spring-data-mongo - optional query parameters?

In addition to Archimedes's answer:
If you need the count of matched documents, replace $where with $expr.

@Query("{ $or : [ { $expr: { $eq: ['?0', 'null'] } } , { field : ?0 } ] }")
Page<Something> findAll(String query, Pageable pageable);

To implement this in Boolean logic I do the following and the conversion to operations that are available in programming languages

:query != null -> field == :query
!(:query != null) || (field == :query)
(:query == null) || (field == :query)

In plain SQL, this is done as

where (null = :query) or (field = :query)

In MongoDB this is done through the $where

{ $where: '?0 == null || this.field == ?0' } 

We can speed this up a little by using Mongo Operations rather than building everything to the function at the expense of some readability. does not work unfortunately.

{ $or : [ { $where: '?0 == null' } , { field : ?0 } ] } 

So what you have is

@Query("{ $or : [ { $where: '?0 == null' } , { field : ?0 } ] }")
List<Something> findAll(String query, Pageable pageable);

This can be further expanded to handle arrays for in/all clauses

@Query("{ $or : [ { $where: '?0.length == 0' } , { field : { $in : ?0 } } ] }")
List<Something> findAll(String query, Pageable pageable);