Type Mapping rules

** Out of date, needs rewriting **

Anything beyond simple mapping to equivalent types requires a way of inferring type information, which includes use of an external schema (like GraphQL), examining the Java classes (the way in which most Java serialization frameworks operate) or providing custom mapping rules. As this is a GraphQL centric service the GraphQL schema system is used to drive all mapping rules, but for this to work smoothly it is important that the Kotlin/Java code, the JSON serializers and the GraphQL schema mappers follow consistent rules.

There are in effect 4 types systems in operation. The rules and reasons are explained below and any common rules are implemented in the ‘mapper’ package.

The types supported by GraphQL

This is a restricted list, although it does allow the addition of custom types. Unfortunately this doesn’t include any support for date / time. This service defines the following additional types by default

• DateTime - A date in (ISO 8601)[https://en.wikipedia.org/wiki/ISO_8601] format (e.g ‘xT07:22Z’) which is also the generally accepted JSON format
• Time - A time in “HH:mm:ss” format, (e.g. ‘13:24:33’)
• Date - Just a date, e.g ‘2012-03-19’
• JavaTimestamp - The Java timestamp format, milliseconds since 1970
• Java8Timestamp - The Java 8 timestamp, made of two longs, allowing just about any date / time to be represented

The types supported by JSON

This is just String,Number,Array,Map,Boolean and null.

There are conversions defined between the GraphQL types and

A natural subset of Java types for use with JSON (JavaJson)

Keeping to basic types such as String, Boolean makes it easy to inter-operate with most Java libraries and frameworks. In this service these are referred to as ‘JavaJson’ types

A full set of Java types

This needs rules to “coerce” into the more limited types above. As the set of Java types is in effect infinite some mechanism to include custom rules is needed