Groovy

Pár shell skritpů jsem už napsal - jak pro Windows tak pro Linux, ale v tomto směru se považuji za naprostou lamu a to se ještě nějakou dobu nezmění. Proto jsem fascinovaně naslouchal Dierk Königovi, který na přednášce pražského CZJUGu zmiňoval použití Groovy pro psaní shell skriptů. Vyměnit jazyk proprietárního shellu, ve kterém toho moc neumím, za multiplatformní Groovy, kde jsem na výrazně pevnější půdě, se zdá jako perfektní nápad. Vše šlo tak hladce, že neváhám podobnou věc doporučit všem, co denně kódují na JVM.

In the last post I described the basic principles I found behind the scenes of GroovyScript refresh. Now imagine that you want to create your own long living Groovy instances with auto-refresh behaviour when source code changes. You can use out-of-the-box Spring support - but there are some limitations I stated in the previous article.

In this post I am going to present an alternative solution that addresses some of the painful issues I noticed. As I stated before, key is to wrap the reference to Groovy instance into an another object managed by the Java class loader and that is exactly the main point of the solution presented.

The first thing one should undestand before he tries to integrate scripting support into his application / framework are class loading issues. One of the main reasons (next to the ability to easily switch from Java) why we have chosen Groovy as our primary scripting language is very good support for live refresh of Groovy classes when source file has changed. But what does Groovy exactly do when it "refreshes" its loaded classes to conform to a newly modified source file? What about existing instances referencing to this class? Is it even possible in JVM to change class structure in runtime? Yes JavaRebel can do this, but it needs special setup and debug mode for hotswap. And how does all this fit into the existing Spring support? From the documentation it seems, that it all just magically works! Dozens of questions ran in my mind when I started to strive for Groovy integration in our product. Those questions gets answered in this article.