public class SDCardDemo extends AppCompatActivity implements View.OnClickListener {
    private static final String TAG = "SDCardDemo";

    private Button btn_sd_card;
    private Button btn_check_sd_card_state;
    private Button btn_check_sd_card_free;
    private static final int REQUEST_EXTERNAL_STORAGE = 1;
    private static String[] PERMISSIONS_STORAGE = {
            "android.permission.READ_EXTERNAL_STORAGE",
            "android.permission.WRITE_EXTERNAL_STORAGE" };
    @RequiresApi(api = Build.VERSION_CODES.M)
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sdcard_demo);
        verifyStoragePermissions(this);
        initView();
        setListener();


    }
    private void setListener(){
        btn_sd_card.setOnClickListener(this);
        btn_check_sd_card_state.setOnClickListener(this);
        btn_check_sd_card_free.setOnClickListener(this);
    }
    private void initView(){
        btn_sd_card = findViewById(R.id.btn_sd_card);
        btn_check_sd_card_state = findViewById(R.id.btn_check_sd_card_state);
        btn_check_sd_card_free = findViewById(R.id.btn_check_sd_card_free);
    }
    public static void verifyStoragePermissions(Activity activity) {

        try {
            //检测是否有写的权限
            int permission = ActivityCompat.checkSelfPermission(activity,
                    "android.permission.WRITE_EXTERNAL_STORAGE");
            if (permission != PackageManager.PERMISSION_GRANTED) {
                // 没有写的权限，去申请写的权限，会弹出对话框
                ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,REQUEST_EXTERNAL_STORAGE);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }


    @Override
    public void onClick(View v) {
        File filepath = Environment.getExternalStorageDirectory();
        switch (v.getId()){
            case R.id.btn_sd_card:
                Log.d(TAG, filepath.toString());
                File file = new File(filepath,"info.text");
                try {
                    FileOutputStream fileOutputStream  = new FileOutputStream(file);
                    fileOutputStream.write("www.baidu.com".getBytes());
                    fileOutputStream.close();
                    Toast.makeText(SDCardDemo.this,"sd卡写入数据成功",Toast.LENGTH_SHORT).show();
                } catch (Exception e) {
                    e.printStackTrace();
                }
                break;
            case R.id.btn_check_sd_card_state:
                String state = Environment.getExternalStorageState();
                if(state.equals(Environment.MEDIA_MOUNTED))
                {
                    Toast.makeText(this,"sdcard已经挂载",Toast.LENGTH_SHORT).show();
                }
                if(state.equals(Environment.MEDIA_UNMOUNTED))
                {
                    Toast.makeText(this,"sdcard已经取消挂载",Toast.LENGTH_SHORT).show();
                }
                break;
            case R.id.btn_check_sd_card_free:
                long freeSpace = filepath.getFreeSpace();
                String s = Formatter.formatFileSize(this, freeSpace);
                Toast.makeText(this,s, Toast.LENGTH_LONG).show();
                break;
        }
    }
}
在androidManifest也加了：
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
 动态权限检查也加了，还是报错：
D/SDCardDemo: /storage/emulated/0
W/System.err: java.io.FileNotFoundException: /storage/emulated/0/info.text: open failed: EACCES (Permission denied)
        at libcore.io.IoBridge.open(IoBridge.java:496)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:235)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:186)
        at com.example.qqlogindemo.SDCardDemo.onClick(SDCardDemo.java:77)