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Java™
by example!
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What happens to System.out and System.err output in a Servlet?
It depends on your servlet engine.
Usually (in JServ or Tomcat or Weblogic) 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.
A piece of servlet code shows the idea:
public void doGet(HttpServletRequest req, HttpServletResponse res) {
// let client browser expect plain text
resp.setContentType("text/plain");
// this line will be seen in the browser
System.out.println("Hey, browser!");
// this line goes to error logs or such
System.err.println("Hey, error log!");
}
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This behaviour is not consistent across servlet engines, so portable calls should be used.
To output text to client, use:
public void doGet(HttpServletRequest req, HttpServletResponse res) {
// get proper PrintWriter
PrintWriter pw = resp.getWriter();
// this line will be seen in the browser
w.println("Hey, browser!");
}
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To output text to servlet engine error logs, use ServletContext.log():
public void doGet(HttpServletRequest req, HttpServletResponse res) {
// get ServletContext
ServletContext sc = getServletContext();
// this line will be seen in servlet engine logs
sc.log("Hey, browser!");
}
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Further Information
Author of answer: hal9000
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In-house 