List
package com.mmall.concurrency.example.commonUnsafe;
import com.mmall.concurrency.annoations.NotThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@NotThreadSafe
public class ArrayListExample {
// 請求總數
public static int clientTotal = 5000;
// 同時併發執行的線程數
public static int threadTotal = 200;
private static List<Integer> list = new ArrayList<>();
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:{}", list.size());
}
private static void update(int i) {
list.add(i);
}
}
// 輸出
size:4988
解決方案一（Vector）
package com.mmall.concurrency.example.syncContainer;
import com.mmall.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.List;
import java.util.Vector;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class VectorExample1 {
// 請求總數
public static int clientTotal = 5000;
// 同時併發執行的線程數
public static int threadTotal = 200;
private static List<Integer> list = new Vector<>();
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:{}", list.size());
}
private static void update(int i) {
list.add(i);
}
}
// 輸出
size:5000
解決方案二（同步容器：synchronizedList）
package com.mmall.concurrency.example.syncContainer;
import com.google.common.collect.Lists;
import com.mmall.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.Collections;
import java.util.List;
import java.util.Vector;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class CollectionsExample1 {
// 請求總數
public static int clientTotal = 5000;
// 同時併發執行的線程數
public static int threadTotal = 200;
private static List<Integer> list = Collections.synchronizedList(Lists.newArrayList());
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:{}", list.size());
}
private static void update(int i) {
list.add(i);
}
}
// 輸出
szie:5000
解決方案三（併發容器：CopyOnWriteArrayList）
package com.mmall.concurrency.example.concurrent;
import com.mmall.concurrency.annoations.ThreadSafe;
import lombok.extern.slf4j.Slf4j;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
@ThreadSafe
public class CopyOnWriteArrayListExample {
// 請求總數
public static int clientTotal = 5000;
// 同時併發執行的線程數
public static int threadTotal = 200;
private static List<Integer> list = new CopyOnWriteArrayList<>();
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:{}", list.size());
}
private static void update(int i) {
list.add(i);
}
}
// 輸出
size:5000
分析
- CopyOnWriteArrayList 寫操作時複製，當有新元素添加到集合中時，從原有的數組中拷貝一份出來，然後在新的數組上作寫操作，然後將原來的數組指向新的數組。整個數組的add操作都是在鎖的保護下進行的，防止併發時複製多份副本。讀操作是在原數組中進行，不需要加鎖。
缺點
- 寫操作時複製消耗內存
- 不能用於實時讀的場景
- 由於複製和add操作等需要時間，故讀取時可能讀到舊值。
- 能做到最終一致性，但無法滿足實時性的要求，更適合讀多寫少的場景。
- 如果無法知道數組有多大，或者add、set操作有多少，慎用此類。
設計思想
- 讀寫分離
- 最終一致性
- 使用時另外開闢空間，防止併發衝突
附1：Vector 線程不安全情景
package com.mmall.concurrency.example.syncContainer;
import com.mmall.concurrency.annoations.NotThreadSafe;
import java.util.Vector;
@NotThreadSafe
public class VectorExample2 {
private static Vector<Integer> vector = new Vector<>();
public static void main(String[] args) {
while (true) {
for (int i = 0; i < 10; i++) {
vector.add(i);
}
Thread thread1 = new Thread() {
public void run() {
for (int i = 0; i < vector.size(); i++) {
vector.remove(i);
}
}
};
Thread thread2 = new Thread() {
public void run() {
for (int i = 0; i < vector.size(); i++) {
vector.get(i);
}
}
};
thread1.start();
thread2.start();
}
}
}
- 輸出結果報錯，因爲 remove 和 get 分別是不同對象調用的不同 sync 方法，很有可能出現 remove 在 get 之前操作。
附2：Vector for & foerach & iterator
package com.mmall.concurrency.example.syncContainer;
import java.util.Iterator;
import java.util.Vector;
public class VectorExample3 {
// java.util.ConcurrentModificationException
private static void test1(Vector<Integer> v1) { // foreach
for(Integer i : v1) {
if (i.equals(3)) {
v1.remove(i);
}
}
}
// java.util.ConcurrentModificationException
private static void test2(Vector<Integer> v1) { // iterator
Iterator<Integer> iterator = v1.iterator();
while (iterator.hasNext()) {
Integer i = iterator.next();
if (i.equals(3)) {
v1.remove(i);
}
}
}
// success
private static void test3(Vector<Integer> v1) { // for
for (int i = 0; i < v1.size(); i++) {
if (v1.get(i).equals(3)) {
v1.remove(i);
}
}
}
public static void main(String[] args) {
Vector<Integer> vector = new Vector<>();
vector.add(1);
vector.add(2);
vector.add(3);
test1(vector);
}
}
- 輸出結果
- 源碼分析，因爲 modCount 只要更新操作會被修改導致這裏不相等拋出異常。
結論
使用foreach，iterator以及for循環對集合類遍歷的同時進行修改
- 在foreach、iterator迭代器循環集合的時候，在遍歷過程中儘量不要做更新操作。
- 如果一定要做的話，在遍歷過程中，只做標記，遍歷完成後再更新。
- 遍歷過程中(foeach,iterator)更新會導致ConcurrentModificationException
- 可以用for循環做遍歷過程中的增刪操作