Adding Customized HTTP Header in HTTPServletRequest in Jetty

Sometimes we need Adding some customized HTTP header in Jetty's Request.

Here is a way to change Request.getHeader() using java.lang.instrument and javassist.

	import java.lang.instrument.*;
	import java.util.*;
	import javassist.*;
	public class HeaderAgent implements ClassFileTransformer {
	public static void premain(String agentArgument,
	Instrumentation instrumentation) {
	instrumentation.addTransformer(new HeaderAgent());
	}
	public byte[] transform(ClassLoader loader, String className,
	Class clazz, java.security.ProtectionDomain domain,
	byte[] bytes) {
	//To adding customized HTTP Header, we need adding some code in Request.getHeader() method
	if (!className.equals("org/mortbay/jetty/Request") && !className.equals("org/apache/catalina/connector/Request")) {
	return bytes;
	}
	System.out.println("[transform jetty class]: " + className);
	return doClass(className, clazz, bytes);
	}
	private byte[] doClass(String name, Class clazz, byte[] b) {
	ClassPool pool = ClassPool.getDefault();
	CtClass cl = null;
	try {
	cl = pool.makeClass(new java.io.ByteArrayInputStream(b));
	if (cl.isInterface() == false) {
	CtBehavior[] methods = cl.getDeclaredBehaviors();
	for (int i = 0; i < methods.length; i++) {
	//Only Modify getHeader behavior
	if (methods[i].getName().equals("getHeader")) {
	doMethod(methods[i]);
	}
	}
	b = cl.toBytecode();
	}
	} catch (Exception e) {
	System.err.println("Could not instrument " + name
	+ ", exception : " + e.getMessage());
	} finally {
	if (cl != null) {
	cl.detach();
	}
	}
	return b;
	}
	private void doMethod(CtBehavior method)
	throws NotFoundException, CannotCompileException {
	//adding code to customize value of "Host" to "hunmr.blogspot.com"
	method.insertBefore("if (\"Host\".equals($1)) { return \"hunmr.blogspot.com\"; }");
	}
	}

view raw Customize HTTP Header in Jetty hosted with ❤ by GitHub

contents of MANIFEST.MF:

	Premain-Class: HeaderAgent
	Boot-Class-Path: /absolute-path-to-javassist-jar-folder/javassist.jar

view raw MANIFEST.MF hosted with ❤ by GitHub

command to compile and pack jar:
javac -classpath ./javassist.jar HeaderAgent.java
jar -cmf MANIFEST.MF sslagent.jar HeaderAgent.class


adding following option to your java command, then Request.getHeader() method will be customized after Request.class loaded.
-javaagent:$HOME/bin/jettySSLPlugin/sslagent.jar

git@github.com:whunmr/jettyHeaderAgent.git