从webview下载pdf以blob格式存储在mysql DB中的pdf

Question

这是我用于webview和下载文件的代码 此代码不适用于我要从保存在Blob中的mysql db下载的文件 这适用于静态网址。 请帮我解决这个问题，我有点卡在这里

package com.example.da;
import androidx.appcompat.app.AlertDialog;
import  androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;

import android.Manifest;
import android.app.DownloadManager;
import android.content.DialogInterface;
import android.content.pm.PackageManager;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.webkit.CookieManager;
import android.webkit.DownloadListener;
import android.webkit.URLUtil;
import android.webkit.WebChromeClient;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
public class MainActivity extends AppCompatActivity {
    private static final String TAG="_";
    WebView daweb;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        daweb = findViewById(R.id.daweb);
        //daweb.getSettings().setPluginState(WebSettings.PluginState.ON);
        //daweb.getSettings().setPluginState(WebSettings.PluginState.ON_DEMAND);

        daweb.getSettings().setJavaScriptEnabled(true);
        daweb.getSettings().setJavaScriptCanOpenWindowsAutomatically(true);
        daweb.getSettings().setSupportMultipleWindows(true);
        daweb.setHorizontalScrollBarEnabled(false);
        daweb.setWebChromeClient(new WebChromeClient());
        daweb.loadUrl("http://192.168.43.254/da/");
        daweb.setWebViewClient(new WebViewClient());

        daweb.setDownloadListener(new DownloadListener() {
            public void onDownloadStart(String url, String userAgent,String contentDisposition, String mimetype,long contentLength) {
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                    if (checkSelfPermission(Manifest.permission.WRITE_EXTERNAL_STORAGE)
                            == PackageManager.PERMISSION_GRANTED) {
                        Log.v(TAG,"Permission is granted");
                        downloadDialog(url,userAgent,contentDisposition,mimetype);

                    } else {

                        Log.v(TAG,"Permission is revoked");
                        //requesting permissions.
                        ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);

                    }
                }
                else {
                    //Code for devices below API 23 or Marshmallow
                    Log.v(TAG,"Permission is granted");
                    downloadDialog(url,userAgent,contentDisposition,mimetype);

                }
            }
        });
    }



    public void downloadDialog(final String url,final String userAgent,String contentDisposition,String mimetype)
    {
        //getting filename from url.
        final String filename = "Diabetes_care";
        //alertdialog
        AlertDialog.Builder builder=new AlertDialog.Builder(MainActivity.this);
        //title of alertdialog
        builder.setTitle("Download");
        //message of alertdialog
        builder.setMessage("Do you want to save " +filename);
        //if Yes button clicks.
        builder.setPositiveButton("Yes", new DialogInterface.OnClickListener()
        {
            @Override
            public void onClick(DialogInterface dialog, int which)
            {
                //DownloadManager.Request created with url.
                DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
                //cookie
                String cookie= CookieManager.getInstance().getCookie(url);
                //Add cookie and User-Agent to request
                request.addRequestHeader("Cookie",cookie);
                request.addRequestHeader("User-Agent",userAgent);
                //file scanned by MediaScannar
                request.allowScanningByMediaScanner();
                //Download is visible and its progress, after completion too.
                request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);
                //DownloadManager created
                DownloadManager downloadManager=(DownloadManager)getSystemService(DOWNLOAD_SERVICE);
                //Saving files in Download folder
                request.setDestinationInExternalPublicDir(Environment.DIRECTORY_DOWNLOADS, filename);
                //download enqued
                downloadManager.enqueue(request);
            }
        });
        builder.setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which)
            {
                //cancel the dialog if Cancel clicks
                dialog.cancel();
            }

        });
        //alertdialog shows.
        builder.create().show();
    }



}

这就是我要下载pdf的方式... 这在webview中不起作用，但在浏览器中可以正常工作

echo "<a download='diabetes_care.PDF' href='data:application/pdf;base64,".base64_encode($row[1])."'>Click here to download</a>"

这在应用程序和浏览器中均能正常工作

echo "<a download='diabetes_care.PDF' href='pdf\sample.pdf'>Click here to download</a>"