j2i.net

Source Code

Like many others my coworkers and I have been called back to work in the office for part of the week. Returning to the office hasn’t been without its challenges, especially since the environment has substantially changed. At the end of one week I was asked to collect some information on ads served to the browser in certain countries. To gather this information, I used a VPN to browse from a different country and I created a web-crawler using JavaScript and Node. It created a browser instance, followed links starting from a specific set of pages, and kept track of resources that the pages loaded, and download content that was accessed from certain domains. The app worked fine and it collected the information that I needed. On Monday, when I was in the office, I was asked to produce a similar dataset as seen from a different country. I started my software to do this tasks only to find that the network now actively blocks VPN connections.

I thought about driving back home to complete the task, but decided to just make a new web crawler to run from my Android tablet. That’s what I did. I made an app with a WebView and had it to load each one of ty starting pages. For each page that loaded, there were two sets of data that I needed to capture; the resources that the page requested, and the links that were in the page. To retrieve this information, I would need a WebViewClient for the WebView. The WebViewClient is an object with a number of methods that get called that let one intercept or get notifications of what the WebView is doing. I was only concerned with a few methods on this object.

• onPageFinished – Fires once a page has finished loading
• onLoadResource – Fires when a page is requesting a resource, such as an image

When a page finishes loading, I grab the links. There is not API specifically for querying the page’s DOM. There is, however, a method on the WebView to execute JavaScript and return the results as a string object. I inject a small function into the page that grabs the links and extract them from the JSON array of strings that comes back. This is the JavaScript.

function extractLinks(){
     var list = Array.from(document.getElementsByTagName('a')); 
     for(var i=0;i<list.length;++i) { 
           list[i] = list[i].href;
     }
     return list;
})()

To execute the JavaScript in the webview, I use the WebView’s evaluateJavascript() method. The method accepts a ValueCallback object. The value is a string of the JSON encoding of the information. I convert that to a String array and save the links. The two references to the dataHandler object are from a class that I defined. The two methods of interest here are LinksExtracted(String[]) and PageLoadComplete(). The LinksExtracted method receives all of the URLs of the links in the page. The dataHandler is responsible for saving those. PageLoadComplete is used to create demarcaction in the data between the pages. Note that this method of capturing links isn’t perfect; it is possible that after a page loads, the page could dynamically adjust the HTML to remove some links and add others. For my application, the result of this apparent oversight is fine.

    override fun onPageFinished(view: WebView?, url: String?) {
        super.onPageFinished(view, url)

        view!!.evaluateJavascript("(function extractLinks(){var list = Array.from(document.getElementsByTagName('a')); for(var i=0;i<list.length;++i) { list[i] = list[i].href};; return list;})()",
            object:ValueCallback<String> {
                override fun onReceiveValue(value: String) {
                    if(value != null && value != "null")
                    {
                        val gson = GsonBuilder().create()
                        val theList = gson.fromJson<ArrayList<String>>(value, object :
                            TypeToken<ArrayList<String>>(){}.type)
                        if(theList != null) {
                            dataHandler.LinksExtracted(theList.toTypedArray());
                        }
                    }
                    dataHandler.PageLoadComplete()
                }
            }
            )
    }

The links are persisted to an SqLite database. To do this, I’ve defined a data class for holding a row of data.

package net.j2i.webcrawler.data
import kotlinx.serialization.Serializable

@Serializable
data class UrlReading(val sessionID:Long=0L, val pageRequestID:Long = 0L, val url:String = "", val timestamp:Long = -1L) {
}

The sessionID will be the same for all values captured during the same run of the program. pageRequestID increments every time a new page loads. urlString contains the information of interest, the URL. And timestamp contains the time at which the URL was captured.

Creation of the database and insertion of data into it fairly plain-vanilla code. I won’t post the code here, but if you would like to see it, it’s on GitHub and can be found through this link: https://github.com/j2inet/sample-webcrawler/blob/main/app/src/main/java/net/j2i/webcrawler/data/UrlReadingDataHelper.kt

When the data is to be extracted, the program will write it to a CSV file with headers. To minimize the memory demand for this, I have a method on the data helper that will write the data as a cursor is reading it.

    fun writeAllRecords(os:OutputStreamWriter):List<UrlReading>  {

        os.write("SessionID, PageRequestID, Timestamp, URL\r\n")

