Component Classes

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 (the case in Tapestry 4).

Classes for pages, for components and for component mixins are all created in an identical way.

Component Class Basics

Creating page and component classes in Tapestry 5 is a breeze.

Unlike Tapestry 4, in Tapestry 5, component classes are not abstract, nor do they extend from framework base classes. They are pure POJOs (Plain Old Java Objects).

There are only a few constraints:

• The classes must be public.
• The classes must be in the correct package, as per the application configuration.
• The class must have a standard public, no arguments constructor.
  Here's a very basic component:

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.");
    }
}

This component's only job is to write out a fixed message. The @BeginRender annotation is a type of component lifecycle annotation, a method annotation that instructs Tapestry when and under what circumstances to invoke methods of your class.

In another departure from Tapestry 4, these methods are not necessarily public; they can have any visibility you like.

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.

For pages, place classes in root.pages. Page names are mapped to classes within this package.

For components, place classes in root.components. Component types are mapped to classes within this package.

For mixins, place classes in root.mixins. Mixin 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.

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 (not user/EditUser). The goal here is to provide shorter, more natural URLs.

Pages vs. Components

The distinction in Tapestry 5 between pages and component is very, very small. The only real difference is the package name: root.pages.PageName for pages, and root.components.ComponentType for components.

In Tapestry 4, there was a much greater distinction between pages and components, which showed up as separate interfaces and a hierarchy of abstract implementations to extend your classes from.

In Tapestry 5, the "page" is still somewhat present, but is really an internal Tapestry class. Page components are simply the root component of a page's component tree.

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 pools pages between requests.

For the most part, these transformations are both sensible and invisible. In a few limited cases, they are maginally leaky – for instance, the requirement that instance variables be private – but we feel that the programming model in general will support very high levels 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

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 occured.

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. Other classes, such as service interfaces and implementations, or 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).

Instance variables must be private. Tapestry must perform runtime class modifications to support instance variables, and may only do so for private variables. You may have non-private variables in your class, but you may then see unexpected behavior in a production application because of how Tapestry pools and reuses pages and components. Tapestry will log an error for each component class that contains fields that are neither static nor private.

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.

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).

If you have a variable that can keep its value between requests and you would like to defeat that reset logic, then you should attach 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 user, a different page instance may be used.

Use persistent fields to hold information from one request to the next.

Further, final fields are (in fact) final, and will not be reset.

Constructors

Tapestry will instantiate your class using the default, no arguments constructor. Other constructors will be ignored.

Injection

Injection of dependencies occurs at the field level, via additional annotations. At runtime, fields that contain injections become read-only.

Parameters

Component parameters are also identified using private fields of your class, with the @Parameter annotation.

Persistent Fields

Fields may be annotated so that they retain their value across requests.

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 the 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 convienience 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 precendence.

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">.