can you redirect System.err to a file?

[excession]

Is it possible to redirect System.err to a file so that all errors are logged?

 

[Diancecht]

Take a look at setErr method at java.lang.System class.

Cheers,

Dian

 

[petrusp]

Yes. All you have to do is set a string object equal to System.err.toString(), get a byte[] of the string and write it to a file.

FileOutputStream file = new FileOutputStream("log.txt");
for (int i = 0; i < bytes.length; i++)
     file.write(bytes[i]);
file.close();

 

[excession]

ah ha, something like:

try {
            PrintStream out =
            new PrintStream(
            new BufferedOutputStream(
            new FileOutputStream("errorlog.txt")));
            
            System.setErr(out);
            System.err.println("testing 1 2 3");
            out.close();
        } catch (IOException e){
            e.printStackTrace();
        }

 

[excession]

if I redirect the System.err in my main class, does this automatically affect all objects it creates?

 

[Diancecht]

AFAIK, there's just an stderr for each virtual machine.

Cheers,

Dian

 

[sedj]

stdout and stderr are OS level file descriptors (in the C sense).

In Java System.err amd System.out are just stream objects basically that map to the OS's stdout and stderr.

So In Java, as in C, if you remap stdout and stderr to some other file descriptor then all calls to those handles will be redirected until the process (in Java, the JVM process) exits, or the streams are redirected/closed again.

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[stefanwagner]

What is the effect, if the user redirects the errorstream on the commandline (let's say, to a different file)?

I guess in most cases, it should be left to the user to redirect the errorstream.
He knows, where it is possible and appropriate to log to, and might wish to perform special handlings to the errorstream like tee-ing, filtering etc.

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