Technological Thoughts by Jerome Kehrli

Following my first article on Bytecode manipulation with Javassist presented there: Bytecode manipulation with Javassist for fun and profit part I: Implementing a lightweight IoC container in 300 lines of code, I am here presenting another example of Bytecode manipulation with Javassist: generating toString method as well as property getters and setters with Javassist.

While the former example was oriented towards understanding how Javassist and bytecode manipulation comes in help with implementing IoC concerns, such as what is done by the spring framework of the pico IoC container, this new example is oriented towards generating boilerplate code, in a similar way to what Project Lombok is doing.
As a matter of fact, generating boilerplate code is another very sound use case for bytecode manipulation.

Boilerplate code refers to portions of code that have to be included or written in the same way in many places with little or no alteration.
The term is often used when referring to languages that are considered verbose, i.e. the programmer must write a lot of code to do minimal job. And Java is unfortunately a clear winner in this regards.
Avoiding boilerplate code is one of the main reasons (but by far not the only one of course !) why developers are moving away from Java in favor of other JVM languages such as Scala.

In addition, as a reminder, a sound understanding of the Java Bytecode and the way to manipulate it are strong prerequisites to software analytics tools, mocking libraries, profilers, etc. Bytecode manipulation is a key possibility in this regards, thanks to the JVM and the fact that bytecode is interpreted.
Traditionally, bytecode manipulation libraries suffer from complicated approaches and techniques. Javassist, however, proposes a natural, simple and efficient approach bringing bytecode manipulation possibilities to everyone.

So in this second example about Javassist we'll see how to implement typical Lombok features using Javassist, in a few dozen lines of code.

Java bytecode is the form of instructions that the JVM executes.
A Java programmer, normally, does not need to be aware of how Java bytecode works.

Understanding the bytecode, however, is essential to the areas of tooling and program analysis, where the applications can modify the bytecode to adjust the behavior according to the application's domain. Profilers, mocking tools, AOP, ORM frameworks, IoC Containers, boilerplate code generators, etc. require to understand Java bytecode thoroughly and come up with means of manipulating it at runtime.
Each and every of these advanced features of what is nowadays standard approaches when programming with Java require a sound understanding of the Java bytecode, not to mention completely new languages running on the JVM such as Scala or Clojure.

Bytecode manipulation is not easy though ... except with Javassist.
Of all the libraries and tools providing advanced bytecode manipulation features, Javassist is the easiest to use and the quickest to master. It takes a few minutes to every initiated Java developer to understand and be able to use Javassist efficiently. And mastering bytecode manipulation, opens a whole new world of approaches and possibilities.

The goal of this article is to present Javassist in the light of a concrete use case: the implementation in a little more than 300 lines of code of a lightweight, simple but cute IoC Container: SCIF - Simple and Cute IoC Framework.