Refactor HTTP Method from Enum to Class

This commit refactors HttpMethod from a Java enum into a class. The
underlying reason being that HTTP methods are not enumerable, but
instead an open range and not limited to the predefined values in the
specifications.

Closes gh-27697

spring-web/src/test/java/org/springframework/http/HttpMethodTests.java

@@ -0,0 +1,76 @@
+/*
+ * Copyright 2002-2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.http;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Arjen Poutsma
+ */
+class HttpMethodTests {
+
+	@Test
+	public void comparison() {
+		HttpMethod method1 = HttpMethod.valueOf("FOO");
+		HttpMethod method2 = HttpMethod.valueOf("FOO");
+		HttpMethod method3 = HttpMethod.valueOf("BAR");
+
+		assertThat(method1).isEqualTo(method2);
+		assertThat(method1).isNotEqualTo(method3);
+
+		assertThat(method1.hashCode()).isEqualTo(method2.hashCode());
+
+		assertThat(method1.compareTo(method2)).isEqualTo(0);
+		assertThat(method1.compareTo(method3)).isNotEqualTo(0);
+	}
+
+	@Test
+	void values() {
+		HttpMethod[] values = HttpMethod.values();
+		assertThat(values).containsExactly(HttpMethod.GET, HttpMethod.HEAD, HttpMethod.POST, HttpMethod.PUT,
+				HttpMethod.PATCH, HttpMethod.DELETE, HttpMethod.OPTIONS, HttpMethod.TRACE);
+
+		// check defensive copy
+		values[0] = HttpMethod.POST;
+		assertThat(HttpMethod.values()).containsExactly(HttpMethod.GET, HttpMethod.HEAD, HttpMethod.POST, HttpMethod.PUT,
+				HttpMethod.PATCH, HttpMethod.DELETE, HttpMethod.OPTIONS, HttpMethod.TRACE);
+	}
+
+	@Test
+	void valueOf() {
+		HttpMethod get = HttpMethod.valueOf("GET");
+		assertThat(get).isSameAs(HttpMethod.GET);
+
+		HttpMethod foo = HttpMethod.valueOf("FOO");
+		HttpMethod other = HttpMethod.valueOf("FOO");
+		assertThat(foo).isEqualTo(other);
+	}
+
+	@Test
+	void name() {
+		HttpMethod method = HttpMethod.valueOf("FOO");
+		assertThat(method.name()).isEqualTo("FOO");
+	}
+
+	@Test
+	void matches() {
+		assertThat(HttpMethod.GET.matches("GET")).isTrue();
+		assertThat(HttpMethod.GET.matches("FOO")).isFalse();
+	}
+}