记一次生产频繁发生FullGC问题

迈不过友情╰ 2023-09-25 16:51 85阅读 0赞

## 问题发现 ##

早上过来，饭都没来的及吃，运维就给我发来信息，说是某个接口调用大量超时。因为最近这个接口调用量是翻倍了，所以我就去检查了下慢SQL，发现确实是有较多的慢SQL，所以我就缩减了查询的时间范围，但是效果并不好。

过了一会发现，这个服务fullGC是有问题的，太频繁了，这个应该是导致接口超时的根本问题，因为时间也是对的上的。

这个是最近三个小时fullGC的监控图：

![dca024f0ec2ad6992cdd1a66faa11ff1.png][]

这个是最近三天fullGC的监控图：

![c3df06ff38dd52175c9038bf3dfb325f.png][]

对比一下，就不难发现，fullGC数量是从3月15号晚上9点开始增加的，也是这个接口对外开放的时间。

## 解决思路 ##

1、首先去服务器上面下载dump文件，分析是哪里造成了内存泄漏，频繁触发fullGC。首先找出服务器内java文件的PID，然后保存dump文件，我们公司java服务是固定端口号：1

使用top命令：

![1e91d963a779432a1a79ba217a55a81d.png][]

然后执行命令：jmap -dump:file=202303160924.dump,format=b 1 ，保存dump文件

![af88f5b51f6b0e1a68cccf4c83f4bbeb.png][]

2、根据dump文件，分析出堆内对象的分布情况

*   *  下载一个可以分析dump文件的工具，这里我下载是Jprofiler
     *  查看大对象的分析，发现是java.lang.ApplicationShutdownHooks的hooks占用太大内存，并且得知改熟悉是一个Map

![c6e26dfc0fed7805c2d44a49d3450957.png][]

*   *  分析这个Map里面的元素引用关系，可以看到这个map里面存的都是线程对象，并且大部分都是一个名为java.util.concurrent.ScheduledThreadPoolExecutor@59648a61的线程池对象，到了这里就定位到问题代码了，是这次新加的接口里面有一个异步操作，用的guava并发包里面的一个超时等待功能的接口，具体思路就是启用一个定时任务线程池去定时去检查在规定时间内，是否返回结果。

![559317d411ec572602243f5f104484a2.png][]

3、看看我的业务代码是哪里出现了问题

//异步执行某个查询方法，并且在规定时间内返回查询结果
    public <T> T asyncWithTimeout(ScheduledThreadPoolExecutor executor, Callable<T> callable,
                                  long time, TimeUnit unit) {
        try {
            ListeningExecutorService listeningExecutorService = MoreExecutors.listeningDecorator(threadPoolExecutor);
            ListenableFuture<T> future = listeningExecutorService.submit(callable);
            //这里是创建一个定时任务线程，去定时检查是否在规定时间内查询完毕，应该就是这个去添加了钩子函数，进去看看
            ScheduledExecutorService scheduledExecutorService = MoreExecutors.getExitingScheduledExecutorService(executor);
            return Futures.withTimeout(future, time, unit, scheduledExecutorService).get(time, unit);
        } catch (InterruptedException | ExecutionException | TimeoutException e) {
            log.warn("异步方法执行失败，error:{}", e.getMessage());
        }
        return null;
    }
    
    //=======================guava并发包代码=======================
    
    @Beta
    @GwtIncompatible // TODO
    public static ScheduledExecutorService getExitingScheduledExecutorService(
        ScheduledThreadPoolExecutor executor) {
      //每次都去创建一个新的对象
      return new Application().getExitingScheduledExecutorService(executor);
    }
    
    final ScheduledExecutorService getExitingScheduledExecutorService(
        ScheduledThreadPoolExecutor executor) {
      return getExitingScheduledExecutorService(executor, 120, TimeUnit.SECONDS);
    }
    
    final ScheduledExecutorService getExitingScheduledExecutorService(
        ScheduledThreadPoolExecutor executor, long terminationTimeout, TimeUnit timeUnit) {
      useDaemonThreadFactory(executor);
      ScheduledExecutorService service = Executors.unconfigurableScheduledExecutorService(executor);
      //添加构造函数的地方，进去看看
      addDelayedShutdownHook(executor, terminationTimeout, timeUnit);
      return service;
    }
    
