What is the Difference Between System.out & System.err output in a Servlet

System.out goes to ‘client side’ and is seen in browser, while System.err goes to ‘server side’ and is visible in error logs and/or on console.

In Java, including Advanced Java, System.out and System.err are both instances of PrintStream that represent standard output and standard error streams, respectively.

In the context of a Servlet:

  1. System.out (Standard Output):
    • It is generally used for normal output, such as regular log messages or responses.
    • The data sent to System.out is typically directed to the console or log files.
    • In a servlet, writing to System.out may result in the output being captured by the servlet container’s logging mechanism.
  2. System.err (Standard Error):
    • It is used for error messages or exceptional situations.
    • The data sent to System.err is typically directed to the console or log files as well, but it is intended for error-related information.
    • In a servlet, writing to System.err is often used to report errors or exceptional conditions.

In summary, the key difference lies in the intended use of these streams. System.out is for regular output, while System.err is for error-related output. When working with servlets, it’s a good practice to use logging frameworks like Log4j or SLF4J for more fine-grained control over logging and to avoid direct use of System.out or System.err.