RandomAccessFile实现断点续传

本文主要是介绍RandomAccessFile实现断点续传，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

本文出自：https://blog.csdn.net/dt235201314/article/details/80911932

一丶概述

断点续传是IO章节的练习，显然实现断点续传知识点不仅仅是IO相关知识，RandomAccessFile倒是主要环节

二丶效果图

三丶RandomAccessFile

1.简介

我们可以看到它的父类是Object，没有继承字节流、字符流家族中任何一个类。并且它实现了DataInput、DataOutput这两个接口，也就意味着这个类既可以读也可以写。

RandomAccessFile既可以读取文件内容，也可以向文件输出数据。同时支持“随机访问”的方式，可以直接跳转到文件的任意地方来读写数据。
与OutputStream、Writer等输出流不同的是，RandomAccessFile允许自由定义文件记录指针，RandomAccessFile可以不从开始的地方开始输出，因此RandomAccessFile可以向已存在的文件后追加内容。如果程序需要向已存在的文件后追加内容，则应该使用RandomAccessFile。
RandomAccessFile的一个重要使用场景就是网络请求中的多线程下载及断点续传。

2.构造方法

RandomAccessFile类有两个构造函数，其实这两个构造函数基本相同，只不过是指定文件的形式不同——一个需要使用String参数来指定文件名，一个使用File参数来指定文件本身。除此之外，创建RandomAccessFile对象时还需要指定一个mode参数，该参数指定RandomAccessFile的访问模式，一共有4种模式：
"r": 以只读方式打开。调用结果对象的任何 write 方法都将导致抛出 IOException。
"rw": 打开以便读取和写入。
"rws": 打开以便读取和写入。相对于 "rw"，"rws" 还要求对“文件的内容”或“元数据”的每个更新都同步写入到基础存储设备。
"rwd" : 打开以便读取和写入，相对于 "rw"，"rwd" 还要求对“文件的内容”的每个更新都同步写入到基础存储设备。

3.重要方法

RandomAccessFile对象包含了一个记录指针，用以标识当前读写处的位置，当程序新创建一个RandomAccessFile对象时，该对象的文件指针记录位于文件头（也就是0处），当读/写了n个字节后，文件记录指针将会后移n个字节。除此之外，RandomAccessFile还可以自由移动该记录指针。下面就是RandomAccessFile具有的两个特殊方法，来操作记录指针，实现随机访问：
long getFilePointer( )：返回文件记录指针的当前位置

void seek(long pos )：将文件指针定位到pos位置

4.demo案例

public static void main(String[] args)
{try
    {insert("d:/out.txt",5,"插入的内容");
    }catch (IOException e){e.printStackTrace();
    }
}private static void insert(String fileName,long pos,String content) throws IOException
{//创建临时空文件
    File tempFile = File.createTempFile("temp",null);
    //在虚拟机终止时，请求删除此抽象路径名表示的文件或目录
    tempFile.deleteOnExit();
    FileOutputStream fos = new FileOutputStream(tempFile);

    RandomAccessFile raf = new RandomAccessFile(fileName,"rw");
    raf.seek(pos);
    byte[] buffer = new byte[4];
    int num = 0;
    while(-1 != (num = raf.read(buffer))){fos.write(buffer,0,num);
    }raf.seek(pos);
    raf.write(content.getBytes());
    FileInputStream fis = new FileInputStream(tempFile);
    while(-1 != (num = fis.read(buffer))){raf.write(buffer,0,num);
    }
}

一个汉字=2个英文字母=2字节杭州G=5个字节

四丶断点续传实现原理

其实断点续传的原理很简单，从字面上理解，所谓断点续传就是从停止的地方重新下载。
断点：线程停止的位置。
续传：从停止的位置重新下载。
用代码解析就是：
断点 ==> 当前线程已经下载完成的数据长度。
续传 ==> 向服务器请求上次线程停止位置之后的数据。
原理知道了，功能实现起来也简单。每当线程停止时就把已下载的数据长度写入记录文件，当重新下载时，从记录文件读取已经下载了的长度。而这个长度就是所需要的断点。
续传的实现也简单，可以通过设置网络请求参数，请求服务器从指定的位置开始读取数据。
而要实现这两个功能只需要使用到httpURLconnection里面的setRequestProperty方法便可以实现.

