在传统的java集合包中的集合中像 ArrayList HashMap等;在同一线程中传统集合是不允许修改的,也就是遍历时不能修改,添加,删除等操作。多个线程就更不行了。
为此java.util.concurrent包中提出了一些解决方法:
使用Collections.synchronizedList() ,new CopyOnWriteArrayList<String>() 等,将集合包装为安全的集合,这样确实是结局了同步的问题。其内部的原理,就是将资源synchronized ,本身没有解决问题,因为ArrayList本身是不支持并发的,这样只会当多个线程都在争用这个资源的时候,效率就会急剧下降。
jdk 1.7 提出了java.util.concurrent.ConcurrentLinkedDeque 非阻塞的线程安全列表
ConcurrentLinkedDeque 是线程安全的,插入,删除和其它的操作可以同时执行,但是不允许插入空值。
注意:如果在进行添加和删除操作的同时也在执行迭代操作,那么此时将不会返回正确的值。
ConcurrentLinkedDeque的主要方法:
add() and offer(): 因为ConcurrentLinkedDeque是线程安全的集合,所有add方法总是返回true,和add一样offer方法也是添加一个元素,总是返回true。
poll() : 获取队列中的第一个元素并同时删除元素,如果没有获取到,返回null(此时并没有)
peek(): 和poll() 一样,不同的是,peek()不会移除元素。
element():和poll()一样,不同的是,element()在没找到时候会抛出NoSuchElementException异常。
简单的示例:
public class ConcurrentLinkedDequeDemo { static ConcurrentLinkedDeque<String> cld = new ConcurrentLinkedDeque<String>(); public static void main(String[] args) { //新建一个线程池 ExecutorService exService = Executors.newFixedThreadPool(2); ThreadOne elementAdd = new ConcurrentLinkedDequeDemo().new ThreadOne(); ThreadTwo elementGet = new ConcurrentLinkedDequeDemo().new ThreadTwo(); //在主线程上执行 add 和 get 线程 exService.execute(elementAdd); exService.execute(elementGet); exService.shutdown(); } class ThreadOne implements Runnable { @Override public void run() { for (int i = 0; i < 5; i++) { cld.add("A" + i); System.out.println("Element added is: A" +i ); } } } class ThreadTwo implements Runnable { @Override public void run() { for (int i = 0; i < 5; i++) { // String s = cld.poll(); String s = cld.peek (); // String s = cld.element (); System.out.println("Element received is: " + s); } } } }
执行结果:
poll(): Element received is: null Element added is: A0 Element received is: A0 Element added is: A1 Element received is: A1 Element added is: A2 Element received is: A2 Element added is: A3 Element received is: A3 Element added is: A4 peek(): Element received is: null Element added is: A0 Element received is: A0 Element added is: A1 Element received is: A0 Element added is: A2 Element received is: A0 Element received is: A0 Element added is: A3 Element added is: A4 element(): Element added is: A0 Element added is: A1 Element added is: A2 Element added is: A3 Element added is: A4 Exception in thread "pool-1-thread-2" java.util.NoSuchElementException at java.util.concurrent.ConcurrentLinkedDeque.screenNullResult(ConcurrentLinkedDeque.java:812) at java.util.concurrent.ConcurrentLinkedDeque.getFirst(ConcurrentLinkedDeque.java:956) at java.util.concurrent.ConcurrentLinkedDeque.element(ConcurrentLinkedDeque.java:1030) at com.dx.java.jdk.ConcurrentLinkedDequeDemo$ThreadTwo.run(ConcurrentLinkedDequeDemo.java:41) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) at java.lang.Thread.run(Thread.java:745)