面试系列之-信号量Semaphore（JAVA基础）

用户4283147

发布于 2023-09-11 15:53:36

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发布于 2023-09-11 15:53:36

概述

控制同一时间并发线程的数目。能够完成对于信号量的控制，可以控制某个资源可被同时访问的个数。提供了两个核心方法——acquire()方法和release()方法。acquire()方法表示获取一个许可，如果没有则等待，release()方法则是在操作完成后释放对应的许可。Semaphore维护了当前访问的个数，通过提供同步机制来控制同时访问的个数。Semaphore可以实现有限大小的链表。

使用场景

Semaphore常用于仅能提供有限访问的资源，比如：数据库连接数。

代码示例

每次获取并释放一个许可，示例代码如下：

package io.binghe.concurrency.example.aqs;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
public class SemaphoreExample {
  private static final int threadCount = 200;
    public static void main(String[] args) throws InterruptedException {
    ExecutorService exec = Executors.newCachedThreadPool();
    final Semaphore semaphore  = new Semaphore(3);
    for (int i = 0; i < threadCount; i++){
      final int threadNum = i;
      exec.execute(() -> {
        try {
          semaphore.acquire();  //获取一个许可
          test(threadNum);
          semaphore.release();  //释放一个许可
       } catch (InterruptedException e) {
          e.printStackTrace();
       }
     });
   }
    exec.shutdown();
 }
  private static void test(int threadNum) throws InterruptedException {
    log.info("{}", threadNum);
    Thread.sleep(1000);
 }
}

每次获取并释放多个许可，示例代码如下:

package io.binghe.concurrency.example.aqs;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
public class SemaphoreExample {
  private static final int threadCount = 200;
  public static void main(String[] args) throws InterruptedException {
    ExecutorService exec = Executors.newCachedThreadPool();
    final Semaphore semaphore  = new Semaphore(3);
    for (int i = 0; i < threadCount; i++){
      final int threadNum = i;
      exec.execute(() -> {
        try {
          semaphore.acquire(3);  //获取多个许可
          test(threadNum);
          semaphore.release(3);  //释放多个许可
       } catch (InterruptedException e) {
          e.printStackTrace();
       }
     });
   }
    log.info("finish");
    exec.shutdown();
 }
  private static void test(int threadNum) throws InterruptedException {
    log.info("{}", threadNum);
    Thread.sleep(1000);
 }
}

假设有这样一个场景，并发太高了，即使使用Semaphore进行控制，处理起来也比较棘手。假设系统当前允许的最高并发数是3，超过3后就需要丢弃，使用Semaphore也能实现这样的场景，示例代码如下：

package io.binghe.concurrency.example.aqs;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
@Slf4j
public class SemaphoreExample {
  private static final int threadCount = 200;
  public static void main(String[] args) throws InterruptedException {
    ExecutorService exec = Executors.newCachedThreadPool();
    final Semaphore semaphore  = new Semaphore(3);
    for (int i = 0; i < threadCount; i++){
      final int threadNum = i;
      exec.execute(() -> {
        try {
          //尝试获取一个许可，也可以尝试获取多个许可，
          //支持尝试获取许可超时设置，超时后不再等待后续线程的执行
          //具体可以参见Semaphore的源码
          if (semaphore.tryAcquire()) {
            test(threadNum);
            semaphore.release();  //释放一个许可
         }
       } catch (InterruptedException e) {
          e.printStackTrace();
       }
     });
   }
    log.info("finish");
    exec.shutdown();
 }
  private static void test(int threadNum) throws InterruptedException {
    log.info("{}", threadNum);
    Thread.sleep(1000);
 }
}

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原始发表：2023-08-23，如有侵权请联系 cloudcommunity@tencent.com 删除

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