public void setRequestProperty(String field, String newValue)

如下所示，便是向服务器请求500-1000之间的500个byte：

conn.setRequestProperty("Range", "bytes=" + 500 + "-" + 1000);

以上只是续传的一部分需求，当我们获取到下载数据时，还需要将数据写入文件，而普通发File对象并不提供从指定位置写入数据的功能，这个时候，就需要使用到RandomAccessFile来实现从指定位置给文件写入数据的功能。

public void seek(long offset)

如下所示，便是从文件的的第100个byte后开始写入数据。

raFile.seek(100);

而开始写入数据时还需要用到RandomAccessFile里面的另外一个方法

public void write(byte[] buffer, int byteOffset, int byteCount)

该方法的使用和OutputStream的write的使用一模一样...

而多线程断点续传便是在单线程的断点续传上延伸的，而多线程断点续传是把整个文件分割成几个部分，每个部分由一条线程执行下载，而每一条下载线程都要实现断点续传功能。
为了实现文件分割功能，我们需要使用到httpURLconnection的另外一个方法：

public int getContentLength()

当请求成功时，可以通过该方法获取到文件的总长度。
每一条线程下载大小 = fileLength / THREAD_NUM

在多线程断点续传下载中，有一点需要特别注意：

由于文件是分成多个部分是被不同的线程的同时下载的，这就需要，每一条线程都分别需要有一个断点记录，和一个线程完成状态的记录；

只有所有线程的下载状态都处于完成状态时，才能表示文件已经下载完成。

五丶核心代码实现

主Activity

public class DownLoadActivity extends Activity {private static final int PROCESSING = 1;
    private static final int FAILURE = -1;

    private EditText pathText;
    private Button downloadButton;
    private Button stopButton;
    private ProgressBar progressBar;
    private Context context;
    Handler handler = new UIHandler(this);

    
    private static class UIHandler extends Handler {private final WeakReference<DownLoadActivity> mActivity;

        //弱引用，避免内存泄露
        UIHandler(DownLoadActivity activity) {mActivity = new WeakReference<>(activity);
        }@Override
        public void handleMessage(Message msg) {DownLoadActivity activity = mActivity.get();
            if (activity != null) {switch (msg.what) {case PROCESSING:ProgressBar progressBar = (ProgressBar) activity.findViewById(R.id.progressBar);
                        TextView resultView = (TextView) activity.findViewById(R.id.resultView);
                        progressBar.setProgress(msg.getData().getInt("size"));
                        float num = (float) progressBar.getProgress() / (float) progressBar.getMax();
                        int result = (int) (num * 100);
                        resultView.setText(result + "%");
                        if (progressBar.getProgress() == progressBar.getMax()) {Toast.makeText(activity, R.string.success, Toast.LENGTH_LONG).show();
                        }break;
                    case FAILURE:Toast.makeText(activity, R.string.error, Toast.LENGTH_LONG).show();
                        break;
                    default:}}}}@Override
    protected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);
        setContentView(R.layout.down_load_activity);
        context = this;
        pathText = (EditText) findViewById(R.id.path);
        downloadButton = (Button) findViewById(R.id.downloadbutton);
        stopButton = (Button) findViewById(R.id.stopbutton);
        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        ButtonClickListener listener = new ButtonClickListener();
        downloadButton.setOnClickListener(listener);
        stopButton.setOnClickListener(listener);
    }private class ButtonClickListener implements View.OnClickListener {@Override
        public void onClick(View v) {switch (v.getId()) {case R.id.downloadbutton:String path = pathText.getText().toString();
                    if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {//外部存储
                        //File savDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_MOVIES);
                        //File savDir = Environment.getExternalStorageDirectory();
                        //内部存储
                        File savDir = new File(context.getCacheDir(), "/cache/exe");
                        download(path, savDir);
                    } else {Toast.makeText(getApplicationContext(), R.string.sdcarderror, Toast.LENGTH_LONG).show();
                    }downloadButton.setEnabled(false);
                    stopButton.setEnabled(true);
                    break;
                case R.id.stopbutton:exit();
                    Toast.makeText(getApplicationContext(), "Now thread is Stopping!!", Toast.LENGTH_LONG).show();
                    downloadButton.setEnabled(true);
                    stopButton.setEnabled(false);
                    break;
                default:}}private DownloadTask task;

        private void exit() {if (task != null) {task.exit();
            }}private void download(String path, File savDir) {task = new DownloadTask(path, savDir);
            new Thread(task).start();
        }class DownloadTask implements Runnable {private String path;
            private File saveDir;
            private FileDownloader loader;

