收集器Collector是jdk8中最为重要的接口之一，一个Collector可分为5个部分（第五个是我自己加上的）：
1、supplier
2、accumulator
3、combiner
4、finisher
5、characteristics
Collector有三个泛型：
public interface Collector<T, A, R> {
/** * A function that creates and returns a new mutable result container. * @return a function which returns a new, mutable result container * 容器提供者 */
Supplier<A> supplier();
/** * A function that folds a value into a mutable result container. * @return a function which folds a value into a mutable result container * 累加操作 */
BiConsumer<A, T> accumulator();
/** * A function that accepts two partial results and merges them. The * combiner function may fold state from one argument into the other and * return that, or may return a new result container. * * @return a function which combines two partial results into a combined * result * 并发的情况将每个线程的中间容器A合并 */
BinaryOperator<A> combiner();
/** * Perform the final transformation from the intermediate accumulation type * {@code A} to the final result type {@code R}. * * <p>If the characteristic {@code IDENTITY_TRANSFORM} is * set, this function may be presumed to be an identity transform with an * unchecked cast from {@code A} to {@code R}. * * @return a function which transforms the intermediate result to the final * result * 终止操作 */
Function<A, R> finisher();
/** * Returns a {@code Set} of {@code Collector.Characteristics} indicating * the characteristics of this Collector. This set should be immutable. * * @return an immutable set of collector characteristics * 收集器特性 */
Set<Characteristics> characteristics();
/** * Returns a new {@code Collector} described by the given {@code supplier}, * {@code accumulator}, and {@code combiner} functions. The resulting * {@code Collector} has the {@code Collector.Characteristics.IDENTITY_FINISH} * characteristic. * * @param supplier The supplier function for the new collector * @param accumulator The accumulator function for the new collector * @param combiner The combiner function for the new collector * @param characteristics The collector characteristics for the new * collector * @param <T> The type of input elements for the new collector * @param <R> The type of intermediate accumulation result, and final result, * for the new collector * @throws NullPointerException if any argument is null * @return the new {@code Collector} */
public static<T, R> Collector<T, R, R> of(Supplier<R> supplier,
BiConsumer<R, T> accumulator,
BinaryOperator<R> combiner,
Characteristics... characteristics) {
Objects.requireNonNull(supplier);
Objects.requireNonNull(accumulator);
Objects.requireNonNull(combiner);
Objects.requireNonNull(characteristics);
Set<Characteristics> cs = (characteristics.length == 0)
? Collectors.CH_ID
: Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH,
characteristics));
return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, cs);
}
/** * Returns a new {@code Collector} described by the given {@code supplier}, * {@code accumulator}, {@code combiner}, and {@code finisher} functions. * * @param supplier The supplier function for the new collector * @param accumulator The accumulator function for the new collector * @param combiner The combiner function for the new collector * @param finisher The finisher function for the new collector * @param characteristics The collector characteristics for the new * collector * @param <T> The type of input elements for the new collector * @param <A> The intermediate accumulation type of the new collector * @param <R> The final result type of the new collector * @throws NullPointerException if any argument is null * @return the new {@code Collector} */
public static<T, A, R> Collector<T, A, R> of(Supplier<A> supplier,
BiConsumer<A, T> accumulator,
BinaryOperator<A> combiner,
Function<A, R> finisher,
Characteristics... characteristics) {
Objects.requireNonNull(supplier);
Objects.requireNonNull(accumulator);
Objects.requireNonNull(combiner);
Objects.requireNonNull(finisher);
Objects.requireNonNull(characteristics);
Set<Characteristics> cs = Collectors.CH_NOID;
if (characteristics.length > 0) {
cs = EnumSet.noneOf(Characteristics.class);
Collections.addAll(cs, characteristics);
cs = Collections.unmodifiableSet(cs);
}
return new Collectors.CollectorImpl<>(supplier, accumulator, combiner, finisher, cs);
}
/** * Characteristics indicating properties of a {@code Collector}, which can * be used to optimize reduction implementations. */
enum Characteristics {
/** * Indicates that this collector is <em>concurrent</em>, meaning that * the result container can support the accumulator function being * called concurrently with the same result container from multiple * threads. * * <p>If a {@code CONCURRENT} collector is not also {@code UNORDERED}, * then it should only be evaluated concurrently if applied to an * unordered data source. */
CONCURRENT,
