JAVA8学习——Stream底层的实现一（学习过程）
Stream底层的实现
Stream接口实现了 BaseStream 接口，我们先来看看BaseStream的定义
BaseStream#
BaseStream是所有流的父类接口。
对JavaDoc做一次解读，了解提供的所有方法。#
/**
* Base interface for streams, which are sequences of elements supporting
* sequential and parallel aggregate operations. The following example
* illustrates an aggregate operation using the stream types {@link Stream}
* and {@link IntStream}, computing the sum of the weights of the red widgets:
*
* <pre>{@code
* int sum = widgets.stream()
* .filter(w -> w.getColor() == RED)
* .mapToInt(w -> w.getWeight())
* .sum();
* }</pre>
*
* See the class documentation for {@link Stream} and the package documentation
* for <a href="package-summary.html">java.util.stream</a> for additional
* specification of streams, stream operations, stream pipelines, and
* parallelism, which governs the behavior of all stream types.
*
* @param <T> the type of the stream elements
* @param <S> the type of of the stream implementing {@code BaseStream}
S 代表中间操作产生的新的流操作。
* @since 1.8
* @see Stream
* @see IntStream
* @see LongStream
* @see DoubleStream
* @see <a href="package-summary.html">java.util.stream</a>
*/
public interface BaseStream<T, S extends BaseStream<T, S>>
extends AutoCloseable {
/**
* Returns an iterator for the elements of this stream.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @return the element iterator for this stream
*/
Iterator<T> iterator(); //迭代器 ，针对于流中元素类型*（T）的迭代器
/**
* Returns a spliterator for the elements of this stream.
*
* <p>This is a <a href="package-summary.html#StreamOps">terminal
* operation</a>.
*
* @return the element spliterator for this stream
*/
Spliterator<T> spliterator(); //分割迭代器， 流中的核心的操作。
/**
* Returns whether this stream, if a terminal operation were to be executed,
* would execute in parallel. Calling this method after invoking an
* terminal stream operation method may yield unpredictable results.
*
* @return {@code true} if this stream would execute in parallel if executed
*/
boolean isParallel(); //是否并行
/**
* Returns an equivalent stream that is sequential. May return
* itself, either because the stream was already sequential, or because
* the underlying stream state was modified to be sequential.，.
返回一个等价的串行流，有可能返回流本身，或者是流修改成串行流的
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @return a sequential stream
*/
S sequential(); //返回值为S：流，新的流对象
/**
* Returns an equivalent stream that is parallel. May return
* itself, either because the stream was already parallel, or because
* the underlying stream state was modified to be parallel.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @return a parallel stream
*/
S parallel();
/**
* Returns an equivalent stream that is
* <a href="package-summary.html#Ordering">unordered</a>. May return
* itself, either because the stream was already unordered, or because
* the underlying stream state was modified to be unordered.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @return an unordered stream
*/
S unordered();
/**
* Returns an equivalent stream with an additional close handler. Close
* handlers are run when the {@link #close()} method
* is called on the stream, and are executed in the order they were
* added. All close handlers are run, even if earlier close handlers throw
* exceptions. If any close handler throws an exception, the first
* exception thrown will be relayed to the caller of {@code close()}, with
* any remaining exceptions added to that exception as suppressed exceptions
* (unless one of the remaining exceptions is the same exception as the
* first exception, since an exception cannot suppress itself.) May
* return itself.
返回值为流。流中带了一个关闭处理器、关闭处理器调用的是 close（）方法。
按照被添加的顺序去关闭。
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param closeHandler A task to execute when the stream is closed
* @return a stream with a handler that is run if the stream is closed
*/
S onClose(Runnable closeHandler);
/**
* Closes this stream, causing all close handlers for this stream pipeline
* to be called.
*
* @see AutoCloseable#close()
*/
@Override
void close();
}
对onClose关闭处理器做单独的认识#
public class StreamTest2 {
public static void main(String[] args) {
List<String> list = Arrays.asList("hello", "world", "hello world");
// list.stream().onClose(()-> System.out.println("aaa")).onClose(()-> System.out.println("bbb")).forEach(System.out::println);
try (Stream<String> stream = list.stream()){
stream.onClose(()-> {
System.out.println("aaa");
throw new NullPointerException("first Exception");
}).onClose(()->{
System.out.println("bbb");
throw new ArithmeticException("first Exception");
}).forEach(System.out::println);
}
}
}
Exception in thread "main" java.lang.NullPointerException: first Exception
at com.dawa.jdk8.StreamTest2.lambda$main$0(StreamTest2.java:21)
at java.util.stream.Streams$1.run(Streams.java:850)
at java.util.stream.AbstractPipeline.close(AbstractPipeline.java:323)
at com.dawa.jdk8.StreamTest2.main(StreamTest2.java:26)
Suppressed: java.lang.ArithmeticException: first Exception
at com.dawa.jdk8.StreamTest2.lambda$main$1(StreamTest2.java:24)
at java.util.stream.Streams$1.run(Streams.java:854)
... 2 more
几种可能的情况
- //运行结果就可以看到 aa,bbb被调用。
- //也可以看到压制异常
- //如果两个地方的异常是相同的异常对象，则第二个异常不会被压制。因为异常是自己不会压制自己的。
- //如果是同一种异常，但是不是同一个异常，还是会压制的。
备注：遇到问题，javadoc里面已经写的很清楚了。往往每个人都伸手可得的内容，容易最被忽视掉。
Stream源码分析#
引入Example，跟源码。
public static void main(String[] args) {
List<String> list = Arrays.asList("hello", "world", "hello world");
list.stream().forEach(System.out::println);
}
1. stream()#
来自Collection接口中的默认方法。
/**
* Returns a sequential {@code Stream} with this collection as its source.
