要从现有的Uri复制文件，可以使用以下代码示例：

import android.content.Context;
import android.net.Uri;
import android.os.Environment;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class FileUtils {

    public static void copyFileFromUri(Context context, Uri uri, String destinationPath) throws IOException {
        InputStream inputStream = context.getContentResolver().openInputStream(uri);
        OutputStream outputStream = new FileOutputStream(destinationPath);

        byte[] buffer = new byte[1024];
        int length;
        while ((length = inputStream.read(buffer)) > 0) {
            outputStream.write(buffer, 0, length);
        }

        outputStream.flush();
        outputStream.close();
        inputStream.close();
    }

    public static void copyFileFromUriToExternalStorage(Context context, Uri uri, String destinationFileName) throws IOException {
        File rootDir = Environment.getExternalStorageDirectory();
        File destinationFile = new File(rootDir, destinationFileName);

        copyFileFromUri(context, uri, destinationFile.getAbsolutePath());
    }
}

使用上述代码，可以通过调用以下方法来复制文件：

1. 复制文件到指定路径：

   FileUtils.copyFileFromUri(context, uri, destinationPath);
   

2. 复制文件到外部存储：

   FileUtils.copyFileFromUriToExternalStorage(context, uri, destinationFileName);
   

请注意，需要在AndroidManifest.xml文件中添加READ_EXTERNAL_STORAGE和WRITE_EXTERNAL_STORAGE权限，以及在运行时请求这些权限。