问题定位1：
可能没有打开访问权限

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"
        tools:ignore="ProtectedPermissions" />
    <uses-permission android:name="android.permission.MOUNT_FORMAT_FILESYSTEMS"
        tools:ignore="ProtectedPermissions" />

问题定位2：
当用户从库中选取文件时，无法保证选取的文件是由其他应用添加或编辑的。因此，如果用户选择的文件，让我们说属于另一个应用程序，我们将遇到权限问题。一个快速的解决方法是将此代码添加到 Android

<manifest ... >
  <application android:requestLegacyExternalStorage="true" ... >
    ...
  </application>
</manifest>

问题定位3：
getExternalStorageDirectory() 废弃导致的问题
全部用getExternalFilesDir（）方法进行代替

public class MainActivity extends AppCompatActivity {

    EditText ed;
    Button saveBtn, readBtn;
    TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ed = findViewById(R.id.main_ed);
        saveBtn = findViewById(R.id.main_save_button);
        readBtn = findViewById(R.id.main_read_button);
        tv = findViewById(R.id.main_tv);


    }

    public void onOpearate(View v) {
        String strPath = getExternalFilesDir(null) + "/imooc.txt";
        Log.d("TAG", strPath);
        switch (v.getId()) {
            case R.id.main_save_button:
                File f = new File(strPath);
                try {
                    if (!f.exists()) {
                        f.createNewFile();
                    }

                    FileOutputStream fos = new FileOutputStream(strPath, true);
                    String str = ed.getText().toString();
                    fos.write(str.getBytes());
                } catch (IOException e) {
                    e.getStackTrace();
                }

                break;
            case R.id.main_read_button:
                try {
                    FileInputStream fis = new FileInputStream(strPath);
                    byte[] bytes = new byte[2048];
                    int len = fis.read(bytes);
                    Log.d("TAG", len+"");
                    String s = new String(bytes, 0, len);
                    tv.setText(s);

                } catch (FileNotFoundException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                break;
        }
    }
}

xml文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    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"
    android:orientation="vertical"
    android:padding="20dp"
    tools:context=".MainActivity">

    <EditText
        android:id="@+id/main_ed"
        android:layout_width="match_parent"
        android:layout_height="300dp"
        android:backgroundTint="#fb7299" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp">

        <Button
            android:id="@+id/main_save_button"
            android:layout_width="100dp"
            android:layout_height="40dp"
            android:layout_marginRight="20dp"
            android:background="#fb7299"
            android:onClick="onOpearate"
            android:text="保存"
            android:textColor="#fff" />

        <Button
            android:id="@+id/main_read_button"
            android:layout_width="100dp"
            android:layout_height="40dp"
            android:background="#fb7299"
            android:onClick="onOpearate"
            android:text="读取"
            android:textColor="#fff" />
    </LinearLayout>

    <TextView
        android:id="@+id/main_tv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>


</LinearLayout>