HashSet、TreeSet
package com.mmall.concurrency.example.commonUnsafe;
import com.mmall.concurrency.annoations.NotThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@NotThreadSafe
public class HashSetExample {
// 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 HashSet<>();
public static void main(String[] args) throws Exception {
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:{}", set.size());
}
private static void update(int i) {
set.add(i);
}
}
// 输出
size:4955解决方案一（同步容器：synchronizedSet）
package com.mmall.concurrency.example.syncContainer;
import com.google.common.collect.Sets;
import com.mmall.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Collections;
import java.util.Set;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class CollectionsExample2 {
// 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 = Collections.synchronizedSet(Sets.newHashSet());
public static void main(String[] args) throws Exception {
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:{}", set.size());
}
private static void update(int i) {
set.add(i);
}
}
// 输出
size:5000解决方案二（并发容器：CopyOnWriteArraySet）（对应 HashSet）
package com.mmall.concurrency.example.concurrent;
import com.mmall.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArraySet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class CopyOnWriteArraySetExample {
// 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 Exception {
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:{}", set.size());
}
private static void update(int i) {
set.add(i);
}
}
// 输出
size:5000分析
- CopyOnWriteArraySet 写操作时复制，当有新元素添加到集合中时，从原有的数组中拷贝一份出来，然后在新的数组上作写操作，然后将原来的数组指向新的数组。整个数组的add操作都是在锁的保护下进行的，防止并发时复制多份副本。读操作是在原数组中进行，不需要加锁。
缺点
- 写操作时复制消耗内存
- 不能用于实时读的场景
- 由于复制和add操作等需要时间，故读取时可能读到旧值。
- 能做到最终一致性，但无法满足实时性的要求，更适合读多写少的场景。
- 如果无法知道数组有多大，或者add、set操作有多少，慎用此类。
- 迭代器不支持可变的remove操作。
设计思想
- 读写分离
- 最终一致性
- 使用时另外开辟空间，防止并发冲突
解决方案三（并发容器：ConcurrentSkipListSet）（对应 TreeSet）
package com.mmall.concurrency.example.concurrent;
import com.mmall.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Set;
import java.util.concurrent.ConcurrentSkipListSet;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class ConcurrentSkipListSetExample {
// 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 Exception {
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:{}", set.size());
}
private static void update(int i) {
set.add(i);
}
}
// 输出
size:5000分析
- 支持自然排序
- 基于Map集合来进一步封装
- contains、add、remove是线程安全的，但是其addAll，containsAll，removeAll不是线程安全的（因为这些批量操作是调用的父类里基础的单个contains、add、remove线程安全方法，所以不能保证原子性）
- 不能存空值
