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JVM Bytecode

Bytecode is computer object code that an interpreter converts into binary machine code so it can be read by a computer's hardware processor. The interpreter is typically implemented as a virtual machine (VM) that translates the bytecode for the target platform.

The full list of bytecode instructions is available at Wikipedia.

Bytecode instructions prefixes/suffixes table:

Prefix/Suffix Type
i integer
l long
f float
d double
s short
b byte
c char
a address/reference

Example: iadd is an integer addition instruction, while ladd is a long addition instruction.

The store and load instructions are used to store and load values from variables, respectively. They also have a suffix of type _n, where n is a number from 0 to 3, which represents the identifier of the variable.

The n for the load and store instructions specifies the index in the local variable array to load from or store to. The aload_0 instruction pushes the object in local variable 0 onto the stack (this is usually the this object). istore_1 stores the integer on the top of the stack into local variable 1. For local variables beyond 3 the suffix is dropped and operands must be used.

The const instruction is used to store a constant value on the stack, and has a suffix of type _n, where n is a number with the value of the variable.

Relevant Instructions

  • store: pop a value from the stack and store it in a local variable;
  • load: load a local variable and push it onto the stack;
  • const: push a constant value onto the stack;
  • return: return the top value on the stack;
  • checkcast: check if the top value on the stack is of the given type;
  • dup: duplicate the top value on the stack;
  • new: allocates storage on Heap, initializes space and the object's header, and returns the reference to newbie object;

Arithmetic Operations

  • add: add the top two values on the stack, and push the result;
  • sub: subtract the top two values on the stack, and push the result;
  • mul: multiply the top two values on the stack, and push the result;
  • div: divide the top two values on the stack, and push the result;
  • rem: remainder of the top two values on the stack, and push the result;
  • neg: negate the top value on the stack, and push the result;

Bitwise Operations

  • shr: shift the top value on the stack right by the number of bits specified by the next value on the stack, and push the result;
  • shl: shift the top value on the stack left by the number of bits specified by the next value on the stack, and push the result;
  • xor: exclusive or the top two values on the stack, and push the result;
  • and: bitwise and the top two values on the stack, and push the result;
  • or: bitwise or the top two values on the stack, and push the result;

Method Invocation

  • invokedynamic: invoke a dynamic method;
  • invokeinterface: invoke an interface method;
  • invokespecial: invoke a special method;
  • invokestatic: invoke a static method;
  • invokevirtual: invoke a virtual method.

The call to the base constructor is made with the invokespecial instruction with the class name and the constructor name (<init>).