References

The Java Virtual Machine Reference

JVM

Easier to generate code for stack-based architecture vs. register-based

Class files

Defined as a stream of 8-bit bytes consisting of a single ClassFile structure.

ClassFile {
	u4 magic; <- MAGIC := 0xCAFEBABE
	u2 minor_version;
	u2 major_version;
	u2 constant_pool_count;
	cp_info
	constant_pool[constant_pool_count-1] <- constants in the program
	u2 access_flags; 
	u2 super_class;
	...
	u2 attributes_count;
	attribute_info attributes[...]
}

Instructions

Loads and stores are between the stack and memory. Arithmetic operations get their operands from the stack, and leave their results on the stack! Instructions consist of a one-byte opcode specifying the operation to be performed, followed by operands supplying args or data used by the operation.

Variables in the JVM are typed. booleans are referred to as ‘Z’. Strings are referred to as Ljava/lang/String;, and so on.

Variables

static, non-static, and local variables

  • static- instructions getstatic and putstatic
  • non-static class members- getfield and putfield
  • local variables- each method invocation causes the creation of a new frame. Frame contains operand stack where most of the work is done, array of local variables