        val readings = mutableListOf<UrlReading>()
        val db = writableDatabase
        val projection = arrayOf(
            BaseColumns._ID,
            UrlReadingsContract.COLUMN_NAME_SESSION_ID,
            UrlReadingsContract.COLUMN_NAME_PAGE_REQUEST_ID,
            UrlReadingsContract.COLUMN_NAME_URL,
            UrlReadingsContract.COLUMN_NAME_TIMESTAMP
        )
        val sortOrder = "${UrlReadingsContract.COLUMN_NAME_TIMESTAMP} ASC"
        val cursor = db.query(
            UrlReadingsContract.TABLE_NAME,
            projection,
            null,
            null,
            null,
            null,
            sortOrder
        )
                    with(cursor) {
            while (moveToNext()) {
                val reading = UrlReading(
                    //source = getString(getColumnIndexOrThrow(BaseColumns._ID)),
                    sessionID = getLong(getColumnIndexOrThrow(UrlReadingsContract.COLUMN_NAME_SESSION_ID)),
                    pageRequestID = getLong(getColumnIndexOrThrow(UrlReadingsContract.COLUMN_NAME_PAGE_REQUEST_ID)),
                    url = getString(getColumnIndexOrThrow(UrlReadingsContract.COLUMN_NAME_URL)),
                    timestamp = getLong(getColumnIndexOrThrow(UrlReadingsContract.COLUMN_NAME_URL)),
                );

                val line = "${reading.sessionID}; ${reading.pageRequestID}; ${reading.timestamp}, ${reading.url}\r\n";
                os.write(line);
                readings.add(reading)
            }
        }
        return readings
    }

The program keeps track of the URLs that it has found links for and ads them to a list. When going to the next page, it randomly selects from this list (and removes the item selected). However, the program will first visit all of the initial set of URLs that it was given before randomly selecting. If I don’t do this, then the links found on the first page loaded might result in the other initial set of pages not being visited or not having a chance of having as much of an impact in the pages visited. Those initial URLs are added to the list and a count of the URLs is saved.

        UrlList.add("https://msn.com")
        UrlList.add("https://yahoo.com");
        linearLoadCount = UrlList.count()

The method for loading random URLs initially dequeues URLs from the beginning of the list. After all of the intial URLs have been read, random reads occur.

    fun openRandomSite() {
        var index = 0;
        if(linearLoadCount>0) {
            --linearLoadCount
            var index = random.nextInt(UrlList.count())
        }
        val nextUrl = UrlList[index];
        UrlList.removeAt(index);
        mainWebView!!.loadUrl(nextUrl)
    }

To keep the pages cycling, in the PageLoadComplete()handler the next call to load a random page is queue (with a delay).

            override fun PageLoadComplete() {
                ++pageSessionID;
                mainHandler.postDelayed(object:Runnable {
                    override fun run() {
                        openRandomSite()
                    }
                },NAVIGATE_DELAY)
            }

It took less time to write this than it would have to drive home. The initial set of URLs in the code are in the source code. This was written to only be used once, so I skipped practices that would have made the program of more general utility. Nevertheless, I think it might be useful to someone. You can find the complete source code on GitHub.

https://github.com/j2inet/sample-webcrawler

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Twitter: @j2inet

On 3 October 2023 around 2:20PM, the USA is testing its emergency alert system. The test will be broadcast over radio (including TV) and mobile phone. Expect phones to be blaring around you around this time. Don’t worry, this is only a test.

If you are likely to be in a situation where you cannot afford or tolerate your phone going off, then you might want to keep your phone powered off around this time. Some environments, such as courthouses, have rules on phones being in silent mode or turned off (I believe a phone going off in court in Atlanta can get someone in trouble for contempt of court). Even if you’ve muted all your settings on your phone, this alert might not respect those settings. While some phones expose settings to silence other alerts, the national alert system’s setting has been unalterable on the phones that I’ve examined over the years.

When the test goes off, don’t be alarmed. If you have one of those emergency tests radios, it might be a good opportunity to see how well it works.

.Net applications can call functions from static DLLs using the [DllImport] attribute. This attribute has as its argument the name of the DLL in which the target is store. But what does one do if the location of the DLL is not in the paths that the system will search? First, let’s consider where the system looks for DLLs in the order that it searches for them.

1. The Application Directory
2. The System Directory
3. The Windows Directory
4. Current Directory
5. Directorys in the PATH environment variable

If the target DLL isn’t in one of those folders, it won’t be found. There is a Win32 function that let’s an application set an additional folder in which the system will look for resolving a DLL location at runtime. The function has the signature HRESULT SetDllDirectory(LPWSTR pathname). When this method is called with a valid path the new search path is as follows.

1. The Application Directory
2. The Directory passed in SetDllDirectory()
3. The System Directory
4. The Windows Directory
5. The Current Directory
6. Directories in the PATH environment variable

The statement for adding a declaration for SetDllDirectory follows.

[DllImport("kernel32.dll", SetLastError = true)]
static extern bool SetDllDirectory(string lpPathName);

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Facebook: @j2inet
YouTube: @j2inet
Telegram: j2inet
Twitter: @j2inet

I was performing enhancements on a video player that read its content at startup, and then would serve from that content on demand. The content, though loaded from the file system, was being put in place by another process. Since the primary application only scanned for content at startup, it would not detect the new content until after the scheduled daily reboot.

The application needed a different behaviour. It needed to detect when content was updated and rescan content accordingly. There are several ways that this could be done, with the application occasionally scanning its content being among the most obvious solutions. There are better solutions. The one I am sharing here is the File System Watcher. I’ll be looking at using the implementations for NodeJS and .NET.

