Improve "reason" message in BindFailureAnalyzer

Update `BindFailureAnalyzer` so that the "Reason" message includes
the root cause exception type and message.

Closes gh-27028

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/diagnostics/analyzer/BindFailureAnalyzer.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -38,6 +38,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Andy Wilkinson
  * @author Madhura Bhave
+ * @author Phillip Webb
  */
 class BindFailureAnalyzer extends AbstractFailureAnalyzer<BindException> {
 
@@ -68,16 +69,33 @@ class BindFailureAnalyzer extends AbstractFailureAnalyzer<BindException> {
 	}
 
 	private String getMessage(BindException cause) {
+		Throwable rootCause = getRootCause(cause.getCause());
 		ConversionFailedException conversionFailure = findCause(cause, ConversionFailedException.class);
 		if (conversionFailure != null) {
-			return "failed to convert " + conversionFailure.getSourceType() + " to "
+			String message = "failed to convert " + conversionFailure.getSourceType() + " to "
 					+ conversionFailure.getTargetType();
+			if (rootCause != null) {
+				message += " (caused by " + getExceptionTypeAndMessage(rootCause) + ")";
+			}
+			return message;
+		}
+		if (rootCause != null && StringUtils.hasText(rootCause.getMessage())) {
+			return getExceptionTypeAndMessage(rootCause);
 		}
-		Throwable failure = cause;
-		while (failure.getCause() != null) {
-			failure = failure.getCause();
+		return getExceptionTypeAndMessage(cause);
+	}
+
+	private Throwable getRootCause(Throwable cause) {
+		Throwable rootCause = cause;
+		while (rootCause != null && rootCause.getCause() != null) {
+			rootCause = rootCause.getCause();
 		}
-		return (StringUtils.hasText(failure.getMessage()) ? failure.getMessage() : cause.getMessage());
+		return rootCause;
+	}
+
+	private String getExceptionTypeAndMessage(Throwable ex) {
+		String message = ex.getMessage();
+		return ex.getClass().getName() + (StringUtils.hasText(message) ? ": " + message : "");
 	}
 
 	private FailureAnalysis getFailureAnalysis(Object description, BindException cause) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/diagnostics/analyzer/BindFailureAnalyzerTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -41,6 +41,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Andy Wilkinson
  * @author Madhura Bhave
+ * @author Phillip Webb
  */
 class BindFailureAnalyzerTests {
 
@@ -76,7 +77,16 @@ class BindFailureAnalyzerTests {
 	void bindExceptionWithNestedFailureShouldDisplayNestedMessage() {
 		FailureAnalysis analysis = performAnalysis(NestedFailureConfiguration.class, "test.foo.value=hello");
 		assertThat(analysis.getDescription()).contains(failure("test.foo.value", "hello",
-				"\"test.foo.value\" from property source \"test\"", "This is a failure"));
+				"\"test.foo.value\" from property source \"test\"", "java.lang.RuntimeException: This is a failure"));
+	}
+
+	@Test // gh-27028
+	void bindExceptionDueToClassNotFoundConvertionFailure() {
+		FailureAnalysis analysis = performAnalysis(GenericFailureConfiguration.class,
+				"test.foo.type=com.example.Missing");
+		assertThat(analysis.getDescription()).contains(failure("test.foo.type", "com.example.Missing",
+				"\"test.foo.type\" from property source \"test\"",
+				"failed to convert java.lang.String to java.lang.Class<?> (caused by java.lang.ClassNotFoundException: com.example.Missing"));
 	}
 
 	private static String failure(String property, String value, String origin, String reason) {
@@ -178,6 +188,8 @@ class BindFailureAnalyzerTests {
 
 		private int value;
 
+		private Class<?> type;
+
 		int getValue() {
 			return this.value;
 		}
@@ -186,6 +198,14 @@ class BindFailureAnalyzerTests {
 			this.value = value;
 		}
 
+		Class<?> getType() {
+			return this.type;
+		}
+
+		void setType(Class<?> type) {
+			this.type = type;
+		}
+
 	}
 
 	@ConfigurationProperties("test.foo")