8

My main question: I know you can generically output class fields with reflection, even if you do not know the variable names, types, or even how many there are. However, is there a way to list all variables within the current function or current scope, assuming I do not know what the variable names are?

In other words:

int x = 5;
int y = 42;
// some more code
//Now I want to println x and y, but assuming I cannot use "x" or "y".

I'd also be happy with an answer to this question: Let's say I'm allowed to store the names of all variables, does that help? e.g.:

Set<String> varNames = new HashSet<String>();
int x = 5;
varNames.add("x");
int y = 42;
varNames.add("y");
// some more code
//Now with varNames, can I output x and y without using "x" or "y"?

Why am I asking this? I am translating XYZ language(s) to java using ANTLR, and I would like to provide a simple method to output the entire state of the program at any point in time.

Third possible solution I'd be happy with: If this is not possible in Java, is there any way I can write byte-code for a function that visits the calling function and examines the stack? This would also solve the problem.

What would be amazing is if Java had the equivalent of Python's eval() or php's get_defined_vars().

If it makes a difference, I'm using Java 6, but anything for Java 5, 6, or 7 should be good.

Thanks!

Arcymag
  • 1,037
  • 1
  • 8
  • 18
  • maybe this question is related (http://stackoverflow.com/questions/2946279/how-do-i-print-the-method-body-reflectively) – user1406062 Sep 30 '12 at 07:28

2 Answers2

5

You can't, as far as I'm aware. At least, not with normal Java code. If you're able to run the bytecode through some sort of post-processor before running it, and assuming you're still building with the debug symbols included, then you could autogenerate the code to do it - but I don't believe there's any way of accessing local variables in the current stack frame via reflection.

Jon Skeet
  • 1,421,763
  • 867
  • 9,128
  • 9,194
  • Interesting. How does something like the eclipse debugger inspect the stack? Does it need to connect via a local network pipe to the JVM, or is it sitting in a thread that has access to the program memory? – Arcymag Sep 30 '12 at 06:45
  • @Arcymag - I don't think the debugger needs to inspect the stack; it relies on symbol files (or something similar) created at compile time. – Richard JP Le Guen Sep 30 '12 at 06:46
1

If you do not want to use this as part of the normal execution path of your program but just for debugging, then use the Java platform debugger architecture (JPDA). Essentially, you would write your own debugger, set a breakpoint and use the JDI API to query the state of the program. Local variables can be listed with StackFrame#visibleVariables().

If the above is not an option, it will be very difficult to achieve. To get the variables names, you could parse the class file and read the local variable table attribute of the method. However, the only way to get the value of a local variable is via the aload/iload/etc. bytecode instructions. These have to be present in the method that you want to analyze, so you cannot put this functionality into a different helper method.

Ingo Kegel
  • 46,523
  • 10
  • 71
  • 102
  • could you tell me where to find examples for JPDA and active online community for JPDA ? – WSS Sep 09 '17 at 17:48