Why is System.out.println bad?

Question

When i run my PMD plugin they say System.out.println has been used. why is System.out.println bad to use and is it a defect when using PMD plugin? and what is the alternative way of over coming this?

possible duplicate of [Logger vs. System.out.println](http://stackoverflow.com/questions/2750346/logger-vs-system-out-println) — jontro, Oct 29 '13 at 11:04
It's a defect because you have a rule enabled that checks for `System.out.println()`. You can suppress the warnings in the places where you think you need output using the methods described on the following page: http://pmd.sourceforge.net/pmd-5.0.5/suppressing.html. — Mad Physicist, Nov 27 '13 at 05:05

score 8 · Answer 1 · answered Oct 29 '13 at 11:07

8

A logger can be turned ON/OFF using a configuration but System.out.println cannot be. Importantly loggers provide different levels of logging and again can be controlled through a configuration file.

Also using loggers you can configure rotation, purging etc but cannot do the same for sysout. This is useful especially in a production environment, executing a lot of code, and generating a lot of logging.

answered Oct 29 '13 at 11:07

Juned Ahsan

67,789
12
98
136

1

What about CLI apps? The usually output to stdout. That is the normal outout of those type of apps. – Tulains Córdova Jul 03 '17 at 18:22
@TulainsCórdova In CLI app u can either use macros or simple if/else to decide log or not to log according to a runtime param – Juned Ahsan Jul 04 '17 at 11:26
1

I mean, if `System.out.println()` is outlawed, what would be the mean for CLI apps to write to the screen? – Tulains Córdova Jul 04 '17 at 11:49

score 5 · Answer 2 · edited Nov 27 '18 at 13:27

5

System.out.println() is considered bad practice for Logging.
Because of

No ability to turn it (ON/OFF)
No ability to set output levels (TRACE, DEBUG, INFO, WARN, ERROR),
without having to recompile your code

Another disadvantage is that the standard output of a program can be redirected, for instance and it's not always clear where the output is actually going, for instance if you do this:

java SomeClass > someFile

In this case the use of a logging API will help you.

But there are situations where you genuinely want to print something to the standard output too, for those occasions there is java.io.Console, which cannot be redirected, so if you're running a command line java program, it gives you confidence that the users are seeing the messages intended to them.

edited Nov 27 '18 at 13:27

Ahmed Nabil

17,392
11
61
88

answered Nov 22 '13 at 00:58

chaos

641
10
21

What about the normal output of a a CLI app? Those app usuarlly write to stdout and that is not considered logging. – Tulains Córdova Jul 03 '17 at 18:23
For that, use System.console(), as it allows you to figure out if you're in a terminal or not, as well as it's more fitting to be used by an user. – chaos Jul 19 '17 at 12:41
Ran `System.console().printf("Hello %s", "world");` in Eclipse and I get NullPointerException. Doc says _"If no console device is available then an invocation of that method will return null. "_ – Tulains Córdova Jul 19 '17 at 13:08
That is considered a bug in Eclipse: https://bugs.eclipse.org/bugs/show_bug.cgi?id=122429 – chaos Jul 25 '17 at 14:07

score 4 · Accepted Answer · edited Feb 24 '18 at 20:00

4

System.out.println..

are usually intended for debugging purposes and can remain in the codebase even in production code. By using a logger one can enable/disable this behaviour at will (and by priority) and avoid clogging the Standard out log.

^{(from SourceMeter Java user guide under "Java Logging Rules")}

Import needed (.jar)

Logger site

Example:

import org.apache.log4j.Logger;

class Foo{
    private static final Logger LOG = Logger.getLogger(Foo.class);
    public void testA () {
        System.out.println("Entering test");
        // Better use this
        LOG.info("Entering test");
    }
}

edited Feb 24 '18 at 20:00

mhpreiman

139
2
4
13

answered Oct 29 '13 at 11:01

Hash

7,726
9
34
53

17

wow, you are asking a question and after some seconds answer to it? good – Admit Oct 29 '13 at 11:08
3

Why you opened a bounty to the question when you have the accepted answer? Surprising ! – Rahul Tripathi Nov 24 '13 at 18:14
Wrap the LOG.info("xyz") line with if(LOG.isInfoEnabled()) { ... } so that any string building/appending within the LOG.info call is not processed if the info logging level is not enabled. – Jason Nov 26 '13 at 06:11
So is there a way to boost rep by doing this? I only ask because I think some of the other answers are better than yours. – Mad Physicist Nov 26 '13 at 16:14
But you still not answered why PMD show sysout as defect – Satheesh Cheveri Nov 26 '13 at 18:51
@Admit It is just fine to ask and answer at exactly the same time. There's even an interface for it. But the question must still meet quality standards. – Andrew Barber Nov 27 '13 at 04:44
@MadPhysicist See my above comment. – Andrew Barber Nov 27 '13 at 04:44
What about CLI apps? The usually output to stdout. – Tulains Córdova Jul 03 '17 at 18:21

score 2 · Answer 4 · answered Nov 25 '13 at 19:04

2

Printing excessive amounts of into to System.out can become a performance bottleneck because it is synchronized, code from PrintStream:

public void println(float x) {
    synchronized (this) {
        print(x);
        newLine();
    }
}

answered Nov 25 '13 at 19:04

SergeyB

9,478
4
33
47

score 1 · Answer 5 · answered Nov 26 '13 at 19:03

This is because, PMD defines a java logging rule named SystemPrintln which detects System.out.println in the code and consider as a defect.

Since: PMD 2.1

System.(out|err).print is used, consider using a logger.

This rule is defined by the following XPath expression:

//Name[
    starts-with(@Image, 'System.out.print')
    or
    starts-with(@Image, 'System.err.print')
    ]

And you should be able to modify above XPath expression to override the behavior( I am not sure though) Below link should give you more insight,

http://pmd.sourceforge.net/pmd-4.2.6/rules/logging-java.html

Why is System.out.println bad?

5 Answers5