线程安全-并发容器J.U.C 线程安全-并发容器J.U.CArrayList -> CopyOnWriteArrayListHashSet、TreeSet -> CopyOnWriteArraySet 、ConcurrentSkipListSetHashMap、TreeMap -> ConcurrentHashMap 、ConcurrentSkipListMapCopyOnWriteArrayList适合读多写少的场景。读的时候再原数组读不需要加锁，写的时候会copy一份会单独加锁。代码示例：package com.yanxizhu.demo.concurrency.concurrent; import com.yanxizhu.demo.concurrency.annotation.ThreadSafety; import com.yanxizhu.demo.concurrency.annotation.UnThreadSafety; import lombok.extern.slf4j.Slf4j; import java.util.ArrayList; import java.util.List; import java.util.concurrent.*; /** * @description: 线程安全容器：CopyOnWriteArrayList * @author: <a href="mailto:batis@foxmail.com">清风</a> * @date: 2022/4/24 9:52 * @version: 1.0 */ @Slf4j @ThreadSafety public class DemoCopyOnWriteArrayList { // 请求总数 public static int clientTotal = 5000; // 同时并发执行的线程数 public static int threadTotal = 200; private static List<Integer> list = new CopyOnWriteArrayList<>(); public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i < clientTotal; i++) { final int count = i; executorService.execute(() -> { try { semaphore.acquire(); update(count); semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("size:{}", list.size()); } private static void update(int i) { list.add(i); } }输出结果：5000CopyOnWriteArraySet线程安全，代码示例package com.yanxizhu.demo.concurrency.concurrent; import com.yanxizhu.demo.concurrency.annotation.ThreadSafety; import lombok.extern.slf4j.Slf4j; import java.util.Set; import java.util.concurrent.*; /** * @description: 线程安全容器：CopyOnWriteArraySet * @author: <a href="mailto:batis@foxmail.com">清风</a> * @date: 2022/4/24 9:52 * @version: 1.0 */ @Slf4j @ThreadSafety public class DemoCopyOnWriteArraySet { // 请求总数 public static int clientTotal = 5000; // 同时并发执行的线程数 public static int threadTotal = 200; private static Set<Integer> set = new CopyOnWriteArraySet<>(); public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i < clientTotal; i++) { final int count = i; executorService.execute(() -> { try { semaphore.acquire(); update(count); semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("set size:{}", set.size()); } private static void update(int i) { set.add(i); } }输出结果：set size:5000，线程安全ConcurrentSkipListSet线程安全，代码示例package com.yanxizhu.demo.concurrency.concurrent; import com.yanxizhu.demo.concurrency.annotation.ThreadSafety; import lombok.extern.slf4j.Slf4j; import java.util.List; import java.util.Set; import java.util.concurrent.*; /** * @description: 线程安全容器：ConcurrentSkipListSet * @author: <a href="mailto:batis@foxmail.com">清风</a> * @date: 2022/4/24 9:52 * @version: 1.0 */ @Slf4j @ThreadSafety public class DemoConcurrentSkipListSet { // 请求总数 public static int clientTotal = 5000; // 同时并发执行的线程数 public static int threadTotal = 200; private static Set<Integer> set = new ConcurrentSkipListSet<>(); public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i < clientTotal; i++) { final int count = i; executorService.execute(() -> { try { semaphore.acquire(); update(count); semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("set size:{}", set.size()); } private static void update(int i) { set.add(i); } }输出结果：5000,线程安全。注意：单纯的add、remove是线程安全的，contains等是不安全的。ConcurrentHashMap线程安全，适合并发量大场景package com.yanxizhu.demo.concurrency.concurrent; import com.yanxizhu.demo.concurrency.annotation.UnThreadSafety; import lombok.extern.slf4j.Slf4j; import java.util.HashMap; import java.util.Map; import java.util.concurrent.*; /** * @description: 线程安全容器：ConcurrentHashMap适合高并发场景 * @author: <a href="mailto:batis@foxmail.com">清风</a> * @date: 2022/4/24 9:52 * @version: 1.0 */ @Slf4j @UnThreadSafety public class DemoConcurrentHashMap { private static Map<Integer, Integer> map = new ConcurrentHashMap<>(); //用户数量 private static final int clientsTotal = 5000; //并发数量 private static final int concurrencyTotal = 200; public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newCachedThreadPool(); //信号量 final Semaphore semaphore = new Semaphore(concurrencyTotal); //闭锁 final CountDownLatch countDownLatch = new CountDownLatch(clientsTotal); for (int i = 0; i < clientsTotal; i++) { final int count = i; executorService.execute(()->{ try { semaphore.acquire(); update(count); semaphore.release(); } catch (InterruptedException e) { log.error("出现错误:【{}】", e.getMessage()); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("map size:{}", map.size()); } /** * 通过线程安全对象dateTimeFormatter处理 */ private static void update(int count){ map.put(count, count); } }输出结果：5000,线程安全ConcurrentSkipListMap线程安全，随着并发量增加性能表现越强，大并发下使用ConcurrentHashMap，并发一直增加推荐使用ConcurrentSkipListMap。package com.yanxizhu.demo.concurrency.concurrent; import com.yanxizhu.demo.concurrency.annotation.UnThreadSafety; import lombok.extern.slf4j.Slf4j; import java.util.Map; import java.util.concurrent.*; /** * @description: 线程安全容器：ConcurrentSkipListMap,随着并发的增加性能表现的越好 * @author: <a href="mailto:batis@foxmail.com">清风</a> * @date: 2022/4/24 9:52 * @version: 1.0 */ @Slf4j @UnThreadSafety public class DemoConcurrentHashMap1 { private static Map<Integer, Integer> map = new ConcurrentSkipListMap<>(); //用户数量 private static final int clientsTotal = 5000; //并发数量 private static final int concurrencyTotal = 200; public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newCachedThreadPool(); //信号量 final Semaphore semaphore = new Semaphore(concurrencyTotal); //闭锁 final CountDownLatch countDownLatch = new CountDownLatch(clientsTotal); for (int i = 0; i < clientsTotal; i++) { final int count = i; executorService.execute(()->{ try { semaphore.acquire(); update(count); semaphore.release(); } catch (InterruptedException e) { log.error("出现错误:【{}】", e.getMessage()); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("map size:{}", map.size()); } /** * 通过线程安全对象dateTimeFormatter处理 */ private static void update(int count){ map.put(count, count); } }输出结果:5000,线程安全。注意：ConcurrentHashMap与ConcurrentSkipListMap的使用场景。小总结：线程限制：一个被线程限制的对象，由线程独占，并且只能被占有它的线程修改共享只读：一个共享只读的对象，在没有额外同步的情况下，可以被多个线程并发访问，但是任何线程都不能修改它。线程安全对象：一个线程安全的对象或者容器，在内部通过同步机制来保证线程安全，所以其他线程无需额外的同步就可以通过公共接口随意访问它被守护对象：被守护对象只能通过获取特定的锁来访问

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。