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Your first filter

Write a Java filter that answers some requests itself and lets the rest through, then the same filter with lambdas. Use this page to learn the filter API; Filters and failure responses covers every hook.

A filter class

Filters keep LittleProxy's shape: an HttpFiltersSource makes an HttpFilters per request, and the HttpFilters overrides the hooks it needs. Returning a response from clientToProxyRequest answers the request without contacting the server; returning null lets it through.

import org.microproxy.FlowContext;
import org.microproxy.HttpFilters;
import org.microproxy.HttpFiltersAdapter;
import org.microproxy.HttpFiltersSourceAdapter;
import org.microproxy.HttpProxyServer;
import org.microproxy.MicroProxy;
import org.microproxy.http.DefaultFullHttpResponse;
import org.microproxy.http.HttpObject;
import org.microproxy.http.HttpRequest;
import org.microproxy.http.HttpResponse;
import org.microproxy.http.HttpResponseStatus;
import org.microproxy.http.HttpVersion;

public class BlockingProxy {
    public static void main(String[] args) {
        HttpProxyServer proxy = MicroProxy.bootstrap()
                .withPort(8080)
                .withFiltersSource(new HttpFiltersSourceAdapter() {
                    @Override
                    public HttpFilters filterRequest(HttpRequest originalRequest, FlowContext ctx) {
                        return new HttpFiltersAdapter(originalRequest, ctx) {
                            @Override
                            public HttpResponse clientToProxyRequest(HttpObject httpObject) {
                                if (httpObject instanceof HttpRequest r && r.uri().contains("/blocked")) {
                                    return new DefaultFullHttpResponse(HttpVersion.HTTP_1_1,
                                            HttpResponseStatus.FORBIDDEN, "blocked by proxy");
                                }
                                return null; // continue proxying
                            }
                        };
                    }
                })
                .start();
        System.out.println("listening on " + proxy.getListenAddress());
    }
}

Run it with the microproxy jar on the class path, and try it:

MICROPROXY_VERSION=0.4.1-SNAPSHOT
java -cp microproxy/target/microproxy-${MICROPROXY_VERSION}.jar BlockingProxy.java

$ curl -s -i -x http://127.0.0.1:8080 http://example.com/blocked
HTTP/1.1 403 Forbidden
...

blocked by proxy

Any other URL is fetched from the server as usual.

The same filter with lambdas

For small filters, HttpFilters.builder() takes one lambda per hook, and HttpFiltersSource is a functional interface. The built filters are also a filters source that returns them for every request (org.microproxy.HttpFiltersBuilder):

HttpFiltersBuilder.Built filters = HttpFilters.builder()
        .onRequest(req -> req.uri().contains("/blocked")
                ? new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.FORBIDDEN, "blocked by proxy")
                : null)
        .build();
MicroProxy.bootstrap().withPort(8080).withFiltersSource(filters).start();

See Filters from lambdas for every builder hook.

Next steps