            DownloadTask(String path, File saveDir) {this.path = path;
                this.saveDir = saveDir;
            }void exit() {if (loader != null) {loader.exit();
                }}//进度监听，通过message机制把传送进度
            DownloadProgressListener downloadProgressListener = new DownloadProgressListener() {@Override
                public void onDownloadSize(int size) {Message msg = new Message();
                    msg.what = PROCESSING;
                    msg.getData().putInt("size", size);
                    handler.sendMessage(msg);
                }};

            @Override
            public void run() {try {//固定三个线程
                    loader = new FileDownloader(getApplicationContext(), path, saveDir, 3);
                    progressBar.setMax(loader.getFileSize());
                    loader.download(downloadProgressListener);
                } catch (Exception e) {e.printStackTrace();
                    handler.sendMessage(handler.obtainMessage(FAILURE));
                }}}}
}

下载线程类

public class DownloadThread extends Thread {private static final String TAG = "DownloadThread";
    private File saveFile;
    private URL downUrl;
    private int block;
    private int threadId = -1;
    private int downloadedLength;
    private boolean finished = false;
    private FileDownloader downloader;

    DownloadThread(FileDownloader downloader, URL downUrl, File saveFile, int block, int downloadedLength, int threadId) {this.downUrl = downUrl;
        this.saveFile = saveFile;
        this.block = block;
        this.downloader = downloader;
        this.threadId = threadId;
        this.downloadedLength = downloadedLength;
    }@Override
    public void run() {if(downloadedLength < block){try {HttpURLConnection http = (HttpURLConnection) downUrl.openConnection();
                http.setConnectTimeout(5 * 1000);
                http.setRequestMethod("GET");
                http.setRequestProperty("Accept", "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");
                http.setRequestProperty("Accept-Language", "zh-CN");
                http.setRequestProperty("Referer", downUrl.toString());
                http.setRequestProperty("Charset", "UTF-8");
                int startPos = block * (threadId - 1) + downloadedLength;
                int endPos = block * threadId -1;
                http.setRequestProperty("Range", "bytes=" + startPos + "-"+ endPos);
                http.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
                http.setRequestProperty("Connection", "Keep-Alive");

                InputStream inStream = http.getInputStream();
                byte[] buffer = new byte[1024];
                int offset = 0;
                print("Thread " + this.threadId + " starts to download from position "+ startPos);
                RandomAccessFile threadFile = new RandomAccessFile(this.saveFile, "rwd");
                threadFile.seek(startPos);

                while (!downloader.getExited() && (offset = inStream.read(buffer, 0, 1024)) != -1) {threadFile.write(buffer, 0, offset);
                    downloadedLength += offset;
                    downloader.update(this.threadId, downloadedLength);
                    downloader.append(offset);
                }threadFile.close();
                inStream.close();

                if(downloader.getExited())print("Thread " + this.threadId + " has been paused");
                else
                    print("Thread " + this.threadId + " download finish");

                this.finished = true;
            } catch (Exception e) {this.downloadedLength = -1;
                print("Thread "+ this.threadId+ ":"+ e);
            }}}private static void print(String msg){Log.i(TAG, msg);
    }public boolean isFinished() {return finished;
    }public long getDownloadedLength() {return downloadedLength;
    }
}

