LittleProxy is a high performance HTTP proxy written in Java atop Trustin Lee's excellent Netty event-based networking library. It's quite stable, performs well, and is easy to integrate into your projects.

One option is to clone LittleProxy and run it from the command line. This is as simple as:

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

You can embed LittleProxy in your own projects through Maven with the following:

    <dependency>
        <groupId>org.littleshoot</groupId>
        <artifactId>littleproxy</artifactId>
        <version>1.1.2</version>
    </dependency>

Once you've included LittleProxy, you can start the server with the following:

HttpProxyServer server =
    DefaultHttpProxyServer.bootstrap()
        .withPort(8080)
        .start();

To intercept and manipulate HTTPS traffic, LittleProxy uses a man-in-the-middle (MITM) manager. LittleProxy's default implementation (SelfSignedMitmManager) has a fairly limited feature set. For greater control over certificate impersonation, browser trust, the TLS handshake, and more, use a the LittleProxy-compatible MITM extension:

• LittleProxy-mitm - A LittleProxy MITM extension that aims to support every Java platform including Android
• mitm - A LittleProxy MITM extension that supports elliptic curve cryptography and custom trust stores

To filter HTTP traffic, you can add request and response filters using a HttpFiltersSource(Adapter), for example:

HttpProxyServer server =
    DefaultHttpProxyServer.bootstrap()
        .withPort(8080)
        .withFiltersSource(new HttpFiltersSourceAdapter() {
            public HttpFilters filterRequest(HttpRequest originalRequest, ChannelHandlerContext ctx) {
                return new HttpFiltersAdapter(originalRequest) {
                    @Override
                    public HttpResponse clientToProxyRequest(HttpObject httpObject) {
                        // TODO: implement your filtering here
                        return null;
                    }

                    @Override
                    public HttpObject serverToProxyResponse(HttpObject httpObject) {
                        // TODO: implement your filtering here
                        return httpObject;
                    }
                };
            }
        })
        .start();

Please refer to the Javadoc of org.littleshoot.proxy.HttpFilters to see the methods you can use.

To enable aggregator and inflater you have to return a value greater than 0 in your HttpFiltersSource#get(Request/Response)BufferSizeInBytes() methods. This provides to you a `FullHttp(Request/Response)' with the complete content in your filter uncompressed. Otherwise you have to handle the chunks yourself.

    @Override
    public int getMaximumResponseBufferSizeInBytes() {
        return 10 * 1024 * 1024;
    }

This size limit applies to every connection. To disable aggregating by URL at *.iso or *dmg files for example, you can return in your filters source a filter like this:

return new HttpFiltersAdapter(originalRequest, serverCtx) {
    @Override
    public void proxyToServerConnectionSucceeded(ChannelHandlerContext serverCtx) {
        ChannelPipeline pipeline = serverCtx.pipeline();
        if (pipeline.get("inflater") != null) {
            pipeline.remove("inflater");
        }
        if (pipeline.get("aggregator") != null) {
            pipeline.remove("aggregator");
        }
        super.proxyToServerConnectionSucceeded(serverCtx);
    }
};

This enables huge downloads in an application, which regular handles size limited FullHttpResponses to modify its content, HTML for example.

A proxy server like LittleProxy contains always a web server, too. If you get an URI without scheme, host and port in originalRequest it's a direct request to your proxy. You can return a HttpFilters implementation which answers responses with HTML content or redirects in clientToProxyRequest like this:

public class AnswerRequestFilter extends HttpFiltersAdapter {
	private final String answer;

	public AnswerRequestFilter(HttpRequest originalRequest, String answer) {
		super(originalRequest, null);
		this.answer = answer;
	}

	@Override
	public HttpResponse clientToProxyRequest(HttpObject httpObject) {
		ByteBuf buffer = Unpooled.wrappedBuffer(answer.getBytes("UTF-8"));
		HttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, buffer);
		HttpHeaders.setContentLength(response, buffer.readableBytes());
		HttpHeaders.setHeader(response, HttpHeaders.Names.CONTENT_TYPE, "text/html");
		return response;
	}
}

On answering a redirect, you should add a Connection: close header, to avoid blocking behavior:

		HttpHeaders.setHeader(response, Names.CONNECTION, Values.CLOSE);

With this trick, you can implement an UI to your application very easy.

If you want to create additional proxy servers with similar configuration but listening on different ports, you can clone an existing server. The cloned servers will share event loops to reduce resource usage and when one clone is stopped, all are stopped.

existingServer.clone().withPort(8081).start()

For examples of configuring logging, see src/test/resources/log4j.xml.

If you have questions, please visit our Google Group here:

https://groups.google.com/forum/#!forum/littleproxy

To subscribe, send an E-Mail to mailto:LittleProxy+subscribe@googlegroups.com. Simply answering, don't clicking the button, bypasses Googles registration process. You will become a member.

Benchmarking instructions and results can be found here.

Many thanks to The Measurement Factory for the use of Co-Advisor for HTTP standards compliance testing.