You could use Spring Data to do that, just use the BasicQuery class instead of Query class. Your code will look like the following:
/* Any arbitrary string that could to parsed to DBObject */
Query q = new BasicQuery("{ filter : true }");
List<Entity> entities = this.template.find(q, Entity.class);
If you want more details:
http://static.springsource.org/spring-data/data-mongo/docs/current/reference/html/#mongo.query