            s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p0.95    sample        0.058            s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p0.99    sample        0.062            s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p0.999   sample        0.068            s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p0.9999  sample        0.068            s/op
CollectorsVsStreamToList.viaCollectors:viaCollectors·p1.00    sample        0.068            s/op
CollectorsVsStreamToList.viaStream                            sample  525   0.039 ±  0.001   s/op
CollectorsVsStreamToList.viaStream:viaStream·p0.00            sample        0.037            s/op
CollectorsVsStreamToList.viaStream:viaStream·p0.50            sample        0.038            s/op
CollectorsVsStreamToList.viaStream:viaStream·p0.90            sample        0.040            s/op
CollectorsVsStreamToList.viaStream:viaStream·p0.95            sample        0.042            s/op
CollectorsVsStreamToList.viaStream:viaStream·p0.99            sample        0.050            s/op
CollectorsVsStreamToList.viaStream:viaStream·p0.999           sample        0.051            s/op
CollectorsVsStreamToList.viaStream:viaStream·p0.9999          sample        0.051            s/op
CollectorsVsStreamToList.viaStream:viaStream·p1.00            sample        0.051            s/op
CollectorsVsStreamToList.viaCollectors                            ss   20   0.060 ±  0.007   s/op
CollectorsVsStreamToList.viaStream                                ss   20   0.043 ±  0.006   s/op

当然，领域专家的第一个问题是基准测试程序是否正确？测试类在 MacOS 上执行。请让我知道所需的任何进一步详细信息。
后续，据我从读数中推断， Stream.toList 的平均时间、吞吐量和采样时间看起来比 Collectors.toList 好。这样的理解正确吗？

Answer 1

Stream::toList 建立在 toArray 之上，而不是 collect 之上。 toArray 中有许多优化，使其可能比收集更快，尽管这在很大程度上取决于细节。如果流管道(从源到最终中间操作)是 SIZED ，则可以预先调整目标数组的大小(而不是像 toList 收集器必须做的那样潜在地重新分配。)如果管道进一步是 SUBSIZED ，那么并行执行不仅可以预先调整结果的大小数组，但可以计算每个分片的精确偏移量，因此每个子任务都可以将其结果放在正确的位置，从而无需将中间结果复制到最终结果中。
因此，根据细节， toList 可能比 collect 快得多。