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.

See Type Coercer Service for the list of build-in coercions.

Parameter Type Coercions

Tapestry automatically handles type coercions for component parameters.

Type coercions occur when a value passed into a parameter (as bound in a template or in an annotation) does not match the type of the parameter.

public class Count
    private int start = 1;

    @Parameter(required = true)
    private int end;

    private int value;
    . . .


Here, the type of all three parameters is int.

However, it is likely that the component will be used as so:

  Merry Christmas: <t:count end="3"> Ho! </t:count>

A bare whole number is interpreted by the prop binding prefix as a long. So this is the long value 3.

Tapestry will automatically coerce the bound value, a long, to the parameter's type, int. This may be a lossy coercion (if the long represents a number larger than can be stored in an int).

TypeCoercer Service

Main Article: Type Coercion

The TypeCoercer service is responsible for this type coercion. This service is part of the tapestry-ioc module. The service is quite extensible, allowing for new types and coercions to be added easily. The TapestryModule contributes a few additional coercions into the TypeCoercer service.