Avoid NPE in ObjectToObjectConverter

Bypass ObjectToObject conversion when source and object types are
identical and protect against a null source object.

Prior to this commit the TypeDescriptor was used to determine if
conversion was necessary.  This caused issues when comparing a
descriptor with annotations to one without.  The updated code
now compares using TypeDescriptor.getType().

The ObjectToObject converter will now no longer attempt to convert
null source objects.

Issue: SPR-9933

spring-core/src/main/java/org/springframework/core/convert/support/ObjectToObjectConverter.java

@@ -48,10 +48,17 @@ final class ObjectToObjectConverter implements ConditionalGenericConverter {
 	}
 
 	public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {
-		return !sourceType.equals(targetType) && hasValueOfMethodOrConstructor(targetType.getType(), sourceType.getType());
+		if (sourceType.getType().equals(targetType.getType())) {
+			// no conversion required
+			return false;
+		}
+		return hasValueOfMethodOrConstructor(targetType.getType(), sourceType.getType());
 	}
 
 	public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
+		if (source == null) {
+			return null;
+		}
 		Class<?> sourceClass = sourceType.getType();
 		Class<?> targetClass = targetType.getType();
 		Method method = getValueOfMethodOn(targetClass, sourceClass);

spring-core/src/test/java/org/springframework/core/convert/support/GenericConversionServiceTests.java

@@ -18,6 +18,8 @@ package org.springframework.core.convert.support;
 
 import java.awt.Color;
 import java.awt.SystemColor;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
@@ -744,6 +746,22 @@ public class GenericConversionServiceTests {
 		assertEquals("1", result);
 	}
 
+	@Test
+	public void convertNullAnnotatedStringToString() throws Exception {
+		DefaultConversionService.addDefaultConverters(conversionService);
+		String source = null;
+		TypeDescriptor sourceType = new TypeDescriptor(getClass().getField("annotatedString"));
+		TypeDescriptor targetType = TypeDescriptor.valueOf(String.class);
+		conversionService.convert(source, sourceType, targetType);
+	}
+
+	@ExampleAnnotation
+	public String annotatedString;
+
+	@Retention(RetentionPolicy.RUNTIME)
+	public static @interface ExampleAnnotation {
+	}
+
 	private static class MyConditionalConverter implements Converter<String, Color>,
 			ConditionalConverter {