Byte Buddy VS Javassist

byte-buddy - runtime code generation for the Java virtual machine - https://github.com/raphw/byte-buddy

> It is stable and in use by distinguished frameworks and tools such as Mockito, Hibernate , Jackson, Google's Bazel build system and many others. Byte Buddy is also used by a large number of commercial products to great result. It is currently downloaded over 75 million times a year.

As seen above, the API exposes the user to low-level bytecode manipulation via byte arrays. It would be unwieldy to do it directly. Hence, real-life projects rely on bytecode manipulation libraries. ASM has been the traditional library for this, but it seems that Byte Buddy has superseded it. Note that Byte Buddy uses ASM but provides a higher-level abstraction.

Just a drive-by comment: ByteBuddy is worth a look https://bytebuddy.net/. It is built on top of ASM.

> can you show an example of how you'd parse, say, a .java.in

The canonical way to do such a thing is through the java annotation processing api [1] and using a tool like java poet [2]. Before you did that, you'd probably decide if you wanted to instead use bytecode generation with a library like bytebuddy [3]

But, assuming for some reason, you wanted to torture yourself and actually consume a java.in file and apply a regex, then you'd probably pull out the "maven-replacer-plugin" [4] and configure that for the task at hand. (or use your favorite templating language plugin. There's a million of them).

Though, to be fair, this really isn't something that comes up in regular java programming due to the nature of the ecosystem. Anything you'd want to codegen likely already has a library and anything you didn't would receive (legitimate) push back.

[1] https://www.baeldung.com/java-annotation-processing-builder

[2] https://github.com/square/javapoet

[3] https://bytebuddy.net/

[4] https://github.com/beiliubei/maven-replacer-plugin

Have a look at byte buddy.

If you've ever used libraries like https://github.com/JakeWharton/hugo or https://hibernate.org/ (if you've ever done some backend development) and wondered how do they seem to add some code/logic into your app just by adding some annotation to some method, or if you ever wondered how mocking frameworks like Mockito can change a class behavior for example, then most likely you're interested in a programming technique that allows to modify existing code, usually known as Aspect oriented programming (also known in Java as Bytecode instrumentation) which, even though it might sound intimidating at first, some really cool tools such as Byte Buddy or AspectJ make it quite easy to accomplish.

In the above program ‘OOMMetaspace’ class’s ‘main()’ method contains an infinite ‘while (true)’ loop. Within the loop, thread uses open-source library javassist to create dynamic classes whose names start with ‘com.buggyapp.MetaspaceObject’. Class names generated by this program will look something like this: ‘com.buggyapp.MetaspaceObjectb7a02000-ff51-4ef8-9433-3f16b92bba78’. When so many such dynamic classes are created, the Metaspace memory region will reach its limit and the JVM will throw ‘java.lang.OutOfMemoryError: Metaspace’.

ClassPool comes from Javaassist library. It gives convenient API to retrieve the line number of Java method.

This problem can be resolved by https://github.com/jboss-javassist/javassist/blob/master/src/main/javassist/util/proxy/DefineClassHelper.java perfectly, I will use some of its code to fix this problem on 0.0.1

Other higher-level abstractions like Javassist: are all based on ASM, but have a nicer and easier-to-understand APIs to deal with.

In the following snippet, the transformer uses Javassist: