java - 我想在android中实现文件选择器，但是当我实现它时它给出了空指针异常

Question

如何从Android设备中选择文件并发送到服务器。这是我的主要 Activity ，运行后会出现onresume Activity 错误。因为选择文件后，它在运行时异常时给出 nullpointer 。

        btnUpload = (Button) findViewById(R.id.btnUploadPhoto);
        btnUpload.setOnClickListener(this);
        btnDownload.setOnClickListener(this);
    }

    private AndroidAuthSession buildSession() {
        AppKeyPair appKeyPair = new AppKeyPair(Constants.DROPBOX_APP_KEY,
                Constants.DROPBOX_APP_SECRET);
        AndroidAuthSession session;

        String[] stored = getKeys();
        if (stored != null) {
            AccessTokenPair accessToken = new AccessTokenPair(stored[0],
                    stored[1]);
            session = new AndroidAuthSession(appKeyPair, Constants.ACCESS_TYPE,
                    accessToken);
        } else {
            session = new AndroidAuthSession(appKeyPair, Constants.ACCESS_TYPE);
        }

        return session;
    }

    private String[] getKeys() {
        SharedPreferences prefs = getSharedPreferences(
                Constants.ACCOUNT_PREFS_NAME, 0);
        String key = prefs.getString(Constants.ACCESS_KEY_NAME, null);
        String secret = prefs.getString(Constants.ACCESS_SECRET_NAME, null);
        if (key != null && secret != null) {
            String[] ret = new String[2];
            ret[0] = key;
            ret[1] = secret;
            return ret;
        } else {
            return null;
        }
    }

    @Override
    public void onClick(View v) {
        if (v == btnDownload) {
            startActivity(new Intent(MainActivity.this, DropboxDownload.class));
        } else if (v == btnUpload) {
            createDir();
            if (mLoggedIn) {
                logOut();
            }
           // Intent intent = new Intent(Intent.ACTION_GET_CONTENT);

          // f = new File(Utils.getPath(),new Date().getTime()+".jpg");
          //  intent.putExtra(MediaStore.EXTRA_OUTPUT, Uri.fromFile(f));

            //  intent.setType("file/*");
          //  startActivityForResult(intent,TAKE_PHOTO);
            //Intent intent = new Intent(MediaStore.ACTION_GET_CONTENT);

         // startActivityForResult(intent, TAKE_PHOTO);


           /* Intent intent = new Intent();
            intent.setType("image/*");
            intent.setAction(Intent.ACTION_GET_CONTENT);
            startActivityForResult(intent, TAKE_PHOTO);    */

            Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
            intent.addCategory(Intent.CATEGORY_OPENABLE);
            intent.setType("*/*");
            Intent f = Intent.createChooser(intent, "File");
            startActivityForResult(f, TAKE_PHOTO);



        }
    }

    private void logOut() {
        mApi.getSession().unlink();

        clearKeys();
    }

    private void clearKeys() {
        SharedPreferences prefs = getSharedPreferences(
                Constants.ACCOUNT_PREFS_NAME, 0);
        Editor edit = prefs.edit();
        edit.clear();
        edit.commit();
    }

    private void createDir() {
        File dir = new File(Utils.getPath());
        if (!dir.exists()) {
            dir.mkdirs();
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == RESULT_OK) {
            if (requestCode == TAKE_PHOTO) {
//              f = new File(Utils.getPath() + "/temp.jpg");
                if (Utils.isOnline(MainActivity.this)) {
                    mApi.getSession().startAuthentication(MainActivity.this);
                    onResume = true;
                } else {
                    Utils.showNetworkAlert(MainActivity.this);
                }
            }
        }
    }

    public void setLoggedIn(boolean loggedIn) {
        mLoggedIn = loggedIn;
        if (loggedIn) {
            UploadFile upload = new UploadFile(MainActivity.this, mApi, DIR, f);
            upload.execute();
            onResume = true;

        }
    }

    private void storeKeys(String key, String secret) {
        SharedPreferences prefs = getSharedPreferences(
                Constants.ACCOUNT_PREFS_NAME, 0);
        Editor edit = prefs.edit();
        edit.putString(Constants.ACCESS_KEY_NAME, key);
        edit.putString(Constants.ACCESS_SECRET_NAME, secret);
        edit.commit();
    }

    private void showToast(String msg) {
        Toast error = Toast.makeText(this, msg, Toast.LENGTH_LONG);
        error.show();
    }

    @Override
    protected void onResume() {

        AndroidAuthSession session = mApi.getSession();

        if (session.authenticationSuccessful()) {
            try {
                session.finishAuthentication();

