Add pathPrefix predicate

Added the `pathPrefix` predicate that tests the start of the request
path against a given pattern.

Issue: SPR-14954

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RequestPredicates.java

@@ -70,7 +70,7 @@ public abstract class RequestPredicates {
 	}
 
 	/**
-	 * Return a {@code RequestPredicate} that tests against the given path pattern.
+	 * Return a {@code RequestPredicate} that tests the request path against the given path pattern.
 	 *
 	 * @param pattern the pattern to match to
 	 * @return a predicate that tests against the given path pattern
@@ -256,6 +256,23 @@ public abstract class RequestPredicates {
 		};
 	}
 
+	/**
+	 * Return a {@code RequestPredicate} that tests the beginning of the request path against the
+	 * given path pattern. This predicate is effectively identical to a
+	 * {@linkplain #path(String) standard path predicate} with path {@code pathPrefixPattern + "/**"}.
+	 * @param pathPrefixPattern the pattern to match against the start of the request path
+	 * @return a predicate that matches if the given predicate matches against the beginning of
+	 * the request's path
+	 */
+	public static RequestPredicate pathPrefix(String pathPrefixPattern) {
+		Assert.notNull(pathPrefixPattern, "'pathPrefixPattern' must not be null");
+
+		if (!pathPrefixPattern.endsWith("/**")) {
+			pathPrefixPattern += "/**";
+		}
+		return path(pathPrefixPattern);
+	}
+
 	/**
 	 * Return a {@code RequestPredicate} that tests the request's query parameter of the given name
 	 * against the given predicate.

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/RequestPredicatesTests.java

@@ -162,7 +162,23 @@ public class RequestPredicatesTests {
 		uri = URI.create("http://localhost/file.foo");
 		request = MockServerRequest.builder().uri(uri).build();
 		assertFalse(predicate.test(request));
+	}
 
+	@Test
+	public void pathPrefix() throws Exception {
+		RequestPredicate predicate = RequestPredicates.pathPrefix("/foo");
+
+		URI uri = URI.create("http://localhost/foo/bar");
+		MockServerRequest request = MockServerRequest.builder().uri(uri).build();
+		assertTrue(predicate.test(request));
+
+		uri = URI.create("http://localhost/foo");
+		request = MockServerRequest.builder().uri(uri).build();
+		assertTrue(predicate.test(request));
+
+		uri = URI.create("http://localhost/bar");
+		request = MockServerRequest.builder().uri(uri).build();
+		assertFalse(predicate.test(request));
 	}
 
 	@Test