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What should I know before getting started?

JVM / Class file format

General concepts

A basic understanding of the JVM / class file format is highly recommended before contributing. Here are some articles that should bring you up to speed:

Terminology

Qualified name: Package separators using the . character. These are names used by runtime functions like Class.forName(name).

For example:

  • java.lang.String
  • com.example.MyClass.InnerClass

Internal name: Package separators using the / character. Inner classes specified with the $ character. These are names how classes are specified internally in the class file.

For example:

  • java/lang/String
  • com/example/MyClass$InnerClass

Primitives (Not the boxed types) use single characters:

Primitive Internal
long J
int I
short S
byte B
boolean Z
float F
double D
void V

Descriptor: Used to describe field and method types. These are essentially the same as internal names, but class names are wrapped in a prefix (L) and suffix character (;).

For example:

  • Ljava/lang/String;
  • I (primitives stay the same)

Method descriptors are formatted like so:

  • double method(int i, String s) = (ILjava/lang/String;)D
  • void method() = ()V

Arrays are prefixed with a [ for each level of the array.

  • int[] = [I
  • String[][] = [[Ljava/lang/String;

Quirks

double and long typed variables take up two slots (On the stack and in the local variable table). For example, declaring two doubles in a static method will use slots 0, then 2. Slots 0-3 are all in-use.