Head First Servlets and JSP, 2nd Edition

You’ve got two choices for output: characters or bytes

This is just plain old java.io, except the ServletResponse interface gives you only two streams to choose from: ServletOutputStream for bytes, or a PrintWriter for character data.

Note

You MUST memorize these methods

You have to know these for the exam. And it’s tricky. Notice that to write to a ServletOutputStream you write(), but to write to a Print-Writer you... println()! It’s natural to assume that you write to a writer, but you don’t. If you already use java.io, then you’ve been down this road. But if you haven’t, just remember:

println() to a PrintWriter

write() to an ServletOutputStream

Make sure you remember that the method names for getting the stream or the writer both drop the first word in the returned type:

ServletOutputStream response.getOutputStream()

PrintWriter response.getWriter()

You need to recognize WRONG method names like:

PrintWriter

Example:

PrintWriter writer = response.getWriter();

writer.println("some text and HTML");

Use it for:

Printing text data to a character stream. Although you can still write character data to an OutputStream, this is the stream that’s designed to handle character data.

OutputStream

Example

ServletOutputStream out = response.getOutputStream();

out.write(aByteArray);

Use it for:

Writing anything else!

Note

FYI: The PrintWriter actually “wraps” the ServletOutputStream. In other words, the PrintWriter has a reference to the ServletOutputStream and delegates calls to it. There’s just ONE output stream back to the client, but the PrintWriter “decorates” the stream by adding higher-level character-friendly methods.