Both are used to send data from your servlet back to the client as part of the HTTP response.

But they are a little different:

Feature	PrintWriter	ServletOutputStream
Data type	Character stream (text data)	Byte stream (binary data)
Best for	Writing textual content (HTML, JSON, plain text)	Writing binary content (images, PDFs, videos, file downloads)
Returned by	response.getWriter()	response.getOutputStream()
Encoding control	Automatically handles encoding (e.g., UTF-8)	You manually control bytes
Ease of use	Easier for text	Needed for non-text (images, files)

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🎯 In simpler words:

• PrintWriter:
  • Works with characters (strings, text).
  • Buffered, convenient for writing HTML, JSON, plain text.
• ServletOutputStream:
  • Works with raw bytes (binary).
  • Needed when you send binary data like files, images, etc.

🛠️ Tiny Example:

✅ Using PrintWriter for text:

protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println("<h1>Hello, World!</h1>");
}

✅ Using ServletOutputStream for binary data:

protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    response.setContentType("application/octet-stream");
    ServletOutputStream out = response.getOutputStream();
    byte[] data = {0x48, 0x65, 0x6C, 0x6C, 0x6F}; // "Hello" in bytes
    out.write(data);
}

⚡ Important Rule:

You can’t use both getWriter() and getOutputStream() in the same response!

• If you call response.getWriter(), you cannot later call response.getOutputStream().
• And vice versa.
• Doing so causes IllegalStateException.

Because:

• Writer and OutputStream are two different ways to manage the response body, and mixing them would break the HTTP response format.

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🔥 Quick Practical Tip:

You want to send…	Use…
HTML page	PrintWriter
JSON response	PrintWriter
Text file (plain text)	PrintWriter
Image (PNG, JPEG)	ServletOutputStream
PDF document	ServletOutputStream
Zip file download	ServletOutputStream

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🚀 Bonus Tip:

If you use PrintWriter, you should always set the character encoding early, for example:

response.setCharacterEncoding("UTF-8");
response.setContentType("text/html; charset=UTF-8");

Otherwise the server default encoding (ISO-8859-1) might be used, causing weird text problems.

🛠️ Quick Summary Table:

Aspect	PrintWriter	ServletOutputStream
Type	Text / Character stream	Binary / Byte stream
Use for	HTML, JSON, plain text	Images, files, binary blobs
Method	response.getWriter()	response.getOutputStream()
Can mix them?	❌ No	❌ No