--- a/src/java.base/share/classes/java/util/stream/AbstractTask.java	Wed Nov 15 11:50:55 2017 -0800
+++ b/src/java.base/share/classes/java/util/stream/AbstractTask.java	Wed Nov 08 10:27:10 2017 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2017, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -27,6 +27,7 @@
 import java.util.Spliterator;
 import java.util.concurrent.CountedCompleter;
 import java.util.concurrent.ForkJoinPool;
+import java.util.concurrent.ForkJoinWorkerThread;
 
 /**
  * Abstract base class for most fork-join tasks used to implement stream ops.
@@ -88,13 +89,7 @@
                             K extends AbstractTask<P_IN, P_OUT, R, K>>
         extends CountedCompleter<R> {
 
-    /**
-     * Default target factor of leaf tasks for parallel decomposition.
-     * To allow load balancing, we over-partition, currently to approximately
-     * four tasks per processor, which enables others to help out
-     * if leaf tasks are uneven or some processors are otherwise busy.
-     */
-    static final int LEAF_TARGET = ForkJoinPool.getCommonPoolParallelism() << 2;
+    private static final int LEAF_TARGET = ForkJoinPool.getCommonPoolParallelism() << 2;
 
     /** The pipeline helper, common to all tasks in a computation */
     protected final PipelineHelper<P_OUT> helper;
@@ -157,6 +152,22 @@
     }
 
     /**
+     * Default target of leaf tasks for parallel decomposition.
+     * To allow load balancing, we over-partition, currently to approximately
+     * four tasks per processor, which enables others to help out
+     * if leaf tasks are uneven or some processors are otherwise busy.
+     */
+    public static int getLeafTarget() {
+        Thread t = Thread.currentThread();
+        if (t instanceof ForkJoinWorkerThread) {
+            return ((ForkJoinWorkerThread) t).getPool().getParallelism() << 2;
+        }
+        else {
+            return LEAF_TARGET;
+        }
+    }
+
+    /**
      * Constructs a new node of type T whose parent is the receiver; must call
      * the AbstractTask(T, Spliterator) constructor with the receiver and the
      * provided Spliterator.
@@ -181,7 +192,7 @@
      * @return suggested target leaf size
      */
     public static long suggestTargetSize(long sizeEstimate) {
-        long est = sizeEstimate / LEAF_TARGET;
+        long est = sizeEstimate / getLeafTarget();
         return est > 0L ? est : 1L;
     }
 

--- a/src/java.base/share/classes/java/util/stream/ForEachOps.java	Wed Nov 15 11:50:55 2017 -0800
+++ b/src/java.base/share/classes/java/util/stream/ForEachOps.java	Wed Nov 08 10:27:10 2017 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2017, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -28,7 +28,6 @@
 import java.util.Spliterator;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CountedCompleter;
-import java.util.concurrent.ForkJoinTask;
 import java.util.function.Consumer;
 import java.util.function.DoubleConsumer;
 import java.util.function.IntConsumer;
@@ -378,7 +377,7 @@
             this.spliterator = spliterator;
             this.targetSize = AbstractTask.suggestTargetSize(spliterator.estimateSize());
             // Size map to avoid concurrent re-sizes
-            this.completionMap = new ConcurrentHashMap<>(Math.max(16, AbstractTask.LEAF_TARGET << 1));
+            this.completionMap = new ConcurrentHashMap<>(Math.max(16, AbstractTask.getLeafTarget() << 1));
             this.action = action;
             this.leftPredecessor = null;
         }

