话不多说上代码

public class MainActivity extends AppCompatActivity {

private Button cunn;

private Button xian;

private ImageView ttt;

//private EditText rrr;

public static final int EXTERNAL_STORAGE_REQ_CODE = 10 ;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

cunn =(Button)findViewById(R.id.cun);

xian=(Button)findViewById(R.id.xians);

ttt=(ImageView)findViewById(R.id.tt);

// rrr=(EditText)findViewById(R.id.rr);

//final Intent intent = new Intent(MainActivity.this,Download.class);

final MyDownloadManager myDownloadManager = new MyDownloadManager();

//final String path= rrr.getText().toString();

final String path = "https://dss3.bdstatic.com/70cFv8Sh_Q1YnxGkpoWK1HF6hhy/it/u=2534506313,1688529724&fm=26&gp=0.jpg";

//final String path1 = "https://ss3.bdstatic.com/70cFv8Sh_Q1YnxGkpoWK1HF6hhy/it/u=1328477591,2018436166&fm=26&gp=0.jpg";

//final String dirname ="/sdcard/";

//final String filename ="rr";

//final Download download = new Download();

// final dd d =new dd();

cunn.setOnClickListener(new View.OnClickListener() {

@RequiresApi(api = Build.VERSION_CODES.O)

@Override

public void onClick(View view) {

myDownloadManager.addDownloadTask(path);

// try {

// d.ddt(path1,dirname,filename);

// } catch (IOException e) {

// e.printStackTrace();

// }

}

});

xian.setOnClickListener(new View.OnClickListener() {

@Override

public void onClick(View view) {

//File file = new File(String.valueOf(myDownloadManager.getDownloadDir()));

//Bitmap bm = BitmapFactory.decodeFile(myDownloadManager.getDownloadDir()+myDownloadManager.getfileName(path));

// ttt.setImageBitmap(bm);

ttt.setImageURI(Uri.fromFile(new File(myDownloadManager.getDownloadDir() +"/"+ myDownloadManager.getfileName(path))));

//ttt.setImageUri(Uri.fromFile(new File("/sdcard/test.jpg")));

}

});

int permission = ActivityCompat.checkSelfPermission(this,

Manifest.permission.WRITE_EXTERNAL_STORAGE);

if (permission != PackageManager.PERMISSION_GRANTED) {

// 请求权限

ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},

EXTERNAL_STORAGE_REQ_CODE);

}

}

public static String MD5(String path) {

// URL url = new URL(path);

//String url = path;

//String path = Environment.getExternalStorageDirectory().getAbsolutePath();

//final long startTime = System.currentTimeMillis();

//String filename=url.substring(url.lastIndexOf("/") + 1);

String filename ="pp.jpg";

return filename;

}

//

public class MyDownloadManager {

public static final int EXTERNAL_STORAGE_REQ_CODE = 10 ;

private static final int REQUEST_EXTERNAL_STORAGE = 1;

private static String[] PERMISSIONS_STORAGE = {

Manifest.permission.READ_EXTERNAL_STORAGE,

Manifest.permission.WRITE_EXTERNAL_STORAGE };

/**

* Checks if the app has permission to write to device storage

*

* If the app does not has permission then the user will be prompted to

* grant permissions

*

* @param activity

*/

public static void verifyStoragePermissions(Activity activity) {

// Check if we have write permission

int permission = ActivityCompat.checkSelfPermission(activity,

Manifest.permission.WRITE_EXTERNAL_STORAGE);

if (permission != PackageManager.PERMISSION_GRANTED) {

// We don't have permission so prompt the user

ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,

REQUEST_EXTERNAL_STORAGE);

}

}

private static final String TAG = "MyDownloadManager";

private File downloadDir; // 文件保存路径

private static MyDownloadManager instance; // 单例

private String fileName;

// 单线程任务队列

public static Executor executor;

private static final ThreadFactory sThreadFactory = new ThreadFactory() {

private final AtomicInteger mCount = new AtomicInteger(1);

public Thread newThread(Runnable r) {

return new Thread(r, "MyDownloadManager #" + mCount.getAndIncrement());

