測(cè)試代碼：

package copyOnWriteArrayListTest;import java.util.ArrayList; import java.util.List; import java.util.concurrent.CopyOnWriteArrayList; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors;class AddThread implements Runnable {private List<Double> list;public AddThread(List<Double> list) {this.list = list;}@Overridepublic void run(){for ( int i = 0; i < 10000; i++){list.add(Math.random());}} }public class CopyOnWriteArrayListTest {public static final int THREAD_POOL_SIZE = 2;public static void main(String[] args) throws InterruptedException {// List<Double> list = new ArrayList<>();List<Double> list = new CopyOnWriteArrayList<>();ExecutorService es = Executors.newFixedThreadPool(THREAD_POOL_SIZE);es.execute(new AddThread(list));es.execute(new AddThread(list));es.shutdown();Thread.sleep(4000);System.out.println("OK, list length: " + list.size());}}

輸出結(jié)果：

OK, list length: 20000

如果把支持寫時(shí)拷貝的list替換成普通的ArrayList：

List list = new ArrayList<>();

因?yàn)閮蓚€(gè)線程同時(shí)對(duì)這個(gè)普通的ArrayList進(jìn)行寫操作，結(jié)果如下：

OK, list length: 17578

看下CopyOnWriteArrayList實(shí)現(xiàn)源代碼里Add方法的實(shí)現(xiàn)：

/*** Appends the specified element to the end of this list.** @param e element to be appended to this list* @return {@code true} (as specified by {@link Collection#add})*/public boolean add(E e) {synchronized (lock) {Object[] es = getArray();int len = es.length;es = Arrays.copyOf(es, len + 1);es[len] = e;setArray(es);return true;}}

lock初始化的地方：

final transient Object lock = new Object();

總結(jié)

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