*
* <p>This method should be overridden when the {@link #spliterator()}
* method cannot return a spliterator that is {@code IMMUTABLE},
* {@code CONCURRENT}, or <em>late-binding</em>. (See {@link #spliterator()}
* for details.)
当这个 spliterator（）无法返回这三个（不可变的，并行的，延迟绑定的）类型中的一个的话，
这个方法需要被重写。
*
* @implSpec
* The default implementation creates a sequential {@code Stream} from the
* collection's {@code Spliterator}.
返回一个针对于当前元素的一个串行流。
*
* @return a sequential {@code Stream} over the elements in this collection
* @since 1.8
*/
default Stream<E> stream() {
return StreamSupport.stream(spliterator(), false);
}
所以，这里就要先了解一下spliterator()这个方法是怎么实现的。
spliterator()的源码实现#
实现方法和stream（）一样，在Collection接口中的默认方法
/**
* Creates a {@link Spliterator} over the elements in this collection.
*
* Implementations should document characteristic values reported by the
* spliterator. Such characteristic values are not required to be reported
* if the spliterator reports {@link Spliterator#SIZED} and this collection
* contains no elements.
Spliterator#SIZED，集合，固定大小，并且没有值。的时候是不用报告的。
--备注：和collectors的characteristic特性值
*
* <p>The default implementation should be overridden by subclasses that
* can return a more efficient spliterator. In order to
* preserve expected laziness behavior for the {@link #stream()} and
* {@link #parallelStream()}} methods, spliterators should either have the
* characteristic of {@code IMMUTABLE} or {@code CONCURRENT}, or be
* <em><a href="Spliterator.html#binding">late-binding</a></em>.
* If none of these is practical, the overriding class should describe the
* spliterator's documented policy of binding and structural interference,
* and should override the {@link #stream()} and {@link #parallelStream()}
* methods to create streams using a {@code Supplier} of the spliterator,
* as in:
默认的实现，应该被子类所重写。为了保留期望的stream（）的延迟行为。分割迭代器的特性值 只有在满足{@code IMMUTABLE} or {@code CONCURRENT}的时候，才是具有延迟行为的。
如果上面条件都无法做的话，重写的类应该去描述这个分割迭代器的文档
并且重写。
用下面的这种方式去定义。
* <pre>{@code
* Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)
* }</pre>
* <p>These requirements ensure that streams produced by the
* {@link #stream()} and {@link #parallelStream()} methods will reflect the
* contents of the collection as of initiation of the terminal stream
* operation.
这些要求确保了由这两个方法生成的流，反应了流的内容 （在终止流操作执行的一瞬间）
*
* @implSpec
* The default implementation creates a
* <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
* from the collections's {@code Iterator}. The spliterator inherits the
* <em>fail-fast</em> properties of the collection's iterator.
默认的实现 从集合的迭代器中，创建一个延迟绑定的分割迭代器。 分割迭代器会继承迭代器的快速失败的属性。
* <p>
* The created {@code Spliterator} reports {@link Spliterator#SIZED}.
创建的分割迭代器，会携带一个 Spliterator#SIZED （固定大小的）的特性值
*
* @implNote
* The created {@code Spliterator} additionally reports
* {@link Spliterator#SUBSIZED}.
还会额外的增加一个Spliterator#SUBSIZED（子大小）的确定。
*
* <p>If a spliterator covers no elements then the reporting of additional
* characteristic values, beyond that of {@code SIZED} and {@code SUBSIZED},
* does not aid clients to control, specialize or simplify computation.
* However, this does enable shared use of an immutable and empty
* spliterator instance (see {@link Spliterators#emptySpliterator()}) for
* empty collections, and enables clients to determine if such a spliterator
* covers no elements.
如果分割迭代器里面没有元素，那么除了 {@code SIZED} and {@code SUBSIZED}之外其他的特性，对于计算的控制是没有帮助作用的。 不过可以促进空的迭代器的共享使用。 参见： Spliterators#emptySpliterator()、
对于一个空的迭代器可以判断是不是没有元素
*
* @return a {@code Spliterator} over the elements in this collection
* @since 1.8
*/
@Override
default Spliterator<E> spliterator() {
return Spliterators.spliterator(this, 0);
}
那么，到底什么是分割迭代器？ Spliterator
到底什么是分割迭代器 —— Spliterator类#
和Collector收集器一样，同时提供了collector接口和 Collectors的工具类
public final class Spliterators {}
public interface Spliterator<T> {}
我们先来看看Spliterator接口的javadoc#
/**
* An object for traversing and partitioning elements of a source. The source
* of elements covered by a Spliterator could be, for example, an array, a
* {@link Collection}, an IO channel, or a generator function.
一个分割迭代器，是一个对象，用于对源中的元素进行遍历和分区。
源可以是：数组，集合，或者IO通道
*
* <p>A Spliterator may traverse elements individually ({@link
* #tryAdvance tryAdvance()}) or sequentially in bulk
* ({@link #forEachRemaining forEachRemaining()}).
一个迭代器可以一个一个的去遍历。tryAdvance()
也可以以块的方式去遍历。 forEachRemaining()
*
* <p>A Spliterator may also partition off some of its elements (using
* {@link #trySplit}) as another Spliterator, to be used in
* possibly-parallel operations. Operations using a Spliterator that
* cannot split, or does so in a highly imbalanced or inefficient
* manner, are unlikely to benefit from parallelism. Traversal
* and splitting exhaust elements; each Spliterator is useful for only a single
* bulk computation.
