AsyncTask RejectedExecution 排查方案

问题

在应用开发的多线程问题中，可能会遇到AsyncTask.execute()时会出现RejectedExecution的场景

java.util.concurrent.RejectedExecutionException: Task android.os.AsyncTask$SerialExecutor$1@7840c01 rejected from java.util.concurrent.ThreadPoolExecutor@5a73928[Running, pool size = 9, active threads = 9, queued tasks = 128, completed tasks = 192]
  at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2014)
  at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:794)
  at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1340)
  at android.os.AsyncTask$SerialExecutor.scheduleNext(AsyncTask.java:247)
  at android.os.AsyncTask$SerialExecutor.execute(AsyncTask.java:241)
  at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:607)
  at android.os.AsyncTask.execute(AsyncTask.java:551)

要从根源上解决以上问题，需要排查抛出异常时队列中任务的具体情况。

如何解决

整体设计如下：

image

1. sDefaultExecutor在原生代码里就是SerialExecutor，而SerialExecutor是串行执行队列，但最终也是通过THREAD_POOL_EXECUTOR完成。为了加大复现概率，可以在Application创建时通过反射，替换AsyncTask的sDefaultExecutor为THREAD_POOL_EXECUTOR

if (BuildConfig.DEBUG) {
    AsyncTaskWithTag.setEnableTaskInfo(true);
    // 线程池监控
    ReflectUtils.setStaticFieldValue(AsyncTask.class, "THREAD_POOL_EXECUTOR", AsyncTaskWithTag.THREAD_POOL_EXECUTOR);
    ReflectUtils.setStaticFieldValue(AsyncTask.class, "sDefaultExecutor", AsyncTaskWithTag.THREAD_POOL_EXECUTOR);
}

2. 使用ThreadPoolExecutorWithTag替换AsyncTask的sDefaultExecutor

//AsyncTaskWithTag.java 

private static final ThreadFactory sThreadFactory = new ThreadFactory() {
    private final AtomicInteger mCount = new AtomicInteger(1);

    public Thread newThread(Runnable runnable) {
        Thread thread = new Thread(runnable, "AsyncTaskTag #" + mCount.getAndIncrement());
        AsyncTaskRunningInfo.addTaskNameToMap(thread.toString(), runnable.toString());
        return thread;
    }
};

private static final RejectedExecutionHandler defaultHandler =
    new RejectedExecutionHandlerWithTag();

public static final Executor THREAD_POOL_EXECUTOR;
static {
    ThreadPoolExecutorWithTag threadPoolExecutor = new ThreadPoolExecutorWithTag(
        CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
        sPoolWorkQueue, sThreadFactory, defaultHandler);
    threadPoolExecutor.allowCoreThreadTimeOut(true);
    THREAD_POOL_EXECUTOR = threadPoolExecutor;
}

其中所有通过ThreadPoolExecutorWithTag创建的Thread，都将信息保存到AsyncTaskRunningInfo的Map集合中。

OOM时拒绝策略，使用自定义的RejectedExecutionHandlerWithTag。

public class RejectedExecutionHandlerWithTag implements RejectedExecutionHandler {

    private static final String TAG = "Rejected";
    private static final String TASK_CREATE_BY = " create by: ";
    private static final String TASK_CREATE_TIME = " create at: ";
    private static final String TASK_COST_TIME = " cost time: ";

    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        if (!executor.isShutdown()) {
            prepareThreadInfo(executor);
            prepareWorksQueueInfo(executor);
        }
        throw new RejectedExecutionException("Task " + r.toString() +
            " rejected from " +
            executor.toString());
    }

    // 工作线程
    private void prepareThreadInfo(ThreadPoolExecutor executor) {

        Object workersValue = ReflectUtils.getFieldValue(executor, "workers");
        if (workersValue instanceof HashSet) {

            long currentTime = System.currentTimeMillis();
            StringBuilder builder = new StringBuilder();

            HashSet