2017-02-20 18:48:46.556  WARN 1608 --- [nio-8081-exec-6] .w.s.m.s.DefaultHandlerExceptionResolver : Failed to read HTTP message: org.springframework.http.converter.HttpMessageNotReadableException: Could not read document: Can not construct instance of de.thkoeln.archilab.quizeditor.entities.Question: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: java.io.PushbackInputStream@1c6c14c; line: 1, column: 346] (through reference chain: de.thkoeln.archilab.quizeditor.entities.Quiz["questions"]->java.util.HashSet[0]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of de.thkoeln.archilab.quizeditor.entities.Question: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: java.io.PushbackInputStream@1c6c14c; line: 1, column: 346] (through reference chain: de.thkoeln.archilab.quizeditor.entities.Quiz["questions"]->java.util.HashSet[0])
2017-02-20 18:48:46.557  WARN 1608 --- [nio-8081-exec-6] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved exception caused by Handler execution: org.springframework.http.converter.HttpMessageNotReadableException: Could not read document: Can not construct instance of de.thkoeln.archilab.quizeditor.entities.Question: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: java.io.PushbackInputStream@1c6c14c; line: 1, column: 346] (through reference chain: de.thkoeln.archilab.quizeditor.entities.Quiz["questions"]->java.util.HashSet[0]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of de.thkoeln.archilab.quizeditor.entities.Question: abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: java.io.PushbackInputStream@1c6c14c; line: 1, column: 346] (through reference chain: de.thkoeln.archilab.quizeditor.entities.Quiz["questions"]->java.util.HashSet[0])