Allow built ServerResponse to throw exception

This commit ensures that ServerResponse.HeadersBuilder::build can throw
an exception, by introducing a separate functional interface that does
allow for exceptions to be thrown.

Closes gh-30818

spring-webmvc/src/main/java/org/springframework/web/servlet/function/AbstractServerResponse.java

@@ -136,6 +136,6 @@ abstract class AbstractServerResponse extends ErrorHandlingServerResponse {
 	@Nullable
 	protected abstract ModelAndView writeToInternal(
 			HttpServletRequest request, HttpServletResponse response, Context context)
-			throws ServletException, IOException;
+			throws Exception;
 
 }

spring-webmvc/src/main/java/org/springframework/web/servlet/function/DefaultServerResponseBuilder.java

@@ -23,7 +23,6 @@ import java.util.Arrays;
 import java.util.LinkedHashSet;
 import java.util.Map;
 import java.util.Set;
-import java.util.function.BiFunction;
 import java.util.function.Consumer;
 
 import jakarta.servlet.http.Cookie;
@@ -174,10 +173,8 @@ class DefaultServerResponseBuilder implements ServerResponse.BodyBuilder {
 	}
 
 	@Override
-	public ServerResponse build(
-			BiFunction<HttpServletRequest, HttpServletResponse, ModelAndView> writeFunction) {
-
-		return new WriterFunctionResponse(this.statusCode, this.headers, this.cookies, writeFunction);
+	public ServerResponse build(WriteFunction writeFunction) {
+		return new WriteFunctionResponse(this.statusCode, this.headers, this.cookies, writeFunction);
 	}
 
 	@Override
@@ -219,12 +216,12 @@ class DefaultServerResponseBuilder implements ServerResponse.BodyBuilder {
 	}
 
 
-	private static class WriterFunctionResponse extends AbstractServerResponse {
+	private static class WriteFunctionResponse extends AbstractServerResponse {
 
-		private final BiFunction<HttpServletRequest, HttpServletResponse, ModelAndView> writeFunction;
+		private final WriteFunction writeFunction;
 
-		public WriterFunctionResponse(HttpStatusCode statusCode, HttpHeaders headers, MultiValueMap<String, Cookie> cookies,
-				BiFunction<HttpServletRequest, HttpServletResponse, ModelAndView> writeFunction) {
+		public WriteFunctionResponse(HttpStatusCode statusCode, HttpHeaders headers, MultiValueMap<String, Cookie> cookies,
+				WriteFunction writeFunction) {
 
 			super(statusCode, headers, cookies);
 			Assert.notNull(writeFunction, "WriteFunction must not be null");
@@ -232,10 +229,10 @@ class DefaultServerResponseBuilder implements ServerResponse.BodyBuilder {
 		}
 
 		@Override
-		protected ModelAndView writeToInternal(
-				HttpServletRequest request, HttpServletResponse response, Context context) {
+		protected ModelAndView writeToInternal(HttpServletRequest request, HttpServletResponse response,
+				Context context) throws Exception {
 
-			return this.writeFunction.apply(request, response);
+			return this.writeFunction.write(request, response);
 		}
 	}
 

spring-webmvc/src/main/java/org/springframework/web/servlet/function/ServerResponse.java

@@ -27,7 +27,6 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CompletionStage;
-import java.util.function.BiFunction;
 import java.util.function.Consumer;
 
 import jakarta.servlet.ServletException;
@@ -457,8 +456,28 @@ public interface ServerResponse {
 		 * Build the response entity with a custom write function.
 		 * @param writeFunction the function used to write to the {@link HttpServletResponse}
 		 */
-		ServerResponse build(BiFunction<HttpServletRequest, HttpServletResponse,
-				ModelAndView> writeFunction);
+		ServerResponse build(WriteFunction writeFunction);
+
+
+		/**
+		 * Defines the contract for {@link #build(WriteFunction)}.
+		 * @since 6.1
+		 */
+		@FunctionalInterface
+		interface WriteFunction {
+
+			/**
+			 * Write to the given {@code servletResponse}, or return a
+			 * {@code ModelAndView} to be rendered.
+			 * @param servletRequest the HTTP request
+			 * @param servletResponse  the HTTP response to write to
+			 * @return a {@code ModelAndView} to render, or {@code null} if handled directly
+			 * @throws Exception in case of Servlet errors
+			 */
+			@Nullable
+			ModelAndView write(HttpServletRequest servletRequest, HttpServletResponse servletResponse) throws Exception;
+
+		}
 
 	}