疑问
- 多个线程执行结束后怎么执行某一特定操作？
- 怎么限制执行某块业务的线程的数量？
CountDownLatch
计数器锁，初始化一个 count（数）锁，每个业务线程依次 countDown （递减），主线程阻塞 await （等待）直至 count 等于 0，或者指定 await 时间：
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
/**
* @author caojiantao
*/
public class Test {
public static void main(String[] args) {
System.out.println("========================= 开始 =========================");
// 工作线程数量
int workCount = 5;
// 模拟耗时范围
int cost = 3000;
CountDownLatch latch = new CountDownLatch(workCount);
for (int i = 0; i < workCount; i++) {
new Thread(() -> {
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 开始执行...");
try {
TimeUnit.MILLISECONDS.sleep(new Random().nextInt(cost));
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 执行完...");
latch.countDown();
}).start();
}
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("========================= 结束 =========================");
}
private static String getTimeFmtString(){
return LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME);
}
}
CyclicBarrier
篱栅，功能与 CountDownLatch 大体相同，但是其特色支持计数器重置，循环使用：
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Random;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.TimeUnit;
/**
* @author caojiantao
*/
public class Test {
public static void main(String[] args) {
System.out.println("========================= 开始 =========================");
// 工作线程数量
int workCount = 5;
// 模拟耗时范围
int cost = 3000;
CyclicBarrier barrier = new CyclicBarrier(workCount, () -> {
System.out.println("========================= 结束 =========================");
});
for (int i = 0; i < workCount; i++) {
new Thread(() -> {
try {
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 开始执行...");
TimeUnit.MILLISECONDS.sleep(new Random().nextInt(cost));
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 执行完...");
barrier.await();
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
}).start();
}
}
private static String getTimeFmtString() {
return LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME);
}
}
注：相比较 CountDownLatch，CyclicBarrier 能够 reset 重置计数器，同时注意只有 await 方法会阻塞当前线程，countDown 并不会。
Semaphore
信号量，保持当前信号量（执行线程数量）最多为 permits：
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Random;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
/**
* @author caojiantao
*/
public class Test {
public static void main(String[] args) {
// 工作线程数量
int workCount = 10;
// 模拟耗时范围
int cost = 3000;
Semaphore semaphore = new Semaphore(5);
for (int i = 0; i < workCount; i++) {
new Thread(() -> {
try {
// 申请执行权限
semaphore.acquire();
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 开始执行...");
TimeUnit.MILLISECONDS.sleep(new Random().nextInt(cost));
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 执行完...");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// 释放权限
semaphore.release();
}
}).start();
}
}
private static String getTimeFmtString() {
return LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME);
}
}
案例
三方平台比价接口
假如存在 A、B、C 三个平台正在出售某商品 goods，现在需要多线程获取三个平台该商品 goods 价格，最终输出最低价格价格信息。
import java.math.BigDecimal;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Map;
import java.util.Random;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
/**
* @author caojiantao
*/
public class Test {
private static Map<String, Double> infoMap = new ConcurrentHashMap<>(3);
public static void main(String[] args) {
CountDownLatch latch = new CountDownLatch(3);
Thread a = new Thread(new Task("A", latch));
Thread b = new Thread(new Task("B", latch));
Thread c = new Thread(new Task("C", latch));
a.start();
b.start();
c.start();
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
Map.Entry<String, Double> lowest = null;
for (Map.Entry<String, Double> entry : infoMap.entrySet()) {
if (lowest == null || entry.getValue() < lowest.getValue()) {
lowest = entry;
}
}
assert lowest != null;
System.out.println(getTimeFmtString() + " 最低价格信息为：" + lowest.getKey() + " " + lowest.getValue());
}
private static String getTimeFmtString() {
return LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME);
}
static class Task implements Runnable {
private String name;
private CountDownLatch latch;
public Task(String name, CountDownLatch latch) {
this.name = name;
this.latch = latch;
}
@Override
public void run() {
int cost = 5000;
try {
TimeUnit.MILLISECONDS.sleep(new Random().nextInt(cost));
double price = new BigDecimal(new Random().nextDouble() * 1000).setScale(2, BigDecimal.ROUND_HALF_UP).doubleValue();
infoMap.put(name, price);
System.out.println(getTimeFmtString() + " " + name + " 报价：" + price);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
latch.countDown();
}
}
}
}
限制接口访问次数
存在某接口 queryData，需要控制在 10 个访问数以内，超过的请求阻塞直至有正在执行的请求已经完成。
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Random;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
/**
* @author caojiantao
*/
public class Service {
private Semaphore semaphore = new Semaphore(10);
public void queryData() {
// 模拟耗时范围
int cost = 3000;
try {
// 申请执行权限
semaphore.acquire();
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 请求...");
TimeUnit.MILLISECONDS.sleep(new Random().nextInt(cost));
System.out.println(getTimeFmtString() + " " + Thread.currentThread().getName() + " 结束");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// 释放权限
semaphore.release();
}
}
private String getTimeFmtString() {
return LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_TIME);
}
}