也可以使用 trySplit（） 对元素进行分区，形成一个新的元素迭代器。也可以以并行的方式去操作。
使用Spliterator的操作，是不能分割，或者效率非常低的分割， 如果用并行的话，不会获得很大的收益。
（比如，100个元素，分区分为 2+98，这种的就是非常低效的。就无法利用并行的优势了。）
每一个分割迭代器，只对自己特定的块有用。
*
* <p>A Spliterator also reports a set of {@link #characteristics()} of its
* structure, source, and elements from among {@link #ORDERED},
* {@link #DISTINCT}, {@link #SORTED}, {@link #SIZED}, {@link #NONNULL},
* {@link #IMMUTABLE}, {@link #CONCURRENT}, and {@link #SUBSIZED}. These may
* be employed by Spliterator clients to control, specialize or simplify
* computation. For example, a Spliterator for a {@link Collection} would
* report {@code SIZED}, a Spliterator for a {@link Set} would report
* {@code DISTINCT}, and a Spliterator for a {@link SortedSet} would also
* report {@code SORTED}. Characteristics are reported as a simple unioned bit
* set.
分割迭代器还会 设置 特性值
{@link #ORDERED},
{@link #DISTINCT},
{@link #SORTED},
{@link #SIZED},
{@link #NONNULL},
{@link #IMMUTABLE},
{@link #CONCURRENT},
{@link #SUBSIZED}.
这些属性用来控制特定的某些计算。
比如说，一个 Collection就需要SIZED特性值
Set需要DISTINCT
*
* Some characteristics additionally constrain method behavior; for example if
* {@code ORDERED}, traversal methods must conform to their documented ordering.
* New characteristics may be defined in the future, so implementors should not
* assign meanings to unlisted values.
不要给没有列出来的值赋予新的含义。
*
* <p><a name="binding">A Spliterator that does not report {@code IMMUTABLE} or
* {@code CONCURRENT} is expected to have a documented policy concerning:
* when the spliterator <em>binds</em> to the element source; and detection of
* structural interference of the element source detected after binding.</a> A
* <em>late-binding</em> Spliterator binds to the source of elements at the
* point of first traversal, first split, or first query for estimated size,
* rather than at the time the Spliterator is created. A Spliterator that is
* not <em>late-binding</em> binds to the source of elements at the point of
* construction or first invocation of any method. Modifications made to the
* source prior to binding are reflected when the Spliterator is traversed.
* After binding a Spliterator should, on a best-effort basis, throw
* {@link ConcurrentModificationException} if structural interference is
* detected. Spliterators that do this are called <em>fail-fast</em>. The
* bulk traversal method ({@link #forEachRemaining forEachRemaining()}) of a
* Spliterator may optimize traversal and check for structural interference
* after all elements have been traversed, rather than checking per-element and
* failing immediately.
并不是说一个迭代器在创建的时候就被绑定到源上面了。而是在满足首次遍历，首次分割，首次查询的时候，才进行绑定。
ConcurrentModificationException，在绑定之前操作，会出现这一行的异常。
叫做 快速失败。
*
* <p>Spliterators can provide an estimate of the number of remaining elements
* via the {@link #estimateSize} method. Ideally, as reflected in characteristic
* {@link #SIZED}, this value corresponds exactly to the number of elements
* that would be encountered in a successful traversal. However, even when not
* exactly known, an estimated value value may still be useful to operations
* being performed on the source, such as helping to determine whether it is
* preferable to split further or traverse the remaining elements sequentially.
*
* <p>Despite their obvious utility in parallel algorithms, spliterators are not
* expected to be thread-safe; instead, implementations of parallel algorithms
* using spliterators should ensure that the spliterator is only used by one
* thread at a time. This is generally easy to attain via <em>serial
* thread-confinement</em>, which often is a natural consequence of typical
* parallel algorithms that work by recursive decomposition. A thread calling
* {@link #trySplit()} may hand over the returned Spliterator to another thread,
* which in turn may traverse or further split that Spliterator. The behaviour
* of splitting and traversal is undefined if two or more threads operate
* concurrently on the same spliterator. If the original thread hands a
* spliterator off to another thread for processing, it is best if that handoff
* occurs before any elements are consumed with {@link #tryAdvance(Consumer)
* tryAdvance()}, as certain guarantees (such as the accuracy of
* {@link #estimateSize()} for {@code SIZED} spliterators) are only valid before
* traversal has begun.
*
serial-thread-confinement : 线程安全围栏
* <p>Primitive subtype specializations of {@code Spliterator} are provided for
* {@link OfInt int}, {@link OfLong long}, and {@link OfDouble double} values.
* The subtype default implementations of
* {@link Spliterator#tryAdvance(java.util.function.Consumer)}
* and {@link Spliterator#forEachRemaining(java.util.function.Consumer)} box
* primitive values to instances of their corresponding wrapper class. Such
* boxing may undermine any performance advantages gained by using the primitive
* specializations. To avoid boxing, the corresponding primitive-based methods
* should be used.
tryAdvance()方法和forEachRemaining()
提供了原生子类型的特化， int, long, doule 等，子类型默认的实现。
避免包装类型装箱拆箱操作。
如下。
For example, 如下特化版本.
* {@link Spliterator.OfInt#tryAdvance(java.util.function.IntConsumer)}
* and {@link Spliterator.OfInt#forEachRemaining(java.util.function.IntConsumer)}
* should be used in preference to
* {@link Spliterator.OfInt#tryAdvance(java.util.function.Consumer)} and
* {@link Spliterator.OfInt#forEachRemaining(java.util.function.Consumer)}.
