Improve forwarded URL support in HtmlUnit integration

Closes gh-28722

Author: rstoyanchev

spring-test/src/main/java/org/springframework/test/web/servlet/htmlunit/ForwardRequestPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,28 +19,42 @@
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.test.web.servlet.request.RequestPostProcessor;
 import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 
 /**
- * {@link RequestPostProcessor} for forward requests.
+ * {@link RequestPostProcessor} to update the request for a forwarded dispatch.
  *
  * @author Rob Winch
  * @author Sam Brannen
+ * @author Rossen Stoyanchev
  * @since 4.2
  */
 final class ForwardRequestPostProcessor implements RequestPostProcessor {
 
-	private final String forwardUrl;
+	private final String forwardedUrl;
 
 
-	public ForwardRequestPostProcessor(String forwardUrl) {
-		Assert.hasText(forwardUrl, "Forward URL must not be null or empty");
-		this.forwardUrl = forwardUrl;
+	public ForwardRequestPostProcessor(String forwardedUrl) {
+		Assert.hasText(forwardedUrl, "Forwarded URL must not be null or empty");
+		this.forwardedUrl = forwardedUrl;
 	}
 
 	@Override
 	public MockHttpServletRequest postProcessRequest(MockHttpServletRequest request) {
-		request.setServletPath(this.forwardUrl);
+		request.setRequestURI(this.forwardedUrl);
+		request.setServletPath(initServletPath(request.getContextPath()));
 		return request;
 	}
 
+	private String initServletPath(String contextPath) {
+		if (StringUtils.hasText(contextPath)) {
+			Assert.state(this.forwardedUrl.startsWith(contextPath), "Forward supported to same contextPath only");
+			return (this.forwardedUrl.length() > contextPath.length() ?
+					this.forwardedUrl.substring(contextPath.length()) : "");
+		}
+		else {
+			return this.forwardedUrl;
+		}
+	}
+
 }