Java how to print all local variables?

Question

I have a method and want to examine variables inside it without debugging - is it possible in Java?

I do not want to write tons of code like:

System.out.println("a: " + a);

I want to something like:

System.out.printLocals();

Also it should be great to have something like:

System.out.printMembersOf(someObjectInstance);

• Ciro Santilli OurBigBook.com: All members has been asked at: stackoverflow.com/questions/1526826/…, let's keep this just for locals.

Answer 1

Well, you can write a method with a varargs parameter and just write:

dump(variable1, variable2, variable3, variable4, ...);

It's not ideal, but it will be enough in some circumstances. There's no way to automatically grab all the local variables from a method and dump them though.

You might consider some sort of bytecode manipulation (e.g. with BCEL) which could do it... but it would be pretty ugly.

• Anurag Awasthi: BCEL hmm, ugly eh? I'd rather use dump(ar1, ar2m,..)

Answer 2

I want to have something like: System.out.printLocals();

Also it should be great to have something like: System.out.printMembersOf(someObjectInstance);

Just about every Java class has a toString method. You override that method, and you can get a string that represents what you're asking for.

Eclipse Helios, among other IDEs, will generate a basic toString method for you.

• denys: this will not work for case: public void foo(int k, String s) { System.out.printLocals(); } // expected output is the same as for System.out.ptintln("k: " + Integer.toString(k)); System.out.ptintln("s: " + s);
• Gilbert Le Blanc: @denys: You're correct. However, there's no reason you couldn't code a toLocalString method for each of the methods you wanted local variables.

Answer 3

You can use System.out.println to print out variables. Example:

int i = 42;
System.out.println("i:" + i);

Answer 4

• Just print them out to the debugger. (Debugging with Eclipse)

OR

• Print out to the console: (System.out.println("var"))

OR

• User Log4J : http://en.wikipedia.org/wiki/Log4j

• Ciro Santilli OurBigBook.com: On jdb there is locals.

Answer 5

What do you mean by "examining" ? Try a System.out.println(yourvariable), or (yourvariable.toString()) if it's an object. It'll display it in the console.

Answer 6

It's not simple to read the values of local variables at runtime, even if you're a debugger. If there are no debug symbols in your class, there is no way at all to see local variables, even if you use a debugger.

The most simple solution to see the values is printing them with System.out.println() or to use logging (slf4j).

If you want to example local variables at runtime without changing the code, you can try AOP (Aspect-oriented programming). Or you can use the same API that a debugger uses to examine the running VM.