httpproxy with littleproxy

great product https://github.com/adamfisk/LittleProxy

git clone https://github.com/adamfisk/LittleProxy
cd LittleProxy
./run.bash


To intercept HTTP requests, edit org.littleshoot.proxy.Launcher and insert the line with "withFiltersSource":

System.out.println("About to start...");
bootstrap.withFiltersSource(new MyHttpFiltersSourceAdapter());
bootstrap.start();

package org.littleshoot.proxy.impl;

import org.littleshoot.proxy.HttpFilters;
import org.littleshoot.proxy.HttpFiltersAdapter;
import org.littleshoot.proxy.HttpFiltersSource;
import java.util.Arrays;

import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.http.HttpObject;
import io.netty.handler.codec.http.HttpRequest;
import io.netty.handler.codec.http.HttpResponse;
import io.netty.handler.codec.http.DefaultHttpRequest;

public class MyHttpFiltersSourceAdapter implements HttpFiltersSource {
    private static final String HTTP_REPO1_MAVEN_ORG_MAVEN2 = "http://repo1.maven.org/maven2/";
    public HttpFiltersAdapter filterRequest(HttpRequest originalRequest, ChannelHandlerContext ctx) {
        return new HttpFiltersAdapter(originalRequest) {
            @Override
            public HttpResponse clientToProxyRequest(HttpObject httpObject) {
                // TODO: implement your filtering here
             System.out.println("clientToProxyRequest invoked");
             System.out.println(httpObject.toString());
             if (httpObject instanceof DefaultHttpRequest) {
              DefaultHttpRequest request = (DefaultHttpRequest)httpObject;
              System.out.println("it's a DefaultHttpRequest with uri:");
              String uri = request.getUri()
              System.out.println(uri);
              System.out.println(request.toString());
              System.out.println(request.headers());
  if (uri.startsWith(HTTP_REPO1_MAVEN_ORG_MAVEN2)) {
   String gavstring = uri.substring(HTTP_REPO1_MAVEN_ORG_MAVEN2.length());
   System.out.println("gavstring=" + gavstring);
   String[] parts = gavstring.split("/");
   int len = parts.length;
   if (len < 4) {
    System.err.println("invalid length for gavstring " + gavstring + " len = " + len + " should be at least 4");
   }
   else {
    String filename =  parts[len-1];
    String version =  parts[len-2];
    String artifactid = parts[len-3];
    String group = String.join(".", Arrays.copyOfRange(parts, 0, len - 3));
    System.out.println("filename=" + filename + " version=" + version + " artifactid=" + artifactid + " group=" + group);
   }
   
   
    }
    }
         System.out.println();
        return null;
    }

    @Override
    public HttpObject serverToProxyResponse(HttpObject httpObject) {
             System.out.println("serverToProxyResponse invoked");
             System.out.println(httpObject.toString());
             System.out.println();
                return httpObject;
            }
        };
    }

    @Override
    public int getMaximumRequestBufferSizeInBytes() {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public int getMaximumResponseBufferSizeInBytes() {
        // TODO Auto-generated method stub
    return 0;
    }
}

This is priceless to analyze what is going on between Nexus repository and Maven central: make sure your maven uses Nexus as a mirror: vi /home/centos/apache-maven-3.5.0/conf/settings.xml

<mirror>
<id>nexus</id>
<mirrorOf>*</mirrorOf>
<url>http://localhost:8081/repository/maven-public/</url>
</mirror>

then make sure that Nexus "maven-central" proxy repo (http://localhost:8081/repository/maven-central/) points to http://repo1.maven.org/maven2/ and not to https://repo1.maven.org/maven2/ (it's easier to trace http traffic than https) then configure Nexus 3.11 (login as admin/admin123, menu System/HTTP, set HTTP proxy to localhost and 8080) let's prepare a Maven project to compile: git clone https://github.com/gabrielf/maven-samples cd maven-samples/ mvn package to make sure a give jar is missing from the m2 repository, delete it before each build: rm -rf /home/centos/.m2/repository/org/codehaus/plexus/plexus-utils/3.0/plexus-utils-3.0.jar and remove it also from Nexus (Search/Maven, search for plexus-utils 3.0, Delete component) What you see in the LittleProxy stdout is:

clientToProxyRequest invoked
DefaultHttpRequest(decodeResult: success, version: HTTP/1.1)
GET http://repo1.maven.org/maven2/org/codehaus/plexus/plexus-utils/3.0/plexus-utils-3.0.jar HTTP/1.1
Host: repo1.maven.org
Proxy-Connection: Keep-Alive
User-Agent: Nexus/3.11.0-01 (OSS; Linux; 3.10.0-693.17.1.el7.x86_64; amd64; 1.8.0_141)
Accept-Encoding: gzip,deflate

As you can read in https://github.com/adamfisk/LittleProxy/blob/master/src/main/java/org/littleshoot/proxy/HttpFilters.java for the method clientToProxyRequest, if you return null the request will be further processed; if you return a io.netty.handler.codec.http.HttpResponse, then you effectively break the flow and you can stop the request. It's worth mentioning also https://github.com/lightbody/browsermob-proxy as a superset of Littleproxy