下载逻辑在android开发中可谓很常见，那么封装一个通用简洁的下载器时很有必要的。如果不想给工程引入一个很重的jar包那么可以直接复用下面的代码即可。

主要对外接口

构造函数 ：     public CommonDownloader(String saveDir, int timeoutMs)

开始下载接口： public void start(String saveFileName, String url)

停止下载接口： public void stop()

结构（十分简单）

下载主要由一个Handler和一个下载线程组成，Handler统一处理结果，下载线程负责将下载并将结果发送给Handler。

 

内部实现

public class CommonDownloader {    /**patch save dir*/    private String mSaveDir;    /**http request timeout*/    private int mTimeoutMs;    /**download listener, see {@link OnDownloadListener}*/    private OnDownloadListener mDownloadListener;    private Thread mDownloadThread;    /**download control tag*/    private boolean isStop;    /**UI event handler, see {@link DownloadHandler}*/    private DownloadHandler mDownloadHandler;    /**     * download event listener     */    public interface OnDownloadListener {        /**start download the callback*/        void onStarted();        /**download success the callback*/        void onSuccess(String file);        /**download failed the callback*/        void onFailed(String errorMsg);    }    public CommonDownloader(String saveDir, int timeoutMs) {        if (TextUtils.isEmpty(saveDir)) {            throw new IllegalArgumentException("mSaveDir is empty! please reset.");        } else {            File file = new File(saveDir);            if (!file.exists() || !file.isDirectory()) {                if (!file.mkdirs()) {                    throw new IllegalArgumentException("failed to create file directory. > " + file.getAbsolutePath());                }            }            this.mSaveDir = saveDir;        }        this.mTimeoutMs = timeoutMs;        mDownloadHandler = new DownloadHandler(this);    }    /**     * start download     * @param patchSaveFileName     * @param url     */    public void start(String patchSaveFileName, String url) {        mDownloadHandler.sendEmptyMessage(DownloadHandler.STATUS_START);        if (TextUtils.isEmpty(patchSaveFileName)) {            Message message = Message.obtain();            message.what = DownloadHandler.STATUS_FAILED;            message.obj = "patchSaveFileName is empty! please reset.";            mDownloadHandler.sendMessage(message);            return;        }        File file = new File(mSaveDir, patchSaveFileName);        if (file.exists() && file.isFile()) {            if (!file.delete()) {                Message message = Message.obtain();                message.what = DownloadHandler.STATUS_FAILED;                message.obj = "try deleted this file failed. >" + file.getAbsolutePath();                mDownloadHandler.sendMessage(message);                return;            }        }        try {            if (!file.createNewFile()) {                Message message = Message.obtain();                message.what = DownloadHandler.STATUS_FAILED;                message.obj = "failed to create the patch file. >" + file.getAbsolutePath();                mDownloadHandler.sendMessage(message);                return;            }        } catch (IOException e) {            Message message = Message.obtain();            message.what = DownloadHandler.STATUS_FAILED;            message.obj = e.getMessage();            mDownloadHandler.sendMessage(message);            Log.e(e);            return;        }        stop();        mDownloadThread = new Thread(new DownloadTask(url, patchSaveFileName, file));        mDownloadThread.start();    }    /**     * stop download     */    public void stop() {        isStop = true;        if (mDownloadThread != null) {            try {                mDownloadThread.join(3000);            } catch (InterruptedException e) {                Log.w(e.getMessage());            }        }    }    /**     * set the download listener     * @param mDownloadListener     */    public void setmDownloadListener(OnDownloadListener mDownloadListener) {        this.mDownloadListener = mDownloadListener;    }    /**     * create file output stream     * @param patchSaveFileName     * @return     */    private OutputStream createOutputStream(String patchSaveFileName) {        FileOutputStream fileOutputStream = null;        try {            fileOutputStream = new FileOutputStream(new File(mSaveDir, patchSaveFileName));        } catch (FileNotFoundException e) {            Message message = Message.obtain();            message.what = DownloadHandler.STATUS_FAILED;            message.obj = e.getMessage();            mDownloadHandler.sendMessage(message);            Log.e(e);        }        return fileOutputStream;    }    /**     * download task     */    private class DownloadTask implements Runnable {        private String urlAddress;        private String patchSaveFileName;        private File downloadFile;        private DownloadTask(String urlAddress, String patchSaveFileName, File downloadFile) {            this.urlAddress = urlAddress;            this.patchSaveFileName = patchSaveFileName;            this.downloadFile = downloadFile;        }        @Override        public void run() {            isStop = false;            HttpURLConnection connection = null;            InputStream inputStream = null;            OutputStream outputStream = null;            try {                URL url = new URL(urlAddress);                connection = (HttpURLConnection)url.openConnection();                connection.setConnectTimeout(mTimeoutMs);                connection.setReadTimeout(mTimeoutMs);                connection.setUseCaches(false);                connection.setDoInput(true);                connection.setRequestProperty("Accept-Encoding", "identity");                connection.setRequestMethod("GET");                inputStream = connection.getInputStream();                byte[] buffer = new byte[100 * 1024];                int length;                outputStream = createOutputStream(patchSaveFileName);                if(outputStream == null)    return;                while (!isStop && (length = inputStream.read(buffer)) != -1) {                    outputStream.write(buffer, 0, length);                }                if (!isStop) {                    Message message = Message.obtain();                    message.what = DownloadHandler.STATUS_SUCCESS;                    message.obj = downloadFile.getAbsolutePath();                    mDownloadHandler.sendMessage(message);                } else {                    Message message = Message.obtain();                    message.what = DownloadHandler.STATUS_FAILED;                    message.obj = "the patch download has been canceled!";                    mDownloadHandler.sendMessage(message);                }            } catch (MalformedURLException e) {                Message message = Message.obtain();                message.what = DownloadHandler.STATUS_FAILED;                message.obj = e.getMessage();                mDownloadHandler.sendMessage(message);                Log.e(e);            } catch (IOException e) {                Message message = Message.obtain();                message.what = DownloadHandler.STATUS_FAILED;                message.obj = e.getMessage();                mDownloadHandler.sendMessage(message);                Log.e(e);            } catch (Exception ex) {                Message message = Message.obtain();                message.what = DownloadHandler.STATUS_FAILED;                message.obj = ex.getMessage();                mDownloadHandler.sendMessage(message);                Log.e(ex);            } finally {                if (connection != null) {                    connection.disconnect();                }                if (inputStream != null) {                    try {                        inputStream.close();                    } catch (IOException e) {                        Log.e(e);                    }                }                if (outputStream != null) {                    try {                        outputStream.close();                    } catch (IOException e) {                        Log.e(e);                    }                }            }        }    }    /**     * download event handler     */    private static class DownloadHandler extends Handler {        private static final int STATUS_START = 0x01;        private static final int STATUS_SUCCESS = 0x02;        private static final int STATUS_FAILED = 0x03;        private WeakReference
     
