TestServlet.java

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import java.io.*;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;
import java.util.function.Supplier;
import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.*;

@WebServlet("/mytest")
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", "bar1, bar2");

        StringBuilder sb = new StringBuilder();

        final InputStream in = req.getInputStream();
        int b;
        while ((b = in.read()) != -1) {
            sb.append((char) b);
        }

        System.out.println("--> body = " + sb.toString());

        if (req.isTrailerFieldsReady()) {
            Map<String, String> reqTrailerFields = req.getTrailerFields();
            sb.append(reqTrailerFields.get("foo1"));
            sb.append(reqTrailerFields.get("foo2"));
            sb.append(reqTrailerFields.size());
        }

        Supplier<Map<String, String>> prevSupplier = res.getTrailerFields();
        res.setTrailerFields(new Supplier<Map<String, String>>() {
            @Override
            public Map<String, String> get() {
                Map<String, String> map = new HashMap<>();
                if (prevSupplier != null) {
                    map.putAll(prevSupplier.get());
                }
                map.put("bar2", "B");
                System.out.println("--> new supplier return: " + map);
                return map;
            }
        });
        PrintWriter writer = res.getWriter();
        writer.write(sb.toString());
    }
}