Servlet 4.0 added Http Trailer() supports, which is a specific collection of HTTP headers comes after response body.
It is useful in some case, such as chunked transfer encoding or implements some specific protocols.
The reading side, HttpServletRequest has a method isTrailerFieldsReady() to check if the trailer fields are available, if it returns true, the trailer fields can be read via getTrailerFields() method.
The writing side, HttpServletResponse has a method setTrailerFields, which accepts a Supplier as it's parameter.
An example of Http Trailer to handle chunked transfer encoding.
@WebServlet("/test")
public class TestServlet extends HttpServlet {
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
res.setContentType("text/plain");
res.addHeader("Transfer-encoding", "chunked");
res.addHeader("TE", "trailers");
res.addHeader("Trailer", "bar");
StringBuilder sb = new StringBuilder();
final InputStream in = req.getInputStream();
int b;
while ((b = in.read()) != -1) {
sb.append((char) b);
}
String foo = null;
int size = -1;
if (req.isTrailerFieldsReady()) {
Map<String, String> reqTrailerFields = req.getTrailerFields();
size = reqTrailerFields.size();
foo = reqTrailerFields.get("foo");
}
final String finalFoo = foo;
final int finalSize = size;
res.setTrailerFields(() -> {
Map<String, String> map = new HashMap<>();
map.put("bar", finalFoo + finalSize);
return map;
});
res.getWriter().write(sb.toString());
}
}
A client to send chunked data.
@WebServlet("")
public class ClientServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse res)
throws ServletException, IOException {
res.setContentType("text/plain");
StringBuilder sb = new StringBuilder();
String hostStr = req.getServerName();
int port = req.getServerPort();
try (
Socket socket = new Socket(hostStr, port);
OutputStream output = socket.getOutputStream();
InputStream input = socket.getInputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(input))
) {
String reqStr = (new StringBuffer("POST /ee8-servlet-trailer/test HTTP/1.1\r\n")).
append("Host: " + hostStr + "\r\n").
append("Transfer-encoding: chunked\r\n").
append("Connection: close\r\n").
append("trailer: foo\r\n").
append("\r\n").
append("5\r\n").
append("hello\r\n").
append("0\r\n").
append("foo: A\r\n").
append("\r\n").
toString();
output.write(reqStr.getBytes(Charset.forName("US-ASCII")));
String line = null;
while ((line = reader.readLine()) != null) {
if (line.startsWith("bar")) {
sb.append(line).append("\r\n");
}
}
}
res.getWriter().write(sb.toString());
}
}