Generating Java source code and jumps

Question

I am generating Java code on the fly which will then be compiled and loaded back in. My issues is that I essentially need jumps because when I return from the function I wish to continue execution from the point in the loop from where I have exited. Hopefully, the code here illustrates my desire. Although the actual code will have some ifs as well and be much deeper.

MyIter a,b,c;

boolean func() {
 jump to correct state here

 state0:

 a = ...;
 while (a.next()) {
   state1: while (b.next()) {
     return true; // start again at state1
   }
   b = ...;
 }

 c = ...;
 state2:
 while (c.next()) {
   return true; // start again at state2
 }
 return false;
}

In C I would probably use a jump table and state variable. Performance is the key here and the code interacts with the Java application.

My best guesses so far have been:

Flipping the nesting of loops and the logic. That became too cumbersome and problematic.
Adding logic to allow the flow to get back to where it was but this may be messy.
I assume the bytecode can jump so I could generate the bytecode myself but that doesn't seem ideal (is there any good libraries for this?)
Continuation passing style to avoid returning but that would involve major restructuring, although currently the likely way to go.

I was wondering if anyone had any thoughts?

I think you'll have a hard time doing jumps even in bytecode - the verifier has all sorts of rules that make bytecode a lot less flexible than you might think. — Tom Anderson, Jan 08 '11 at 21:17

score 2 · Accepted Answer · answered Jan 08 '11 at 20:35

2

The simplest option is to use a switch block (or if you have to, nested switch statements)

enum State {
   state1, state2, state3;
}

State state = State.state1;

public boolean func() {
   while(true) {
     switch(state) {
        case state1:
            if(test()) return true;
            state = state2;
            break;
        case state2:
            if(test2()) return false;
            state = state3;
            break;
        case state3:
            if(test3()) return true;
            state = state1;
            break;
      }
   }
}

answered Jan 08 '11 at 20:35

Peter Lawrey

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Thanks. After playing and thinking a little more this looks the best option for my purposes. A quick implementation is working well, although sadly it will be nested at times. – Molehill Jan 09 '11 at 03:23
You may be able to flatten the states by having state3_1, state3_2, ... state3_N. That may or may not make things clearer. – Peter Lawrey Jan 09 '11 at 08:44

score 0 · Answer 2 · edited May 23 '17 at 12:18

0

A Java coroutine library might suit your purposes. See this related StackOverflow question for some pointers.

edited May 23 '17 at 12:18

Community

1
1

answered Jan 08 '11 at 20:34

Brian Clapper

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Generating Java source code and jumps

2 Answers2