Improve checks on URI string in MockMvc request builder

Closes gh-24556

spring-test/src/main/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -145,7 +145,14 @@ public class MockHttpServletRequestBuilder
 	 * @param vars zero or more URI variables
 	 */
 	MockHttpServletRequestBuilder(HttpMethod httpMethod, String url, Object... vars) {
-		this(httpMethod.name(), UriComponentsBuilder.fromUriString(url).buildAndExpand(vars).encode().toUri());
+		this(httpMethod.name(), initUri(url, vars));
+	}
+
+	private static URI initUri(String url, Object[] vars) {
+		Assert.notNull(url, "'url' must not be null");
+		Assert.isTrue(url.startsWith("/") || url.startsWith("http://") || url.startsWith("https://"), "" +
+				"'url' should start with a path or be a complete HTTP URL: " + url);
+		return UriComponentsBuilder.fromUriString(url).buildAndExpand(vars).encode().toUri();
 	}
 
 	/**