       weakReference;        private DownloadHandler(CommonDownloader patchDownloader) {            super(Looper.getMainLooper());            weakReference = new WeakReference
      
       (patchDownloader);        }        @Override        public void handleMessage(Message msg) {            super.handleMessage(msg);            int status = msg.what;            CommonDownloader patchDownloader = weakReference.get();            switch (status) {                case STATUS_START:                    if(patchDownloader != null && patchDownloader.mDownloadListener != null) {                        patchDownloader.mDownloadListener.onStarted();                    }                    break;                case STATUS_SUCCESS:                    if(patchDownloader != null && patchDownloader.mDownloadListener != null) {                        patchDownloader.mDownloadListener.onSuccess((String)msg.obj);                    }                    break;                case STATUS_FAILED:                    if (patchDownloader != null && patchDownloader.mDownloadListener != null) {                        patchDownloader.mDownloadListener.onFailed((String)msg.obj);                    }                    break;                default:                    break;            }        }    }}
      
     

细节分析：

1. Hanlder中弱引用的使用：

当下载器已经被回收时，Listener也不会再收到回调结果

可以参考这篇关于Activity中Handler防止内存泄漏的方法： 

2. 停止下载的方法：

首先将标记为  isStop 置为true，这样下载就不再进行（DownloadThread里面写数据时进行了判断），同时调用join方法等待线程停止。 （join方法含义可以参考：）

断点续传

断点续传支持从文件上次中断的地方开始传送数据，而并非是从文件开头传送。

http协议支持： http请求头部可以带上请求文件的开始到结束字节。

http协议首部有四种：

• 通用首部字段
• 请求首部字段（首部“Range”，可以设置需要下载的字节开始和结束字节,格式如下所示）

Range: bytes=5001-10000

• 响应首部字段
• 实体首部字段

下面贴出支持断点续传的下载器：

public class DownloadInfo {    public static final long TOTAL_ERROR = -1;//获取进度失败    private String url;    private long total;    private long progress;    private String fileName;    public DownloadInfo(String url) {        this.url = url;    }    public String getUrl() {        return url;    }    public String getFileName() {        return fileName;    }    public void setFileName(String fileName) {        this.fileName = fileName;    }    public long getTotal() {        return total;    }    public void setTotal(long total) {        this.total = total;    }    public long getProgress() {        return progress;    }    public void setProgress(long progress) {        this.progress = progress;    }}

public  abstract class DownLoadObserver implements Observer
      
        {    protected Disposable d;//可以用于取消注册的监听者    protected DownloadInfo downloadInfo;    @Override    public void onSubscribe(Disposable d) {        this.d = d;    }    @Override    public void onNext(DownloadInfo downloadInfo) {        this.downloadInfo = downloadInfo;    }    @Override    public void onError(Throwable e) {        e.printStackTrace();    }}
      

