Add request binding to functional endpoints

This commit introduces form binding to ServerRequest in both WebMVC.fn
and WebFlux.fn's, in the form of a bind(Class) method.

Closes gh-25943

spring-webmvc/src/test/java/org/springframework/web/servlet/function/DefaultServerRequestTests.java

@@ -49,6 +49,7 @@ import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.validation.BindException;
 import org.springframework.web.HttpMediaTypeNotSupportedException;
 import org.springframework.web.servlet.handler.PathPatternsTestUtils;
 import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
@@ -304,6 +305,71 @@ class DefaultServerRequestTests {
 		assertThat(request.principal()).contains(principal);
 	}
 
+	@Test
+	void bindToConstructor() throws BindException {
+		MockHttpServletRequest servletRequest = PathPatternsTestUtils.initRequest("GET", "/", true);
+		servletRequest.addParameter("foo", "FOO");
+		servletRequest.addParameter("bar", "BAR");
+
+		DefaultServerRequest request = new DefaultServerRequest(servletRequest, this.messageConverters);
+
+		ConstructorInjection result = request.bind(ConstructorInjection.class);
+		assertThat(result.getFoo()).isEqualTo("FOO");
+		assertThat(result.getBar()).isEqualTo("BAR");
+	}
+
+	@Test
+	void bindToProperties() throws BindException {
+		MockHttpServletRequest servletRequest = PathPatternsTestUtils.initRequest("GET", "/", true);
+		servletRequest.addParameter("foo", "FOO");
+		servletRequest.addParameter("bar", "BAR");
+
+		DefaultServerRequest request = new DefaultServerRequest(servletRequest, this.messageConverters);
+
+		PropertyInjection result = request.bind(PropertyInjection.class);
+		assertThat(result.getFoo()).isEqualTo("FOO");
+		assertThat(result.getBar()).isEqualTo("BAR");
+	}
+
+	@Test
+	void bindToMixed() throws BindException {
+		MockHttpServletRequest servletRequest = PathPatternsTestUtils.initRequest("GET", "/", true);
+		servletRequest.addParameter("foo", "FOO");
+		servletRequest.addParameter("bar", "BAR");
+
+		DefaultServerRequest request = new DefaultServerRequest(servletRequest, this.messageConverters);
+
+		MixedInjection result = request.bind(MixedInjection.class);
+		assertThat(result.getFoo()).isEqualTo("FOO");
+		assertThat(result.getBar()).isEqualTo("BAR");
+	}
+
+	@Test
+	void bindCustomizer() throws BindException {
+		MockHttpServletRequest servletRequest = PathPatternsTestUtils.initRequest("GET", "/", true);
+		servletRequest.addParameter("foo", "FOO");
+		servletRequest.addParameter("bar", "BAR");
+
+		DefaultServerRequest request = new DefaultServerRequest(servletRequest, this.messageConverters);
+
+		PropertyInjection result = request.bind(PropertyInjection.class, dataBinder -> dataBinder.setAllowedFields("foo"));
+		assertThat(result.getFoo()).isEqualTo("FOO");
+		assertThat(result.getBar()).isNull();
+	}
+
+	@Test
+	void bindError() throws BindException {
+		MockHttpServletRequest servletRequest = PathPatternsTestUtils.initRequest("GET", "/", true);
+		servletRequest.addParameter("foo", "FOO");
+
+		DefaultServerRequest request = new DefaultServerRequest(servletRequest, this.messageConverters);
+
+		assertThatExceptionOfType(BindException.class).isThrownBy(() ->
+			request.bind(ErrorInjection.class)
+		);
+	}
+
+
 	@ParameterizedHttpMethodTest
 	void checkNotModifiedTimestamp(String method) throws Exception {
 		MockHttpServletRequest servletRequest = PathPatternsTestUtils.initRequest(method, "/", true);
@@ -487,4 +553,83 @@ class DefaultServerRequestTests {
 	@interface ParameterizedHttpMethodTest {
 	}
 
+	private static final class ConstructorInjection {
+		private final String foo;
+
+		private final String bar;
+
+		public ConstructorInjection(String foo, String bar) {
+			this.foo = foo;
+			this.bar = bar;
+		}
+
+		public String getFoo() {
+			return this.foo;
+		}
+
+		public String getBar() {
+			return this.bar;
+		}
+	}
+
+	private static final class PropertyInjection {
+		private String foo;
+
+		private String bar;
+
+		public String getFoo() {
+			return this.foo;
+		}
+
+		public void setFoo(String foo) {
+			this.foo = foo;
+		}
+
+		public String getBar() {
+			return this.bar;
+		}
+
+		public void setBar(String bar) {
+			this.bar = bar;
+		}
+	}
+
+	private static final class MixedInjection {
+		private final String foo;
+
+		private String bar;
+
+		public MixedInjection(String foo) {
+			this.foo = foo;
+		}
+
+		public String getFoo() {
+			return this.foo;
+		}
+
+		public String getBar() {
+			return this.bar;
+		}
+
+		public void setBar(String bar) {
+			this.bar = bar;
+		}
+	}
+
+	private static final class ErrorInjection {
+
+		private int foo;
+
+		public int getFoo() {
+			return this.foo;
+		}
+
+		public void setFoo(int foo) {
+			this.foo = foo;
+		}
+	}
+
+
+
+
 }