Removing access to System.out in java

Question

I maintain an application which acts as a container for multiple individual programs. These programs have their own dedicated logging facility, i.e. everything they log does to a special log file.

Nevertheless, application developers seem to love to throw System.out.println and e.printStackTrace calls all over, making it impossible to maintain a clean console when running the container.

How can I prevent these applications from polluting System.out and System.err?

Implementation notes:

the applications use Log4j for logging;
the container also uses the console for logging, but it is strictly reserved for lifecycle events and problems, so I still need the console;
the applications are loaded using a custom classloader, but no security checks are applied.

Update:

Simply redirecting System.out would not work since it redirects all output, so something like this fails:

    System.setOut(new PrintStream(new OutputStream() {

        @Override
        public void write(int b) {

            throw new Error("Not on my watch you don't");

        }
    }));

    Logger logger = Logger.getLogger(Runner.class);
    logger.info("My log message");

This should succeed.

Update 2:

The applications are loaded and configured using code similar to

App app = new UrlClassLoader(...).loadClass(className)).newInstance();
app.setLogger(loggerForClass(app));

Log4j is loaded from the system class loader.

This isn't really an answer, but: as a developer I tend to do this as well because it's easier to check the console rather than dig through log files. So when I check in, I first do a find/replace: search for System.out (replace with log.debug()), search for System.err (replace with log.info()), search for printStackTrace (delete). I even think this might be automated for svn for example (not sure how, but I think it can be done) — laura
– laura, Commented Nov 11, 2009 at 9:51
Hm, in a systems language the idiom would be to fork(), redirect the child process's stdout and stderr, and then exec(), but I'm not sure what the equivalent is with Java classloaders. Could you go into more detail about how you are loading the applications? — Ted Kaplan
– Ted Kaplan, Commented Nov 11, 2009 at 19:40
Is the container using log4j or printing directly to the console? — Fedearne
– Fedearne, Commented Nov 11, 2009 at 20:54

Alexander Torstling · Accepted Answer · 2009-11-10 20:34:44Z

12

Assuming that you can control your containers output you can do the following:

import java.io.*;
public class SysOut {
    public static void main(String[] args) throws Exception {
            PrintStream pw = new PrintStream(new FileOutputStream("a.txt"));
            PrintStream realout = System.out;
            System.setOut(pw);
            System.out.println("junk");
            realout.print("useful");
}
}

$ java SysOut 
useful
$ cat a.txt 
junk

answered Nov 10, 2009 at 20:34

Alexander Torstling

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Ted Kaplan · Accepted Answer · 2009-11-10 20:27:25Z

10

You can use System.setOut() and System.setErr() to redirect stdout and stderr to instances of PrintStream.

answered Nov 10, 2009 at 20:27

Ted Kaplan

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Comments

Mike Clark · Accepted Answer · 2010-04-21 13:56:15Z

6

While Java defines a standard System.out and System.err, these can be overwritten with your own streams. See http://www.devx.com/tips/Tip/5616

Basically you can set up new streams that either pipe to the logging, or simply let data flop off into nothingness. My preference would be the latter, as it would instantly cure developers from relying on System.out and err as anything they write there just disappears.

**Update: I just reread your stipulations in the question and see you still need the console for the container application. This might still work if you write a wrapper around the standard stream so you can check each call and see if it is coming from the parent application (and pass it on) or a child application (and block it)

edited Apr 21, 2010 at 13:56

answered Nov 10, 2009 at 20:27

Mike Clark

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djna · Accepted Answer · 2009-11-10 20:27:10Z

5

Use aversion therapy. A visit from "The Inspectors" is scheduled whenever any code is checked in containing unpleasant constructs.

Nice cubicle you got ere, be shame if anyfing appened to it.

answered Nov 10, 2009 at 20:27

djna

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2 Comments

Andrzej Doyle Over a year ago

Wow, this suggestion solved my completely unrelated problem too... thanks! ;-)

djna Over a year ago

I also have another useful pattern: "Reward", it involves Red Wine, party hats, and a visit from "The Choir". "Halleluia!"

Michael Wiles · Accepted Answer · 2009-11-11 08:55:12Z

3

If you have a headless build mechanism, ant or such like then you could add CheckStyle to the build and configure checkstyle to fail the build if it finds any System.out.println or e.printStackTrace in the code.

