Re: Incremental Java Compile

From:

Joshua Maurice <joshuamaurice@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 12 May 2010 14:35:34 -0700 (PDT)

Message-ID:

<366e548c-f45d-4c56-a58e-f60c2204589f@6g2000prg.googlegroups.com>

Bad news. It appears that class files do not contain the necessary
dependency information for my goal of not rebuilding all java files
downstream. Ex:

//AA.java
public class AA { public final int x = 1; }

//BB.java
public class BB { public int x = new AA().x; }

//javap -verbose -classpath . BB
Compiled from "BB.java"
public class BB extends java.lang.Object
  SourceFile: "BB.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method #7.#19; // java/lang/Object."<init>":()V
const #2 = class #20; // AA
const #3 = Method #2.#19; // AA."<init>":()V
const #4 = Method #7.#21; // java/lang/Object.getClass:()Ljava/
lang/Class
;
const #5 = Field #6.#22; // BB.x:I
const #6 = class #23; // BB
const #7 = class #24; // java/lang/Object
const #8 = Asciz x;
const #9 = Asciz I;
const #10 = Asciz <init>;
const #11 = Asciz ()V;
const #12 = Asciz Code;
const #13 = Asciz LineNumberTable;
const #14 = Asciz LocalVariableTable;
const #15 = Asciz this;
const #16 = Asciz LBB;;
const #17 = Asciz SourceFile;
const #18 = Asciz BB.java;
const #19 = NameAndType #10:#11;// "<init>":()V
const #20 = Asciz AA;
const #21 = NameAndType #25:#26;// getClass:()Ljava/lang/Class;
const #22 = NameAndType #8:#9;// x:I
const #23 = Asciz BB;
const #24 = Asciz java/lang/Object;
const #25 = Asciz getClass;
const #26 = Asciz ()Ljava/lang/Class;;

{
public int x;

public BB();
  Code:
   Stack=3, Locals=1, Args_size=1
   0: aload_0
   1: invokespecial #1; //Method java/lang/Object."<init>":()V
   4: aload_0
   5: new #2; //class AA
   8: dup
   9: invokespecial #3; //Method AA."<init>":()V
   12: invokevirtual #4; //Method java/lang/Object.getClass:()Ljava/
lang/Clas
s;
   15: pop
   16: iconst_1
   17: putfield #5; //Field x:I
   20: return
  LineNumberTable:
   line 1: 0

  LocalVariableTable:
   Start Length Slot Name Signature
   0 21 0 this LBB;

}
/////
/////

Specifically note that the instructions to initialize BB.x involve
"iconst_1", which, as I understand it, puts the constant 1 on the
stack. javac, even with -g, inlined the value of a final not-static
int field. If it can do this, I don't know what else it can do, and I
don't want to try to write tests to catch all possible variations. So,
I'm back to calling javac -verbose once per java file (but still only
one JVM), and parsing the verbose output to get the actual class
compile dependencies. From initial testing, this slows down a clean
build by a factor of 4x to 5x for my code base.

So, I'm back to square one. Anyone know a way to get the dependency
information javac -verbose would supply per java file without calling
javac -verbose (using the tools.jar API) once per java file?

Anyone have an inkling of how hard this would be to just modify javac
itself to output the information I need? I assume the change would be
quite minor, relatively speaking. It already prints out a message when
it loads a class file, and it has to know which java file it's loading
it for; it's just that it only prints the message the first time
loading the class file (presumably caching its contents), and does not
print out the java file for which it's being loaded.