java中StreamAPI的Collector原理分析

前言

StreamAPI是java8提供的一种方便，高效操作容器的工具。

简单使用

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Client {

  public static void main(String[] args) {
    List<String> list = Arrays.asList("hello", "world");
    //字符串转大写
    list = list.stream().map(String::toUpperCase).collect(Collectors.toList());
    System.out.println(list);
  }

}

Stream对象调用collect()方法，根据不同的Collector返回不同的结果。

/**
* 收集器接口
*/
public interface Collector<T, A, R> {
    /**
     * 创建一个可变的结果容器
     */
    Supplier<A> supplier();

    /**
     * 将一个元素添加到结果容器中
     */
    BiConsumer<A, T> accumulator();

    /**
     * 合并两个容器
     */
    BinaryOperator<A> combiner();

    /**
     * 进行结果的转换，如果设置了IDENTITY_FINISH标识，该方法不被调用
     */
    Function<A, R> finisher();

    /**
     * 返回该收集器的标识不可变集合 
     */
    Set<Characteristics> characteristics();

                                       
    /**
     * 标识的定义
     */
    enum Characteristics {
        /**
         * 表示支持并行操作，结果容器要支持多线程
         */
        CONCURRENT,

        /**
         * 表示结果无序
         */
        UNORDERED,

        /**
         * 表示finisher方法是否可以省略
         */
        IDENTITY_FINISH
    }
}

定义一个自己的Collector

import java.util.ArrayList;
import java.util.Collections;
import java.util.EnumSet;
import java.util.List;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

public class MyListCollector<T> implements Collector<T, List<T>, List<T>> {

  @Override
  public Supplier<List<T>> supplier() {
    System.out.println("supplier() invoked");
    return () -> {
      System.out.println("new a ArrayList");
      return new ArrayList<>();
    };
  }

  @Override
  public BiConsumer<List<T>, T> accumulator() {
    System.out.println("accumulator() invoked");
    return (list, item) -> {
      System.out.println("list add a item");
      list.add(item);
    };
  }

  @Override
  public BinaryOperator<List<T>> combiner() {
    System.out.println("combiner() invoked");
    return (left, right) -> {
      System.out.println("left list addAll right");
      left.addAll(right);
      return left;
    };
  }

  @Override
  public Function<List<T>, List<T>> finisher() {
    System.out.println("finisher() invoked");
    return Function.identity();
  }

  @Override
  public Set<Characteristics> characteristics() {
    return Collections.unmodifiableSet(EnumSet.of(Characteristics.IDENTITY_FINISH));
  }
}

import java.util.Arrays;
import java.util.List;

public class Client {

  public static void main(String[] args) {
    List<String> list = Arrays.asList("hello", "world");
    //字符串转大写
    list = list.stream().map(String::toUpperCase).collect(new MyListCollector<>());
    System.out.println(list);
  }
  
}

输出打印结果为

supplier() invoked
accumulator() invoked
combiner() invoked
new a ArrayList
list add a item
list add a item
[HELLO, WORLD]

finisher()方法没有被执行，在包含IDENTITY_FINISH标识时，会进行强制类型转换，没有时才会调用。

combiner()方法在并行流且没有CONCURRENT标识时才会执行，这是因为包含CONCURRENT标识时，并行流只会操作同一个结果容器，不包含时，会创建多个结果容器进行合并。

源码分析

/**
     * Simple implementation class for {@code Collector}.
     *
     * @param <T> the type of elements to be collected
     * @param <R> the type of the result
     */
    static class CollectorImpl<T, A, R> implements Collector<T, A, R> {
        private final Supplier<A> supplier;
        private final BiConsumer<A, T> accumulator;
        private final BinaryOperator<A> combiner;
        private final Function<A, R> finisher;
        private final Set<Characteristics> characteristics;

        CollectorImpl(Supplier<A> supplier,
                      BiConsumer<A, T> accumulator,
                      BinaryOperator<A> combiner,
                      Function<A,R> finisher,
                      Set<Characteristics> characteristics) {
            this.supplier = supplier;
            this.accumulator = accumulator;
            this.combiner = combiner;
            this.finisher = finisher;
            this.characteristics = characteristics;
        }

        CollectorImpl(Supplier<A> supplier,
                      BiConsumer<A, T> accumulator,
                      BinaryOperator<A> combiner,
                      Set<Characteristics> characteristics) {
            this(supplier, accumulator, combiner, castingIdentity(), characteristics);
        }

        @Override
        public BiConsumer<A, T> accumulator() {
            return accumulator;
        }

        @Override
        public Supplier<A> supplier() {
            return supplier;
        }

        @Override
        public BinaryOperator<A> combiner() {
            return combiner;
        }

        @Override
        public Function<A, R> finisher() {
            return finisher;
        }

        @Override
        public Set<Characteristics> characteristics() {
            return characteristics;
        }
    }

java中的收集器默认实现，一个收集器包括：供应器，累加器，合并器，完成器，标识列表。

/**
     * Returns a {@code Collector} that accumulates the input elements into a
     * new {@code List}. There are no guarantees on the type, mutability,
     * serializability, or thread-safety of the {@code List} returned; if more
     * control over the returned {@code List} is required, use {@link #toCollection(Supplier)}.
     *
     * @param <T> the type of the input elements
     * @return a {@code Collector} which collects all the input elements into a
     * {@code List}, in encounter order
     */
    public static <T>
    Collector<T, ?, List<T>> toList() {
        return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
                                   (left, right) -> { left.addAll(right); return left; },
                                   CH_ID);
    }

我们常用的toList()收集器，返回一个ArrayList，标识列表包含IDENTITY_FINISH，表示中间类型和结果类型相同，可以强制类型转换。