Question

What’s the difference between calling :

res.flushBuffer();

versus

res.getOutputStream().flush();  

Do these methods flush the same buffer ?

If so, can you give me a clue on how this buffer is managed by the servlet container?

Answer 1

They would flush the same buffer if you have been using getOutputStream to write to the body. The other alternative is getWriter for non-binary data. If you had been using that, then calling res.getOutputStream().flush(); probably wouldn’t work.

The way the buffer is managed is implementation-specific but take one of the Tomcat implementations for example. You can see that there are some fields like this:

/**
 * The associated output buffer.
 */
protected OutputBuffer outputBuffer;
/**
 * The associated output stream.
 */
protected CoyoteOutputStream outputStream;
/**
 * The associated writer.
 */
protected CoyoteWriter writer;

Calling getOutputStream() creates a CoyoteOutputStream that uses the outputBuffer field that is shown there and likewise for getWriter(). So they both would use that outputBuffer depending on which you use. flushBuffer simply does this:

@Override
public void flushBuffer()
    throws IOException {
    outputBuffer.flush();
}