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POTD: no more tears

December 14th, 2013

In a modular Java program or in a large Java project that has lots of dependencies, you often end up a version of library that’s different from the version used to compile the code.

This often results in LinkageError, where a method/field that was present when the code was compiled do not exist any more in the version being loaded at the runtime. This restriction applies to seemingly trivial safe changes, such as changing the return type of a method to the subtype of what it used to be.

Previously, the only way to deal with this is not to remove any signatures that matter. In other words, you count on library/module developers to be more disciplined. Over the time, Java programmers have accepted this as a way of life, but there are some notorious offenders (Guava and ASM, I’m looking at you.) Besides, it makes it difficult to evolve code.

I have dealt with this in multiple different ways in the past.

The bridge method injector is an example of static compile-time transformation. This kind of tool is non-intrusive to the users of a module, which is good, but it’s still a tool for library developers to be diligent, and processing at compile time means it has only limited information to operate on.

The bytecode compatibility transformer is an example of runtime transformation. This has a lot more information to let it do the right transformation, but modifying class files on the fly requires a custom classloader, which limits its applicability.

On the way back from my recent trip, I realized there is the 3rd way to achieve the same effect — invokedynamic. You see, invokedynamic is really just a mechanism of deferring the linking to the runtime. This allows me to combine the benefit of two approaches. I can transform class files at the compile time without really deciding how the references are linked. Then at runtime, I can decide how they actually get linked but without a need of runtime transformation. The only downside is that it requires Java7.

But in any case, I thought the idea was clever, so I implemented it as my “project of the day”. Please let me know what you think.

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