eval4j: a JVM bytecode interpreter written in Java, used to implement "Evaluate Expression" functionality in debuggers for JVM Languages Video: http://medianetwork.oracle.com/video/player/3731152022001

1. eval4j
compiling expressions to be
evaluated by the debugger
andrey.breslav@ .com

3. Stop at a
breakpoint
Inspect memoryInspect call stacks

4. Evaluate given
expression
Evaluate given
expression

6. Working Horse: Bare JDI
• Access Variables
• Access Fields
• Call Methods
• No Evaluate Expression
out-of-the-box

7. How it’s done for Java
• IntelliJ has an intepreter for Java expressions
– written from scratch
– yet another definition of semantics
– may disagree with the compiler’s semantics
• Well, IntelliJ’s not very intimate with javac

8. When you have your own compiler?
• Eclipse Java DT still has an interpreter
• Scala IDE (Eclipse) — no Evaluate Expression
• Ceylon IDE (Eclipse) — no Evaluate Expression
• There’s some room for improvement…

9. Reuse the Compiler (Take 1)
a + b
.class
file
Debugee
process
compile
load
run

10. Eval4j: Bytecode Interpreter
• foo.x + 2
– ALOAD 1
– GETFIELD Foo.x : I
– ICONST_2
– IADD
– INVOKESTATIC kotlin/io/IoPackage.println (I)V
• https://github.com/abreslav/eval4j
• Re-uses ASM’s Interpreter

11. Compiling an expression
free variables
Extract Method Refactoring

12. Summary so far
Extract Method
+
Compiler
+
eval4j
=
Evaluate Expression

13. Challenge: Lambdas/Anonymous
Classes
• Not supported for Java/Scala
• Problems:
– New class that is not available in the debugee
process
– That class may access privates that it’s not allowed
to access

14. Challenge: New Classes
private?
• Also: How do you load a class into the
debugee?

15. Loading a class…
• protected Class<?> defineClass(byte[], …)

16. How Privates Work Normally
• Special synthetic accessor methods
– INVOKESTATIC Foo.access$001()
– can’t insert a new method into the outer class

17. Prepare the methods of the lambda
• Evaluate the body of the method
public Object invoke() {
Object result = null;
Throwable exception = null;
if (exception == null)
return result;
else throw exception;
}

18. What happens to your thread
• You stopped on a bp1
• You evaluated an expression
– Set another bp
– Stopped on it
– Computed something
– Resulmed
– => you are not on bp1 any more

19. Workaround
• Start a special thread
while (true) {
nop();
}
• To stop:
boolean done = false;
while (!done) {
nop();
}

20. Summary
• Prerequisites
– Extract method
– Ability to compile with some tweaks
• Results
– Any expressions/statements
– Lambdas/local classes
• Use it for your language!
– https://github.com/abreslav/eval4j