You can execute decision services using the Java API using the ExecuteDecisionBuilder. This builder can be obtained via the DmnDecisionService.
You can make use of the executeWithSingleResult() or the executeWithAuditTrail() if you want to execute either a DecisionService or a Decision(Table) with a set key.
Or use explicit methods like executeDecisionService() or executeDecision() to execute either a DecisionService or a DecisionTable.
Currently the REST API exposes only executing decision tables. We will add support for Decision Services in the next release.