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爲什麼會有CopyOnWriteArrayList?
我們知道ArrayList和LinkedList實現的List都是非線程安全的,於是就有了Vector,它是基於ArrayList的線程安全集合,但Vector無論是add方法還是get方法都加上了synchronized修飾,當多線程讀寫List必須排隊執行,很顯然這樣效率比較是低下的,那有沒有一種辦法讓效率提升,讓當讀List的時候線程是異步的,當寫List是同步的呢?答案是CopyOnWriteArrayList,他是讀寫分離的,好處是提高線程訪問效率,下面我們對比下CopyOnWriteArrayList和Vector執行效率。
import java.util.Vector;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CountDownLatch;
/**
* @author :jiaolian
* @date :Created in 2021-01-18 15:28
* @description:安全list性能對比
* @modified By:
*/
public class SafeListTest {
private static Vector<String> safeList = new Vector<>();
//private static CopyOnWriteArrayList<String> safeList = new CopyOnWriteArrayList<>();
private static CountDownLatch countDownLatch = new CountDownLatch(2);
public static void main(String[] args) throws InterruptedException {
//初始化
safeList.add("叫練");
MySerive fishSerive = new MySerive();
long start = System.currentTimeMillis();
new Thread(()->{
fishSerive.read();
countDownLatch.countDown();
},"叫練讀線程").start();
new Thread(()->{
fishSerive.write();
countDownLatch.countDown();
},"叫練寫線程").start();
countDownLatch.await();
System.out.println("花費:"+(System.currentTimeMillis()-start));
}
private static class MySerive {
//讀
public void read() {
for (int i=0 ;i<1000000; i++) {
safeList.get(0);
}
}
//寫
public void write() {
for (int i=0 ;i<100000; i++) {
safeList.add("叫練");
}
}
}
}
如上代碼:當安全集合用Vector時,執行時長是100毫秒,當安全集合用CopyOnWriteArrayList時,執行時長是5000毫秒,神碼?你不是說CopyOnWriteArrayList的效率要高麼?但執行情況CopyOnWriteArrayList執行的時長竟然是Vector的50倍!
通過翻看源碼,我們發現當CopyOnWriteArrayList寫元素時是通過備份數組的方式實現的,當多線程同步激烈,數據量較大時會不停的複製數組,內存浪費嚴重。這就是時間長的原因!但是我們還是認可讀寫分離思想!
什麼是弱一致性
import java.util.Iterator;
import java.util.Vector;
import java.util.concurrent.CopyOnWriteArrayList;
/**
* @author :jiaolian
* @date :Created in 2021-01-18 16:40
* @description:CopyOnWriteArrayList弱一致性
* @modified By:
*/
public class WeekCopyOnWriteArrayListTest {
private static CopyOnWriteArrayList<String> safeList = new CopyOnWriteArrayList<>();
//private static Vector<String> safeList = new Vector<>();
public static void main(String[] args) throws InterruptedException {
safeList.add("叫");
safeList.add("練");
Iterator<String> iterator = safeList.iterator();
Thread thread = new Thread(()->{
//刪除下標爲0的元素
safeList.remove(0);
});
thread.start();
//主線程等待thread執行完成;
thread.join();
while (iterator.hasNext()) {
System.out.println(iterator.next());
}
}
}
如上代碼:主線程等待thread子線程執行完畢,循環打印safeList元素,最終執行結果如下圖所示
你可能會有疑問,thread不是已經刪除“叫”嗎?控制檯不是應該只打印一個“練”字嗎?爲什麼還會打出“叫練”兩個字,原因是main線程在執行Iterator<String> iterator = safeList.iterator();保存了元素快照,所以能看到這樣的執行結果,當thread線程執行完畢後,此時JVM內存狀態如下圖所示!
fail-safe特性
提到fail-safe,會先提到fail-fast,字面上翻譯快速失敗,它是集合快速檢測失敗機制,防止集合不正確操作!一般情況下,如果線程通過iterator方式循環集合時,另外一個線程也修改了這個集合,我們測試下,如上述測試弱一致性的代碼,將private static CopyOnWriteArrayList<String> safeList = new CopyOnWriteArrayList<>();換成private static Vector<String> safeList = new Vector<>();會發生什麼情況呢?
如上圖,java.util.ConcurrentModificationException,集合併發修改錯誤,但換成CopyOnWriteArrayList執行正常,原因是CopyOnWriteArrayList刪除數據時會有集合快照。
所以他是fail-safe,而Vector是fail-fast!
總結
總結下吧,我們用代碼簡述說明了CopyOnWriteArrayList的讀寫分離,弱一致性,fail-safe,fail-safe等概念,並簡述了實現原理。喜歡的請點贊加關注哦。