public class DownloadManager {    private static final AtomicReference
      
        INSTANCE = new AtomicReference<>();    private HashMap
       
         downCalls;//用来存放各个下载的请求    private OkHttpClient mClient;//OKHttpClient;    //获得一个单例类    public static DownloadManager getInstance() {        for (; ; ) {            DownloadManager current = INSTANCE.get();            if (current != null) {                return current;            }            current = new DownloadManager();            if (INSTANCE.compareAndSet(null, current)) {                return current;            }        }    }    private DownloadManager() {        downCalls = new HashMap<>();        mClient = new OkHttpClient.Builder().build();    }    /**     * 开始下载     *     * @param url              下载请求的网址     * @param downLoadObserver 用来回调的接口     */    public void download(String url, DownLoadObserver downLoadObserver) {        Observable.just(url)                .filter(s -> !downCalls.containsKey(s))//call的map已经有了,就证明正在下载,则这次不下载                .flatMap(s -> Observable.just(createDownInfo(s)))                .map(this::getRealFileName)//检测本地文件夹,生成新的文件名                .flatMap(downloadInfo -> Observable.create(new DownloadSubscribe(downloadInfo)))//下载//                .observeOn(AndroidSchedulers.mainThread())//在主线程回调                .subscribeOn(Schedulers.io())//在子线程执行                .subscribe(downLoadObserver);//添加观察者    }    public void cancel(String url) {        Call call = downCalls.get(url);        if (call != null) {            call.cancel();//取消        }        downCalls.remove(url);    }    /**     * 创建DownInfo     *     * @param url 请求网址     * @return DownInfo     */    private DownloadInfo createDownInfo(String url) {        DownloadInfo downloadInfo = new DownloadInfo(url);        long contentLength = getContentLength(url);//获得文件大小        downloadInfo.setTotal(contentLength);        String fileName = url.substring(url.lastIndexOf("/"));        downloadInfo.setFileName(fileName);        return downloadInfo;    }    private DownloadInfo getRealFileName(DownloadInfo downloadInfo) {        String fileName = downloadInfo.getFileName();        long downloadLength = 0, contentLength = downloadInfo.getTotal();        File file = new File(MyApp.sContext.getFilesDir(), fileName);        if (file.exists()) {            //找到了文件,代表已经下载过,则获取其长度            downloadLength = file.length();        }        //之前下载过,需要重新来一个文件        int i = 1;        while (downloadLength >= contentLength) {            int dotIndex = fileName.lastIndexOf(".");            String fileNameOther;            if (dotIndex == -1) {                fileNameOther = fileName + "(" + i + ")";            } else {                fileNameOther = fileName.substring(0, dotIndex)                        + "(" + i + ")" + fileName.substring(dotIndex);            }            File newFile = new File(MyApp.sContext.getFilesDir(), fileNameOther);            file = newFile;            downloadLength = newFile.length();            i++;        }        //设置改变过的文件名/大小        downloadInfo.setProgress(downloadLength);        downloadInfo.setFileName(file.getName());        return downloadInfo;    }    private class DownloadSubscribe implements ObservableOnSubscribe
        
          {        private DownloadInfo downloadInfo;        public DownloadSubscribe(DownloadInfo downloadInfo) {            this.downloadInfo = downloadInfo;        }        @Override        public void subscribe(ObservableEmitter
         
           e) throws Exception {            String url = downloadInfo.getUrl();            long downloadLength = downloadInfo.getProgress();//已经下载好的长度            long contentLength = downloadInfo.getTotal();//文件的总长度            //初始进度信息            e.onNext(downloadInfo);            Request request = new Request.Builder()                    //确定下载的范围,添加此头,则服务器就可以跳过已经下载好的部分                    .addHeader("RANGE", "bytes=" + downloadLength + "-" + contentLength)                    .url(url)                    .build();            Call call = mClient.newCall(request);            downCalls.put(url, call);//把这个添加到call里,方便取消            Response response = call.execute();            File file = new File(MyApp.sContext.getFilesDir(), downloadInfo.getFileName());            InputStream is = null;            FileOutputStream fileOutputStream = null;            try {                is = response.body().byteStream();                fileOutputStream = new FileOutputStream(file, true);                byte[] buffer = new byte[2048];//缓冲数组2kB                int len;                while ((len = is.read(buffer)) != -1) {                    fileOutputStream.write(buffer, 0, len);                    downloadLength += len;                    downloadInfo.setProgress(downloadLength);                    e.onNext(downloadInfo);                }                fileOutputStream.flush();                downCalls.remove(url);            } finally {                //关闭IO流                IOUtil.closeAll(is, fileOutputStream);            }            e.onComplete();//完成        }    }    /**     * 获取下载长度     *     * @param downloadUrl     * @return     */    private long getContentLength(String downloadUrl) {        Request request = new Request.Builder()                .url(downloadUrl)                .build();        try {            Response response = mClient.newCall(request).execute();            if (response != null && response.isSuccessful()) {                long contentLength = response.body().contentLength();//                response.close();                return contentLength == 0 ? DownloadInfo.TOTAL_ERROR : contentLength;            }        } catch (IOException e) {            e.printStackTrace();        }        return DownloadInfo.TOTAL_ERROR;    }}
         
        
       
      

主要流程如下:(具体过程查看代码)

参考：