The document discusses the eval4j project, which enables evaluating expressions within the debugger for various programming languages, highlighting its functions and integration with compilers. Challenges addressed include support for lambdas, anonymous classes, and accessing private members. It also emphasizes the project's potential for enhancements and invites users to utilize it in their own languages.

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