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

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]

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