[CONF] Apache Tapestry > IoC - coerce

[confluence]

IoC - coerce Page added by Ulrich Stärk Type Coercion Tapestry frequently must coerce objects from one type to another. By coercion, we mean to convert an object of some type into a new object of a different type with similar content: a common example is coercing a string into an integer or a double. Although these types of coercions happens more inside tapestry-core (inlcuding coercions of [component parameters|../tapestry-core/guide/coercion.html]), this may also happen inside tapestry-ioc, such as when injecting a value, rather than a service, into a builder method. Like everything else in Tapestry, type coercions are extensible. At the root is the [TypeCoercer|../apidocs/org/apache/tapestry5/ioc/services/TypeCoercer.html] service. Its configuration consists of a number of [CoercionTuple|../apidocs/org/apache/tapestry5/ioc/services/CoercionTuple.html]s. Each tuple defines how to coerce from one type to another. The initial set of coercions is focused primarily on coercions between different numeric types: !images/type-coercer.png!Default Type Coercions There's a few special coercions related to null there; Object --> List wraps a lone object as a singleton list, we then need null --> List to ensure that null stays null (rather than a singleton list whose lone element is a null). Tapestry can interpolate necessary coercions. For example, say it is necessary to coerce a StringBuffer to an Integer; the TypeCoercer will chain together a series of coercions: Object --> String String --> Long Long --> Integer Coercing from null Coercing from null is special; it is not a spanning search as with the other types. Either there is a specific coercion from null to the desired type, or no coercion takes places (and the coerced value is null). The only built-in null coercion is from null to boolean (which is always false). Contributing new Coercions TypeCoercer is extensible, you may add new coercions as desired. For example, let's say you have a Money type that represents an amount of some currency, and you want to be able to convert from BigDecimal to Money. Further, let's assume that Money has a constructor that accepts a BigDecimal as its parameter. We'll use a little Tapestry IOC configuration jujitsu to inform the TypeCoercer about this coercion. public static void contributeTypeCoercer(Configuration<CoercionTuple> configuration) { Coercion<BigDecimal, Money> coercion = new Coercion<BigDecimal, Money>() { public Money coerce(BigDecimal input) { return new Money(input); } }; configuration.add(new CoercionTuple<BigDecimal, Money>(BigDecimal.class, Money.class, coercion)); } Further, since TypeCoercer knows how to convert Double to BigDecimal, or even Integer (to Long to Double) to BigDecimal, all of those coercions would work as well. When creating a coercion from null, use Void.class as the source type. For example, the builtin coercion from null to Boolean is implemented as: configuration.add(new CoercionTuple(void.class, Boolean.class, new Coercion<Void, Boolean>() { public Boolean coerce(Void input) { return false; } })); Change Notification Preferences View Online