文件下载类

public class FileDownloader {private static final String TAG = "FileDownloader";
    private Context context;
    private FileService fileService;
    private boolean exited;
    private int downloadedSize = 0;
    private int fileSize = 0;
    private DownloadThread[] threads;
    private File saveFile;
    private Map<Integer, Integer> data = new ConcurrentHashMap<Integer, Integer>();
    private int block;
    private String downloadUrl;

    public int getThreadSize() {return threads.length;
    }public void exit() {this.exited = true;
    }public boolean getExited() {return this.exited;
    }public int getFileSize() {return fileSize;
    }protected synchronized void append(int size) {downloadedSize += size;
    }protected synchronized void update(int threadId, int pos) {this.data.put(threadId, pos);
        this.fileService.update_tyc(this.downloadUrl, threadId, pos);
        //this.fileService.update(this.downloadUrl, this.data);
    }public FileDownloader(Context context, String downloadUrl, File fileSaveDir, int threadNum) {try {this.context = context;
            this.downloadUrl = downloadUrl;
            fileService = new FileService(this.context);
            URL url = new URL(this.downloadUrl);
            if (!fileSaveDir.exists())fileSaveDir.mkdirs();
            this.threads = new DownloadThread[threadNum];

            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setConnectTimeout(5 * 1000);
            conn.setRequestMethod("GET");
            conn.setRequestProperty("Accept","image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");
            conn.setRequestProperty("Accept-Language", "zh-CN");
            conn.setRequestProperty("Referer", downloadUrl);
            conn.setRequestProperty("Charset", "UTF-8");
            conn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
            conn.setRequestProperty("Connection", "Keep-Alive");
            conn.connect();
            printResponseHeader(conn);

            if (conn.getResponseCode() == 200) {this.fileSize = conn.getContentLength();
                if (this.fileSize <= 0)throw new RuntimeException("Unkown file size ");

                String filename = getFileName(conn);
                this.saveFile = new File(fileSaveDir, filename);
                Map<Integer, Integer> logdata = fileService.getData(downloadUrl);

                if (logdata.size() > 0) {for (Map.Entry<Integer, Integer> entry : logdata.entrySet())data.put(entry.getKey(), entry.getValue());
                }if (this.data.size() == this.threads.length) {for (int i = 0; i < this.threads.length; i++) {this.downloadedSize += this.data.get(i + 1);
                    }print("已经下载的长度" + this.downloadedSize + "个字节");
                }this.block = (this.fileSize % this.threads.length) == 0 ? this.fileSize / this.threads.length : this.fileSize / this.threads.length + 1;
            } else {print("服务器响应错误:" + conn.getResponseCode() + conn.getResponseMessage());
                throw new RuntimeException("server response error ");
            }} catch (Exception e) {print(e.toString());
            throw new RuntimeException("Can't connection this url");
        }}private String getFileName(HttpURLConnection conn) {String filename = this.downloadUrl.substring(this.downloadUrl.lastIndexOf('/') + 1);

        if (filename == null || "".equals(filename.trim())) {for (int i = 0;; i++) {String mine = conn.getHeaderField(i);
                if (mine == null)break;
                if ("content-disposition".equals(conn.getHeaderFieldKey(i).toLowerCase())) {Matcher m = Pattern.compile(".*filename=(.*)").matcher(mine.toLowerCase(Locale.getDefault()));
                    if (m.find())return m.group(1);
                }}filename = UUID.randomUUID() + ".tmp";
        }return filename;
    }public int download(DownloadProgressListener listener) throws Exception {try {RandomAccessFile randOut = new RandomAccessFile(this.saveFile, "rwd");
            if (this.fileSize > 0)randOut.setLength(this.fileSize);
            randOut.close();
            URL url = new URL(this.downloadUrl);