* Traversal of primitive values using boxing-based methods
* {@link #tryAdvance tryAdvance()} and
* {@link #forEachRemaining(java.util.function.Consumer) forEachRemaining()}
* does not affect the order in which the values, transformed to boxed values,
* are encountered.
*
* @apiNote
* <p>Spliterators, like {@code Iterator}s, are for traversing the elements of
* a source. The {@code Spliterator} API was designed to support efficient
* parallel traversal in addition to sequential traversal, by supporting
* decomposition as well as single-element iteration. In addition, the
* protocol for accessing elements via a Spliterator is designed to impose
* smaller per-element overhead than {@code Iterator}, an d to avoid the inherent
* race involved in having separate methods for {@code hasNext()} and
* {@code next()}.
Spliterator支持高效的，并行的操作。
支持解耦，分解，氮元素的遍历。
此外，通过accessing协议。。。 相对于 Iterator，遍历元素的时候成本更低。
原因： 之前的{@code hasNext()} and {@code next()}.搭配使用存在竞争。
现在直接使用一个tryAdvance()方法就解决了这两个方法实现的事情。
*
* <p>For mutable sources, arbitrary and non-deterministic behavior may occur if
* the source is structurally interfered with (elements added, replaced, or
* removed) between the time that the Spliterator binds to its data source and
* the end of traversal. For example, such interference will produce arbitrary,
* non-deterministic results when using the {@code java.util.stream} framework.
如果源在结构上被修改了（增删改），在绑定迭代器之后和执行完毕之前这段时间内进行任意修改。
行为就是不确定的了。
所以在使用流框架的时候，要求源是不可变的
*
* <p>Structural interference of a source can be managed in the following ways
* (in approximate order of decreasing desirability):
源结构上的修改，是可以通过如下几个方式去修改的
* <ul>
* <li>The source cannot be structurally interfered with.
如，源是不允许被修改的。
* <br>For example, an instance of
* {@link java.util.concurrent.CopyOnWriteArrayList} is an immutable source.
* A Spliterator created from the source reports a characteristic of
* {@code IMMUTABLE}.</li>
如：CopyOnWriteArrayList是一个不可变的源。
先拷贝，再追加。 （但是效率会下降。）适合读多写少的操作。
* <li>The source manages concurrent modifications.
源本身自己去管理并发。
* <br>For example, a key set of a {@link java.util.concurrent.ConcurrentHashMap}
* is a concurrent source. A Spliterator created from the source reports a
* characteristic of {@code CONCURRENT}.</li>
如：ConcurrentHashMap 。 创建的是并发源
* <li>The mutable source provides a late-binding and fail-fast Spliterator.
* <br>Late binding narrows the window during which interference can affect
* the calculation; fail-fast detects, on a best-effort basis, that structural
* interference has occurred after traversal has commenced and throws
* {@link ConcurrentModificationException}. For example, {@link ArrayList},
* and many other non-concurrent {@code Collection} classes in the JDK, provide
* a late-binding, fail-fast spliterator.</li>
可变的源提供了延迟绑定和快速失败的迭代分割器。
会限制时间点的缩短。
如果在遍历中修改，则会抛出ConcurrentModificationException。
* <li>The mutable source provides a non-late-binding but fail-fast Spliterator.
* <br>The source increases the likelihood of throwing
* {@code ConcurrentModificationException} since the window of potential
* interference is larger.</li>
* <li>The mutable source provides a late-binding and non-fail-fast Spliterator.
* <br>The source risks arbitrary, non-deterministic behavior after traversal
* has commenced since interference is not detected.
* </li>
* <li>The mutable source provides a non-late-binding and non-fail-fast
* Spliterator.
* <br>The source increases the risk of arbitrary, non-deterministic behavior
* since non-detected interference may occur after construction.
* </li>
总结。
1. 源是不是并发的
2. 是不是快速绑定的，是不是快速失败的（2*2 组合的四种情况。）
* </ul>
*
串行案例
* <p><b>Example.</b> Here is a class (not a very useful one, except
* for illustration) that maintains an array in which the actual data
* are held in even locations, and unrelated tag data are held in odd
* locations. Its Spliterator ignores the tags.
如：类维护了一个数组。 实际的数据是在偶数的位置上存放。不想管的标签数据是存放在奇数位置上。
*
* <pre> {@code
* class TaggedArray<T> {
* private final Object[] elements; // immutable after construction
* TaggedArray(T[] data, Object[] tags) {
* int size = data.length;
* if (tags.length != size) throw new IllegalArgumentException();
* this.elements = new Object[2 * size];
* for (int i = 0, j = 0; i < size; ++i) {
* elements[j++] = data[i];
* elements[j++] = tags[i];
* }
* }
*
* public Spliterator<T> spliterator() {
* return new TaggedArraySpliterator<>(elements, 0, elements.length);
* }
*
* static class TaggedArraySpliterator<T> implements Spliterator<T> {
* private final Object[] array;
* private int origin; // current index, advanced on split or traversal
* private final int fence; // one past the greatest index
*
* TaggedArraySpliterator(Object[] array, int origin, int fence) {
* this.array = array; this.origin = origin; this.fence = fence;
* }
*
* public void forEachRemaining(Consumer<? super T> action) {
* for (; origin < fence; origin += 2)
* action.accept((T) array[origin]);
* }
*
* public boolean tryAdvance(Consumer<? super T> action) {
* if (origin < fence) {
* action.accept((T) array[origin]);
* origin += 2;
* return true;
* }
* else // cannot advance
* return false;
* }
*
* public Spliterator<T> trySplit() {
* int lo = origin; // divide range in half
* int mid = ((lo + fence) >>> 1) & ~1; // force midpoint to be even
* if (lo < mid) { // split out left half
* origin = mid; // reset this Spliterator's origin
* return new TaggedArraySpliterator<>(array, lo, mid);
* }
* else // too small to split
* return null;
* }
*
* public long estimateSize() {
* return (long)((fence - origin) / 2);
* }
*
* public int characteristics() {
* return ORDERED | SIZED | IMMUTABLE | SUBSIZED;
* }
* }
* }}</pre>
*
并行案例：
* <p>As an example how a parallel computation framework, such as the
* {@code java.util.stream} package, would use Spliterator in a parallel
* computation, here is one way to implement an associated parallel forEach,
* that illustrates the primary usage idiom of splitting off subtasks until
* the estimated amount of work is small enough to perform
* sequentially. Here we assume that the order of processing across
* subtasks doesn't matter; different (forked) tasks may further split
* and process elements concurrently in undetermined order. This
* example uses a {@link java.util.concurrent.CountedCompleter};
* similar usages apply to other parallel task constructions.