/** * Indicates that the collection operation does not commit to preserving * the encounter order of input elements. (This might be true if the * result container has no intrinsic order, such as a {@link Set}.) */
UNORDERED,
/** * Indicates that the finisher function is the identity function and * can be elided. If set, it must be the case that an unchecked cast * from A to R will succeed. */
IDENTITY_FINISH
}
}
Javadoc对前边四个做了说明：
*
A {@code Collector} is specified by four functions that work together to
* accumulate entries into a mutable result container, and optionally perform
* a final transform on the result. They are:
- creation of a new result container ({@link #supplier()}) *
- incorporating a new data element into a result container ({@link #accumulator()}) *
- combining two result containers into one ({@link #combiner()}) *
- performing an optional final transform on the container ({@link #finisher()}) *
*
supplier:是一个容器提供者，提供容器A,比如：List list = new ArrayList()；
accumulator:是要操作的集合的每个元素以怎样的形式添加到supplier提供的容器A当中，即做累加操作，比如：List.add(item)；
combiner:用于在多线程并发的情况下，每个线程都有一个supplier和，如果有N个线程那么就有N个supplier提供的容器A，执行的是类似List.addAll(listB)这样的操作,只有在characteristics没有被设置成CONCURRENT并且是并发的情况下 才会被调用。ps：characteristics被设置成CONCURRENT时，整个收集器只有一个容器，而不是每个线程都有一个容器，此时combiner()方法不会被调用，这种情况会出现java.util.ConcurrentModificationException异常,此时需要使用线程安全的容器作为supplier返回的对象。
finisher:是终止操作，如果收集器的characteristics被设置成IDENTITY_FINISH，那么会将中间集合A牵制转换为结果R类型，如果A和R没有父子之类的继承关系，会报类型转换失败的错误，如果收集器的characteristics没有被设置成IDENTITY_FINISH，那么finisher()方法会被调用，返回结果类型R。
关于枚举Characteristics是用来控制收集器的相关特性，它在Collector接口内部：
/** * Characteristics indicating properties of a {@code Collector}, which can * be used to optimize reduction implementations. */
enum Characteristics {
/** * Indicates that this collector is <em>concurrent</em>, meaning that * the result container can support the accumulator function being * called concurrently with the same result container from multiple * threads. *如果一个收集器被标记为concurrent特性，那么accumulator 方法可以被多线程并发的的调用，并且只使用一个容器A * <p>If a {@code CONCURRENT} collector is not also {@code UNORDERED}, * then it should only be evaluated concurrently if applied to an * unordered data source. * 如果收集器被标记为concurrent，但是要操作的集合失败有序的，那么最终得到的结果不能保证原来的顺序 */
CONCURRENT,
/** * Indicates that the collection operation does not commit to preserving * the encounter order of input elements. (This might be true if the * result container has no intrinsic order, such as a {@link Set}.) * 适用于无序的集合 */
UNORDERED,
/** * Indicates that the finisher function is the identity function and * can be elided. If set, it must be the case that an unchecked cast * from A to R will succeed. * 如果收集器特性被设置IDENTITY_FINISH，那么会强制将中间容器A类型转换为结果类型R */
IDENTITY_FINISH
}下面我们写一个自定义的收集器了解他的原理：
package com.ceaser.jdk8lambda.stream2;
import java.util.*;
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;
import java.util.stream.Collectors;
import static java.util.stream.Collector.Characteristics.CONCURRENT;
import static java.util.stream.Collector.Characteristics.IDENTITY_FINISH;
import static java.util.stream.Collector.Characteristics.UNORDERED;
/** * Created by CeaserWang on 2017/2/27. * 此收集器的作用是将Set集合转换为Map */
public class MyCollectorA<T> implements Collector<T,Set<T>,Map<T,T>> {
@Override
public Supplier<Set<T>> supplier() {
System.out.println("supplier invoked...");
return HashSet::new;//实例化一个存放中间结果的集合Set
}
@Override
public BiConsumer<Set<T>,T> accumulator() {
System.out.println("accumulator invoked...");
return (item1,item2) -> {
/** * * A a1 = supplier.get(); * accumulator.accept(a1, t1); * accumulator.accept(a1, t2); * R r1 = finisher.apply(a1); // result without splitting * * A a2 = supplier.get(); * accumulator.accept(a2, t1); * A a3 = supplier.get(); * accumulator.accept(a3, t2); * R r2 = finisher.apply(combiner.apply(a2, a3)); // result with splitting */
// System.out.println("current thread : "+item1+" , "+Thread.currentThread().getName());
item1.add(item2);//将遍历的每个元素加入到Set当中
};
}
@Override
public BinaryOperator<Set<T>> combiner() {
System.out.println("combiner invoked...");
return (item1,item2) -> {
item1.addAll(item2);//多线程下，集合Set的合并操作
System.out.println("--------");
return item1;
};
}
@Override
public Function<Set<T>,Map<T,T>> finisher() {
System.out.println("finisher invoked...");
return (item1) ->{
Map<T,T> rm = new HashMap<T,T>();
item1.stream(). forEach( (bean) -> rm.put(bean,bean) );//将Set集合的每个元素加入到新的map之中
return rm;
};
}
@Override
public Set<Characteristics> characteristics() {
System.out.println("characteristics invoked...");
return Collections.unmodifiableSet(EnumSet.of(UNORDERED,CONCURRENT));//支持并发操作，并且是不能保证原始集合的顺序。
}
}测试类：
/** * Created by Ceaser Wang on 2017/2/27. */
public class MyCollectorATest {
public static void main(String[] args) {
List<String> list = Arrays.asList("hello","world","welcome","helloworld","helloworldA");
Set<String> set = new HashSet<>();
set.addAll(list);
Map<String,String> maped = set.parallelStream().collect(new MyCollectorA<>());
}
}输入出结果：
characteristics invoked...
