What's the method representation in memory?

Question

While thinking a little bit about programming in Java/C# I wondered about how methods which belong to objects are represented in memory and how this fact does concern multi

Answer 1

The Java specifications don't dictate how to do memory layout, and different implementations can do whatever they like, providing it meets the spec where it matters.

Having said that, the mainstream Oracle JVM (HotSpot) works off of things called oops - Ordinary Object Pointers. These consist of two words of header followed by the data which comprises the instance member fields (stored inline for primitive types, and as pointers for reference member fields).

One of the two header words - the class word - is a pointer to a klassOop. This is a special type of oop which holds pointers to the instance methods of the class (basically, the Java equivalent of a C++ vtable). The klassOop is kind-of a VM-level representation of the Class object corresponding to the Java type.

If you're curious about the low-level detail, you can find out a lot more by looking in the OpenJDK source for the definition of some of the oop types (klassOop is a good place to start).

tl;dr Java holds one blob of code for each method of each type. The blobs of code are shared among each instance of the type, and hidden this pointers are used to know which instance's members to use.