ThreadPoolExecutor中的竞态条件问题排查与修复求助

代码要求 #

• 同步执行Bot#run方法

需要修复的问题 #

• 不同Bot实例与线程间的资源共享（如Bot.responseData等）冲突

Bot.run()执行流程 #

1. 发起HTTP请求从服务器获取随机值（如f8c3de3d-1fea-4d7c-a8b0-29f63c4c3454，服务器始终返回随机值）
2. 将值存入Bot.responseData
3. 以Bot.responseData为参数发起HTTP请求，获取JSON结果
4. 将JSON存入Bot.statistics
5. 清空Bot.responseData = null

相关代码 #

Bot实体 #

public class Bot 
{
    private final BotStatistic statistic = new BotStatistic();
    private final Object mutex = new Object();
    private String responseData;    

    void run() 
    {
        synchronized(mutex) 
        {
          String myHost = "http://myhost";
          firstCall(myHost);
          secondCall(myHost);
          cleanup();
        }
    }

    private void firstCall(String host) 
    {
        // first http call -> responseData = "f8c3de3d-1fea-4d7c-a8b0-29f63c4c3454";
    }

    private void secondCall(String host) 
    {
        // second http call(responseData) -> 
        // JSON {"success": true, "requestValue": "f8c3de3d-1fea-4d7c-a8b0-29f63c4c3454"}
        statistic.add(json);
    }

    private void cleanup() 
    {
        this.responseData=null;
    }
}

BotRunner实体 #

public class BotRunner implements Runnable 
{
    private final Bot bot;
    
    public BotRunner(Bot bot) 
    {
        this.bot = bot;
    }
    
    @Override
    public void run() 
    {
       bot.run();
    }
}

执行方式 #

//init executor
LocalDateTime termination = LocalDateTime.now().plusSeconds(5L);
while (LocalDateTime.now().isBefore(termination)) 
{
    for (Bot bot : bots) 
    {
        executor.execute(new BotRunner(bot));
    }
}
//shutdown & close executor

当前现象 #

执行后发现1-2%的不同Bot实例出现重复的Bot.responseData，示例如下：

thread-1 : start (Bot@44g35)
thread-1 : f8c3de3d-1fea-4d7c-a8b0-29f63c4c3454
thread-2 : start (Bot@898g)
thread-2 : f8c3de3d-1fea-4d7c-a8b0-29f63c4c3454
thread-1 : end
thread-2 : end

期望结果 #

每个Bot实例获取唯一的随机值，示例如下：

thread-1 : start (Bot@44g35)
thread-1 : f8c3de3d-1fea-4d7c-a8b0-29f63c4c3454
thread-2 : start (Bot@898g)
thread-2 : 018b2f19-e79e-7d6a-a56d-29feb6211b04
thread-1 : end
thread-2 : end

问题分析与修复方案 #

问题根源 #

当前Bot类的synchronized(mutex)已保证单个实例的run()不会被多线程并发执行，但不同实例出现重复随机值的核心原因是HTTP请求工具存在线程不安全的共享状态——比如复用了线程不安全的请求对象、共享了未隔离的响应处理逻辑，导致不同线程的请求互相干扰，拿到重复响应值。实例变量responseData的同步保护无法解决HTTP工具本身的线程问题。

修复步骤 #

1. 确保HTTP请求工具线程安全
   使用线程安全的HTTP客户端（如Apache HttpClient的CloseableHttpClient可全局复用），但每个请求必须新建独立的请求对象（HttpGet/HttpPost），禁止共享请求实例。示例修正firstCall：

   private void firstCall(String host) {
       try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
           HttpGet request = new HttpGet(host + "/get-random-value");
           try (CloseableHttpResponse response = httpClient.execute(request)) {
               this.responseData = EntityUtils.toString(response.getEntity());
           }
       } catch (IOException e) {
           e.printStackTrace();
       }
   }
   

2. 移除实例变量，改用局部变量传递状态
   把responseData改为run()方法的局部变量，彻底消除实例内的状态共享风险，同时可移除不必要的同步锁：

   public class Bot 
   {
       private final BotStatistic statistic = new BotStatistic();
       
       void run() 
       {
           String myHost = "http://myhost";
           String randomValue = firstCall(myHost);
           if (randomValue != null) {
               secondCall(myHost, randomValue);
           }
       }
   
       private String firstCall(String host) 
       {
           try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
               HttpGet request = new HttpGet(host + "/get-random-value");
               try (CloseableHttpResponse response = httpClient.execute(request)) {
                   return EntityUtils.toString(response.getEntity());
               }
           } catch (IOException e) {
               e.printStackTrace();
               return null;
           }
       }
   
       private void secondCall(String host, String randomValue) 
       {
           try (CloseableHttpClient httpClient = HttpClients.createDefault()) {
               HttpPost request = new HttpPost(host + "/submit-value");
               request.setEntity(new StringEntity("{\"value\":\"" + randomValue + "\"}"));
               request.setHeader("Content-Type", "application/json");
               try (CloseableHttpResponse response = httpClient.execute(request)) {
                   String json = EntityUtils.toString(response.getEntity());
                   statistic.add(json);
               }
           } catch (IOException e) {
               e.printStackTrace();
           }
       }
   }
   

   局部变量为线程私有，无需同步即可避免状态冲突。

3. 优化任务提交逻辑
   原循环批量提交任务可能导致请求过于密集，引发连接池异常。改为每个Bot实例独立循环执行，控制请求频率：

   // 替代原执行逻辑
   ExecutorService executor = Executors.newFixedThreadPool(bots.size());
   LocalDateTime termination = LocalDateTime.now().plusSeconds(5L);
   for (Bot bot : bots) {
       executor.submit(() -> {
           while (LocalDateTime.now().isBefore(termination)) {
               bot.run();
               Thread.sleep(100); // 可选：添加间隔避免请求过载
           }
       });
   }
   executor.shutdown();
   executor.awaitTermination(6, TimeUnit.SECONDS);
   

验证 #

修复后运行测试，检查不同Bot实例的随机值是否唯一，同时监控HTTP请求日志，确认每个请求都获取到服务器返回的独立随机值。

内容的提问来源于stack exchange，提问作者IndependenceCR