*
* <pre>{@code
* static <T> void parEach(TaggedArray<T> a, Consumer<T> action) {
* Spliterator<T> s = a.spliterator();
* long targetBatchSize = s.estimateSize() / (ForkJoinPool.getCommonPoolParallelism() * 8);
* new ParEach(null, s, action, targetBatchSize).invoke();
* }
*
* static class ParEach<T> extends CountedCompleter<Void> {
* final Spliterator<T> spliterator;
* final Consumer<T> action;
* final long targetBatchSize;
*
* ParEach(ParEach<T> parent, Spliterator<T> spliterator,
* Consumer<T> action, long targetBatchSize) {
* super(parent);
* this.spliterator = spliterator; this.action = action;
* this.targetBatchSize = targetBatchSize;
* }
*
* public void compute() {
* Spliterator<T> sub;
* while (spliterator.estimateSize() > targetBatchSize &&
* (sub = spliterator.trySplit()) != null) {
* addToPendingCount(1);
* new ParEach<>(this, sub, action, targetBatchSize).fork();
* }
* spliterator.forEachRemaining(action);
* propagateCompletion();
* }
* }}</pre>
*
* @implNote
* If the boolean system property {@code org.openjdk.java.util.stream.tripwire}
* is set to {@code true} then diagnostic warnings are reported if boxing of
* primitive values occur when operating on primitive subtype specializations.
*
* @param <T> the type of elements returned by this Spliterator
*
* @see Collection
* @since 1.8
*/
- serial-thread-confinement
我们再来看看Spliterator类中的方法#
- tryAdvance() 尝试遍历，对元素执行动作。
/**
* If a remaining element exists, performs the given action on it,
* returning {@code true}; else returns {@code false}. If this
* Spliterator is {@link #ORDERED} the action is performed on the
* next element in encounter order. Exceptions thrown by the
* action are relayed to the caller.
*
* @param action The action
* @return {@code false} if no remaining elements existed
* upon entry to this method, else {@code true}.
* @throws NullPointerException if the specified action is null
*/
boolean tryAdvance(Consumer<? super T> action);
- forEachRemaining() 。通过函数式接口 调用tryAdvance().
/**
* Performs the given action for each remaining element, sequentially in
* the current thread, until all elements have been processed or the action
* throws an exception. If this Spliterator is {@link #ORDERED}, actions
* are performed in encounter order. Exceptions thrown by the action
* are relayed to the caller.
*
* @implSpec
* The default implementation repeatedly invokes {@link #tryAdvance} until
* it returns {@code false}. It should be overridden whenever possible.
*
* @param action The action
* @throws NullPointerException if the specified action is null
*/
default void forEachRemaining(Consumer<? super T> action) {
do { } while (tryAdvance(action));
}
- trySplit() 尝试进行分割
/**
* If this spliterator can be partitioned, returns a Spliterator
* covering elements, that will, upon return from this method, not
* be covered by this Spliterator.
如果这个分割迭代器能够被进行分割。就会返回一个 涵盖这个元素的Spliterator，
分割出来的新的Spliterator可能会被继续分割，剩下的继续又当前的Spliterator涵盖
*
* <p>If this Spliterator is {@link #ORDERED}, the returned Spliterator
* must cover a strict prefix of the elements.
如果 Spliterator is {@link #ORDERED}。返回的必须是ORDERED的
*
* <p>Unless this Spliterator covers an infinite number of elements,
* repeated calls to {@code trySplit()} must eventually return {@code null}.
除非这个 Spliterator 涵盖的事一个无限的元素。
否则，必须被确认返回个数是确定的。
重复的去继续分割，分割到不能再分割。 （一定会有这样的情况。）
* Upon non-null return:
* <ul>
* <li>the value reported for {@code estimateSize()} before splitting,
* must, after splitting, be greater than or equal to {@code estimateSize()}
* for this and the returned Spliterator; and</li>
* <li>if this Spliterator is {@code SUBSIZED}, then {@code estimateSize()}
* for this spliterator before splitting must be equal to the sum of
* {@code estimateSize()} for this and the returned Spliterator after
* splitting.</li>
* </ul>
如果不会空：
分割前的 estimateSize()的返回值，必须大于等于分割之后estimateSize()的返回值。
如果 Spliterator is {@code SUBSIZED}，那么 分割之前 estimateSize()的大小，必须等于 分割之后的 estimateSize() 和返回来的值的大小。（分割前后：必须 8 = 4+4.）
*
* <p>This method may return {@code null} for any reason,
* including emptiness, inability to split after traversal has
* commenced, data structure constraints, and efficiency
* considerations.
