说明：最近碰到一个需求，根据后端返回的倒计时时间，展示一个定时器，要求页面切换和刷新的时候，定时器保持运行，

step1:

package com.unayshah.myapplication;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import java.lang.reflect.Type;
import java.util.List;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;

public class HomeActivity extends AppCompatActivity {
    private EditText et_home;
    private Button btn_start;
    private TextView tv_home;
    private String etString;
    private int etInt=0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        et_home = findViewById(R.id.et_home);
        btn_start = findViewById(R.id.btn_start);
        tv_home = findViewById(R.id.tv_home);

        btn_start.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                etString = et_home.getText().toString();
                etInt= Integer.parseInt(etString);
                mScheduledExecutorService.scheduleAtFixedRate(new Runnable() {
                    @Override
                    public void run() {
                        etInt--;
                        try {
                            Message msg = new Message();
                            msg.what = 402;
                            msg.obj =etInt;
                            mHandler.sendMessage(msg);
                            Thread.sleep(1000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }, 1, 1, TimeUnit.SECONDS);
            }
        });
    }

    private ScheduledExecutorService mScheduledExecutorService = Executors.newScheduledThreadPool(0, new ThreadFactory() {
        @Override
        public Thread newThread(@NonNull Runnable r) {

            return new Thread(r) {
                @Override
                public void run() {
                    Log.e("lzp", "newThread");
                    super.run();
                    Log.e("lzp", "newThread over");
                }
            };
        }
    });

    private Handler mHandler = new Handler(new Handler.Callback() {
        @Override
        public boolean handleMessage(@NonNull Message message) {
            if (message.what == 402) {
                int post = (int) message.obj;
                Log.e("lzp", "newThread"+post);
                if (post<1){
                    tv_home.setVisibility(View.INVISIBLE);
                }else{
                    tv_home.setVisibility(View.VISIBLE);
                }
                tv_home.setText(String.valueOf(post));
            }
            return false;
        }
    });

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.e("lzp", "onDestroy注销:");
        //mScheduledExecutorService.shutdown(); 这一句可以结束定时循环任务
    }
}