Learn Java bytecode.

Official source: https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html

Practical huge instruction table: http://en.wikipedia.org/wiki/Java_bytecode_instruction_listings

Lots of common usage for the instructions: http://cs.au.dk/~mis/dOvs/jvmspec/ref-Java.html

Rotate between:

• read the specs: https://docs.oracle.com/javase/specs/jvms/se7/html/index.html. Chapters 2, 4, and 6 are the meat.
• javac compile small Java examples, then understand every byte of hd Class.class. Then javap Class.class in every language construct.
• make your own bytecode with an assembler like Jasmin

The class file contains:

• a header, which specifies the number of entries for all further parts
• a constant table, which contains everything else, including the instructions

The header makes top-level references into the constant table.

Entries in the constant table can then reference other entries in the constant table.

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.11.1

long and double take up two stack entries.

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.3.3

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.3.2:

• B: byte: unsigned byte
• C: char: UTF-16 encoded in BMP
• D: double
• F: float
• I: int
• J: long
• LClassName;: instance of ClassName
• S: short
• Z: boolean
• [: array

For the return type, there is also void:

• V: void

void method(int i, float f, String s, Integer i)

gives:

(IFLjava/lang/String;java/lang/Integer)V

Breakdown:

( I F Ljava/lang/String; java/lang/Integer ) V
  ^ ^ ^                  ^                   ^
  1 2 3                  4                   5

1. int
2. float
3. String
4. Integer
5. void return type

Any byte sequence that does not contain . ; [ / < > :, thus much less restricted than the Java rules.

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.6

There are 3 main data regions you have to consider:

• constant pool
• stack
• locals

Holds all constants to be used by the program.

This includes: method names and signatures as modified UTF8 strings http://docs.oracle.com/javase/6/docs/api/java/io/DataInput.html#modified-utf-8

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.4

Constant pool entries do not have identical sizes: e.g. UTF-8 entries have variable length. Nonetheless, instructions refer to them by table entry number, e.g.:

invokestatic 00 08

uses entry number 8 of the table.

There is no constant table index built-in to the .class.

So:

• when Java loads those files, it must build a index on the fly by parsing the entries
• you can modify constant table entries manually without changing anything else

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.6.1

Both exist per stack frame (method call).

Each method declares the maximum numbers of both it will need on the code attribute: https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.3

The stack is analogous to the CPU registers: they are where operators take input, and save output to.

The main difference between the stack and CPU registers is that the size of the stack is not limited.

Operations can only affect the top of the stack.

A prototypical operation is iadd, which adds two integers. Before iadd, the stack looks like:

1 <-- top
2
... <-- more entries

After iadd it becomes:

3 <-- top
... <-- more entries

Local variables are just a random access array accessed by index, analogous to RAM memory allocated per function call in x86 programming.

Arguments are passed as local variables: so you need at least as many local variables as you have arguments.

Extra ones can be used.

this is always the first argument for member methods.

TODO move all of this to a Jasmin cheat.

Duplicate the top of the stack.

Discard one or two values from the top of the stack.

Swap top two values of the stack.

Push null into stack.

Push constants to the stack.

Why only up to 5? Why do they exist at all, given bipush? To generate shorter code, and because there was instruction space left?

Push byte to stack.

Push short to stack.

new int[]

compiles to:

iconst_2
newarray int

array[0] = 1

compiles to:

iconst_0
iconst_1
iastore

array[0]

(array access) compiles to:

iconst_0
iaload

array.length

compiles to:

arraylength

TODO call methods

Return from method

TODO

TODO

http://stackoverflow.com/questions/5871190/why-are-jsr-ret-deprecated-java-bytecode

Not used in Oracle's Java compiler since Java 6.

http://stackoverflow.com/questions/399546/synthetic-class-in-java

Do nothing.

Does not get normally generated by javac.

• http://stackoverflow.com/questions/10422086/what-is-the-nop-in-jvm-bytecode-used-for
• http://stackoverflow.com/questions/10422086/what-is-the-nop-in-jvm-bytecode-used-for

TODO reserved for debuggers.

TODO check that these are the concurrency instructions.

http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html#jvms-6.5.monitorenter

There are no thread creation instructions on Java bytecode. The key Java Thread methods like creation and shutdown are native.