求助：android studio3.5测试XMl序列化写入sd卡权限报错被拒绝

求助各位大神,我的mainactivity的代码如下：
public class MainActivity extends AppCompatActivity {
    private Button btn;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btn=findViewById(R.id.btn);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                createXmlFile();
            }
        });
    }
    private void createXmlFile() {
        XmlSerializer xmlSerializer = null;
        FileOutputStream  fileOutputStream = null;
        try {
            //获取xmlSerializer
            xmlSerializer = Xml.newSerializer();
            File file = new File(Environment.getExternalStorageDirectory(),"book.xml");
            fileOutputStream = new FileOutputStream(file);
            String encoding = "utf-8";
            xmlSerializer.setOutput(fileOutputStream, encoding);
            xmlSerializer.startDocument(encoding, true);
            //根节点开始
            xmlSerializer.startTag(null, "books");            //内容结点
            xmlSerializer.startTag(null, "book");
            xmlSerializer.attribute(null, "id", "1");
            xmlSerializer.text("java网络编程");
            xmlSerializer.endTag(null, "book");            //内容结点
            xmlSerializer.startTag(null, "book");
            xmlSerializer.attribute(null, "id", "2");
            xmlSerializer.text("head first java");
            xmlSerializer.endTag(null, "book");            //内容结点
            xmlSerializer.startTag(null, "book");
            xmlSerializer.attribute(null, "id", "3");
            xmlSerializer.text("java编程思想");
            xmlSerializer.endTag(null, "book");            //根节点结束
            xmlSerializer.endTag(null, "books");
            xmlSerializer.endDocument();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (fileOutputStream != null) {
                try {
                    fileOutputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}在androidmanifest.xml文件中配置文件如下：
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="cn.ynou.xmltes">
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"
        tools:ignore="ProtectedPermissions"/>
    <application
        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">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application></manifest>运行时报错: java.io.FileNotFoundException: wu.xml: open failed: EROFS (Read-only file system)
怎么都找不到错误，求助各位高手帮忙看看，3.5以后sd卡怎么写入文件啊！