    final void addDelayedShutdownHook(
        final ExecutorService service, final long terminationTimeout, final TimeUnit timeUnit) {
      checkNotNull(service);
      checkNotNull(timeUnit);
      //继续点进去
      addShutdownHook(
          MoreExecutors.newThread(
              //线程名字对上了，就在对象引用的截图里面出现过
              "DelayedShutdownHook-for-" + service,
              new Runnable() {
                @Override
                public void run() {
                  try {
                    // We'd like to log progress and failures that may arise in the
                    // following code, but unfortunately the behavior of logging
                    // is undefined in shutdown hooks.
                    // This is because the logging code installs a shutdown hook of its
                    // own. See Cleaner class inside {@link LogManager}.
                    service.shutdown();
                    service.awaitTermination(terminationTimeout, timeUnit);
                  } catch (InterruptedException ignored) {
                    // We're shutting down anyway, so just ignore.
                  }
                }
              }));
    }
    
    @VisibleForTesting
    void addShutdownHook(Thread hook) {
      Runtime.getRuntime().addShutdownHook(hook);
    }
    
    //=======================guava并发包代码=======================
    
    public void addShutdownHook(Thread hook) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            sm.checkPermission(new RuntimePermission("shutdownHooks"));
        }
        //定位到问题了，就是这里添加的钩子函数
        ApplicationShutdownHooks.add(hook);
    }
    
    static synchronized void add(Thread hook) {
        if(hooks == null)
            throw new IllegalStateException("Shutdown in progress");
    
        if (hook.isAlive())
            throw new IllegalArgumentException("Hook already running");
    
        if (hooks.containsKey(hook))
            throw new IllegalArgumentException("Hook previously registered");
    
        //存在到 hooks 这个map对象里面，就是这个大对象
        hooks.put(hook, hook);
    }

## 问题解决 ##

经过上面问题的排查，造成hooks大对象的原因找到了，就是每次调用接口的时候，都会往hooks里面put一个对象。

所以，解决办法很简单，就是不用每次都去生成一个ScheduledExecutorService对象，类初始化的时候创建一次就行了

改造后的代码如下：

private ListeningExecutorService listeningExecutorService;
    private ScheduledExecutorService scheduledExecutorService;
    
    public static AsyncUtils getInstance() {
        return ThreadHolder.INSTANCE.getAsyncWithCallback();
    }
    
    @SuppressWarnings("UnstableApiUsage")
    private AsyncUtils() {
        listeningExecutorService = MoreExecutors.listeningDecorator(ThreadPoolConstant.THREAD_POOL_EXECUTOR);
        scheduledExecutorService = MoreExecutors.getExitingScheduledExecutorService(ThreadPoolConstant.SCHEDULED_THREAD_POOL_EXECUTOR);
    }
    
    @SuppressWarnings("UnstableApiUsage")
    public <T> T asyncWithTimeout(Callable<T> callable,
                                  long time, TimeUnit unit) {
        try {
            ListenableFuture<T> future = listeningExecutorService.submit(callable);
            return Futures.withTimeout(future, time, unit, scheduledExecutorService).get(time, unit);
        } catch (InterruptedException | ExecutionException | TimeoutException e) {
            log.warn("异步方法执行失败，error:{}", e.getMessage());
        }
        return null;
    }
    
    private enum ThreadHolder {
        /**
         * 线程持有类 INSTANCE
         */
        INSTANCE;
        private final AsyncUtils asyncUtils;
    
        ThreadHolder() {
            asyncUtils = new AsyncUtils();
        }
    
        public AsyncUtils getAsyncWithCallback() {
            return asyncUtils;
        }
    }

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[af88f5b51f6b0e1a68cccf4c83f4bbeb.png]: https://img-blog.csdnimg.cn/img_convert/af88f5b51f6b0e1a68cccf4c83f4bbeb.png
[c6e26dfc0fed7805c2d44a49d3450957.png]: https://img-blog.csdnimg.cn/img_convert/c6e26dfc0fed7805c2d44a49d3450957.png
[559317d411ec572602243f5f104484a2.png]: https://img-blog.csdnimg.cn/img_convert/559317d411ec572602243f5f104484a2.png