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Java并发编程 - 线程不安全类与解决方案(Set)


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

  1. 写操作时复制消耗内存
  2. 不能用于实时读的场景
  3. 由于复制和add操作等需要时间,故读取时可能读到旧值。
  4. 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。
  5. 如果无法知道数组有多大,或者add、set操作有多少,慎用此类。
  6. 迭代器不支持可变的remove操作。

设计思想

  1. 读写分离
  2. 最终一致性
  3. 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

  1. 支持自然排序
  2. 基于Map集合来进一步封装
  3. 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)
  4. 不能存空值


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