A Web Server is usually running as a background process without connected stdio streams. Even if the server is running in a console, a Servlet can not expect to access that console with the System.in, System.out and System.err streams. Instead all messages should be sent to a server log file with one of ServletContext’s log methods:

• public void log(String msg) writes the specified message to the log file.
• public void log(String msg, Throwable t) [API 2.1] writes the specified message and a stack trace of the Throwable object to the log file. The message will usually be an explanation of an error and the Throwable an exception that caused the error.
• public void log(Exception exception, String msg) does the same as the previous method. It is available in earlier API versions and deprecated in version 2.1.

Two convenience methods are implemented in GenericServlet. They automatically prefix the messages with the right Servlet name and then call the appropriate ServletContext method:

• public void log(String msg)
• public void log(String msg, Throwable cause) [API 2.1]

Exceptions which cause one of the Servlet lifecycle methods (or a do... method in an HttpServlet) to fail do not need to be caught and logged. They are handled automatically by the Servlet Engine.

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