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.
Related Articles
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.
Creating a Trivial Component
Creating a page or component in Tapestry 5 is a breeze. There are only a few constraints:
- There must be a public Java class.
- The class must be in the correct package (see below).
- The class must have a public, no-arguments constructor. (The default one provided by the compiler is fine.)
Here's a minimal component that outputs a fixed message, using a template with a matching file name:
package org.example.myapp.components;
public class HelloWorld
{
}
<html>
Bonjour from HelloWorld component.
</html>
In the example above, the HelloWorld class contains no code at all (except what it inherits from the Object class and what Tapestry adds invisibly).
And here's a component that does the same thing, but without needing a template:
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.");
}
}
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.
These methods are not necessarily public; they can have any access level you like (unlike in Tapestry 4). By convention they usually have package-private access level (the default).
Component Packages
Component classes must exist within an appropriate package (this is necessary for runtime code transformation and class reloading to operate).
These packages exist under the application's root package, as follows:
- For pages, place classes in root.pages. Page names are mapped to classes within this package.
- For mixins, place classes in root.mixins. Mixin types are mapped to classes within this package.
- For other components, place classes in root.components. Component types are mapped to classes within this package.
In addition, it is common for an application to have base classes, often abstract base classes, that should not be directly referenced. These should not go in the pages, components or mixins packages, because they then look like valid pages, components or mixins. Instead, use the root.base package to store such base classes.
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 (anonymous or not), which are loaded by the same class loader as the component class loader, but not transformed as components.
Sub-Folders / Sub-Packages
Classes do not have to go directly inside the package (pages, components, mixins, etc.). It is valid to create a sub-package to store some of the classes. The sub-package name becomes part of the page name or component type. Thus you might define a page component com.example.myapp.pages.admin.CreateUser and the logical page name (which often shows up inside URLs) will be admin/CreateUser.
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 (instead of user/EditUser). The goal here is to provide shorter, more natural URLs.
Index Pages
One special simplification 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/.
In previous versions of Tapestry there was also the concept of a start page configured with the tapestry.start-page-name configuration symbol (defaults to "start"). If a page with a name as configured with that symbol exists at the root level, this page is used as the root URL. This has precedence over an existing Index page. If for example you have a page class com.example.myapp.pages.Start it will map to /.
Use of start-pages is discouraged and support for it will eventually be removed. Use an Index page instead.
Pages vs. Components
The distinction between pages and component is very, very small. The 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.
For Tapestry 4 users: there was a much greater distinction in Tapestry 4 between pages and components, which showed up as separate interfaces and a hierarchy of abstract implementations to extend your classes from.
Class Transformation
Tapestry uses your class as a starting point. It transforms your class at runtime. This is necessary for a number of reasons, including to address how Tapestry shares pages between requests.
For the most part, these transformations are both sensible and invisible. In a few limited cases, they comprise a marginally leaky abstraction – for instance, the scope restrictions on instance variables described below – but the programming model in general supports a very high level of developer productivity.
Because transformation doesn't occur until runtime, the build stage of your application is not affected by the fact that you are creating a Tapestry application. Further, your classes are absolutely simple POJOs during unit testing.
Live Class Reloading
Main Article: Class Reloading
Component classes are monitored for changes by the framework. Classes are reloaded when changed. This allows you to build your application with a speed approaching that of a scripting environment, without sacrificing any of the power of the Java platform.
And it's fast! You won't even notice that this magic class reloading has occurred.
The net result: super productivity — change your class, see the change instantly. This is designed to be a blend of the best of scripting environments (such as Python or Ruby) with all the speed and power of Java backing it up.
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.
Instance Variables
Tapestry components may have instance variables (unlike Tapestry 4, where you had to use abstract properties).
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 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
Unless an instance variable is decorated with an annotation, it will be a transient instance variable. This means that its value resets to its default value at the end of reach request (when the page is detached from the request).
About initialization
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.
Use persistent fields to hold client-specific information from one request to the next.
Further, final fields are (in fact) final, and will not be reset between requests.
Constructors
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.
@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 private fields of your component class annotated with @Parameter. 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
Most fields in component classes are automatically cleared at the end of each request. However, fields may be annotated so that they retain their value across requests, using the @Persist annotation.
Embedded Components
Components often contain other components. Components inside another component's template are called embedded components. The containing component's template will contain special elements, in the Tapestry namespace, identifying where the the embedded components go.
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:
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;
}
The above defines a component whose embedded id is "count" (this id is derived from the name of the field and an element with that id must be present in the corresponding template, otherwise an error is displayed (see below)). The type of the component is org.example.app.components.Count. The start and end parameters of the Count component are bound to literal values, and the value parameter of the Count component is bound to the countValue property of the Countdown component.
Technically, the start and end parameters should be bound to properties, just 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.
You may specify additional parameters inside the component template, but parameters in the component class take precedence.
TODO: May want a more complex check; what if user uses prop: in the template and there's a conflict?
You may override the default component id (as derived from the field name) using the id() attribute of the Component annotation.
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">.