Allow treating empty @RequestParam as missing value

If type conversion turns an empty request parameter value (i.e. "") to
null, we should treat it as a missing value. By default the
ConversionService doesn't change empty strings and therefore one must
explicitly convert them to null for example by registering a
StringTrimmerEditor with emptyAsNull=true.

Issue: SPR-10402

spring-web/src/main/java/org/springframework/web/method/annotation/AbstractNamedValueMethodArgumentResolver.java

@@ -74,6 +74,7 @@ public AbstractNamedValueMethodArgumentResolver(ConfigurableBeanFactory beanFact
 		this.expressionContext = (beanFactory != null) ? new BeanExpressionContext(beanFactory, new RequestScope()) : null;
 	}
 
+	@Override
 	public final Object resolveArgument(
 			MethodParameter parameter, ModelAndViewContainer mavContainer,
 			NativeWebRequest webRequest, WebDataBinderFactory binderFactory)
@@ -96,11 +97,17 @@ else if ("".equals(arg) && (namedValueInfo.defaultValue != null)) {
 			arg = resolveDefaultValue(namedValueInfo.defaultValue);
 		}
 
+		boolean emptyArgValue = "".equals(arg);
+
 		if (binderFactory != null) {
 			WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
 			arg = binder.convertIfNecessary(arg, paramType, parameter);
 		}
 
+		if (emptyArgValue && (arg == null)) {
+			handleMissingValue(namedValueInfo.name, parameter);
+		}
+
 		handleResolvedValue(arg, namedValueInfo.name, parameter, mavContainer, webRequest);
 
 		return arg;

spring-web/src/test/java/org/springframework/web/method/annotation/RequestParamMethodArgumentResolverTests.java

@@ -16,13 +16,6 @@
 
 package org.springframework.web.method.annotation;
 
-import static org.junit.Assert.assertArrayEquals;
-import static org.junit.Assert.assertEquals;
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertNull;
-import static org.junit.Assert.assertTrue;
-import static org.junit.Assert.fail;
-
 import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.List;
@@ -32,6 +25,7 @@
 
 import org.junit.Before;
 import org.junit.Test;
+import org.springframework.beans.propertyeditors.StringTrimmerEditor;
 import org.springframework.core.LocalVariableTableParameterNameDiscoverer;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ParameterNameDiscoverer;
@@ -41,13 +35,20 @@
 import org.springframework.mock.web.test.MockMultipartHttpServletRequest;
 import org.springframework.mock.web.test.MockPart;
 import org.springframework.web.bind.MissingServletRequestParameterException;
+import org.springframework.web.bind.WebDataBinder;
 import org.springframework.web.bind.annotation.RequestParam;
 import org.springframework.web.bind.annotation.RequestPart;
+import org.springframework.web.bind.support.WebDataBinderFactory;
+import org.springframework.web.bind.support.WebRequestDataBinder;
 import org.springframework.web.context.request.NativeWebRequest;
 import org.springframework.web.context.request.ServletWebRequest;
 import org.springframework.web.multipart.MultipartException;
 import org.springframework.web.multipart.MultipartFile;
 
+import static org.junit.Assert.*;
+import static org.mockito.BDDMockito.*;
+import static org.mockito.Mockito.*;
+
 /**
  * Test fixture with {@link org.springframework.web.method.annotation.RequestParamMethodArgumentResolver}.
  *
@@ -242,6 +243,23 @@ public void missingRequestParam() throws Exception {
 		fail("Expected exception");
 	}
 
+	// SPR-10402
+
+	@Test(expected = MissingServletRequestParameterException.class)
+	public void missingRequestParamEmptyValueConvertedToNull() throws Exception {
+
+		WebDataBinder binder = new WebRequestDataBinder(null);
+		binder.registerCustomEditor(String.class, new StringTrimmerEditor(true));
+
+		WebDataBinderFactory binderFactory = mock(WebDataBinderFactory.class);
+		given(binderFactory.createBinder(webRequest, null, "stringNotAnnot")).willReturn(binder);
+
+		this.request.addParameter("stringNotAnnot", "");
+
+		resolver.resolveArgument(paramStringNotAnnot, null, webRequest, binderFactory);
+		fail("Expected exception");
+	}
+
 	@Test
 	public void resolveSimpleTypeParam() throws Exception {
 		request.setParameter("stringNotAnnot", "plainValue");