                TokenPair tokens = session.getAccessTokenPair();
                storeKeys(tokens.key, tokens.secret);
                setLoggedIn(onResume);
            } catch (IllegalStateException e) {
                showToast("Couldn't authenticate with Dropbox:"
                        + e.getLocalizedMessage());
            }
        }
        super.onResume();
    }
}

这是我的崩溃报告:

01-25 13:30:20.629 9008-9008/com.example.pranit.cloudexample W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x419eacf8)
01-25 13:30:20.629 9008-9008/com.example.pranit.cloudexample W/dalvikvm: threadid=1: uncaught exception occurred
01-25 13:30:20.630 9008-9008/com.example.pranit.cloudexample W/System.err: java.lang.RuntimeException: Unable to resume activity {com.example.pranit.cloudexample/com.example.pranit.cloudexample.MainActivity}: java.lang.NullPointerException
01-25 13:30:20.631 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3003)
01-25 13:30:20.631 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3032)
01-25 13:30:20.631 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1384)
01-25 13:30:20.632 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:110)
01-25 13:30:20.632 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.os.Looper.loop(Looper.java:193)
01-25 13:30:20.632 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5333)
01-25 13:30:20.632 9008-9008/com.example.pranit.cloudexample W/System.err:     at java.lang.reflect.Method.invokeNative(Native Method)
01-25 13:30:20.632 9008-9008/com.example.pranit.cloudexample W/System.err:     at java.lang.reflect.Method.invoke(Method.java:515)
01-25 13:30:20.632 9008-9008/com.example.pranit.cloudexample W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
01-25 13:30:20.632 9008-9008/com.example.pranit.cloudexample W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645)
01-25 13:30:20.633 9008-9008/com.example.pranit.cloudexample W/System.err:     at dalvik.system.NativeStart.main(Native Method)
01-25 13:30:20.633 9008-9008/com.example.pranit.cloudexample W/System.err: Caused by: java.lang.NullPointerException
01-25 13:30:20.638 9008-9008/com.example.pranit.cloudexample W/System.err:     at com.example.pranit.cloudexample.UploadFile.<init>(UploadFile.java:76)
01-25 13:30:20.638 9008-9008/com.example.pranit.cloudexample W/System.err:     at com.example.pranit.cloudexample.MainActivity.setLoggedIn(MainActivity.java:162)
01-25 13:30:20.638 9008-9008/com.example.pranit.cloudexample W/System.err:     at com.example.pranit.cloudexample.MainActivity.onResume(MainActivity.java:194)
01-25 13:30:20.639 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1197)
01-25 13:30:20.639 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.app.Activity.performResume(Activity.java:5422)
01-25 13:30:20.639 9008-9008/com.example.pranit.cloudexample W/System.err:     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2989)
01-25 13:30:20.640 9008-9008/com.example.pranit.cloudexample W/System.err:  ... 10 more
01-25 13:30:20.640 9008-9008/com.example.pranit.cloudexample W/dalvikvm: threadid=1: calling UncaughtExceptionHandler
01-25 13:30:20.647 9008-9008/com.example.pranit.cloudexample E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.pranit.cloudexample, PID: 9008
java.lang.RuntimeException: Unable to resume activity {com.example.pranit.cloudexample/com.example.pranit.cloudexample.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3003)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3032)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1384)
    at android.os.Handler.dispatchMessage(Handler.java:110)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:5333)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at com.example.pranit.cloudexample.UploadFile.<init>(UploadFile.java:76)
    at com.example.pranit.cloudexample.MainActivity.setLoggedIn(MainActivity.java:162)
    at com.example.pranit.cloudexample.MainActivity.onResume(MainActivity.java:194)
    at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1197)
    at android.app.Activity.performResume(Activity.java:5422)
    at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2989)
    at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3032) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1384) 
    at android.os.Handler.dispatchMessage(Handler.java:110)
    at android.os.Looper.loop(Looper.java:193) 
    at android.app.ActivityThread.main(ActivityThread.java:5333) 
    at java.lang.reflect.Method.invokeNative(Native Method) 
    at java.lang.reflect.Method.invoke(Method.java:515) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645) 
    at dalvik.system.NativeStart.main(Native Method) 

Answer 1

如果您的设备/模拟器 > api 24

此崩溃是由于 android M 和 N 的新规则的权限问题而产生的。因此，如果您正在使用文件选择器，请输入代码以向用户请求有关 android.permission.WRITE_EXTERNAL_STORAGE 和 android.permission.READ_EXTERNAL_STORAGE

的权限

或引用Android 6.0 Marshmallow. Cannot write to SD Card

否则分享崩溃日志。