单例双式以及端点续传

最新推荐文章于 2025-09-20 10:16:26 发布

原创最新推荐文章于 2025-09-20 10:16:26 发布 · 154 阅读

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CC 4.0 BY-SA版权

本文详细解析了单例模式的两种实现方式——饿汉式与懒汉式，并探讨了它们在线程安全方面的考量。此外，还介绍了如何在Android应用中实现文件的断点续传功能，包括通过HTTP请求获取文件总长度和断点位置，以及使用随机流进行文件下载。

单例模式

总体思路：先创建一个私有的private static 的对象，再创建一个私有的构造，防止外部调用修改；最后创建一个对外开放的静态的对象实例，返回值为return 对象；
饿汗式是先new出对象，等待调用；懒汉式是先创建对象为null，之后等待调用，只有当被调用时才会new出对象

饿汗式

 //饿汗式
    private static Utils utils = new Utils();
    
    //私有构造
    private Utils(){
        
    }
    
    //提供对外公开的方法
    public static Utils getInstance(){
        return utils;
    }

懒汉式

需要上锁，双重校验锁，否则会造成线程安全问题

 //懒汉式
    private static Utils utils = null;

    //私有构造
    private Utils(){

    }

    //提供对外公开的方法
    private static Utils getInstance(){
        if (utils == null) {
            synchronized (Object.class){
                if (utils == null) {
                    utils = new Utils();
                }
            }
        }
        return utils;
    }

断点续传

2次请求，第一次获取文件总长度，第二次获取从SD卡中获取的文件长度并继续用随机流下载

package com.example.work;

import android.os.Handler;
import android.os.Message;
import android.util.Log;

import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

public class MyThread extends Thread {

    private static final String TAG = "---";

    private String path;
    private String str_url;
    private Handler handler;

    public MyThread(String path, String str_url, Handler handler) {
        this.path = path;
        this.str_url = str_url;
        this.handler = handler;
    }
    private long end; //总长度
    private long start; //SD卡文件的长度
    @Override
    public void run() {

        try {
            URL url = new URL(str_url);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            int responseCode = connection.getResponseCode();
            Log.i(TAG, "code: "+responseCode);

            if (responseCode == 200) {
                end = connection.getContentLength();
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
        File file = new File(path);
        if(file.exists()){
            start = file.length();
        }else{
            start = 0;
        }


        try {
            URL url = new URL(str_url);
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

            connection.setRequestProperty("Range","bytes="+start+"-"+end);

            connection.connect();
            int responseCode = connection.getResponseCode();
            Log.i(TAG, "code: "+responseCode);
            Log.i(TAG, "run: "+start+" - "+end);
            if (responseCode == 206) {
                int len = 0;
                byte[] b = new byte[1024];
                InputStream is = connection.getInputStream();

                RandomAccessFile randomAccessFile = new RandomAccessFile(path,"rw");
                randomAccessFile.seek(start);

                long count = start;

                while((len = is.read(b)) != -1){
                    randomAccessFile.write(b, 0, len);

                    count += len;
                    int values = (int) ((float)count/end*100);
                    Message msg = Message.obtain();
                    msg.what = 0;
                    msg.obj = values;
                    Log.i(TAG, "values: "+values);
                    handler.sendMessage(msg);
                }
                is.close();
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

综合技能

package com.example.work;

import android.Manifest;
import android.app.ProgressDialog;
import android.content.DialogInterface;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import com.bumptech.glide.Glide;
import com.google.gson.Gson;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private ListView listView;
    private MyAdapter adapter;

    private String path2 = "http://www.qubaobei.com/ios/cf/dish_list.php?stage_id=1&limit=20&page=1";
    public static List<String> isLoad = new ArrayList<>();

    private List<JavaBean.DataBean> dataList = new ArrayList<>();
    public static List<String> name = new ArrayList<>();
    private int index = 0;

    private ProgressDialog progressDialog;
    private File directory;
    private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            if (msg.what == 0) {
                isLoad.set(index,"已下载");
                adapter.notifyDataSetChanged();
            } else if (msg.what == 1) {
                progressDialog.setProgress((Integer) msg.obj);
                progressDialog.dismiss();
                if((Integer) msg.obj == 100){
                    Toast.makeText(MainActivity.this, "下载成功！", Toast.LENGTH_SHORT).show();
                }
            } else if (msg.what == 2) {
                Toast.makeText(MainActivity.this, "已经下载过了", Toast.LENGTH_SHORT).show();
                progressDialog.dismiss();
            }
        }
    };

    private long start = 0;
    private long end = 0;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        requestPermissions(new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE, Manifest.permission.READ_EXTERNAL_STORAGE}, 1000);

        initView();
    }

    private void initView() {
        listView = (ListView) findViewById(R.id.list_view);

        adapter = new MyAdapter(dataList,MainActivity.this);
        listView.setAdapter(adapter);

        new MyAsyncTask(dataList,adapter).execute(path2);

        listListener();
    }

    private void listListener() {
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, final int position, long id) {

                MyDialog(position);
            }
        });

    }

    private void MyDialog(final int position) {
        AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);

        builder.setMessage("是否下载？");
        builder.setTitle("提示信息");

        builder.setPositiveButton("确定", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {

                progressDialog = new ProgressDialog(MainActivity.this);
                progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
                progressDialog.setMessage("下载中。。。");
                index = position;
                new Thread(new Runnable() {
                    @Override
                    public void run() {

                        try {
                            URL url = new URL(dataList.get(position).getPic());
                            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

                            if (connection.getResponseCode() == 200) {
                                end = connection.getContentLength();
                            }

                        } catch (Exception e) {
                            e.printStackTrace();
                        }
                        directory = Environment.getExternalStorageDirectory();
                        File file = new File(directory, name.get(position));
                        if (file.exists()) {
                            start = file.length();
                        }else{
                            start = 0;
                        }
                        if(start == end){
                            handler.sendEmptyMessage(2);
                            return;
                        }

                        try {
                            URL url = new URL(dataList.get(position).getPic());
                            HttpURLConnection connection = (HttpURLConnection) url.openConnection();

                            connection.setRequestProperty("Range","bytes="+start+"-"+end);
                            connection.connect();
                            if (connection.getResponseCode() == 206) {
                                int len = 0;
                                byte[] b = new byte[1024];
                                InputStream is = connection.getInputStream();
                                FileOutputStream fos = new FileOutputStream(new File(directory,name.get(position)));

                                long count = start;

                                while ((len = is.read(b)) != -1) {
//                                    Thread.sleep(1000);

                                    fos.write(b, 0, len);
                                    count += len;

                                    int values = (int) ((float)count/end*100);

                                    Message obtain = Message.obtain();
                                    obtain.what = 1;
                                    obtain.obj = values;
                                    handler.sendMessage(obtain);
                                }

                                handler.sendEmptyMessage(0);

                                connection.disconnect();
                                is.close();
                                fos.close();
                            }

                        } catch (Exception e) {
                            e.printStackTrace();
                        }

                    }
                }).start();

                progressDialog.show();
            }

        });

        builder.setNegativeButton("取消", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {

            }
        });

        AlertDialog alertDialog = builder.create();
        alertDialog.show();
    }


}