Eval4j @ JVMLS 2014
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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.
![Loading a class…
• protected Class<?> defineClass(byte[], …)](https://image.slidesharecdn.com/eval4j-140730133656-phpapp02/85/Eval4j-JVMLS-2014-15-320.jpg)
Eval4j @ JVMLS 2014
- 1. eval4j compiling expressions to be evaluated by the debugger andrey.breslav@ .com
- 3. Stop at a breakpoint Inspect memoryInspect call stacks
- 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