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Comment: Added list of available coercions (obtained from a "Could not find a coercion" exception report)

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Type Coercion

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Page: TypeCoercer Service

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 service 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).

List of Coercions

As of Tapestry versions 5.1 and 5.2, the following coercions are available:

Double --> Float
Float --> Double
Long --> Boolean
Long --> Byte
Long --> Double
Long --> Integer
Long --> Short
Number --> Long
Object --> Object[]
Object --> String
Object --> java.util.List
Object[] --> java.util.List
String --> Boolean
String --> Double
String --> Long
String --> java.io.File
String --> java.math.BigDecimal
String --> java.math.BigInteger
String --> java.text.DateFormat
String --> java.util.regex.Pattern
String --> org.apache.tapestry5.Renderable
String --> org.apache.tapestry5.SelectModel
String --> org.apache.tapestry5.corelib.ClientValidation
String --> org.apache.tapestry5.corelib.LoopFormState
String --> org.apache.tapestry5.corelib.SubmitMode
String --> org.apache.tapestry5.corelib.data.BlankOption
String --> org.apache.tapestry5.corelib.data.GridPagerPosition
String --> org.apache.tapestry5.corelib.data.InsertPosition
String --> org.apache.tapestry5.ioc.Resource
String --> org.apache.tapestry5.ioc.util.TimeInterval
boolean[] --> java.util.List
byte[] --> java.util.List
char[] --> java.util.List
double[] --> java.util.List
float[] --> java.util.List
int[] --> java.util.List
java.math.BigDecimal --> Double
java.util.Collection --> Boolean
java.util.Collection --> Object[]
java.util.Collection --> org.apache.tapestry5.grid.GridDataSource
java.util.Date --> java.util.Calendar
java.util.List --> org.apache.tapestry5.SelectModel
java.util.Map --> org.apache.tapestry5.SelectModel
long[] --> java.util.List
null --> Boolean
null --> org.apache.tapestry5.grid.GridDataSource
org.apache.tapestry5.ComponentResources --> org.apache.tapestry5.PropertyOverrides
org.apache.tapestry5.PrimaryKeyEncoder --> org.apache.tapestry5.ValueEncoder
org.apache.tapestry5.Renderable --> org.apache.tapestry5.Block
org.apache.tapestry5.Renderable --> org.apache.tapestry5.runtime.RenderCommand
org.apache.tapestry5.ioc.util.TimeInterval --> Long
org.apache.tapestry5.runtime.ComponentResourcesAware --> org.apache.tapestry5.ComponentResources
short[] --> java.util.List



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;
            }
        }));



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