B4J Tutorial [Server] Building web servers with B4J

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Edit: Web Apps tutorial: http://www.b4x.com/android/forum/threads/webapp-web-apps-overview.39811/

The server implementation is based on a popular and powerful Java server library named Jetty (license)
The database connection pooling implementation is based on c3p0 (license).

B4J Server features:
  • High performance web server
  • Automatic handling of static files
  • Simple to develop custom "handlers"
  • Support for GET, POST, DELETE and PUT requests
  • Support for multipart requests (file uploads)
  • Standard logging
  • Each handler can either be multi-threaded or single-threaded
  • Simple and powerful database connections pooling
  • Server can run on Windows, Mac and Linux (including board computers such as Raspberry Pi)

Possible usages
  • Internal or external web sites
  • Backend solutions (with or without UI) that can optionally connect to any type of database

If you never worked with web servers then it is recommended to start with a HTTP tutorial such as this one: http://net.tutsplus.com/tutorials/t...rotocol-every-web-developer-must-know-part-1/

I also recommend you to use a proper browser with debugging plugin such as FireFox + FireBug. The ability to inspect the requests and responses is very useful.

How it works

A server program is a non-UI program.
In the Main module you should create a Server object, configure it, start it and call StartMessageLoop:
Sub Process_Globals
   Private srvr As Server
End Sub

Sub AppStart (Args() As String)
   srvr.Port = 8888
   srvr.StaticFilesFolder = File.Combine(File.DirApp, "www")
   srvr.AddHandler("/hello", "HelloPage", False)
   srvr.AddHandler("/FormExampleHelper", "FormExampleHelper", False)
   srvr.AddHandler("/FileUpload", "FileUpload", False)
End Sub

As you can see in the above code, the port is set to 8888.
StaticFilesFolder sets the folder (and subfolders) that will be used to serve static files. Any file that is not generated by your code is a static file (html, js, css, images, etc.).
Files outside of this folder cannot be accessed. This is important for security reasons.
Note that if the url points to a folder then the index.html file in that folder will be served (if it exists).


A handler is a B4J class that is mapped to a URL. A handler class is responsible for getting the request and providing the response.

Each request is handled by a new instance of the handler class. A handler class should have an empty Initialize and a Handle sub with the following signature:
Sub Handle(req As ServletRequest, resp As ServletResponse)

You should read the data from the ServletRequest and write the response to the ServletResponse.

For example:
Sub Handle(req As ServletRequest, resp As ServletResponse)
resp.Write("Hello World!")
End Sub

The attached example includes three handlers:
HelloPage - A simple handler that creates the following page:


Note that this page was served from a Raspberry Pi board :)

FormExampleHelper - This handler receives a submitted form and prints the form's fields. It is used by two html pages. A simple form page and a page that uses Ajax (with JQuery) to send the form.

FileUpload - This handler receives a multipart form with a file and other fields. It logs the fields and the file size.


When you add a handler you need to specify whether you want it to be a single threaded handler or a multithreaded handler. By default you should use multithreaded handlers (third parameter should be false).

A multithreaded handler means that your handler code will be run by a thread from the server threads pool. Multiple instances of the same handler and other handlers can be executed in the same time. As long as you don't access any global variable out of the current handler instance you should be safe.

A single threaded handler will always be executed by the main thread. This means that if there are multiple requests they will be queued and executed one by one. Single threaded handlers can be useful in many cases. Some examples:
- A handler that accesses a SQLite database, which is less suitable for concurrent connections. See this tutorial for concurrent access to SQLite: https://www.b4x.com/android/forum/t...ent-access-to-sqlite-databases.39904/#content
- A handler that writes to a specific file.
- A handler that sends a job to the printer

Note that when you run your code in Debug mode the handlers will always run in the main thread.

Datebase Connections Pooling

Many web servers are based on databases. Databases, such as MySQL and others, can properly handle concurrent queries and transactions. The ConnectionPool object handles a pool of database connections. You can then get a connection (SQL object) from the pool, work with it and close it, which will actually return it to the pool.

The pool is responsible for maintaining the connections.
An alternative for a connections pool is a process global SQL variable. However for it to work properly all the handlers should be single threaded handlers.

As explained in the SQL tutorial you should use #AdditionalJar attribute to add a reference to the JDBC jar. You should then make a public process global pool variable and use it from the handler.

To try this example, run it from B4J and point the browser to
This will load the index.html file that is available in the www folder.

Examples & Tutorials:

Online examples: https://www.b4x.com:51041
Other server tutorials: http://www.b4x.com/android/forum/pages/results/?query=[server]&page=1&prefix=0


  • ServerHelloWorld.zip
    79.8 KB · Views: 6,293
Last edited:


New Member

I was wondering... i would like to post the file to 000webhost
but im unsure where is the file i need to download to the site...

could anyone help?




New Member

yea...can you suggest such a place to begin with?

I want to put a bj4 server on the net and I could not find any guides on how to do that....




Licensed User
Longtime User
Hello. In File Upload, How can change name (MultiPart.........) for original name of file?


Active Member
Licensed User
Longtime User

When I try your code on my raspberry pi 2 I get following error... see screenshot...

Any ideas?


  • webservererror.PNG
    7.3 KB · Views: 544


Licensed User
Longtime User
Hi, I have just one question as I not found response for it.

Is possible to set File.DirAssets as srvr.StaticFilesFolder???

I tried it, but it always return 404... as /. was not found. I added everything to Files folder, all file names are also lower case.

I need to have all files except log directory, config file and sqlite file located in assets folder as it will be then easy to distribute my app as only one executable archive without possibility to manually change anything in static html code...
Last edited:


Licensed User
Longtime User
It is not possible.

Note that the jar file is a simple zip file so it will not add any real protection anyway.

Hi Erel, I know that jar file is simple zip file, but my final thinking was to provide whole app as packaged exe file, where it will not be so simple to "hack" anything...

Second question - when I add also subfolders to assets ("Files") directory, then I found that compiled jar contains only files I added via IDE - there is not possible to add subfolders... Is this bug? As at somewhere at this forum I found it may work.
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