代码
1 public static void main (String[] args) {
2 Stream.of("d2", "a2", "b1", "b3", "c")
3 .sorted((s1, s2) -> {
4 System.out.printf("sort: %s; %s\n", s1, s2);
5 return s1.compareTo(s2);
6 })
7 .forEach(System.out::println);
8 }
执行结果
sort: a2; d2 sort: b1; a2 sort: b1; d2 sort: b1; a2 sort: b3; b1 sort: b3; d2 sort: c; b3 sort: c; d2 a2 b1 b3 c d2
看到结果不淡定了，因此决定调试一下看看内部包装了哪种排序算法，这一调试不得了，发现stream的调用链有点奇怪：
以上这段代码利用了java8中的stream概念，在实际调试过程中，你会发现并不能从sorted()这里直接进入排序部分，由此引出本文。
分析
如果把上面代码中的 .forEach(System.out::println) 去掉，你会发现sorted()函数会被忽略因而根本不会执行，这就涉及到了stream的执行原理。
调用链记录为两大部分：
第一部分:sorted()
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/stream/ReferencePipeline.java
public final Stream<P_OUT> sorted(Comparator<? super P_OUT> comparator) {
return SortedOps.makeRef(this, comparator);
}
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/stream/SortedOps.java
static <T> Stream<T> makeRef(AbstractPipeline<?, T, ?> upstream,
Comparator<? super T> comparator) {
return new OfRef<>(upstream, comparator);
}
OfRef(AbstractPipeline<?, T, ?> upstream, Comparator<? super T> comparator) {
super(upstream, StreamShape.REFERENCE,
StreamOpFlag.IS_ORDERED | StreamOpFlag.NOT_SORTED);
this.isNaturalSort = false;
this.comparator = Objects.requireNonNull(comparator);
}
第二部分：foreach()
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/stream/ReferencePipeline.java
public void forEach(Consumer<? super P_OUT> action) {
evaluate(ForEachOps.makeRef(action, false));
}
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/stream/AbstractPipeline.java
final <R> R evaluate(TerminalOp<E_OUT, R> terminalOp) {
assert getOutputShape() == terminalOp.inputShape();
if (linkedOrConsumed)
throw new IllegalStateException(MSG_STREAM_LINKED);
linkedOrConsumed = true;
return isParallel()
? terminalOp.evaluateParallel(this, sourceSpliterator(terminalOp.getOpFlags()))
: terminalOp.evaluateSequential(this, sourceSpliterator(terminalOp.getOpFlags()));
}
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/stream/ForEachOps.java
public <S> Void evaluateSequential(PipelineHelper<T> helper,
Spliterator<S> spliterator) {
return helper.wrapAndCopyInto(this, spliterator).get();
}
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/stream/AbstractPipeline.java
final <P_IN, S extends Sink<E_OUT>> S wrapAndCopyInto(S sink, Spliterator<P_IN> spliterator) {
copyInto(wrapSink(Objects.requireNonNull(sink)), spliterator);
return sink;
}
final <P_IN> Sink<P_IN> wrapSink(Sink<E_OUT> sink) {
Objects.requireNonNull(sink);
for ( @SuppressWarnings("rawtypes") AbstractPipeline p=AbstractPipeline.this; p.depth > 0; p=p.previousStage) {
sink = p.opWrapSink(p.previousStage.combinedFlags, sink);
}
return (Sink<P_IN>) sink;
}
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/stream/SortedOps.java
public Sink<T> opWrapSink(int flags, Sink<T> sink) {
Objects.requireNonNull(sink);
// If the input is already naturally sorted and this operation
// also naturally sorted then this is a no-op
if (StreamOpFlag.SORTED.isKnown(flags) && isNaturalSort)
return sink;
else if (StreamOpFlag.SIZED.isKnown(flags))
return new SizedRefSortingSink<>(sink, comparator);
else
return new RefSortingSink<>(sink, comparator);
}
SizedRefSortingSink(Sink<? super T> sink, Comparator<? super T> comparator) {
super(sink, comparator);
}
public void end() {
Arrays.sort(array, 0, offset, comparator);
downstream.begin(offset);
if (!cancellationWasRequested) {
for (int i = 0; i < offset; i++)
downstream.accept(array[i]);
}
else {
for (int i = 0; i < offset && !downstream.cancellationRequested(); i++)
downstream.accept(array[i]);
}
downstream.end();
array = null;
}
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/Arrays.java
public static <T> void sort(T[] a, int fromIndex, int toIndex,
Comparator<? super T> c) {
if (c == null) {
sort(a, fromIndex, toIndex);
} else {
rangeCheck(a.length, fromIndex, toIndex);
if (LegacyMergeSort.userRequested)
legacyMergeSort(a, fromIndex, toIndex, c);
else
TimSort.sort(a, fromIndex, toIndex, c, null, 0, 0);
}
}
/Library/Java/JavaVirtualMachines/jdk1.8.0_171.jdk/Contents/Home/src.zip!/java/util/TimSort.java
static <T> void sort(T[] a, int lo, int hi, Comparator<? super T> c,
T[] work, int workBase, int workLen) {
assert c != null && a != null && lo >= 0 && lo <= hi && hi <= a.length;
int nRemaining = hi - lo;
if (nRemaining < 2)
return; // Arrays of size 0 and 1 are always sorted
// If array is small, do a "mini-TimSort" with no merges
if (nRemaining < MIN_MERGE) {
int initRunLen = countRunAndMakeAscending(a, lo, hi, c);
binarySort(a, lo, hi, lo + initRunLen, c);
return;
}
/**
* March over the array once, left to right, finding natural runs,
* extending short natural runs to minRun elements, and merging runs
* to maintain stack invariant.
*/
TimSort<T> ts = new TimSort<>(a, c, work, workBase, workLen);
int minRun = minRunLength(nRemaining);
do {
// Identify next run
int runLen = countRunAndMakeAscending(a, lo, hi, c);
// If run is short, extend to min(minRun, nRemaining)
if (runLen < minRun) {
int force = nRemaining <= minRun ? nRemaining : minRun;
binarySort(a, lo, lo + force, lo + runLen, c);
runLen = force;
}
// Push run onto pending-run stack, and maybe merge
ts.pushRun(lo, runLen);
ts.mergeCollapse();
// Advance to find next run
lo += runLen;
nRemaining -= runLen;
} while (nRemaining != 0);
// Merge all remaining runs to complete sort
assert lo == hi;
ts.mergeForceCollapse();
assert ts.stackSize == 1;
}
第3部分：compareTo()
这部分就不贴了，比较器
从以上调用链可以看出，sorted()之后首先进入foreach()，然后在foreach()中调用了Timsort()排序算法，最后又调用比较器；很明显，这个调用过程和程序顺序不一样。由于刚刚接触，就先把调用链记录下来，以后再补充深层原理。
原文地址：https://www.cnblogs.com/xiaoxi666/p/9495202.html
时间： 2024-08-22 15:34:51