ClassCastException in MappingMongoConverter when having Map<String, Object> as a field type.

[dinerotah]

After upgrading to Spring Boot from 2.4.4 to 2.5.2(spring-data-mongodb 3.1.6 --> 3.2.2) I am having the following exception:

Caused by: java.lang.ClassCastException: java.util.LinkedHashMap incompatible with org.bson.conversions.Bson
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter$ConversionContext.convert(MappingMongoConverter.java:1905)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter$MongoDbPropertyValueProvider.getPropertyValue(MappingMongoConverter.java:1636)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.readProperties(MappingMongoConverter.java:477)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.populateProperties(MappingMongoConverter.java:392)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:371)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.readDocument(MappingMongoConverter.java:341)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:277)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:273)
	at org.springframework.data.mongodb.core.convert.MappingMongoConverter.read(MappingMongoConverter.java:102)
	at org.springframework.data.mongodb.core.MongoTemplate$ReadDocumentCallback.doWith(MongoTemplate.java:3178)
	at org.springframework.data.mongodb.core.MongoTemplate.executeFindMultiInternal(MongoTemplate.java:2813)
	at org.springframework.data.mongodb.core.MongoTemplate.doFind(MongoTemplate.java:2543)
	at org.springframework.data.mongodb.core.MongoTemplate.doFind(MongoTemplate.java:2525)
	at org.springframework.data.mongodb.core.MongoTemplate.find(MongoTemplate.java:847)
	at org.springframework.data.mongodb.repository.support.SimpleMongoRepository.findAll(SimpleMongoRepository.java:428)
	at org.springframework.data.mongodb.repository.support.SimpleMongoRepository.findAllById(SimpleMongoRepository.java:162)

In the domain object there is a the following property:
protected Map<String, Object> _flexFields = new HashMap<>()

When MappingMongoConverter try to convert it in line 1905, the exception is thrown even the map is empty.
The 3.1.x MappingMongoConverter implementation works fine.
Please some fix.

[christophstrobl]

Thanks for reporting can you please add the domain types involved as well as the raw document representation. Ideally you have a complete minimal sample (something that we can unzip or git clone, build, and deploy) that reproduces the problem.
This would help us to better understand the issue.

[dinerotah]

I will try but in 3.1.x there was no cast to Bson and this the problem when using Object as value in a map.

if (typeHint.isMap()) { return (S) mapConverter.convert(this, (Bson) source, typeHint); }

[dinerotah]

minimal_sample.zip
Here is a minimal sample to reproduce the error. It works with 3.1.x but not with 3.2.2
Server port is 9999

In the following link, near the end of the article, you could find Postman collection with all crud ops.

[christophstrobl]

The java driver uses org.bson.Document to represent Map structures. the _flexFields is actually not present in the source when reading from MongoDB. However the FlexDocumentListener puts in a LinkedHashMap via the afterLoad event which then in turn leads to the error you see.

I admit the restriction in place is a kinda hard one and I'll take this to the team to see if we want it to be softened to something like:

if (typeHint.isMap()) {
	if(source instanceof Bson) {
		return (S) mapConverter.convert(this, (Bson) source, typeHint);
	}
	if(source instanceof Map) {
		return (S) mapConverter.convert(this, new Document((Map<String,Object>)source), typeHint);
	}
	throw new IllegalArgumentException(String.format("Expected map like structure but found %s", source.getClass()));
}

For the time being please org.bson.Document when adding map like structures to an existing document.

[mp911de]

Closing as duplicate of #3702