To add to my post on unit testing code that calls System.exit() I've wrote a little JUnit @Rule that allows you to set expectations in a simplified way.

The code is available on github.com/kay/assert-exit

Usage:

It sometimes comes up in the course of unit testing where a unit of code calls System.exit(status). If you've a sane codebase this is normally within a main class. Consider an insane codebase:

Consider

public class DisgruntledPenguin {
    public static boolean happy() {
        System.exit(5);
    }
}

Looking through the unanswered Python questions on StackOverflow, I found one that seemed interesting.. "Python Get Screen Pixel Value in OS X" - how to access screen pixel values, without the overhead of calling the screencapture command, then loading the resulting image.

After a bit of searching, the best supported way of grabbing a screenshot is provided by the CoreGraphics API, part of Quartz, specifically CGWindowListCreateImage.

Since CoreGraphics is a C-based API, the code map almost directly to Python function calls. It's also simplified a bit, because PyObjC handles most of the memory-management (when the wrapping Python object goes out of scope, the underlying object is freed)

Memory leaks are difficult to trace down. A nice description of finding one is "A server memory leak", a story about tracing a memory leak in a Django application.

I'm trying to work out why a server process is slowly using more and more memory. The server parses a stream of text, and shoves stuff into a database. It's written in Python, and about the only external code it uses is SQLAlchemy

I found a combination of Meliae and objgraph useful. Pympler also seems useful (the tracker class particularly). Dowser also looks useful, and might have made things a bit easier..

Click on an image for more details

A common problem we all have to deal with is fixing memory leaks. In garbage collecting languages like Java we typically expect not to ever need to worry about memory management. However the limitations of how a garbage collector works means that we can still create leaks. Which defeats the advantage of a garbage collector.

How do leaks get formed?

The Java GC determines which objects to collect by looking for references to the object. If there is no references, then the object is no longer in use and can be safely destroyed without making the application unstable. If an object is now unused but still referenced, a leak occurs. Let's create a leak:

There are four distinct forms of references in the JVM, and indeed many of these apply to other garbage collected languages.

• Strong references
• Soft references
• Weak references
• Phantom references

It's important to know the differences, what affect they have on the collector and when you should be using them.

I like to organise my files by when they were taken. So I wrote a script to do it for me. You will need the jhead tool installed. On Debian/Ubuntu/etc this is in the jhead package.

#!/bin/bash
 
rm renameJPEG.restore.sh
 
for file in *.jpg *.jpeg *.JPG *.JPEG; do
        if [ ! -e "$file" ]; then
                continue
        fi
        echo "Inspecting $file"
        DateTime=`jhead "$file" | grep "Date/Time"`
        FoundCount=`jhead "$file" | grep "Date/Time" | wc -l`
        if [ $FoundCount -gt 1 ]; then
                echo "Found too many results for: $file"
                continue
        elif [ $FoundCount -eq 0 ]; then
                echo "No valid headers found."
                continue
        else
            BaseFile=`echo $DateTime | awk '{ print $3 }' | tr ':' '-'`
            for i in `seq 1000`; do
                if [ ! -e "$BaseFile - $i.jpg" ]; then
                    echo "New file: $BaseFile - $i.jpg"
                    mv "$file" "$BaseFile - $i.jpg"
                    echo "mv '$BaseFile - $i.jpg' '$file'" >> renameJPEG.restore.sh
                    break
                fi
            done
        fi
done

This will rename all the files to the format YY-MM-DD - i.jpg where i is a uniquifier. Note that if you've some files with bad headers then you will get the wrong output.

Over the past month, my colleague Kurtis and I have been engineering a fully automated deployment system for all of our projects at work. This system was created to remove the painful deployment process from our development environment. This article discusses the requirements that we had for our deployment system. I eventually hope to discuss the entire thing, how we built it, and what revisions we make.

Why Deploying Can Be Painful

Most programmers hate deploying code. I certainly did until this past year. Deploying code is (often times) much harder than writing the code itself. Deploying requires a great deal of knowledge: OS configuration and setup, cloud provider APIs or physical server installation quirks, numerous software components, system monitoring, access lists, and a lot of other things.

It's a pain.

NEVERFEAR.org is now on Github. We're slowly putting stuff up there in a central organisation. In the long term we hope to replace the horribly ugly projects page with our activities on Github.

We invite you to wander over to https://github.com/organizations/NEVERFEAR where you will find our main page.

Want to participate? If you want to join the team by all means comment on this post and we'll get back to you.