Are you learning about JVM bytecode? This post explains bytecode compatibility, presents two java libraries for bytecode manipulation and gives you some helpful resources.

This chapter of JVM Specification describes three phases of life of a JVM class: loading, linking and initializing. Good read A good read from the perspective of a Java programmer is chapter 12 of Java Language Specification which describes the execution of Java program starting from a main method of the starting class (also called: main class). In order to start executing main method (public void main(String[] args)) of the main (or: initial) class the JVM needs to have a runtime representation of the class.

The weather is still “very february”, so I grab a cup coffee ☕, start reading and taking notes. This post completes Chapter 4 of the JVM Specification - it quickly goes through format checking, shows two types of Code attribute checks and lists JVM limitations. Format Checking Before the class is loaded, JVM needs to ensure that the .class file has appropriate format. There are five basic integrity checks that are performed on a .

Attributes in the .class file layout In previous post I made some notes about constant_pool. Looking at the order in which data is layed out in the class file, constant_pool is followed by some general information specific to the class like its name, names of implemented interfaces or its superclass (see the post JVM: Class file structure). Information about fields and methods comes next. Both fields as well as methods are represented with the same general structure:

It’s snowy, windy and dark outside. It is a perfect weather to read a technical spec, isn’t it?

Let’s read about a .classfile’s constant pool today.

This post is a note taken during reading 4.2 and 4.3 sections of chapter 4. Section 4.2 describes the rules of representing class and interface names, field and method names, module and package names. Section 4.3 describes descriptors and provides a short grammar according to which a representation of a type is constructed. Names Binary class and interface names those names always appear in a fully qualified form known as binary name (see JLS 13.