Java Assembly Language

There is currently no standard syntax for representing Java bytecodes in human-readable form. Sun's javap utility provides one representation, but its output is complicated by code offsets and constant pool index numbers. Thus, a simpler syntax of coding in Java bytecodes was created.

Note: This is documentation on the syntax only. To understand what the operands mean and their usage in programming, look at the Java Virtual Machine Specification. Most of the documentation is beta, meaning some of it is out-of-date. Look for the book by Tim Lindholm and Frank Yellin (to be available sometime this summer by Addison-Wesley) for the latest documentation. Excerpts are available at the above link.

The Class File Format

[abstract] [final] [public] [interface] class classname 
[extends superclassname]
[implements interfacename [interfacename [...] ] ]
{
  [field_declarations]
  [method_declarations]
  [sourcefile sourcefilename]
}

• classname is a valid class name for this class
• superclassname is a valid class name representing the class that this class inherits from. If superclassname is missing, the default java.lang.Object superclass is used.
• interfacename is a valid class name representing an interface that this class implements.
• sourcefilename is a quoted string literal of the name of the source file (if this assembly program is generated by a compiler).

Field Declarations

field [access_specifier] [static] [final] [transient] [volatile] fieldname [= constantvalue]

• fieldname is an identifier representing the name of the field.
• constantvalue is a constant (appropriate to the type of the field) representing the constant value of this field.

Method Declarations

method [access_specifier] [static] [abstract] [final] [native] [synchronized] returntype methodname ( [arg1 [, arg2 [, ...] ] ] )
[throws exceptionname [exceptionname [...] ] ]
max_stack value1
[max_locals value2]
{
  [code]
  [exceptiontable]
  [linenumbertable]
  [localvariabletable]
}

• returntype is either void or a valid type.
• methodname is an identifier representing the method's name.
• arg1, arg2, ... are valid types followed by an optional identifier representing the name of that local variable.
• exceptionname is a valid class name identifying an exception that this method throws.
• value1 is an integer constant representing the maximum size of the stack in this method.
• value2 is an integer constant representing the maximum number of local variable slots needed for this method. If max_locals is not given, the assembler calculates max_locals based on local variable declarations.

Access Specifier

An access specifer (used in field declarations and method declarations) is one of the following:

private
private protected
protected
public 

Code

[label] operation

[label] vartype localvarname

• label is an identifier appended with a colon (':') and is used to mark that position in the code.
• vartype is a valid type denoting the type of the local variable.
• localvarname is an identifier representing the name of a local variable.

Operation

no_arg_operand

constant_arg_operand constantval

field_ref_operand fieldtype fieldname

method_ref_operand returntype methodname ( [arg1 [,arg2 [...]]] )

class_ref_operand classname

label_operand labelname

local_var_operand localvararg

anewarray anewarrayname

iinc localvararg incrementvalue

lookupswitch 
default defaultlabel
{
  value1 : gotolabel1 
  value2 : gotolabel2
  ...
}

multianewarray multianewarraytype dimensions

newarray newarraytype

tableswitch beginval to endval
default defaultlabel
{
  gotolabel1
  gotolabel2
  ...
} 

• constantval is a valid constant.
• fieldtype is a valid type.
• fieldname is a valid field name.
• returntype is either void or a valid type.
• methodname is a valid method name.
• arg1, arg2, ... are valid types.
• classname is a valid class name.
• labelname, defaultlabel, gotolabel1, gotolabel2 are identifiers corresponding to a label names.
• localvararg is either an integer representing a slot number or an identifier representing an already defined local variable.
• incrementvalue, value1, value2, dimensions, beginval, endval are integer constants.
• anewarraytype, multianewarraytype are either valid class names or array declarations which are valid types followed by one or more bracket pairs ("[]").
• newarraytype is one of the following: boolean, char, float, double, byte, short, int, long.

Exception Table

exceptions 
{
  startpc1 endpc1 handlerpc1 catchtype1
  startpc2 endpc2 handlerpc2 catchtype2
  ...
}

• startpc1, startpc2, endpc1, endpc2, handlerpc1, handlerpc2 are identifiers corresponding to label names.
• catchtype1, catchtype2 are either valid class names or the integer 0.

Line Number Table

linenumbertable 
{
  startpc1 linenumber1
  startpc2 linenumber2
  ...
}

Local Variable Table

localvariabletable 
{
  startpc1 endpc1 type1 localvar1 slotnum1
  startpc2 endpc2 type2 localvar2 slotnum2
  ...
}

• startpc1, startpc2, endpc1, endpc2, are identifiers corresponding to label names which represent the range during which the local variables will have a value.
• type1, type2 are valid types denoting the types of the given local variables.
• localvar1, localvar2 are identifiers representing the names of the given local variables.
• slotnum1, slotnum2 are integer constants representing the slots of the local variables in the method's frame.

Class Name

part1[.part2[.part3[...]]]

Field Name

[aclass].afield

• aclass is a valid class name. If aclass is missing, the current class is used.
• afield is an identifier representing the field name.

Method Name

[aclass].amethod

• aclass is a valid class name. If aclass is missing, the current class is used.
• amethod is an identifier representing the method name.

Constant

Type

basetype[bracketpair[bracketpair[...]]]

• basetype is either a class name or one of the following: byte, char, double, float, int, long, short, boolean.
• bracketpair is a matching pair of brackets ("[]") and denotes array dimensions.

No Arg Operand

Constant Arg Operand

Field Ref Operand

Method Ref Operand

Class Ref Operand

Label Operand

Local Var Operand