聊聊Elasticsearch的Iterables

序

本文主要研究一下 Elasticsearch 的 Iterables

Iterables

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/common/util/iterable/Iterables.java

public class Iterables {public static <T> Iterable<T> concat(Iterable<T>... inputs) {Objects.requireNonNull(inputs);
        return new ConcatenatedIterable<>(inputs);
    }

    static class ConcatenatedIterable<T> implements Iterable<T> {private final Iterable<T>[] inputs;

        ConcatenatedIterable(Iterable<T>[] inputs) {this.inputs = Arrays.copyOf(inputs, inputs.length);
        }

        @Override
        public Iterator<T> iterator() {
            return Stream
                    .of(inputs)
                    .map(it -> StreamSupport.stream(it.spliterator(), false))
                    .reduce(Stream::concat)
                    .orElseGet(Stream::empty).iterator();}
    }

    /** Flattens the two level {@code Iterable} into a single {@code Iterable}.  Note that this pre-caches the values from the outer {@code
     *  Iterable}, but not the values from the inner one. */
    public static <T> Iterable<T> flatten(Iterable<? extends Iterable<T>> inputs) {Objects.requireNonNull(inputs);
        return new FlattenedIterables<>(inputs);
    }

    static class FlattenedIterables<T> implements Iterable<T> {
        private final Iterable<? extends Iterable<T>> inputs;

        FlattenedIterables(Iterable<? extends Iterable<T>> inputs) {List<Iterable<T>> list = new ArrayList<>();
            for (Iterable<T> iterable : inputs) {list.add(iterable);
            }
            this.inputs = list;
        }

        @Override
        public Iterator<T> iterator() {
            return StreamSupport
                    .stream(inputs.spliterator(), false)
                    .flatMap(s -> StreamSupport.stream(s.spliterator(), false)).iterator();}
    }

    public static <T> T get(Iterable<T> iterable, int position) {Objects.requireNonNull(iterable);
        if (position < 0) {throw new IllegalArgumentException("position >= 0");
        }
        if (iterable instanceof List) {List<T> list = (List<T>)iterable;
            if (position >= list.size()) {throw new IndexOutOfBoundsException(Integer.toString(position));
            }
            return list.get(position);
        } else {Iterator<T> it = iterable.iterator();
            for (int index = 0; index < position; index++) {if (!it.hasNext()) {throw new IndexOutOfBoundsException(Integer.toString(position));
                }
                it.next();}
            if (!it.hasNext()) {throw new IndexOutOfBoundsException(Integer.toString(position));
            }
            return it.next();}
    }
}

• Iterables 提供了 concat、flatten、get 三个静态方法，其中 concat 返回的是 ConcatenatedIterable；flatten 返回的是 FlattenedIterables；get 方法会先判断是否是 List 类型是的话直接通过 position 取值返回，不是则遍历 iterable 获取指定 index 的元素

实例

elasticsearch-7.0.1/server/src/test/java/org/elasticsearch/common/util/iterable/IterablesTests.java

public class IterablesTests extends ESTestCase {public void testGetOverList() {test(Arrays.asList("a", "b", "c"));
    }

    public void testGetOverIterable() {Iterable<String> iterable = () ->
                new Iterator<String>() {
                    private int position = 0;

                    @Override
                    public boolean hasNext() {return position < 3;}

                    @Override
                    public String next() {if (position < 3) {
                            String s = position == 0 ? "a" : position == 1 ? "b" : "c";
                            position++;
                            return s;
                        } else {throw new NoSuchElementException();
                        }
                    }
                };
        test(iterable);
    }

    public void testFlatten() {List<List<Integer>> list = new ArrayList<>();
        list.add(new ArrayList<>());

        Iterable<Integer> allInts = Iterables.flatten(list);
        int count = 0;
        for(@SuppressWarnings("unused") int x : allInts) {count++;}
        assertEquals(0, count);
        list.add(new ArrayList<>());
        list.get(1).add(0);

        // changes to the outer list are not seen since flatten pre-caches outer list on init:
        count = 0;
        for(@SuppressWarnings("unused") int x : allInts) {count++;}
        assertEquals(0, count);

        // but changes to the original inner lists are seen:
        list.get(0).add(0);
        for(@SuppressWarnings("unused") int x : allInts) {count++;}
        assertEquals(1, count);
    }

    private void test(Iterable<String> iterable) {
        try {Iterables.get(iterable, -1);
            fail("expected IllegalArgumentException");
        } catch (IllegalArgumentException e) {assertThat(e, hasToString("java.lang.IllegalArgumentException: position >= 0"));
        }
        assertEquals("a", Iterables.get(iterable, 0));
        assertEquals("b", Iterables.get(iterable, 1));
        assertEquals("c", Iterables.get(iterable, 2));
        try {Iterables.get(iterable, 3);
            fail("expected IndexOutOfBoundsException");
        } catch (IndexOutOfBoundsException e) {assertThat(e, hasToString("java.lang.IndexOutOfBoundsException: 3"));
        }
    }
}

• flatten 用于将 Iterable 类型的 Iterable 进行 flat 操作；get 方法则获取 iterable 中指定 index 的元素

小结

Iterables 提供了 concat、flatten、get 三个静态方法，其中 concat 返回的是 ConcatenatedIterable；flatten 返回的是 FlattenedIterables；get 方法会先判断是否是 List 类型是的话直接通过 position 取值返回，不是则遍历 iterable 获取指定 index 的元素

doc

• Iterables