Category: Programming

Snake in Groovy

Series: Groovy, Ruby, BASIC, Dart, Elm, Python3+Qt5

I’m starting a series where I write the game Snake in lots of programming languages.

I almost always use writing Snake as my way in to understand a new language, so I’ll share my thoughts about each language as I go.

Slides: Snake in Groovy

If you want to, you can Support me on Patreon.

Programmatic equivalents of web.xml sections for Tomcat

Most documentation for J2EE configuration is based on having a web.xml file, but I want to configure my Tomcat programmatically. Here are some of the things I have found out.

Please use the comments below to correct what I got wrong, and mention equivalents for other parts of web.xml.

Getting started

<web-app ...

in code becomes something like:

import org.apache.catalina.Context;
import org.apache.catalina.startup.Tomcat;
...
Tomcat tomcat = new Tomcat();
Context context = tomcat.addContext( "", "WebContent" );

Adding a Servlet

<web-app ...
    <servlet>
        ...
        <servlet-name>MyServlet</servlet-name>
        <servlet-class>com.example.MyServlet</servlet-class>
        ...
    </servlet>
    <servlet-mapping>
        <servlet-name>MyServlet</servlet-name>
        <url-pattern>/foo/*</url-pattern>
    </servlet-mapping>

in code becomes something like:

...
Class servletClass = MyServlet.class;
// MyServlet extends javax.servlet.http.HttpServlet 
String servletName = servletClass.getSimpleName(); // Or something else if you like
Tomcat.addServlet( context, servletName, servletClass.getName() );
context.addServletMapping( "/foo/*", servletName );

Adding a filter

<web-app ...
    <filter>
        <filter-name>MyFilter</filter-name>
        <filter-class>com.example.MyFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>MyFilter</filter-name>
        <url-pattern>/bar/*</url-pattern>
    </filter-mapping>

in code becomes something like:

...
import org.apache.catalina.deploy.FilterDef;
import org.apache.catalina.deploy.FilterMap;
...
Class filterClass = MyFilter.class;
// MyFilter implements javax.servlet.Filter
String filterName = filterClass.getSimpleName(); // Or something else if you like
FilterDef def = new FilterDef();
def.setFilterName( filterName );
context.addFilterDef( def );
FilterMap map = new FilterMap();
map.setFilterName( filterName );
map.addURLPattern( "/bar/*" );
context.addFilterMap( filterMap );

Adding a Listener

<web-app ...
    <listener>
    	<listener-class>com.example.MyContextListener</listener-class>
    </listener>

in code becomes something like:

...
context.addApplicationListener( MyContextListener.class.getName() );
// MyContextListener implements javax.servlet.ServletContextListener

This is for a ServletContextListener: it may be similar for other listeners, but I’m not sure.

Snowflake Christmas card web page on the Raspberry Pi

In this video I will show you how to make an electronic Christmas card for your friends or family using HTML and JavaScript, which means it will be a little web site that anyone can see by going to it in their Internet browser.

I’m doing this on the Raspberry Pi, but you can do the same thing on almost any computer that exists. All you need is a web browser (like Firefox or Internet Explorer) and a text editor (like Notepad or Gedit).

If this looks very difficult, try the video I made making a similar card using Scratch, which is a lot easier: Snowflake Christmas card in Scratch on the Raspberry Pi.

If you’d like to use the snowflake picture I drew, right-click this link and choose “Save link as…” or similar: snowflake.svg.

If you’d like to compare my code against yours, right-click this link and choose “Save link as…” or similar: snowflakes.html.

If you’d like to see what the finished product looks like, just left-click on snowflakes.html above, instead of right-clicking.

Snowflake Christmas card in Scratch on the Raspberry Pi

In this video I will show you how to make an electronic Christmas card for your friends or family using Scratch.

Scratch can work on most computers – you can download it from http://scratch.mit.edu/.

Scratch is already installed on your Raspberry Pi if you’ve got Raspian or NOOBs on your SD card.

(If you want to get a Pi try my Raspberry Pi: Before we start video.)

If you want to try something more advanced, you can make a similar card as a web page: Snowflake Christmas card web page on the Raspberry Pi.

Encoding URLs in Java

To encode a URL in Java, create a new instance of java.net.URI and then call toString() or toASCIIString() on it.

DO NOT use java.net.URLEncoder. It is for HTML form encoding, not encoding URLs, despite its name.

Each part of a URL must be escaped differently, so use the multi-argument constructors for URI where you have multiple parts you want to stick together. You can pass in null for arguments you want to omit, and it works the way you’d hope.

This program:

class EncodeUrls
{
    public static void main( String[] args ) throws java.net.URISyntaxException
    {
        url( "http", "example.com", "/x/y/x", "x=y", "pos" );

        url(
            "http",
            "exa\uD83D\uDCA9mple.com",
            "/x/\uD83D\uDCA9/x",
            "x=\uD83D\uDCA9",
            "po\uD83D\uDCA9"
        );
    }

    private static void url(
        String scheme,
        String host,
        String path,
        String query,
        String fragment
    )
    throws java.net.URISyntaxException
    {
        java.net.URI uri = new java.net.URI(
            scheme, host, path, query, fragment );

        System.out.println( "." + uri.toString() );
        System.out.println( "." + uri.toASCIIString() );
        System.out.println();
    }
}

prints:

.http://example.com/x/y/x?x=y#pos
.http://example.com/x/y/x?x=y#pos

.http://exa💩mple.com/x/💩/x?x=💩#po💩
.http://exa%F0%9F%92%A9mple.com/x/%F0%9F%92%A9/x?x=%F0%9F%92%A9#po%F0%9F%92%A9