这个放个出于以下原因，都会返回Null值
1. emptiness
*
* @apiNote
* An ideal {@code trySplit} method efficiently (without
* traversal) divides its elements exactly in half, allowing
* balanced parallel computation. Many departures from this ideal
* remain highly effective; for example, only approximately
* splitting an approximately balanced tree, or for a tree in
* which leaf nodes may contain either one or two elements,
* failing to further split these nodes. However, large
* deviations in balance and/or overly inefficient {@code
* trySplit} mechanics typically result in poor parallel
* performance.
@API文档
一种理想的trySplit()方法，会恰好将元素分为两半。允许平衡的并行计算。
很多情况下违背了这种理想的情况。
比如说：只是分割一个嫉妒不平衡的一个数，数中只有两个节点。等。不能再次进行分割。
然而，很不平衡的这种机制，会导致并发效率的极度降低。
*
* @return a {@code Spliterator} covering some portion of the
* elements, or {@code null} if this spliterator cannot be split
返回一个Spliterator
*/
Spliterator<T> trySplit();
- estimateSize() 估算大小。
/**
* Returns an estimate of the number of elements that would be
* encountered by a {@link #forEachRemaining} traversal, or returns {@link
* Long#MAX_VALUE} if infinite, unknown, or too expensive to compute.
返回元素数量的估算值。（会被forEachRemaining 遍历的元素）
infinite, unknown, or too expensive to compute.这几种情况会返回：MAX_VALUE
*
* <p>If this Spliterator is {@link #SIZED} and has not yet been partially
* traversed or split, or this Spliterator is {@link #SUBSIZED} and has
* not yet been partially traversed, this estimate must be an accurate
* count of elements that would be encountered by a complete traversal.
* Otherwise, this estimate may be arbitrarily inaccurate, but must decrease
* as specified across invocations of {@link #trySplit}.
如果Spliterator是SIZED 或者是SUBSIZED 。那个 这个元素的estimate值一定是精确的。
然而，必须要减少 trySplit 的调用。
*
* @apiNote
* Even an inexact estimate is often useful and inexpensive to compute.
* For example, a sub-spliterator of an approximately balanced binary tree
* may return a value that estimates the number of elements to be half of
* that of its parent; if the root Spliterator does not maintain an
* accurate count, it could estimate size to be the power of two
* corresponding to its maximum depth.
甚至一个不太精确的估算，也是有用的。
*
* @return the estimated size, or {@code Long.MAX_VALUE} if infinite,
* unknown, or too expensive to compute.
*/
long estimateSize();
- getExactSizeIfKnown() 如果知道的话就会返回确定的大小。
/**
* Convenience method that returns {@link #estimateSize()} if this
* Spliterator is {@link #SIZED}, else {@code -1}.
如果Spliterator是SIZED的话， estimateSize就会返回确定的大小。
* @implSpec
* The default implementation returns the result of {@code estimateSize()}
* if the Spliterator reports a characteristic of {@code SIZED}, and
* {@code -1} otherwise.
*
* @return the exact size, if known, else {@code -1}.
*/
default long getExactSizeIfKnown() {
return (characteristics() & SIZED) == 0 ? -1L : estimateSize();
}
- characteristics() 特性值。
/**
* Returns a set of characteristics of this Spliterator and its
* elements. The result is represented as ORed values from {@link
* #ORDERED}, {@link #DISTINCT}, {@link #SORTED}, {@link #SIZED},
* {@link #NONNULL}, {@link #IMMUTABLE}, {@link #CONCURRENT},
* {@link #SUBSIZED}. Repeated calls to {@code characteristics()} on
* a given spliterator, prior to or in-between calls to {@code trySplit},
* should always return the same result.
返回这个Spliterator的特性值的集合。
{@link#ORDERED},
{@link #DISTINCT},
{@link #SORTED},
{@link #SIZED},
{@link #NONNULL},
{@link #IMMUTABLE},
{@link #CONCURRENT},
{@link #SUBSIZED}
这8个，在下面有定义。
*
* <p>If a Spliterator reports an inconsistent set of
* characteristics (either those returned from a single invocation
* or across multiple invocations), no guarantees can be made
* about any computation using this Spliterator.
*
* @apiNote The characteristics of a given spliterator before splitting
* may differ from the characteristics after splitting. For specific
* examples see the characteristic values {@link #SIZED}, {@link #SUBSIZED}
* and {@link #CONCURRENT}.
具体的例子看下面的说明。
*
* @return a representation of characteristics
*/
int characteristics();
- hasCharacteristics(int characteristics) 查看是否包含给定的特性值
/**
* Returns {@code true} if this Spliterator's {@link
* #characteristics} contain all of the given characteristics.
*
* @implSpec
* The default implementation returns true if the corresponding bits
* of the given characteristics are set.
*
* @param characteristics the characteristics to check for
* @return {@code true} if all the specified characteristics are present,
* else {@code false}
*/
default boolean hasCharacteristics(int characteristics) {
return (characteristics() & characteristics) == characteristics;
}
- getComparator() ：抛出一个不可实现的状态异常。
/**
* If this Spliterator's source is {@link #SORTED} by a {@link Comparator},
* returns that {@code Comparator}. If the source is {@code SORTED} in
* {@linkplain Comparable natural order}, returns {@code null}. Otherwise,
* if the source is not {@code SORTED}, throws {@link IllegalStateException}.
如果源是有序的，返回用于排序的 Comparator
如果是按照自然排序的，就返回空 （就不需要比较器）
否则就抛出异常，
*
* @implSpec
* The default implementation always throws {@link IllegalStateException}.
*
* @return a Comparator, or {@code null} if the elements are sorted in the
* natural order.
* @throws IllegalStateException if the spliterator does not report
* a characteristic of {@code SORTED}.