            if (this.data.size() != this.threads.length) {this.data.clear();
                for (int i = 0; i < this.threads.length; i++) {this.data.put(i + 1, 0);
                }this.downloadedSize = 0;
            }for (int i = 0; i < this.threads.length; i++) {int downloadedLength = this.data.get(i + 1);
                if (downloadedLength < this.block && this.downloadedSize < this.fileSize) {this.threads[i] = new DownloadThread(this, url, this.saveFile, this.block, this.data.get(i + 1), i + 1);
                    this.threads[i].setPriority(7);
                    this.threads[i].start();
                } else {this.threads[i] = null;
                }}this.fileService.delete(this.downloadUrl);
            this.fileService.save(this.downloadUrl, this.data);
            boolean notFinished = true;
            while (notFinished) {Thread.sleep(900);
                notFinished = false;
                for (int i = 0; i < this.threads.length; i++) {if (this.threads[i] != null && !this.threads[i].isFinished()) {notFinished = true;
                        if (this.threads[i].getDownloadedLength() == -1) {this.threads[i] = new DownloadThread(this, url, this.saveFile, this.block, this.data.get(i + 1), i + 1);
                            this.threads[i].setPriority(7);
                            this.threads[i].start();
                        }}}if (listener != null)listener.onDownloadSize(this.downloadedSize);
            }if (downloadedSize == this.fileSize)this.fileService.delete(this.downloadUrl);
        } catch (Exception e) {print(e.toString());
            throw new Exception("File downloads error");
        }return this.downloadedSize;
    }public static Map<String, String> getHttpResponseHeader(HttpURLConnection http) {Map<String, String> header = new LinkedHashMap<String, String>();
        for (int i = 0;; i++) {String mine = http.getHeaderField(i);
            if (mine == null)break;
            header.put(http.getHeaderFieldKey(i), mine);
        }return header;
    }public static void printResponseHeader(HttpURLConnection http) {Map<String, String> header = getHttpResponseHeader(http);
        for (Map.Entry<String, String> entry : header.entrySet()) {String key = entry.getKey() != null ? entry.getKey() + ":" : "";
            print(key + entry.getValue());
        }}private static void print(String msg) {Log.i(TAG, msg);
    }
}

Android内存处理类

public class FileService {private DBOpenHelper openHelper;

    public FileService(Context context) {openHelper = new DBOpenHelper(context);
    }@SuppressLint("UseSparseArrays")public Map<Integer, Integer> getData(String path) {SQLiteDatabase db = openHelper.getReadableDatabase();
        Cursor cursor = db.rawQuery("select threadid, downlength from filedownlog where downpath=?", new String[]{path});
        Map<Integer, Integer> data = new HashMap<Integer, Integer>();
        while (cursor.moveToNext()) {data.put(cursor.getInt(0), cursor.getInt(1));
            data.put(cursor.getInt(cursor.getColumnIndexOrThrow("threadid")), cursor.getInt(cursor.getColumnIndexOrThrow("downlength")));
        }cursor.close();
        db.close();
        return data;
    }public void save(String path, Map<Integer, Integer> map) {SQLiteDatabase db = openHelper.getWritableDatabase();
        db.beginTransaction();
        try {for (Map.Entry<Integer, Integer> entry : map.entrySet()) {db.execSQL("insert into filedownlog(downpath, threadid, downlength) values(?,?,?)", new Object[]{path, entry.getKey(), entry.getValue()});
            }db.setTransactionSuccessful();
        } finally {db.endTransaction();
        }db.close();
    }public void update_tyc(String path, int threadId, int pos) {SQLiteDatabase db = openHelper.getWritableDatabase();
        db.execSQL("update filedownlog set downlength=? where downpath=? and threadid=?", new Object[]{pos, path, threadId});
        db.close();
    }public void update(String path, Map<Integer, Integer> map) {SQLiteDatabase db = openHelper.getWritableDatabase();
        db.beginTransaction();
        try {for (Map.Entry<Integer, Integer> entry : map.entrySet()) {db.execSQL("update filedownlog set downlength=? where downpath=? and threadid=?", new Object[]{entry.getValue(), path, entry.getKey()});
            }db.setTransactionSuccessful();
        } finally {db.endTransaction();
        }db.close();
    }public void delete(String path) {SQLiteDatabase db = openHelper.getWritableDatabase();
        db.execSQL("delete from filedownlog where downpath=?", new Object[]{path});
        db.close();
    }
}

数据辅助类

public class DBOpenHelper extends SQLiteOpenHelper {private static final String DBNAME = "eric.db";
    private static final int VERSION = 1;

    public DBOpenHelper(Context context) {super(context, DBNAME, null, VERSION);
    }@Override
    public void onCreate(SQLiteDatabase db) {db.execSQL("CREATE TABLE IF NOT EXISTS filedownlog (id integer primary key autoincrement, downpath varchar(100), threadid INTEGER, downlength INTEGER)");
    }@Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {db.execSQL("DROP TABLE IF EXISTS filedownlog");
        onCreate(db);
    }
}