The File System Watchers keeps track of files in specific paths and notifies an application when a change of interest occurs. Once could watch an entire folder or watch only specific files. If any files change, the application receives a notification.

Let’s consider how this feature is used in NodeJS first. You’ll need to import the file system object. The file system object has a function named watch that accepts a file path. The object that is returned is used to received notifications when an item within that path is created or updated.

const fs = require('fs')
const readline = require('readline');
const path = require('path');

var watcher;
let watchPath = path.join(__dirname, 'config');
console.log(`Watch path: ${watchPath}`);
watcher = fs.watch(watchPath)
watcher.on('change', (event, filename)=> {
	console.log(event);
	console.log(filename);
});

console.log('asset watcher activated');

When a configuration file is change, how that is handled depends on the logic of how your application works.

In the .Net environment there’s a class named FileSystemWatcher that accepts a directory name and a file filter. The file filter is the pattern for the file names that you won’t considered. Use *.* to monitor for any file. You can also filter for notifications of the file attributes changing. Instances of FileSystemWatcher exposes several events for different types of file system events.

• Renamed
• Deleted
• Changed
• Created

When an event occurs, the application receives a FileSystemEventArgs object. It provides three properties about the change that has occurred.

• ChangeType – Type of event that occurred
• FullPath – The full path to the file system object affected
• Name – the name of the file system object affected

These should tell you most of the information that you need to know the nature of the change.

Whether in NodeJS or .Net, using the file system watcher provides a simple and efficient method for detecting when vital files have been updated. If you decide to add features to your application to ensure it is responsive to changes in files, you’ll want to use it in your solutions.

Find the source code for sample apps here

https://github.com/j2inet/FileSystemWatcherDemo

.Net Sample App

The .Net Sample App monitors the executable directory for the files content.txt and title.txt. The application has a title area and a content area. If the contents of the files are changed, the application UI updates accordingly. I made this a WPF app because the binding features makes it especially easy to present the value of a variable with minimal custom code. I did make use of some custom base classes to keep the app-specific code simple.

using System;
using System.Collections.Generic;
using System.DirectoryServices;
using System.IO;
using System.Linq;
using System.Security.Policy;
using System.Text;
using System.Threading.Tasks;

namespace FileSystemWatcherSample.ViewModels
{
    public  class MainViewModel: ViewModelBase
    {
        public MainViewModel() {
            var assemblyFile = new FileInfo(this.GetType().Assembly.Modules.FirstOrDefault().FullyQualifiedName);
            var parentDirectory = assemblyFile.Directory;
            Directory.SetCurrentDirectory(parentDirectory.FullName);

            FileSystemWatcher fsw = new FileSystemWatcher(parentDirectory.FullName);
            fsw.Filter = "*.txt";
            fsw.Created += FswCreatedOrChanged;
            fsw.Changed += FswCreatedOrChanged;
            fsw.NotifyFilter = NotifyFilters.CreationTime | NotifyFilters.LastWrite | NotifyFilters.FileName;
            fsw.EnableRaisingEvents = true;
        }


        void FswCreatedOrChanged(object sender, FileSystemEventArgs e)
        {
            var name = e.Name.ToLower();
            switch (name)
            {
                case "contents.txt":
                    try
                    {
                        Content = File.ReadAllText(e.FullPath);
                    }catch(IOException exc)
                    {
                        Content = "<unreadable>";
                    }
                    break;
                case "title.txt":
                    try
                    {
                        Title = File.ReadAllText(e.FullPath);
                    } catch(IOException exc)
                    {
                        Title = "<unreadable>";
                    }
                    break;
                default:
                    break;
            }
        }

        private string _title = "<<empty>>";
        public string Title
        {
            get => _title;
            set => SetValueIfChanged(() => Title, () => _title, value);
        }

        string _content = "<<empty>>";
        public string Content
        {
            get => _content;
            set => SetValueIfChanged(()=>Content, ()=>_content, value);
        }
    }
}

Node Sample App

The Node Sample App runs from the console. In operation, it is much more simple than the .Net application. When a file is updated it prints a notification to the screen.

const fs = require('fs')
const readline = require('readline');
const path = require('path');


function promptUser(query) {
    const rl = readline.createInterface({
        input: process.stdin,
        output: process.stdout,
    });

    return new Promise(resolve => rl.question(query, ans => {
        rl.close();
        resolve(ans);
    }))
}



var watcher;
let watchPath = path.join(__dirname, 'config');
console.log(watchPath);
watcher = fs.watch(watchPath)
watcher.on('change', (event, fileName)=> {
    console.log(event);
    console.log(fileName);
    if(fileName == 'asset-config.js') {
      targetWindow.webContents.send('ASSET_UPDATE', fileName);
    }
  })
  console.log('asset watcher activated');



var result = promptUser("press [Enter] to terminate program.")