*/
default Comparator<? super T> getComparator() {
throw new IllegalStateException();
}
- 8个特性值
ORDERED
DISTINCT
SORTED
SIZED
NONNULL
IMMUTABLE
CONCURRENT
SUBSIZED
//更多的是用在并发的时候，指定执行哪些内容。
我们再来看看Spliterator中的8种Characteristic#
/**
* Characteristic value signifying that an encounter order is defined for
* elements. If so, this Spliterator guarantees that method
* {@link #trySplit} splits a strict prefix of elements, that method
* {@link #tryAdvance} steps by one element in prefix order, and that
* {@link #forEachRemaining} performs actions in encounter order.
*
* <p>A {@link Collection} has an encounter order if the corresponding
* {@link Collection#iterator} documents an order. If so, the encounter
* order is the same as the documented order. Otherwise, a collection does
* not have an encounter order.
*
* @apiNote Encounter order is guaranteed to be ascending index order for
* any {@link List}. But no order is guaranteed for hash-based collections
* such as {@link HashSet}. Clients of a Spliterator that reports
* {@code ORDERED} are expected to preserve ordering constraints in
* non-commutative parallel computations.
*/
public static final int ORDERED = 0x00000010;
/**
* Characteristic value signifying that, for each pair of
* encountered elements {@code x, y}, {@code !x.equals(y)}. This
* applies for example, to a Spliterator based on a {@link Set}.
*/
public static final int DISTINCT = 0x00000001;
/**
* Characteristic value signifying that encounter order follows a defined
* sort order. If so, method {@link #getComparator()} returns the associated
* Comparator, or {@code null} if all elements are {@link Comparable} and
* are sorted by their natural ordering.
*
* <p>A Spliterator that reports {@code SORTED} must also report
* {@code ORDERED}.
*
* @apiNote The spliterators for {@code Collection} classes in the JDK that
* implement {@link NavigableSet} or {@link SortedSet} report {@code SORTED}.
*/
public static final int SORTED = 0x00000004;
/**
* Characteristic value signifying that the value returned from
* {@code estimateSize()} prior to traversal or splitting represents a
* finite size that, in the absence of structural source modification,
* represents an exact count of the number of elements that would be
* encountered by a complete traversal.
在执行遍历或者分割之前，由estimateSize返回的值，表示一个有序的大小。
表示元素的数量的精确的值。
*
* @apiNote Most Spliterators for Collections, that cover all elements of a
* {@code Collection} report this characteristic. Sub-spliterators, such as
* those for {@link HashSet}, that cover a sub-set of elements and
* approximate their reported size do not.
大部分对于Collections的分割迭代器，一般都会有这个特性值。
*/
public static final int SIZED = 0x00000040;
/**
* Characteristic value signifying that the source guarantees that
* encountered elements will not be {@code null}. (This applies,
* for example, to most concurrent collections, queues, and maps.)
*/
public static final int NONNULL = 0x00000100;
/**
* Characteristic value signifying that the element source cannot be
* structurally modified; that is, elements cannot be added, replaced, or
* removed, so such changes cannot occur during traversal. A Spliterator
* that does not report {@code IMMUTABLE} or {@code CONCURRENT} is expected
* to have a documented policy (for example throwing
* {@link ConcurrentModificationException}) concerning structural
* interference detected during traversal.
指定元素的源是不能被修改的，不能被（be added, replaced, or removed）。
在执行的时候，如果发现被修改，没有返回，则会抛出ConcurrentModificationException并发修改异常。
*/
public static final int IMMUTABLE = 0x00000400;
/**
* Characteristic value signifying that the element source may be safely
* concurrently modified (allowing additions, replacements, and/or removals)
* by multiple threads without external synchronization. If so, the
* Spliterator is expected to have a documented policy concerning the impact
* of modifications during traversal.
表示元素的源能够安全的被并发修改。允许 modified (allowing additions, replacements, and/or removals)。
不需要外部的同步化的操作。Spliterator的提供了允许被修改的策略。
*
* <p>A top-level Spliterator should not report both {@code CONCURRENT} and
* {@code SIZED}, since the finite size, if known, may change if the source
* is concurrently modified during traversal. Such a Spliterator is
* inconsistent and no guarantees can be made about any computation using
* that Spliterator. Sub-spliterators may report {@code SIZED} if the
* sub-split size is known and additions or removals to the source are not
* reflected when traversing.
顶层的Spliterator 不应该同时返回：{@code CONCURRENT} and {@code SIZED}。
因为两者之间存在一定的矛盾性。
这个的Spliterator 是不一直到，
得到的Sub-spliterators 可能会返回SIZED。
*
* @apiNote Most concurrent collections maintain a consistency policy
* guaranteeing accuracy with respect to elements present at the point of
* Spliterator construction, but possibly not reflecting subsequent
* additions or removals.
大多是的这种并发性的集合，都会被维护一定的策略。
：原有的Spliterator ，不会去影响子的Spliterator
*/
public static final int CONCURRENT = 0x00001000;
/**
* Characteristic value signifying that all Spliterators resulting from
* {@code trySplit()} will be both {@link #SIZED} and {@link #SUBSIZED}.
* (This means that all child Spliterators, whether direct or indirect, will
* be {@code SIZED}.)
*
* <p>A Spliterator that does not report {@code SIZED} as required by
* {@code SUBSIZED} is inconsistent and no guarantees can be made about any
* computation using that Spliterator.
A Spliterator如果没有返回要求的SIZED。 是没有明确的保证的。
*
* @apiNote Some spliterators, such as the top-level spliterator for an
* approximately balanced binary tree, will report {@code SIZED} but not
* {@code SUBSIZED}, since it is common to know the size of the entire tree
* but not the exact sizes of subtrees.
