Java Cookbook

Duplicating a Stream as It Is Written

Problem

You want anything written to a stream, such as the standard output System.out or the standard error System.err, to appear there but also be logged into a file.

Solution

Subclass PrintStream and have its write( ) methods write to two streams. Then use system.setErr( ) or setOut( ) as in Section 9.7 to replace the existing standard stream with this “tee” PrintStream subclass.

Discussion

Classes are meant to be subclassed. Here we’re just subclassing PrintStream and adding a bit of functionality: a second PrintStream! I wrote a class called TeePrintStream, named after the ancient Unix command tee. That command allowed you to duplicate, or “tee off,” a copy of the data being written on a "pipeline” between two programs.

The original Unix tee command is used like this: the | character creates a pipeline in which the standard output of one program becomes the standard input to the next. This often-used example of pipes shows how many users are logged into a Unix server:

who | wc -l

This runs the who program (which lists who is logged into the system, one name per line along with the terminal port and login time) with its output, instead of going to the terminal, going into the standard input of the word count (wc) program. Here wc is being asked to count lines, not words; hence the -l option. To tee a copy of the intermediate data into a file, you might say:

who | tee wholist | wc -l

which creates a file wholist containing the data. For the curious, ...