问在Android姜饼之后，异步任务到底发生了什么变化？

EN

package com.sample.asynctask;

import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;

public class AsyncTaskTestActivity extends Activity {
    private static final String TAG = "AsyncTaskTestActivity";

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        //ExecutorService executorService = Executors.newFixedThreadPool(1);
        for (int i = 1; i <= 20; i++) {
            TestTask testTask = new TestTask(i);
            testTask.execute();
        }
    }

    private static class TestTask extends AsyncTask<Void, Integer, Void> {
        int i;
        public TestTask(int i) {
            Log.i(TAG, "Constructor for " + i);
            this.i = i;
        }

        @Override
        protected void onPreExecute() {
            // TODO Auto-generated method stub
            super.onPreExecute();
            Log.i(TAG, "onPreExecute for " + i);
        }

        @Override
        protected Void doInBackground(Void... params) {
            Log.i(TAG, i + " Thread goes to sleep");
            try {
                Thread.sleep(20000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            Log.i(TAG, i + " Thread wakes up");
            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);
            Log.i(TAG, "onPostExecute for " + i);
        }
    }
}