Java and The Hobbit!

Wednesday, 26 January 2005

The Hobbit

At the start of The Hobbit Bilbo is listening to tales of the Dwarfs and he blurts out “Struck by lightening”.

I’ve wondered about that for years – what was that about?????

Java

Once upon a time I did a lot of Java coding, and now it seems I must do some more. The first thing I did was look at some of my old Java code.

Once upon a time I wrote a simple Java Shell.

It was a toy to learn about reflection and Java class-loaders, no job control or anything fancy.

I came across a big problem – if I loaded a class and tried to treat it as an application, invoking main when the program called exit my shell would exit too.

My solution was I think even now pretty inspired.

I added a SecurityManager and trapped calls to System.exit effectively ignoring it.

This worked wonderfully, I could load arbitary .class files, and invoke them as if they had been executed normally. But there was a new problem. My shell wouldn’t ever exit itself!

What did I do? I told my new SecurityManager class to only allow the System.exit() call if the exit value was a magic constant, a number that no loaded program would ever use, but that I could tell the Shell itself to use when it wanted to terminate.

The number? It was my home phone number at the time .. I wonder how many people realised?

I wouldn’t do that again ..!

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