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CopyOnWriteArrayList(对应ArrayList)
- CopyOnWriteArrayList使用了一种叫写时复制的方法，当有新元素添加到CopyOnWriteArrayList时，先从原有的数组中拷贝一份出来，然后在新的数组做写操作，写完之后，再将原来的数组引用指向到新数组。
- CopyOnWriteArrayList的整个add操作都是在锁的保护下进行的。
这样做是为了避免在多线程并发add的时候，复制出多个副本出来,把数据搞乱了，导致最终的数组数据不是我们期望的。 - 通过上面的分析，CopyOnWriteArrayList 有几个缺点：
1、由于写操作的时候，需要拷贝数组，会消耗内存，如果原数组的内容比较多的情况下，可能导致young gc或者full gc
2、不能用于实时读的场景，像拷贝数组、新增元素都需要时间，所以调用一个set操作后，读取到数据可能还是旧的,虽然CopyOnWriteArrayList 能做到最终一致性,但是还是没法满足实时性要求.
package com.alan.concurrency.example.concurrent;
import com.alan.concurrency.annoations.NotThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;
@Slf4j
public class CopyOnWriteArrayListExample {
//请求数1000
public static int clientTotal = 5000;
//同时并发执行的线程数
public static int threadTotal = 200;
public static List list = new CopyOnWriteArrayList<>();
private static void update(){
list.add(1);
}
public static void main(String[] args) throws InterruptedException {
//定义线程池ExecutorService接口
ExecutorService executorService = Executors.newCachedThreadPool();
//定义信号量,传入并发线程数 final修饰不允许重新赋值
final Semaphore semaphore = new Semaphore(threadTotal);
//定义计数器闭锁。传入请求总数
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
for (int i = 0; i < clientTotal; i++) {
//通过匿名内部类方式
executorService.execute(new Runnable() {
@Override
public void run() {
try {
//semaphore控制并发数量
semaphore.acquire();
update();
semaphore.release();
} catch (InterruptedException e) {
log.error("exception",e);
}
//每次执行计数器减掉一个
countDownLatch.countDown();
}
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}",list.size());
}
}
CopyOnWriteArraySet(对应HashSet) ConcurrentSkipListSet(对应TreeSet)
package com.alan.concurrency.example.concurrent;
import lombok.extern.slf4j.Slf4j;
import java.util.Set;
import java.util.concurrent.*;
@Slf4j
public class CopyOnWriteHashSetExample {
//请求数1000
public static int clientTotal = 5000;
//同时并发执行的线程数
public static int threadTotal = 200;
public static Set set = new CopyOnWriteArraySet<>();
private static void update(int i){
set.add(i);
}
public static void main(String[] args) throws InterruptedException {
//定义线程池ExecutorService接口
ExecutorService executorService = Executors.newCachedThreadPool();
//定义信号量,传入并发线程数 final修饰不允许重新赋值
final Semaphore semaphore = new Semaphore(threadTotal);
//定义计数器闭锁。传入请求总数
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
for (int i = 0; i < clientTotal; i++) {
//通过匿名内部类方式
int count = i;
executorService.execute(new Runnable() {
@Override
public void run() {
try {
//semaphore控制并发数量
semaphore.acquire();
update(count);
semaphore.release();
} catch (InterruptedException e) {
log.error("exception",e);
}
//每次执行计数器减掉一个
countDownLatch.countDown();
}
});
}
countDownLatch.await();
executorService.shutdown();
log.info("size:{}", set.size());
}
}
ConcurrentHashMap(对应HashMap) ConcurrentSkipListMap(对应TreeMap)
- ConcurrentHashMap
1、Java5中新增加的一个线程安全的Map集合，可以用来替代HashTable。对于ConcurrentHashMap是如何提高其效率的，
2、它使用了多个锁代替HashTable中的单个锁，也就是锁分离技术（Lock Stripping）
package com.alan.concurrency.example.concurrent;
import com.alan.concurrency.annoations.NotThreadSafe;
import com.alan.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.*;
@Slf4j
@ThreadSafe
public class ConcurrentHashMapExample {
// 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 Map map = new ConcurrentHashMap<>();
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:{}", map.size());
}
private static void update(int i) {
map.put(i, i);
}
}
- ConcurrentSkipListMap
1、TreeMap使用红黑树按照key的顺序（自然顺序、自定义顺序）来使得键值对有序存储，但是只能在单线程下安全使用；多线程下想要使键值对按照key的顺序来存储，则需要使用ConcurrentSkipListMap。
2、ConcurrentSkipListMap的底层是通过跳表来实现的。跳表是一个链表，但是通过使用“跳跃式”查找的方式使得插入、读取数据时复杂度变成了O（logn）
package com.alan.concurrency.example.concurrent;
import com.alan.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Map;
import java.util.concurrent.*;
@Slf4j
@ThreadSafe
public class ConcurrentSkipListMapExample {
// 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 Map map = new ConcurrentSkipListMap<>();
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:{}", map.size());
}
private static void update(int i) {
map.put(i, i);
}
}
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