Introduce HttpStatusCode interface

This commit introduces HttpStatusCode, an interface implemented by
HttpStatus. Instances of HttpStatusCode are obtained via static
valueOf(int) factory method, returning a HttpStatus enum entry if
available, and a default implementation otherwise.

The underlying reason behind this change is HTTP status codes are not
enumerable, but instead range from 100-999.

Closes gh-28214

spring-web/src/main/java/org/springframework/http/DefaultHttpStatusCode.java

@@ -0,0 +1,105 @@
+/*
+ * 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.
+ * 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 java.io.Serializable;
+
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Default implementation of {@link HttpStatusCode}.
+ *
+ * @author Arjen Poutsma
+ * @since 6.0
+ */
+final class DefaultHttpStatusCode
+		implements HttpStatusCode, Comparable<HttpStatusCode>, Serializable {
+
+	private static final long serialVersionUID = 7017664779360718111L;
+
+	private final int value;
+
+
+	public DefaultHttpStatusCode(int value) {
+		this.value = value;
+	}
+
+	@Override
+	public int value() {
+		return this.value;
+	}
+
+	@Override
+	public boolean is1xxInformational() {
+		return hundreds() == 1;
+	}
+
+	@Override
+	public boolean is2xxSuccessful() {
+		return hundreds() == 2;
+	}
+
+	@Override
+	public boolean is3xxRedirection() {
+		return hundreds() == 3;
+	}
+
+	@Override
+	public boolean is4xxClientError() {
+		return hundreds() == 4;
+	}
+
+	@Override
+	public boolean is5xxServerError() {
+		return hundreds() == 5;
+	}
+
+	@Override
+	public boolean isError() {
+		int hundreds = hundreds();
+		return hundreds == 4 || hundreds == 5;
+	}
+
+	private int hundreds() {
+		return this.value / 100;
+	}
+
+	@Override
+	public int compareTo(@NotNull HttpStatusCode o) {
+		return Integer.compare(this.value, o.value());
+	}
+
+	@Override
+	public int hashCode() {
+		return this.value;
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (obj instanceof HttpStatusCode other) {
+			return this.value == other.value();
+		}
+		else {
+			return false;
+		}
+	}
+
+	@Override
+	public String toString() {
+		return Integer.toString(this.value);
+	}
+}

spring-web/src/main/java/org/springframework/http/HttpStatus.java

@@ -31,7 +31,7 @@ import org.springframework.lang.Nullable;
  * @see <a href="https://www.iana.org/assignments/http-status-codes">HTTP Status Code Registry</a>
  * @see <a href="https://en.wikipedia.org/wiki/List_of_HTTP_status_codes">List of HTTP status codes - Wikipedia</a>
  */
-public enum HttpStatus {
+public enum HttpStatus implements HttpStatusCode {
 
 	// 1xx Informational
 
@@ -436,9 +436,7 @@ public enum HttpStatus {
 	}
 
 
-	/**
-	 * Return the integer value of this status code.
-	 */
+	@Override
 	public int value() {
 		return this.value;
 	}
@@ -458,70 +456,32 @@ public enum HttpStatus {
 		return this.reasonPhrase;
 	}
 
-	/**
-	 * Whether this status code is in the HTTP series
-	 * {@link org.springframework.http.HttpStatus.Series#INFORMATIONAL}.
-	 * <p>This is a shortcut for checking the value of {@link #series()}.
-	 * @since 4.0
-	 * @see #series()
-	 */
+	@Override
 	public boolean is1xxInformational() {
 		return (series() == Series.INFORMATIONAL);
 	}
 
-	/**
-	 * Whether this status code is in the HTTP series
-	 * {@link org.springframework.http.HttpStatus.Series#SUCCESSFUL}.
-	 * <p>This is a shortcut for checking the value of {@link #series()}.
-	 * @since 4.0
-	 * @see #series()
-	 */
+	@Override
 	public boolean is2xxSuccessful() {
 		return (series() == Series.SUCCESSFUL);
 	}
 
-	/**
-	 * Whether this status code is in the HTTP series
-	 * {@link org.springframework.http.HttpStatus.Series#REDIRECTION}.
-	 * <p>This is a shortcut for checking the value of {@link #series()}.
-	 * @since 4.0
-	 * @see #series()
-	 */
+	@Override
 	public boolean is3xxRedirection() {
 		return (series() == Series.REDIRECTION);
 	}
 
