Binary Class

The binary class format is defined in the JVM spec. So compatible class files can be produced from any language definition.
Every binary class starts with the "cafe babe" signature.
Class format version numbers - minor and major.
Bytecode is machine code for the JVM.
The JVM uses a stack architecture with instruction operands being loaded onto an internal stack.
Early JVM's were interpreters - Next gen JVM's added JIT to compile bytecode to native code.

Loading the classes

C/C++ require a linker to merge all compiled code from various source files, alongwith code from libraries to form a single executable.
In case of Java, it's different. Linking compiled classes is performed by the JVM when it loads the classes into memory.
E.g. Apps can be written to use interfaces with the actual implementation loaded at runtime.
Basic principle is that classes are loaded when needed.
A lot happens inside the JVM when a class is loaded and initialized :
- decoding the binary class format
- checking compatibility with other classes
- verifying the sequence of bytecode operations
- finally constructing a java.lang.Class instance to represent the new class