Java Stream Head的构建 (一)

这里以集合、数组为源构建流，只讨论串行而先忽略并行方式，如果你对Lambda表达式和函数式接口不是很了解，可以参考这篇

//以集合为数据源
Collection.java
     default Stream<E> stream() {
         return StreamSupport.stream(spliterator(), false);
     }
     会被子类复写
     default Spliterator<E> spliterator() {
             return Spliterators.spliterator(this, 0);
     }

以ArrayList为列，调用spliterator()方法生成一个可分割迭代器（splitable iterator）,用于对数据源进行遍历和分区

ArrayList.java
public Spliterator<E> spliterator() {
        return new ArrayListSpliterator<>(this, 0, -1, 0);
    }

ArrayListSpliterator(ArrayList<E> list, int origin, int fence,int expectedModCount) {
            this.list = list; // OK if null unless traversed
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }

迭代器在串流使用到的主要方法就是遍历

//ArrayList.java
public void forEachRemaining(Consumer<? super E> action) {
            int i, hi, mc; // hoist accesses and checks from loop
            ArrayList<E> lst; Object[] a;
            if (action == null)
                throw new NullPointerException();
            if ((lst = list) != null && (a = lst.elementData) != null) {
                if ((hi = fence) < 0) {
                    mc = lst.modCount;
                    hi = lst.size;
                }
                else
                    mc = expectedModCount;
                if ((i = index) >= 0 && (index = hi) <= a.length) {
                    for (; i < hi; ++i) {
                        @SuppressWarnings("unchecked") E e = (E) a[i];
                        action.accept(e);
                    }
                    if (lst.modCount == mc)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }

再看以可变长参数为数据源

Stream.java
public static<T> Stream<T> of(T... values) {
        return Arrays.stream(values);
    }

Arrays.java
public static <T> Stream<T> stream(T[] array, int startInclusive, int endExclusive) {
        return StreamSupport.stream(spliterator(array, startInclusive, endExclusive), false);
    }

public static <T> Spliterator<T> spliterator(T[] array, int startInclusive, int endExclusive) {
        return Spliterators.spliterator(array, startInclusive, endExclusive,Spliterator.ORDERED | Spliterator.IMMUTABLE);
    }

由Spliterators中方法实现

Spliterators.java
public static <T> Spliterator<T> spliterator(Object[] array, int fromIndex, int toIndex,
                                                 int additionalCharacteristics) {
        ...
        return new ArraySpliterator<>(array, fromIndex, toIndex, additionalCharacteristics);
    }
    
public ArraySpliterator(Object[] array, int origin, int fence, int additionalCharacteristics) {
        this.array = array;
        this.index = origin;
        this.fence = fence;
        this.characteristics = additionalCharacteristics | Spliterator.SIZED | Spliterator.SUBSIZED;
    }
 //ArraySpliterator的迭代方式
public void forEachRemaining(Consumer<? super T> action) {
            Object[] a; int i, hi; // hoist accesses and checks from loop
            if (action == null)
                throw new NullPointerException();
            
            if ((a = array).length >= (hi = fence) &&
                (i = index) >= 0 && i < (index = hi)) {
                //挨个发射数组中的数据
                do { action.accept((T)a[i]); } while (++i < hi);
            }
        }

除了这两种，Stream还能使用iterate、generate方法构造无限流，暂不讨论。
上述都以返回的Spliterator作为参数，使用工具类StreamSupport的方法stream构建一个源头。

StreamSupport.java
public static <T> Stream<T> stream(Spliterator<T> spliterator, boolean parallel) {
        ...
        //Head是ReferencePipeline的静态内部类,也是其子类，ReferencePipeline是AbstractPipeline子类
        return new ReferencePipeline.Head<>(spliterator,StreamOpFlag.fromCharacteristics(spliterator),parallel);
    }

看一下组织结构,参考https://www.cnblogs.com/CarpenterLee/p/6637118.html

stream_hierarch.png

Head构造方法调用父类AbstractPipeline的构造方法来初始化源头

AbstractPipeline(Spliterator<?> source,int sourceFlags, boolean parallel) {
        this.previousStage = null;
        //源头持用Spliterator引用
        this.sourceSpliterator = source;
        //sourceStage就是源头的引用，这里使用自己初始化
        this.sourceStage = this;
        //先忽略
        this.sourceOrOpFlags = sourceFlags & StreamOpFlag.STREAM_MASK;
        this.combinedFlags = (~(sourceOrOpFlags << 1)) & StreamOpFlag.INITIAL_OPS_VALUE;
        this.depth = 0;
        this.parallel = parallel;
    }

再看一张全家福，出自https://www.jianshu.com/p/ee5712de3927。和Stream同一级别的还有几个定制的基础类型，ReferencePipeline也是一样，

java_stream.png

到此，数据源的迭代方式和源头的构建就完成了，下面来看看Stream 的中间操作（二）。