How to make Java compiler generate line numbers in compiled code

Question

I have a hello world program called A.class. It was compiled using command javac A.java. All it does is print "hello world".

Next, I compiled using javac -g A.java. I am expecting to see line numbers, but can't see. Any idea what happened?

I do see very minor differences in some kind of special characters between .class file of javac compiled, and javac -g compiled. But I can't see any line numbers.

My curiosity for this is because I want to find what kind of impact line numbers may have on performance. Second, I want to know how log4j etc maintain line numbers for logging. Thanks.

-g is for generating debug information. class files are bytecode, not human readable. You can use javap to disassemble a class file. — OldProgrammer
– OldProgrammer, Commented Apr 12, 2017 at 18:04
Have you actually pasted the contents of the .class file into the question? Have you really done that? And if I may ask, why have you done it twice? — Mike Nakis
– Mike Nakis, Commented Apr 12, 2017 at 18:08
First one is with javac. Second one is compiled with javac -g <<<. Notice the "g". No line number! Javap is great, but still no line number! — Apurva Singh
– Apurva Singh, Commented Apr 12, 2017 at 18:10
Uh. Wow. What are you trying to achieve? Forget about your solution and tell us why you would want this. — Gray
– Gray, Commented Apr 12, 2017 at 18:20
I want this to see if there is any kind of performance impact in maintaining line numbers, and secondly, I want to see how log4j (and other loggers) manage to obtain line number for logging. So I need this real bad. But I am not able to see line numbers in javac -g generated bytecode. Help! — Apurva Singh
– Apurva Singh, Commented Apr 12, 2017 at 18:24

Stephan Herrmann · Accepted Answer · 2017-04-13 12:43:31Z

6

The compile command is good, -g turns on generation of debug information, indeed. BTW: line numbers are generated by default, need -g:none or similar to turn this off.

What's missing is a way to meaningfully inspect the generated .class file, similar to how a tool would use it. Try:

$ javap -l -c A.class

-l turns on printing of line number tables. -c turns on printing of disassembled bytecode instructions (might be interesting since line number tables relate source line numbers to bytecode instructions).

answered Apr 13, 2017 at 12:43

Stephan Herrmann

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