理解 servlet 中的 out.println

Question

How in Servlets, out.println writes the html text to container's response object where normally in System.out.println() , it writes the text to the console??

This is creating a problem for me conceptually.

Answer 1

Neither PrintStream nor PrintWriter is specified in terms of a console.

A PrintStream adds functionality to another output stream, namely the ability to print representations of various data values conveniently. PrintStream

Prints formatted representations of objects to a text-output stream. PrintWriter

They are both about formatting data and passing the formatted data to an output stream. Any output stream. By default, in certain environments, the PrintStream objects initially referenced by each of System.out and System.err send their data to some sort of console, but that is just those particular instances.

Even those can be changed. For example, it can be useful in some types of testing to replace System.out with a PrintStream that sends its data to a ByteArrayOutputStream .

Answer 2

It uses a ServletOutputStream from the Javadoc,

Provides an output stream for sending binary data to the client. A ServletOutputStream object is normally retrieved via the ServletResponse.getOutputStream() method.

If you look at System.out you can see that it's a PrintStream which is a different implementation of OutputStream .

Answer 3

If someplace in a servlet an invocation of out.println(...) writes to the response object, then out must be a reference to the response object's OutputStream , or a wrapper stream and it. If System.out sends output someplace else (as normally it would do) then it is a reference to a different OutputStream .

A servlet method's (or any other method's) local variable named out is completely unrelated to System.out , barring you initializing the one as a reference to the other.