Find the location in code of a system.out.println

Question

Lets say I'm working in a very large project, and have noticed an empty print line, so I'm assuming there is a System.out.println(""); located somewhere in the code. How would I go about trying to figure out where it is, short of just searching the entire project for all occurrences of System.out.println?

Answer 1

You could implement your own PrintStream and use System.setOut to replace the default stdout. Then either put a debugging marker inside the class (if an empty string is printed), or print out the method name through the call stack (throw and catch an exception and get the stack information).

Answer 2

If you're using Java 8+, Durian has a StackDumper class which makes it easy to find where a given line is being printed:

StackDumper.dumpWhenSysOutContains("SomeTrigger")

When "SomeTrigger" is printed, this will get dumped to System.err:

+----------\
| Triggered by SomeTrigger
| at package.MyClass.myMethod(MyClass.java:62)
| (the rest of the stacktrace)
+----------/

For your case (looking for an empty string), it's a little more complicated:

PrintStream sysOutClean = System.out;
StringPrinter sysOutReplacement = new StringPrinter(StringPrinter.stringsToLines(line -> {
    if (line.isEmpty()) {
        StackDumper.dump("Found empty line");
    }
    sysOutClean.println(line);
}));
System.setOut(sysOutReplacement.toPrintStream());

Now if there's something like this:

System.out.println("ABC");
System.out.println("123");
System.out.println("");
System.out.println("DEF");

Then your console will look like this:

ABC
123
+----------\
| Found empty line
| at package.MyClass.myMethod(MyClass.java:62)
| (the rest of the stacktrace)
+----------/

DEF