diff --git a/src/main/java/moe/yushi/authlibinjector/httpd/URLFilter.java b/src/main/java/moe/yushi/authlibinjector/httpd/URLFilter.java
index 46e8cdc..fdecb6e 100644
--- a/src/main/java/moe/yushi/authlibinjector/httpd/URLFilter.java
+++ b/src/main/java/moe/yushi/authlibinjector/httpd/URLFilter.java
@@ -6,8 +6,24 @@ import java.util.Optional;
 import moe.yushi.authlibinjector.internal.fi.iki.elonen.IHTTPSession;
 import moe.yushi.authlibinjector.internal.fi.iki.elonen.Response;
 
+/**
+ * A URLFilter filters the URLs in the bytecode, and intercepts those he is interested in.
+ */
 public interface URLFilter {
 
+	/**
+	 * Returns true if the filter MAY be interested in the given URL.
+	 *
+	 * The URL is grabbed from the bytecode, and it may be different from the URL being used at runtime.
+	 * Therefore, the URLs may be incomplete or malformed, or contain some template symbols. eg:
+	 * https://api.mojang.com/profiles/ (the actual one being used is https://api.mojang.com/profiles/minecraft)
+	 * https://sessionserver.mojang.com/session/minecraft/profile/<uuid> (template symbols)
+	 *
+	 * If this method returns true for the given URL, the URL will be intercepted.
+	 * And when a request is sent to this URL, handle() will be invoked.
+	 * If it turns out that the filter doesn't really want to intercept the URL (handle() returns empty),
+	 * the request will be reverse-proxied to the original URL, as if nothing happened.
+	 */
 	boolean canHandle(String domain, String path);
 
 	Optional<Response> handle(String domain, String path, IHTTPSession session) throws IOException;
diff --git a/src/main/java/moe/yushi/authlibinjector/httpd/URLProcessor.java b/src/main/java/moe/yushi/authlibinjector/httpd/URLProcessor.java
index 7e8005d..893c7f3 100644
--- a/src/main/java/moe/yushi/authlibinjector/httpd/URLProcessor.java
+++ b/src/main/java/moe/yushi/authlibinjector/httpd/URLProcessor.java
@@ -39,6 +39,16 @@ public class URLProcessor {
 		this.redirector = redirector;
 	}
 
+	/**
+	 * Transforms the input URL(which is grabbed from the bytecode).
+	 *
+	 * If any filter is interested in the URL, the URL will be redirected to the local HTTP server.
+	 * Otherwise, the URLRedirector will be invoked to determine whether the URL should be modified
+	 * and pointed to the customized authentication server.
+	 * If none of above happens, empty is returned.
+	 *
+	 * @return the transformed URL, or empty if it doesn't need to be transformed
+	 */
 	public Optional<String> transformURL(String inputUrl) {
 		Matcher matcher = URL_REGEX.matcher(inputUrl);
 		if (!matcher.find()) {
diff --git a/src/main/java/moe/yushi/authlibinjector/httpd/URLRedirector.java b/src/main/java/moe/yushi/authlibinjector/httpd/URLRedirector.java
index 6688f64..41e8923 100644
--- a/src/main/java/moe/yushi/authlibinjector/httpd/URLRedirector.java
+++ b/src/main/java/moe/yushi/authlibinjector/httpd/URLRedirector.java
@@ -2,6 +2,10 @@ package moe.yushi.authlibinjector.httpd;
 
 import java.util.Optional;
 
+/**
+ * A URLRedirector modifies the URLs found in the bytecode,
+ * and points them to the customized authentication server.
+ */
 public interface URLRedirector {
 	Optional<String> redirect(String domain, String path);
 }