}

};

private static final BlockingQueue sPoolWorkQueue = new LinkedBlockingQueue<>(128);

public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(1, 1, 1,

TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

public MyDownloadManager() {

// 初始化下载路径

downloadDir = new File("/sdcard/");

if (!downloadDir.exists()) {

downloadDir.mkdirs();

}

executor = new SerialExecutor();

}

/**

* 顺序执行下载任务

*/

private static class SerialExecutor implements Executor {

final ArrayDeque mTasks = new ArrayDeque();

Runnable mActive;

public synchronized void execute(final Runnable r) {

mTasks.offer(new Runnable() {

public void run() {

try {

r.run();

} finally {

scheduleNext();

}

}

});

if (mActive == null) {

scheduleNext();

}

}

protected synchronized void scheduleNext() {

if ((mActive = mTasks.poll()) != null) {

THREAD_POOL_EXECUTOR.execute(mActive);

}

}

}

/**

* 获取单例对象

*

* @return

*/

public static MyDownloadManager getInstance() {

if (instance == null) {

instance = new MyDownloadManager();

}

return instance;

}

/**

* 添加下载任务

*

* @param path

*/

public void addDownloadTask(final String path) {

Log.i(TAG, "进入run00");

executor.execute(new Runnable() {

@Override

public void run() {

Log.i(TAG, "进入run");

download(path);

}

});

}

/**

* 下载文件

*

* @param path

*/

private void download(String path) {

String fileName = MainActivity.MD5(path);

File savePath = new File(downloadDir, fileName); // 下载文件路径

//File finallyPath = new File(downloadDir, fileName + ".png"); // 下载完成后加入

if (savePath.exists()) { // 文件存在则已下载

Log.i(TAG, "file is existed");

return;

}

// 如果是Wifi则开始下载

if (savePath.exists() && savePath.delete()) { // 如果之前存在文件，证明没有下载完成，删掉重新创建

savePath = new File(downloadDir, fileName);

Log.i(TAG, "download 111111");

}

Log.i(TAG, "download start");

try {

byte[] bs = new byte[1024];

int len;

URL url = new URL(path);

InputStream is = url.openStream();

OutputStream os = new FileOutputStream(savePath,true);

Log.i(TAG, "download 22222");

while ((len = is.read(bs)) != -1) {

os.write(bs, 0, len);

}

is.close();

os.close();

} catch (IOException e) {

e.printStackTrace();

}

//if (savePath.renameTo(finallyPath)) { // 下载完成后重命名为

// Log.i(TAG, "download end");

// }

}

/**

* 添加删除任务

*

* @param path

*/

public void addDeleteTask(final String path) {

executor.execute(new Runnable() {

@Override

public void run() {

delete(path);

}

});

}

/**

* 删除本地文件

*

* @param path

*/

private void delete(String path) {

String fileName = MainActivity.MD5(path);

File savePath = new File(downloadDir, fileName + ".png");

Log.i(TAG, savePath.getPath());

if (savePath.exists()) {

if (savePath.delete()) {

Log.i(TAG, "file is deleted");

}

}

}

/**

* 返回下载路径

*

* @return

*/

public File getDownloadDir() {

return downloadDir;

}

public String getfileName(String path){

String f =MainActivity.MD5(path);

return f;

}

}

//

xmlns:app="http://schemas.android.com/apk/res-auto"

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

tools:context=".MainActivity"

android:orientation="vertical"

>

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="保存"

android:id="@+id/cun"

/>

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="显示"

android:id="@+id/xians"

/>

android:id="@+id/tt"

android:layout_width="match_parent"

android:layout_height="match_parent"/>

//

xmlns:tools="http://schemas.android.com/tools"

package="com.example.myapplication">

tools:ignore="ProtectedPermissions" />

android:allowBackup="true"

android:icon="@mipmap/ic_launcher"

android:label="@string/app_name"

android:roundIcon="@mipmap/ic_launcher_round"

android:supportsRtl="true"

android:theme="@style/AppTheme"

android:requestLegacyExternalStorage="true"

>

代码中我有很多尝试的地方 注释的地方可以忽略 或者可以参考

来源：oschina

链接：https://my.oschina.net/u/4404541/blog/4497783