CopyOnWriteArrayList遍历集合 package com.yanxizhu; import java.util.*; /** * @description: synchronizedList遍历错误 * @date: 2022/3/19 11:30 * @version: 1.0 */ public class ConCurrentHashMapTest { public static void main(String[] args) { conCurrentDemo conCurrentDemo = new conCurrentDemo(); for(int i=0;i<5;i++){ new Thread(conCurrentDemo).start(); } } public static class conCurrentDemo implements Runnable{ //并发线程安全，修改报错 private static List<String> list = Collections.synchronizedList(new ArrayList<>()); static { list.add("apple"); list.add("xiaomi"); list.add("huawei"); } @Override public void run() { Iterator<String> iterator = list.iterator(); while(iterator.hasNext()){ System.out.println(iterator.next()); list.add("iphone"); } } } }线程安全报错：Exception in thread "Thread-0" Exception in thread "Thread-3" Exception in thread "Thread-4" Exception in thread "Thread-2" Exception in thread "Thread-1" java.util.ConcurrentModificationException at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1043) at java.base/java.util.ArrayList$Itr.next(ArrayList.java:997) at com.yanxizhu.ConCurrentHashMapTest$conCurrentDemo.run(ConCurrentHashMapTest.java:35) at java.base/java.lang.Thread.run(Thread.java:834) java.util.ConcurrentModificationException at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1043) at java.base/java.util.ArrayList$Itr.next(ArrayList.java:997) at com.yanxizhu.ConCurrentHashMapTest$conCurrentDemo.run(ConCurrentHashMapTest.java:35) at java.base/java.lang.Thread.run(Thread.java:834)通过用CopyOnWriteArrayList修改：package com.yanxizhu; import java.util.*; import java.util.concurrent.CopyOnWriteArrayList; /** * @description: CopyOnWriteArrayList遍历集合 * @date: 2022/3/19 11:30 * @version: 1.0 */ public class ConCurrentHashMapTest { public static void main(String[] args) { conCurrentDemo conCurrentDemo = new conCurrentDemo(); for(int i=0;i<5;i++){ new Thread(conCurrentDemo).start(); } } public static class conCurrentDemo implements Runnable{ //并发线程安全，修改报错 // private static List<String> list = Collections.synchronizedList(new ArrayList<>()); private static CopyOnWriteArrayList<String> list = new CopyOnWriteArrayList<>(); static { list.add("apple"); list.add("xiaomi"); list.add("huawei"); } @Override public void run() { Iterator<String> iterator = list.iterator(); while(iterator.hasNext()){ System.out.println(iterator.next()); //CopyOnWriteArrayList，每次会重新复制一个新的，添加。 list.add("iphone"); } } } }

高并发中集合问题 第一代线程安全集合类Vector、Hashtable是怎么保证线程安排的：使用synchronized修饰方法缺点：效率低下第二代线程非安全集合类ArrayList、HashMap线程不安全，但是性能好，用来替代Vector、Hashtable使用ArrayList、HashMap，需要线程安全怎么办呢？使用Collections.synchronizedList（list）；Collections.synchronizedMap（m）；底层使用synchronized代码块锁虽然也是锁住了所有的代码，但是锁在方法里边，并所在方法外边性能可以理解为稍有提高吧。毕竟进方法本身就要分配资源的第三代线程安全集合类在大量并发情况下如何提高集合的效率和安全呢？java.util.concurrent.*ConcurrentHashMap：CopyOnWriteArrayList：CopyOnWriteArraySet：注意不是CopyOnWriteHashSet*底层大都采用Lock锁（1.8的ConcurrentHashMap不使用Lock锁），保证安全的同时，性能也很高。