THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
Component Classes
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A Component class is A component class is the class associated with a page, component or mixin in your Tapestry web application. Classes for pages, components and mixins are all created in an identical way. They are pure POJOs (Plain Old Java Objects), typically with annotations and conventionally named methods. They are not abstract, nor do they need to extend base classes or implement interfaces.
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In most cases, each component class will have a corresponding component template. However, it is also possible for a component class to emit all of its markup itself, without using a template.
For Tapestry 4 Users: Component classes in Tapestry 5 are much easier than in Tapestry 4. There are no base classes to extend from, the classes are concrete (not abstract), and there's no XML file. There is still a bit of configuration in the form of Java annotations, but those now go directly onto fields of your class, rather than on abstract getters and setters.
In most cases, each component class will have a corresponding component template. However, it is also possible for a component class to emit all of its markup itself, without using a template.
Creating a Trivial Component
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Here's a minimal component that outputs a fixed message, using a template with a matching file name:
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And here's a component that does the same thing, but without needing a template:
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package org.example.myapp.components;
import org.apache.tapestry5.MarkupWriter;
import org.apache.tapestry5.annotations.BeginRender;
public class HelloWorld
{
@BeginRender
void renderMessage(MarkupWriter writer)
{
writer.write("Bonjour from HelloWorld component.");
}
}
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In this example, just like the first one, the component's only job is to write out a fixed message. The @BeginRender annotation is a type of render phase annotation
, a method annotation that instructs Tapestry when and under what circumstances to invoke methods of your class.
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Only component classes should go in any of these controlled packages; classes representing data, or interfaces, or anything that isn't precisely a component class, must go elsewhere. Any top-level class in any of the controlled packages will be transformed at runtime. The only exception is inner classes |
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Tapestry performs some simple optimizations of the logical page name (or component type, or mixin type). It checks to see if the package name is either a prefix or a suffix of the unqualified class name (case insensitively, of course) and removes the prefix or suffix if so. The net result is that a class name such as com.example.myapp.pages.user.EditUser will have a page name of user/Edit (not instead of user/EditUser). The goal here is to provide shorter, more natural URLs.
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One special simplification are exists for Index pages: if the logical page name is Index after removing the package name from the unqualified class name, it will map to the root of that folder. A class such as com.example.myapp.pages.user.IndexUser or com.example.myapp.pages.user.UserIndex will have a page name of user/.
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The distinction between pages and component is very, very small. The only real primary difference is the package name: root.pages.PageName for pages, and root.components.ComponentType for components. Conceptually, page components are simply the root component of a page's component tree.
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For the most part, these transformations are both sensible and invisible. In a few limited cases, they are marginally leaky – comprise a marginally leaky abstraction – for instance, the scope restrictions on instance variables described below – but we feel that the programming model in general will support supports a very high levels level of developer productivity.
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However, class reloading only applies to component classes (pages, components and mixins) and, starting in 5.2, Tapestry IOC-based service implementations (with some restrictions). Other classes, such as service interfaces, entity/model classes, and other data objects, are loaded by the normal class loader and not subject to live class reloading.
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Tapestry components may have instance variables (unlike Tapestry 4, where you had to use abstract properties).
Instance variables must be private.. These scope restrictions are necessary in order for Tapestry to perform runtime class modifications to support instance variables. You may have instance variables with other scopes in your class, but you may then see unexpected behavior in a production application because of how Tapestry shares and reuses pages and components. Tapestry will log an error for each component class that contains instance variables that violate these scope restrictions.
Be aware that you will need to provide getter and setter methods to access your classes' instance variables. Tapestry does not do this automatically unless you provide the @Property annotation on the field.
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Since release 5.3.2, instance variables may be protected, or package private (that is, no access modifier). Under specific circumstances they may even be public (public fields must either be final, or have the @Retain
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annotation).
Be aware that you will need to either provide getter and setter methods to access your classes' instance variables, or else annotate the fields with @Property.
Transient Instance Variables
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Never initialize an instance field to a mutable object at the point of declaration. If this is done, the instance created from that initializer becomes the default value for that field and is reused inside the component on every request. This could cause state to inadvertently be shared between different sessions in an application. |
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| For Tapestry 5.1 and earlier, in the rare event that you have a variable that can keep its value between requests and you would like to defeat that reset logic, then you can add a @a @Retain annotation to the field. You should take care that no client-specific data is stored into such a field, since on a later request the same page instance may be used for a different user. Likewise, on a later request for the same client, a different page instance may be used. |
Use persistent fields to hold client-specific information from one request to the next.
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Tapestry will instantiate your class using the default, no arguments constructor. Other constructors will be ignored.
Injection
Main Article: Injection
Injection of dependencies occurs at the field level, via additional annotations. At runtime, fields that contain injections become read-only.
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@Inject // inject a resource private ComponentResources componentResources; @Inject // inject a block private Block foo; @Inject // inject an asset @Path("context:images/top_banner.png") private Asset banner; @Inject // inject a service private AjaxResponseRenderer ajaxResponseRenderer; |
Parameters
Main Article: Component Parameters
Component parameters are also identified using private fields of your component class , annotated with the @Parameter annotation. Component parameters represent a two-way binding of a field of your component and a property or resource of its containing component or page.
Persistent Fields
Main Article: Persistent Page Data
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You can define the type of component inside template, or you can create an instance variable for the component and use the @Component annotation to define the component type and parameters.
Example:
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package org.example.app.pages;
import org.apache.tapestry5.annotations.Component;
import org.apache.tapestry5.annotations.Property;
import org.example.app.components.Count;
public class Countdown
{
@Component(parameters =
{ "start=5", "end=1", "value=countValue" })
private Count count;
@Property
private int countValue;
}
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Technically, the start and end parameters should be bound to properties, just the like the value parameter. However, certain literal values, such as the numeric literals in the example, are accepted by the prop: binding prefix even though they are not actually properties (this is largely as a convenience to the application developer). We could also use the literal: prefix, "start=literal:5", which accomplishes largely the same thing.
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If you define a component in the component class, and there is no corresponding element in the template, Tapestry will log an error. In the example above that would be the case if the template for the Countdown page didn't contain an element with <t:count t:id="count">.
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