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：

这个是最近三天fullGC的监控图：

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。

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：

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

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

• 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。

    

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);
}

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;
    }
}