Capturing Console Output (println) in Java

Quoting (with my valuable additions) from here >>>>>>> This tip demonstrates how you can record in a log file everything you print to standard out and standard error. This is especially useful if you deploy an application and your users encounter problems. You can have the users send you the log file for analysis.

A SaveOutput object is a PrintStream object that acts like a tee. Any characters it receives are forwarded to two places: the log file and the underlying print streams.

SaveOutput extends PrintStream, and is constructed on an existing PrintStream, plus a second existing PrintStream, and acts like a tee. Any characters it receives are forwarded to two places: the log stream and the underlying print stream.

The PrintStream class is deprecated vs PrintWriter, therefore ALL constructors produce a deprecated notice! But System still requires PrintStream for stdout and stderr, so there is nothing we can do about it.

SaveOutput has two static methods - start () and stop ().

• start () creates a new log file, or empties an existing log file. It copies into the log file, any characters printed to standard output and standard error. The start () method saves the current standard output and standard error print streams. These print streams will be restored when stop () is called.
• stop () restores the original standard output and standard error. It then closes the log file.

The underlying PrintStreams are the original standard output and standard error; these are supplied to the SaveOutput constructor.

The redirection applies to all output sent to standard output and standard error, regardless of what ".java" compile unit it appears in.

The write methods do not throw exceptions. Instead, they set a flag in the print stream if some problem occurs. They set the flag by calling setError (). If the client of the PrintStream wants to check if an error occurred, it can call checkError.

Wish List:

• ability to set debug levels, zero being off, to allow finer control
• redirecting output to a file upon request;
• implementing an ASSERT (or VALID) mechanism if possible;
• minimizing the overhead when it is turned off.
• have each object know how to dump itself to a supplied Debug object.

Although the Sun example doesn't show further elaborations,

• wrappers could provide debug levels and assertion triggers, while any "deeper" prints would still use the tee mechanism,
• could output to a message window instead of (or plus) a log file,
• could support filtering of what lines make it to which places.

Sources

• LogDemo.java
• SaveOutput.java

This software is provided free of charge with no support. Email me at morris_hirsch@brown.edu if you do have a problem, and I will try to help, but I cannot promise to.

Your comments, suggestions, bug reports, bug fixes, and enhancements, are welcome!

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