supplier invoked...
accumulator invoked...
characteristics invoked...
finisher invoked...并发陷阱分析：
可以看到收集器的特性被设置成CONCURRENT，并且是parallelStream，执行过程中没有调用combiner()方法。因为只有一个公用的容器没必要再去掉combiner()合并中间结果。PS：在单线程模式下，并且特性设置成CONCURRENT，combiner()会被调用。
接下来我们将accumulator的这行注释放开：
System.out.println("current thread : "+item1+" , "+Thread.currentThread().getName());再去运行，会报错（如果一次没有出现，多运行几次）：
Exception in thread "main" java.util.ConcurrentModificationException: java.util.ConcurrentModificationException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at java.util.concurrent.ForkJoinTask.getThrowableException(ForkJoinTask.java:593)
at java.util.concurrent.ForkJoinTask.reportException(ForkJoinTask.java:677)
at java.util.concurrent.ForkJoinTask.invoke(ForkJoinTask.java:735)
at java.util.stream.ForEachOps$ForEachOp.evaluateParallel(ForEachOps.java:160)
at java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateParallel(ForEachOps.java:174)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:233)
at java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:418)
at java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:583)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:496)
at com.ceaser.jdk8lambda.stream2.MyCollectorATest.main(MyCollectorATest.java:17)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
Caused by: java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437)
at java.util.HashMap$KeyIterator.next(HashMap.java:1461)
at java.util.AbstractCollection.toString(AbstractCollection.java:461)
at java.lang.String.valueOf(String.java:2994)
at java.lang.StringBuilder.append(StringBuilder.java:131)
at com.ceaser.jdk8lambda.stream2.MyCollectorA.lambda$accumulator$0(MyCollectorA.java:43)
at java.util.stream.ReferencePipeline.lambda$collect$1(ReferencePipeline.java:496)
at java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:184)
at java.util.HashMap$KeySpliterator.forEachRemaining(HashMap.java:1548)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
at java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:291)
at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)为什么呢？
我们只是输出了中间容器的内容，我们看下这行错误信息对应的代码：
at java.util.AbstractCollection.toString(AbstractCollection.java:461)对应的是AbstractCollection的toString：
public String toString() {
Iterator<E> it = iterator();
if (! it.hasNext())
return "[]";
StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
**E e = it.next();**//这样代码是对Set集合进行遍历
sb.append(e == this ? "(this Collection)" : e);
if (! it.hasNext())
return sb.append(']').toString();
sb.append(',').append(' ');
}
}E e = it.next();
此行代码是对集合进行遍历，在多线程下对未同步的集合同时遍历和修改操作会导致ConcurrentModificationException这种异常（其他的HashMap多线程下回出现死循环问题），
为此我们需要替换使用线程安全的集合，比如ConcurrentHashMap等等。
IDENTITY_FINISH特性：
打开collect方法的源码我们看到：
@Override
@SuppressWarnings("unchecked")
public final <R, A> R collect(Collector<? super P_OUT, A, R> collector) {
A container;
if (isParallel()
&& (collector.characteristics().contains(Collector.Characteristics.CONCURRENT))
&& (!isOrdered() || collector.characteristics().contains(Collector.Characteristics.UNORDERED))) {
container = collector.supplier().get();
BiConsumer<A, ? super P_OUT> accumulator = collector.accumulator();
forEach(u -> accumulator.accept(container, u));
}
else {
container = evaluate(ReduceOps.makeRef(collector));
}
return collector.characteristics().contains(Collector.Characteristics.IDENTITY_FINISH)
? (R) container
: collector.finisher().apply(container);
}当收集器的特性包含IDENTITY_FINISH特性时，会把收集器内部的中间集合A强制转换为R（当中间容器类型A和结果类型R不同时，但是又设置了IDENTITY_FINISH特性，那么会抛出java.lang.ClassCastException），否则才会调用收集器的finisher()方法。