Many students reach out to me asking for project ideas they can do as a summer project for learning. Here is an interesting project idea - implement your own java disassembler (and expand it to a VM later).

1. © 2015. Copyright Ganesh Samarthyam. All Rights Reserved.
Disassembler
for
Java
(with
API)
Many students contact me asking for project ideas. If you are a Bachelor or Master’s
student in computer science/IT and looking for a good summer project idea, this is for
you. You can complete this very interesting project in 2 months.
javap
is
a
Java
“disassembler”
tool
that
reads
the
Java
class
files
and
dumps
in
the
info
in
a
human
readable
form.
The
javap
is
tool
is
available
as
part
of
the
JDK
(Java
Development
Kit)
in
the
bin
directory
–
the
same
directory
in
which
the
javac
–
the
Java
compiler
is
available.
Java
programs
are
compiled
into
intermediate
code
known
as
byte
codes
and
are
stored
as
".class"
files.
These
".class"
files
have
these
byte
codes
and
more
-­‐
this
"additional"
information
is
known
as
metadata.
JVM
uses
these
byte
codes
and
metadata
to
execute
the
program.
Here
is
how
you
execute
the
tool
in
command-­‐line.
Consider
this
simple
program:
C:>
type
Hello.java
class
Hello
{
public
static
void
main(String
[]args)
{
System.out.println("hello
world");
}
}
C:>
javap
-­‐c
Hello.class
Compiled
from
"Hello.java"
class
Hello
{
Hello();
Code:
0:
aload_0
1:
invokespecial
#1
//
Method
java/lang/Object."<init>":()V
4:
return
public
static
void
main(java.lang.String[]);
Code:
0:
getstatic
#2
//
Field
java/lang/System.out:Ljava/io/PrintStream;
3:
ldc
#3
//
String
hello
world
5:
invokevirtual
#4
//
Method
java/io/PrintStream.println:(Ljava/lang/String;)V
8:
return
}
Here
is
a
quick
intro
on
bytecodes.
The
intermediate
language
for
Java
is
byte-­‐codes.
They
are
assembly
language
code
for
a
imaginary
stack
based
machine
(that
is
the
reason
why
the
Java
runtime
is
called
as
a
‘virtual
machine’).
The
size
of
an
instruction
is
one
byte,
and
hence
the
name
‘byte
code’.
Let
us
take
a
simple
example
and
understand
how
the
bytecodes
work.
Here
is
the
code
that
swaps
values
of
two
integer
variables
named
x
and
y:
int
x
=
10,
y
=
20;
int
temp
=
x;
x
=
y;

2. © 2015. Copyright Ganesh Samarthyam. All Rights Reserved.
y
=
temp;
Here
is
the
generated
bytecode,
obtained
by
using
javap
tool:
0:
bipush
10
2:
istore_1
3:
bipush
20
5:
istore_2
6:
iload_1
7:
istore_3
8:
iload_2
9:
istore_1
10:
iload_3
11:
istore_2
There
are
only
a
few
instructions
that
we
need
to
understand
to
interpret
how
this
instruction
sequence
works.
The
instruction
bipush
loads
(or
pushes)
the
argument
to
the
stack.
The
instruction
prefized
with
i
–
as
in
iload
and
istore
–
indicates
that
the
instruction
is
meant
for
integers
(remember
that
x
and
y
are
integers).
The
instructions
iload
and
istore
loads
and
stores
an
integer
to
and
from
the
‘runtime
execution
stack’
and
‘local
memory
area
in
the
stack’.
Here
the
suffix
_1
and
_2
indicates
the
first
two
local
variables
x
and
x;
the
suffix
_3
indicates
the
emporary
variable
temp.
Now
let
us
simulate
the
execution
of
instructions.
In
top
is
the
execution
stack
and
its
state;
in
the
bottom
is
the
value
for
memory
locations
x,
y,
and
temp
and
are
indicated
by
‘_1’,
‘_2’,
and
‘_3’.