If you don't have a headless build I would recommend that you build one as it means you have repeatable, predictable builds.

answered Nov 11, 2009 at 8:55

Michael Wiles

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Andrzej Doyle · Accepted Answer · 2009-11-11 10:08:24Z

System.setOut will redirect all output - but the PrintStream you supply can decide how the output is handled. Thus I'm sure you could provide such a stream that would only actually print statements from your application.

The only tricky part really is being able to detect what's a valid call and what's not. A working, but likely very slow way to do this, would be to call Thread.currentThread().getStackTrace() and see what code (or package, at least) is calling you (simply returning if it's not a valid one). I wouldn't recommend this though as the performance hit would be staggering, especially doing this on every byte read.

A better idea might be to set a ThreadLocal flag in all of your valid, container threads. Then you can implement the PrintStream something like the following:

public class ThreadValidity extends ThreadLocal<Boolean>
{
    private static final INSTANCE = new ThreadValidity();

    @Override Boolean initialValue() { return false; }
    public static ThreadValidity getInstance() { return INSTANCE; }
}

class VerifyingPrintStream extends PrintStream
{
    private boolean isValidThread()
    {
        return ThreadValidity.instance().get();
    }

    public void println(String s)
    {
        if (!isValidThread()) return;
        super.println(s);
    }

    public void println(Object o)
    {
        if (!isValidThread()) return;
        super.println(o);
    }

    // etc
}

Alternatively, if you're able to change the printlns in the container code, things get easier. You can hand off all the console writes to a specific worker; and have this worker "steal" System.out (store it in its own field and use it directly for writing output) while setting the actual System.out to a no-op writer.

Robert Munteanu · Accepted Answer · 2009-11-18 07:58:10Z

2

The key here is to configure log4j before redirecting the output streams, e.g.

BasicConfigurator.configure();
System.setOut(...);
System.setErr(...);

System.out.println("I fail");
Logger.getLogger(...).info("I work");

answered Nov 18, 2009 at 7:58

Robert Munteanu

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Thorbjørn Ravn Andersen · Accepted Answer · 2009-11-10 20:41:12Z

1

Convert the System.out and System.err streams to special implementations that throw a RuntimeException("Use logging instead of System.out") every time a character is written.

If your container is important, they will get the idea pretty quickly :)

(For extra bonus throw OutOfMemoryException instead ;-))

answered Nov 10, 2009 at 20:41

Thorbjørn Ravn Andersen

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2 Comments

Thorbjørn Ravn Andersen Over a year ago

In that case please give an example of how to distinguish between output you like, and output you do not like. Are some classes allowed to do this and others not?

Thorbjørn Ravn Andersen Over a year ago

If you have the source for the container then why not refactor it to save and use the original streams, THEN you can replace System.out and System.err?

Peter Lawrey · Accepted Answer · 2009-11-10 20:58:07Z

1

What I have done is redirect the PrintStream for System.out and System.err to commons-logging as INFO and ERROR level logging respectively.

This gets trickier if you want some threads to be able to write to the console or you want logs to go to the console as well but it can be done.

answered Nov 10, 2009 at 20:58

Peter Lawrey

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2 Comments

Peter Lawrey Over a year ago

You can make the code reentrant. i.e. set a flag when OutputStream.write is called. if the flag is not set, print to the loggers. If it is set, print as normal as it must have come from the logger.

Peter Lawrey Over a year ago

Note: You have to unset the flag in a finally block. ;)

Bill K · Accepted Answer · 2009-11-10 21:04:01Z

1

You can actually get and store System.out/err before replacing them.

OutputStream out=System.getOut();  // I think the names are right
System.setOut(some predefined output stream, null won't work);
out.println("Hey, this still goes to the output");
System.out.println("Oh noes, this does not");

I've used this to intercept all the System.out.println's in the codebase and prefix each line of output with the method name/line number it came from.

answered Nov 10, 2009 at 21:04

Bill K

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Javamann · Accepted Answer · 2009-11-10 21:05:52Z

1

Close the System.out and System.err streams.

answered Nov 10, 2009 at 21:05

Javamann

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1 Comment

Raedwald Over a year ago

It can be argued that as the JVM created those streams, only it should close them.

M1EK · Accepted Answer · 2009-11-10 21:03:51Z

0

We use the log4j trick but log to separate files (stdout.log, stderr.log). It's not useful to have their output mixed in with the parts that actually understand logging...

answered Nov 10, 2009 at 21:03

M1EK

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