[CONF] Apache Tapestry > IoC - coerce

[confluence]

IoC - coerce Page edited by Bob Harner Comment: Fixed broken image, spelling, formatting, reorganized lead paragraph Changes (15) ... {float} 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. *Type Coercion* is the conversion of one type of object to a new object of a different type with similar content. Tapestry frequently must coerce objects from one type to another. A common example is the coercion of a string into an integer or a double. Although these types of coercions happens more inside tapestry-core (inlcuding (including coercions of [component parameters|IoC - coerce]), parameters|Component Parameters]), 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|http://tapestry.apache.org/current/apidocs/org/apache/tapestry5/ioc/services/TypeCoercer.html] service. Its configuration consists of a number of [CoercionTuple|http://tapestry.apache.org/current/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: !type-coercer.png! !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). h2. Default Type Coercions There are 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: ... The only built-in null coercion is from null to boolean (which is always false). h2. Contributing nNew 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. {code:java} ... }{code} 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. 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: When creating a coercion from {{null}}, use {{Void.class}} as the source type. For example, the built-in coercion from {{null}} to {{Boolean}} is implemented as: {code:java} ... return false; } }));{code} {code} Full Content Type Coercion Related Articles Parameter Type Coercion Type Coercion is the conversion of one type of object to a new object of a different type with similar content. Tapestry frequently must coerce objects from one type to another. A common example is the coercion of a string into an integer or a double. Although these types of coercions happens more inside tapestry-core (including coercions of component parameters), 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 service. Its configuration consists of a number of CoercionTuples. 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: Default Type Coercions There are 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 built-in 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 | View Changes