--- a/src/java.base/share/classes/java/util/stream/StreamSpliterators.java	Wed Nov 15 11:50:55 2017 -0800
+++ b/src/java.base/share/classes/java/util/stream/StreamSpliterators.java	Wed Nov 08 10:27:10 2017 -0800
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2016, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2017, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -897,7 +897,7 @@
      * Note: The source spliterator may report {@code ORDERED} since that
      * spliterator be the result of a previous pipeline stage that was
      * collected to a {@code Node}. It is the order of the pipeline stage
-     * that governs whether the this slice spliterator is to be used or not.
+     * that governs whether this slice spliterator is to be used or not.
      */
     abstract static class UnorderedSliceSpliterator<T, T_SPLITR extends Spliterator<T>> {
         static final int CHUNK_SIZE = 1 << 7;
@@ -914,7 +914,7 @@
             this.unlimited = limit < 0;
             this.skipThreshold = limit >= 0 ? limit : 0;
             this.chunkSize = limit >= 0 ? (int)Math.min(CHUNK_SIZE,
-                ((skip + limit) / AbstractTask.LEAF_TARGET) + 1) : CHUNK_SIZE;
+                                                        ((skip + limit) / AbstractTask.getLeafTarget()) + 1) : CHUNK_SIZE;
             this.permits = new AtomicLong(limit >= 0 ? skip + limit : skip);
         }
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/jdk/java/util/stream/CustomFJPoolTest.java	Wed Nov 08 10:27:10 2017 -0800
@@ -0,0 +1,154 @@
+/*
+ * Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @summary Tests stream execution in a custom ForkJoinPool
+ * @bug 8190974
+ * @run testng/othervm CustomFJPoolTest
+ * @run testng/othervm -Djava.util.concurrent.ForkJoinPool.common.parallelism=0 CustomFJPoolTest
+ */
+
+import org.testng.annotations.Test;
+
+import java.util.Comparator;
+import java.util.Spliterator;
+import java.util.concurrent.ForkJoinPool;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.function.Consumer;
+import java.util.stream.IntStream;
+import java.util.stream.StreamSupport;
+
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertTrue;
+
+@Test
+public class CustomFJPoolTest {
+
+    // A Spliterator that counts the number of spliterators created
+    // including itself, thus the count starts at 1
+    static class SplitCountingSpliterator<T> implements Spliterator<T> {
+        final Spliterator<T> s;
+        final AtomicInteger nsplits;
+
+        // Top-level constructor
+        public SplitCountingSpliterator(Spliterator<T> s) {
+            this.s = s;
+            nsplits = new AtomicInteger(1);
+        }
+
+        // Splitting constructor
+        SplitCountingSpliterator(Spliterator<T> s, AtomicInteger nsplits) {
+            this.s = s;
+            this.nsplits = nsplits;
+        }
+
+        int splits() {
+            return nsplits.get();
+        }
+
+        @Override
+
+        public boolean tryAdvance(Consumer<? super T> action) {
+            return s.tryAdvance(action);
+        }
+
+        @Override
+        public void forEachRemaining(Consumer<? super T> action) {
+            s.forEachRemaining(action);
+        }
+
+        @Override
+        public Spliterator<T> trySplit() {
+            var split = s.trySplit();
+            if (split != null) {
+                nsplits.incrementAndGet();
+                return new SplitCountingSpliterator<>(split, nsplits);
+            }
+            else {
+                return null;
+            }
+        }
+
+        @Override
+        public long estimateSize() {
+            return s.estimateSize();
+        }
+
+        @Override
+        public long getExactSizeIfKnown() {
+            return s.getExactSizeIfKnown();
+        }
+
+        @Override
+        public int characteristics() {
+            return s.characteristics();
+        }
+
+        @Override
+        public boolean hasCharacteristics(int characteristics) {
+            return s.hasCharacteristics(characteristics);
+        }
+
+        @Override
+        public Comparator<? super T> getComparator() {
+            return s.getComparator();
+        }
+    }
+
+    public void testCustomPools() throws Exception {
+        int splitsForP1 = countSplits(new ForkJoinPool(1));
+        int splitsForP2 = countSplits(new ForkJoinPool(2));
+        assertEquals(splitsForP2, splitsForP1 * 2);
+
+        int commonParallelism = ForkJoinPool.getCommonPoolParallelism();
+        if (commonParallelism > 1 && commonParallelism < 128) {
+            int splitsForPHalfC = countSplits(new ForkJoinPool(commonParallelism / 2));
+            int splitsForPC = countSplits(ForkJoinPool.commonPool());
+
+            assertTrue(splitsForPHalfC < splitsForPC);
+            assertEquals(splitsForPC / splitsForPHalfC,
+                         nearestPowerOfTwo(commonParallelism) / nearestPowerOfTwo(commonParallelism / 2));
+        }
+    }
+
+    static int countSplits(ForkJoinPool fjp) throws Exception {
+        // The number of splits will be equivalent to the number of leaf nodes
+        // and will be a power of 2
+        var fInteger = fjp.submit(() -> {
+            var s = IntStream.range(0, 1024).boxed().parallel().spliterator();
+            var cs = new SplitCountingSpliterator<>(s);
+            StreamSupport.stream(cs, true).forEach(e -> {});
+            return cs.splits();
+        });
+        return fInteger.get();
+    }
+
+    static int nearestPowerOfTwo(int i) {
+        return (i & (i - 1)) == 0
+               ? i
+               : 1 << (32 - Integer.numberOfLeadingZeros(i));
+    }
+}