3. © 2015. Copyright Ganesh Samarthyam. All Rights Reserved.
The
basic
unit
of
input
to
it
is
a
.java
file.
The
Java
compiler
is
no
different
from
compilers
of
other
languages
that
it
is
firmly
based
on
the
conventional
compiler
techniques,
but
the
code
it
generates
in
not
the
conventional
object
code
for
that
particular
platform
but
platform
independent
.class
file
that
is
generated
for
each
and
every
class
or
interface
defined.
Bytecodes
-­‐
the
instructions
for
a
hypothetical
microprocessor
-­‐
are
platform
independent
codes
that
form
the
basis
of
the
platform
independence
by
Java.
They
are
targeted
at
the
stack-­‐oriented
approach.
All
the
evaluation
is
done
in
the
stack
by
execution
of
the
byte
codes.
For
example,
given
the
integer
values
a
and
b,
the
statement:
b
=
a
+
10;
may
be
converted
to
Java
bytecodes
as
follows,
iload_1
//
stands
for
integer
load
the
first
local
//
variable
into
the
operand
stack
bipush
10
//
bipush
stands
for
push
‘byte
integer’
10
into
//
the
operand
stack.
iadd
//
add
the
topmost
two
arguments
in
the
stack
istore_2
//
store
the
result
in
top
of
the
stack
to
the
//
second
variable
b
A
Java
compiler,
irrespective
of
the
machine
it
is
compiled,
generates
the
same
code.
All
the
information
that
you
give
inside
the
class
definition
are
converted
to
some
form
and
get
stored
to
form
the
.class
file
for
that
corresponding
class.
This
Intermediate
Class
File
Format
is
a
specification
given
by
Sun.
It
is
a
well
documented
format.
This
file
format
is
understood
by
the
Java
virtual
machine
that
operates
on
and
executes
it.
This
holds
the
information
like
the
methods
declared
along
with
the
byte
codes,
the
class
variables
used
along
with
initializing
values
(if
any),
super
class
information,
signatures
of
variables
and
methods,
etc.
This
is
similar
to
the
.EXE
code
that
is
organized
in
a
particular
format
that
could
be
understood
by
the
underlying
operating
system.
It
also
has
a
constant
pool
-­‐
a
per
class
runtime
data
structure,
which
is
similar
to
the
symbol
table
created
by
the
compiler.
The
Java
class
file
format
is
very
compact
and
plays
very
important
role
in
making
Java
platform
independent.
The
contents
of
a
.class
file
are
written
in
a
specific
file
format,
described
in
the
specification
of
the
Java
Virtual
Machine.
Don’t
worry,
we’re
not
going
to
understand
this
complex
file
format,
but
we’ll
just
check
its
“magic

4. © 2015. Copyright Ganesh Samarthyam. All Rights Reserved.
number”.
Each
file
format
has
a
“magic
number”
used
for
quickly
checking
the
file
format.
For
example
“.MZ”
is
the
magic
number
(or
more
properly,
“magic
string”)
for
“.exe”
files
in
Windows.
Similarly,
the
.class
files
have
the
magic
number
“0xCAFEBABE”
written
as
a
hexa
decimal
value.
These
magic
numbers
are
typically
written
as
first
few
bytes
of
a
variable
length
file
format.
Let
us
just
check
if
the
given
file
starts
with
the
magic
number
“0xCAFEBABE”
–
if
so,
it
could
be
a
valid
.class
file,
or
else
it’s
certainly
not
a
.class
file.
import
java.io.FileInputStream;
import
java.io.FileNotFoundException;
import
java.io.IOException;
import
java.util.Arrays;
//
check
if
the
passed
file
is
a
valid
.class
file
or
not.
//
note
that
this
is
an
elementary
version
of
a
checker
that
checks
if
the
given
file
//
is
a
valid
file
that
is
written
according
to
the
JVM
specification
//
it
checks
only
the
magic
number
class
ClassFileMagicNumberChecker
{
public
static
void
main(String
[]args)
{
if(args.length
!=
1)
{
System.err.println("Pass
a
valid
file
name
as
argument");
System.exit(-­‐1);
}
String
fileName
=
args[0];
//
create
a
magicNumber
byte
array
with
values
for
four
bytes
in
0xCAFEBABE
//
you
need
to
have
an
explicit
down
cast
to
byte
since
//
the
hex
values
like
0xCA
are
of
type
int
byte
[]magicNumber
=
{(byte)
0xCA,
(byte)0xFE,
(byte)0xBA,
(byte)0xBE};
try
(FileInputStream
fis
=
new
FileInputStream(fileName))
{
//
magic
number
is
of
4
bytes
–
//
use
a
temporary
buffer
to
read
first
four
bytes
byte[]
u4buffer
=
new
byte[4];
//
read
a
buffer
full
(4
bytes
here)
of
data
from
the
file
if(fis.read(u4buffer)
!=
-­‐1)
{
//
if
read
was
successful
//
the
overloaded
method
equals
for
two
byte
arrays
//
checks
for
equality
of
contents
if(Arrays.equals(magicNumber,
u4buffer))
{
System.out.printf("The
magic
number
for
passed
file
%s
matches
that
of
a
.class
file",
fileName);
}
else
{
System.out.printf("The
magic
number
for
passed
file
%s
does
not
match
that
of
a
.class
file",
fileName);
}
}
}
catch(FileNotFoundException
fnfe)
{
System.err.println("file
does
not
exist
with
the
given
file
name
");
}
catch(IOException
ioe)
{
System.err.println("an
I/O
error
occurred
while
processing
the
file");
}
}
}
Let’s
first
see
if
it
works
by
passing
the
source
(.java)
file
and
the
.class
file
for
the
same
program:

5. © 2015. Copyright Ganesh Samarthyam. All Rights Reserved.
D:>
java
ClassFileMagicNumberChecker
ClassFileMagicNumberChecker.java
The
magic
number
for
passed
file
ClassFileMagicNumberChecker.java
does
not
match
that
of
a
.class
file
D:>
java
ClassFileMagicNumberChecker
ClassFile
MagicNumberChecker.class
The
magic
number
for
passed
file
ClassFileMagicNumberChecker.class
matches
that
of
a
.class
file
You
need
to
extend
this
program
by
reading
the
file
format
and
print
the
content
in
a
readable
format.
The
output
should
be
same
as
the
javap
output.
Testcases:
Test
your
tool
implementation
by
disassembling
the
class
files
that
are
provided
as
part
of
the
Java
runtime
library
(i.e.,
test
your
tool
for
the
class
files
provided
within
“rt.jar”
file
that
is
available
as
part
of
the
JDK/JRE
folder).
Resources
for
the
project:
• Wikipedia
overview
page
for
the
Java
class
file
format:
http://en.wikipedia.org/wiki/Java_class_file
• Wikipedia
overview
page
for
Java
bytecodes:
http://en.wikipedia.org/wiki/Java_bytecode
• The
Java
class
file
format
is
documented
here:
http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-­‐
4.html#jvms-­‐4.4
• The
Java
Virtual
Machine
specification
book
is
available
for
download
for
free(!)
here:
http://docs.oracle.com/javase/specs/jvms/se7/jvms7.pdf
.
This
book
includes
the
specification
for
the
class
file
format.
Resources
for
learning
Java:
• If
you
are
new
to
Java,
you
can
get
started
from
here:
http://www.oracle.com/technetwork/topics/newtojava/overview/index.html
• The
free
online
Oracle
Java
tutorial
is
an
excellent
resource
to
learn
Java:
http://docs.oracle.com/javase/tutorial/.
It
is
a
also
available
as
a
free
book
as
part
of
the
tutorial
bundle;
download
it
here:
http://download.oracle.com/javase/tutorial/
• When
you
are
learning
Java
and
writing
code,
you’ll
need
to
consult
the
API
of
the
Java
standard
library.
The
documentation
is
available
here:
http://download.oracle.com/javase/7/docs/api/
• “Head
first
Java”
by
Kathy
Sierra
and
Bert
Bates
is
a
fun
and
easy
book
to
learn
Java;
download
the
free
e-­‐
book
version
here:
http://it-­‐ebooks.info/book/255/.
• “Thinking
in
Java”
by
Bruce
Eckel
is
a
very
good
book
to
learn
Java
well.
Download
it
here:
http://www.mindviewinc.com/Books/downloads.html
• If
you
have
queries
or
doubts,
you
can
get
them
clarified
in
“Beginning
Java”
CodeRanch
forum:
http://www.coderanch.com/forums/f-­‐33/java