如果加上 Characteristics.CONCURRENT.
@Override public Set<Characteristics> characteristics() { System.out.println("characteristics invoked"); return Collections.unmodifiableSet(EnumSet.of(Characteristics.UNORDERED,Characteristics.CONCURRENT)); }则可能会出来一个异常
Caused by: java.util.ConcurrentModificationException如果不加 ,则不会出现异常
多执行几次,会有一定的发现.
查看属性的源码.
/** * 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,出现问题的原因:是在打印了set集合.
/** * This exception may be thrown by methods that have detected concurrent * modification of an object when such modification is not permissible. * <p> * For example, it is not generally permissible for one thread to modify a Collection * while another thread is iterating over it. In general, the results of the * iteration are undefined under these circumstances. Some Iterator * implementations (including those of all the general purpose collection implementations * provided by the JRE) may choose to throw this exception if this behavior is * detected. Iterators that do this are known as <i>fail-fast</i> iterators, * as they fail quickly and cleanly, rather that risking arbitrary, * non-deterministic behavior at an undetermined time in the future. * <p> * Note that this exception does not always indicate that an object has * been concurrently modified by a <i>different</i> thread. If a single * thread issues a sequence of method invocations that violates the * contract of an object, the object may throw this exception. For * example, if a thread modifies a collection directly while it is * iterating over the collection with a fail-fast iterator, the iterator * will throw this exception. * * <p>Note that fail-fast behavior cannot be guaranteed as it is, generally * speaking, impossible to make any hard guarantees in the presence of * unsynchronized concurrent modification. Fail-fast operations * throw {@code ConcurrentModificationException} on a best-effort basis. * Therefore, it would be wrong to write a program that depended on this * exception for its correctness: <i>{@code ConcurrentModificationException} * should be used only to detect bugs.</i> * * @author Josh Bloch * @see Collection * @see Iterator * @see Spliterator * @see ListIterator * @see Vector * @see LinkedList * @see HashSet * @see Hashtable * @see TreeMap * @see AbstractList * @since 1.2 */ public class ConcurrentModificationException extends RuntimeException { }并发修改异常.
因为如果加上这个属性,那么这个就有一个结果集
并行的时候,会对set进行操作,但是你同时又在遍历打印, 两个赶到一起了.然后就会抛出这个异常.
这就是抛出这个异常的根本原因.
注意:如果是并行的话,千万要避免 打印遍历 你要操作的对象.
如果不加这个属性,那么combiner()方法的中间结果集就会被调用,所以就不会出现抢占资源的现象.
扩展: sequential() && parallerl()方法的调用. Set<String> collect = list.stream().parallel().sequential().sequential().parallel().collect(new MySetCollector<>());只有最后一个会生效.
sequential() /** * 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. * * <p>This is an <a href="package-summary.html#StreamOps">intermediate * operation</a>. * * @return a sequential stream */ S sequential(); parallerl() /** * 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(); 关于Supplier()容器的定义.