-	/**
-	 * Whether this status code is in the HTTP series
-	 * {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR}.
-	 * <p>This is a shortcut for checking the value of {@link #series()}.
-	 * @since 4.0
-	 * @see #series()
-	 */
+	@Override
 	public boolean is4xxClientError() {
 		return (series() == Series.CLIENT_ERROR);
 	}
 
-	/**
-	 * Whether this status code is in the HTTP series
-	 * {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}.
-	 * <p>This is a shortcut for checking the value of {@link #series()}.
-	 * @since 4.0
-	 * @see #series()
-	 */
+	@Override
 	public boolean is5xxServerError() {
 		return (series() == Series.SERVER_ERROR);
 	}
 
-	/**
-	 * Whether this status code is in the HTTP series
-	 * {@link org.springframework.http.HttpStatus.Series#CLIENT_ERROR} or
-	 * {@link org.springframework.http.HttpStatus.Series#SERVER_ERROR}.
-	 * <p>This is a shortcut for checking the value of {@link #series()}.
-	 * @since 5.0
-	 * @see #is4xxClientError()
-	 * @see #is5xxServerError()
-	 */
+	@Override
 	public boolean isError() {
 		return (is4xxClientError() || is5xxServerError());
 	}

spring-web/src/main/java/org/springframework/http/HttpStatusCode.java

@@ -0,0 +1,96 @@
+/*
+ * 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.springframework.util.Assert;
+
+/**
+ * Represents an HTTP response status code. Implemented by {@link HttpStatus},
+ * but defined as interface to allow for values not in that enumeration.
+ *
+ * @author Arjen Poutsma
+ * @since 6.0
+ * @see <a href="https://www.iana.org/assignments/http-status-codes">HTTP Status Code Registry</a>
+ * @see <a href="https://en.wikipedia.org/wiki/List_of_HTTP_status_codes">List of HTTP status codes - Wikipedia</a>
+ */
+public sealed interface HttpStatusCode permits DefaultHttpStatusCode, HttpStatus {
+
+	/**
+	 * Return the integer value of this status code.
+	 */
+	int value();
+
+	/**
+	 * Whether this status code is in the Informational class ({@code 1xx}).
+	 * @see <a href="https://datatracker.ietf.org/doc/html/rfc2616#section-10.1">RFC 2616</a>
+	 */
+	boolean is1xxInformational();
+
+	/**
+	 * Whether this status code is in the Successful class ({@code 2xx}).
+	 * @see <a href="https://datatracker.ietf.org/doc/html/rfc2616#section-10.2">RFC 2616</a>
+	 */
+	boolean is2xxSuccessful();
+
+	/**
+	 * Whether this status code is in the Redirection class ({@code 3xx}).
+	 * @see <a href="https://datatracker.ietf.org/doc/html/rfc2616#section-10.3">RFC 2616</a>
+	 */
+	boolean is3xxRedirection();
+
+	/**
+	 * Whether this status code is in the Client Error class ({@code 4xx}).
+	 * @see <a href="https://datatracker.ietf.org/doc/html/rfc2616#section-10.4">RFC 2616</a>
+	 */
+	boolean is4xxClientError();
+
+	/**
+	 * Whether this status code is in the Server Error class ({@code 5xx}).
+	 * @see <a href="https://datatracker.ietf.org/doc/html/rfc2616#section-10.5">RFC 2616</a>
+	 */
+	boolean is5xxServerError();
+
+	/**
+	 * Whether this status code is in the Client or Server Error class
+	 * @see <a href="https://datatracker.ietf.org/doc/html/rfc2616#section-10.4">RFC 2616</a>
+	 * @see <a href="https://datatracker.ietf.org/doc/html/rfc2616#section-10.3">RFC 2616</a>
+	 * ({@code 4xx} or {@code 5xx}).
+	 * @see #is4xxClientError()
+	 * @see #is5xxServerError()
+	 */
+	boolean isError();
+
+
+	/**
+	 * Return an {@code HttpStatusCode} object for the given integer value.
+	 * @param code the status code as integer
+	 * @return the corresponding {@code HttpStatusCode}
+	 * @throws IllegalArgumentException if {@code code} is not a three-digit
+	 * positive number
+	 */
+	static HttpStatusCode valueOf(int code) {
+		Assert.isTrue(code >= 100 && code <= 999, "Code '" + code + "' should be a thee-digit positive integer");
+		HttpStatus status = HttpStatus.resolve(code);
+		if (status != null) {
+			return status;
+		}
+		else {
+			return new DefaultHttpStatusCode(code);
+		}
+	}
+
+}