有一些Spliterator。如二叉树的整个树的大小，我们得知总的数，但是不知道子的数。
*/
public static final int SUBSIZED = 0x00004000;
以上就是关于spliterator的interface所有内容。
Spliterator都支持哪些事情？上面的8个方法。就是具体功能的实现。
OfPrimitive#
专门针对于原生的迭代器（int, long, double）
/**
* A Spliterator specialized for primitive values.
*
* @param <T> the type of elements returned by this Spliterator. The
* type must be a wrapper type for a primitive type, such as {@code Integer}
* for the primitive {@code int} type.
* @param <T_CONS> the type of primitive consumer. The type must be a
* primitive specialization of {@link java.util.function.Consumer} for
* {@code T}, such as {@link java.util.function.IntConsumer} for
* {@code Integer}.
* @param <T_SPLITR> the type of primitive Spliterator. The type must be
* a primitive specialization of Spliterator for {@code T}, such as
* {@link Spliterator.OfInt} for {@code Integer}.
*
* @see Spliterator.OfInt
* @see Spliterator.OfLong
* @see Spliterator.OfDouble
* @since 1.8
*/
public interface OfPrimitive<T, T_CONS, T_SPLITR extends Spliterator.OfPrimitive<T, T_CONS, T_SPLITR>>
extends Spliterator<T> {
@Override
T_SPLITR trySplit();
/**
* If a remaining element exists, performs the given action on it,
* returning {@code true}; else returns {@code false}. If this
* Spliterator is {@link #ORDERED} the action is performed on the
* next element in encounter order. Exceptions thrown by the
* action are relayed to the caller.
*
* @param action The action
* @return {@code false} if no remaining elements existed
* upon entry to this method, else {@code true}.
* @throws NullPointerException if the specified action is null
*/
@SuppressWarnings("overloads")
boolean tryAdvance(T_CONS action);
/**
* Performs the given action for each remaining element, sequentially in
* the current thread, until all elements have been processed or the
* action throws an exception. If this Spliterator is {@link #ORDERED},
* actions are performed in encounter order. Exceptions thrown by the
* action are relayed to the caller.
*
* @implSpec
* The default implementation repeatedly invokes {@link #tryAdvance}
* until it returns {@code false}. It should be overridden whenever
* possible.
*
* @param action The action
* @throws NullPointerException if the specified action is null
*/
@SuppressWarnings("overloads")
default void forEachRemaining(T_CONS action) {
do { } while (tryAdvance(action));
}
}
提供了三个特化版本。实现了OfPrimitive接口。
- OfInt
- OfLong
- OfDouble
OfInt
public interface OfInt extends OfPrimitive<Integer, IntConsumer, OfInt> {
@Override
OfInt trySplit();
@Override
boolean tryAdvance(IntConsumer action);
@Override
default void forEachRemaining(IntConsumer action) {
do { } while (tryAdvance(action));
}
/**
* {@inheritDoc}
* @implSpec
* If the action is an instance of {@code IntConsumer} then it is cast
* to {@code IntConsumer} and passed to
* {@link #tryAdvance(java.util.function.IntConsumer)}; otherwise
* the action is adapted to an instance of {@code IntConsumer}, by
* boxing the argument of {@code IntConsumer}, and then passed to
* {@link #tryAdvance(java.util.function.IntConsumer)}.
*/
@Override
default boolean tryAdvance(Consumer<? super Integer> action) {
if (action instanceof IntConsumer) {
return tryAdvance((IntConsumer) action);
}
else {
if (Tripwire.ENABLED)
Tripwire.trip(getClass(),
"{0} calling Spliterator.OfInt.tryAdvance((IntConsumer) action::accept)");
return tryAdvance((IntConsumer) action::accept);
}
}
/**
* {@inheritDoc}
* @implSpec
* If the action is an instance of {@code IntConsumer} then it is cast
* to {@code IntConsumer} and passed to
* {@link #forEachRemaining(java.util.function.IntConsumer)}; otherwise
* the action is adapted to an instance of {@code IntConsumer}, by
* boxing the argument of {@code IntConsumer}, and then passed to
* {@link #forEachRemaining(java.util.function.IntConsumer)}.
*/
@Override
default void forEachRemaining(Consumer<? super Integer> action) {
if (action instanceof IntConsumer) {
forEachRemaining((IntConsumer) action);
}
else {
if (Tripwire.ENABLED)
Tripwire.trip(getClass(),
"{0} calling Spliterator.OfInt.forEachRemaining((IntConsumer) action::accept)");
forEachRemaining((IntConsumer) action::accept);
}
}
}
问题：要知道Consumer和IntConsumer是没有任何继承关系的话，他们是怎么实现类型转换的呢？
default boolean tryAdvance(Consumer<? super Integer> action) {
if (action instanceof IntConsumer) {
return tryAdvance((IntConsumer) action);
}
如果是纯粹的面向对象的，这种现象是完全不能够存在的。
但是如果是在这函数式编程的情况下，是能够存在的。
原因如下：
- java中存在自动装箱和拆箱的操作 （int->Integer）
- 强制类型的转换在纯粹的面向对象是一定要存在继承关系的，根本原因还在于函数式编程的lambda上面
- lambda的一切信息都是通过上下文推断出来的。（对于同一个lambda表达式，在不同类型中可能推断出来的结果是不同的。在函数式编程中，这种现象是存在的。）
用代码来解释。
作者： dawa大娃bigbaby
出处：https://www.cnblogs.com/dawabigbaby/p/12159495.html
版权：本文采用「署名-非商业性使用-相同方式共享 4.0 国际」知识共享许可协议进行许可。
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