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
image.png
三丶RandomAccessFile
1.简介
image
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);
}
}
image
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 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 data = new ConcurrentHashMap();
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 logdata = fileService.getData(downloadUrl);
if (logdata.size() > 0) {
for (Map.Entry 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 getHttpResponseHeader(HttpURLConnection http) {
Map header = new LinkedHashMap();
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 header = getHttpResponseHeader(http);
for (Map.Entry 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 getData(String path) {
SQLiteDatabase db = openHelper.getReadableDatabase();
Cursor cursor = db.rawQuery("select threadid, downlength from filedownlog where downpath=?", new String[]{path});
Map data = new HashMap();
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 map) {
SQLiteDatabase db = openHelper.getWritableDatabase();
db.beginTransaction();
try {
for (Map.Entry 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 map) {
SQLiteDatabase db = openHelper.getWritableDatabase();
db.beginTransaction();
try {
for (Map.Entry 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);
}
}
六丶提升学习
这里只是入门简单的用HttpURLConnection实现，现在流行的RxRetrofit大神封装见
RxRetrofit - 终极封装 - 深入浅出 & 断点续传
七丶参考文章
深入理解JAVA I/O系列四：RandomAccessFile
Android 多线程断点续传下载
源码下载：
https://github.com/JinBoy23520/CoderToDeveloperByTCLer
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