Core Java: How To Get Java Source Code Line Number & File Name in Code

By Angsuman Chakraborty, Gaea News Network
Monday, September 8, 2008

While debugging code Java programmers often use System.out.println(). It is important to write separate message in each System.out.println() so you can understand from the output where the problem lies.

Now it is time-consuming and somewhat tedious to invent new message for each System.out.println() debug message. What if you could call methods which allows you to print the current file name and line number?

That would automatically ensure unique message in every System.out.println(). Also it will help you to immediately pinpoint the offending code. You can copy-paste something like this anywhere in your code (embellish it with more topical information as needed) and be able to pinpoint its location:

System.out.println(getFileName() + “:” + getClassName() + “:” + getMethodName() + “:” + getLineNumber());

I will show the implementation of getLineNumber() below and leave the rest as an exercise:


/** Get the current line number.
 * @return int - Current line number.
 */
public static int getLineNumber() {
    return Thread.currentThread().getStackTrace()[2].getLineNumber();
}

Have you noticed the magic number - 2? Can you explain it?
Filed under: Headline News, How To, Java Software, Open Source Software, Programming

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Discussion

Content-based image retrieval
March 3, 2010: 7:39 am

please give me source code for “Content-based image retrieval”.

Don
January 21, 2010: 5:10 pm

Thanks for the really useful trick. I wish I had had this 10 years ago.

Arlando Bloom
June 29, 2009: 12:59 am

Thanks for this wonderful addition. Really enjoyed your article. Appreciate people taking the time to write quality work. The
internet is full of bad articles,

iamy
June 11, 2009: 9:53 am

I would like to know about following given code

import java.*.*
Is there any statement in core java.
please explain me.

sravan
March 2, 2009: 6:05 am

write program to convert number to name?
ex:
input: 7
output: seven

Angsuman Chakraborty
September 15, 2008: 9:15 pm

That is correct. As the code is within a method we need to use the stacktrace element of the calling code.

Bharat Kondeti
September 15, 2008: 1:34 pm

This is what I understood

Java Doc for ‘getStackTrace’

‘If the returned array is of non-zero length then the first element of the array represents the top of the stack, which is the most recent method invocation in the sequence.’

when I print class name and method name for all the stackTraceElements for current thread

java.lang.Thread
dumpThreads

java.lang.Thread
getStackTrace

com.bharat.SimpleTest
main

Third element is where ‘Thread.currentThread().getStackTrace()’ is called.
so we are using the number 2

I believe by using the above statement we are accessing the Exception stack trace which would have been thrown if an exception would had occurred.

Mark O'Connor
September 15, 2008: 7:34 am

Enabling debug when compiling the Java code will also help to understand where an exception is being thrown from in a java stack trace/

The following is the ANT “javac” task:

Shubhashish
September 15, 2008: 3:25 am

Probably first is the Main calling class itself and other is the other which we want to debug …

I am not sure

Angsuman Chakraborty
September 9, 2008: 7:42 am

Is trolling still alive?

BlogTactic
September 9, 2008: 4:42 am

Java is still alive ?

sharfah
September 8, 2008: 7:46 am

If you’re coding in Eclipse, you can use “systrace” to print out the Class.Method().
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