Refine ModelAttributeMethodProcessor Kotlin exception handling

This commit refines ModelAttributeMethodProcessor Kotlin exception
handling in order to throw a proper MethodArgumentNotValidException
instead of a NullPointerException when Kotlin null-safety constraints
are not fulfilled, translating to an HTTP error with 400 status code
(Bad Request) instead of 500 (Internal Server Error).

Closes gh-23846

spring-web/src/main/java/org/springframework/web/bind/MethodArgumentNotValidException.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.bind;
 
+import java.lang.reflect.Executable;
 import java.util.ArrayList;
 import java.util.LinkedHashMap;
 import java.util.List;
@@ -42,13 +43,18 @@ import org.springframework.web.ErrorResponse;
  *
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
+ * @author Sebastien Deleuze
  * @since 3.1
  */
 @SuppressWarnings("serial")
 public class MethodArgumentNotValidException extends BindException implements ErrorResponse {
 
+	@Nullable
 	private final MethodParameter parameter;
 
+	@Nullable
+	private final Executable executable;
+
 	private final ProblemDetail body;
 
 
@@ -60,6 +66,19 @@ public class MethodArgumentNotValidException extends BindException implements Er
 	public MethodArgumentNotValidException(MethodParameter parameter, BindingResult bindingResult) {
 		super(bindingResult);
 		this.parameter = parameter;
+		this.executable = null;
+		this.body = ProblemDetail.forStatusAndDetail(getStatusCode(), "Invalid request content.");
+	}
+
+	/**
+	 * Constructor for {@link MethodArgumentNotValidException}.
+	 * @param executable the executable that failed validation
+	 * @param bindingResult the results of the validation
+	 */
+	public MethodArgumentNotValidException(Executable executable, BindingResult bindingResult) {
+		super(bindingResult);
+		this.parameter = null;
+		this.executable = executable;
 		this.body = ProblemDetail.forStatusAndDetail(getStatusCode(), "Invalid request content.");
 	}
 
@@ -83,9 +102,16 @@ public class MethodArgumentNotValidException extends BindException implements Er
 
 	@Override
 	public String getMessage() {
-		StringBuilder sb = new StringBuilder("Validation failed for argument [")
-				.append(this.parameter.getParameterIndex()).append("] in ")
-				.append(this.parameter.getExecutable().toGenericString());
+		StringBuilder sb = new StringBuilder("Validation failed ");
+		if (this.parameter != null) {
+			sb.append("for argument [")
+					.append(this.parameter.getParameterIndex()).append("] in ")
+					.append(this.parameter.getExecutable().toGenericString());
+		}
+		else {
+			sb.append("in ")
+					.append(this.executable.toGenericString());
+		}
 		BindingResult bindingResult = getBindingResult();
 		if (bindingResult.getErrorCount() > 1) {
 			sb.append(" with ").append(bindingResult.getErrorCount()).append(" errors");

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -36,6 +36,7 @@ import org.apache.commons.logging.LogFactory;
 import org.springframework.beans.BeanInstantiationException;
 import org.springframework.beans.BeanUtils;
 import org.springframework.beans.TypeMismatchException;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodParameter;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
@@ -47,6 +48,7 @@ import org.springframework.util.StringUtils;
 import org.springframework.validation.BindException;
 import org.springframework.validation.BindingResult;
 import org.springframework.validation.Errors;
+import org.springframework.validation.ObjectError;
 import org.springframework.validation.SmartValidator;
 import org.springframework.validation.Validator;
 import org.springframework.validation.annotation.ValidationAnnotationUtils;
@@ -329,7 +331,21 @@ public class ModelAttributeMethodProcessor implements HandlerMethodArgumentResol
 			throw new MethodArgumentNotValidException(parameter, result);
 		}
 
-		return BeanUtils.instantiateClass(ctor, args);
+		try {
+			return BeanUtils.instantiateClass(ctor, args);
+		}
+		catch (BeanInstantiationException ex) {
+			Throwable cause = ex.getCause();
+			if (KotlinDetector.isKotlinType(ctor.getDeclaringClass()) && cause instanceof NullPointerException) {
+				BindingResult result = binder.getBindingResult();
+				ObjectError error = new ObjectError(ctor.getName(), cause.getMessage());
+				result.addError(error);
+				throw new MethodArgumentNotValidException(ctor, result);
+			}
+			else {
+				throw ex;
+			}
+		}
 	}
 
 	/**