Symbols

Symbols

Symbols are named configuration settings for Tapestry IOC-based services. Tapestry provides mechanisms for easy access to symbols from within such services.

Syntax

The syntax of symbols is based on Ant expressions. That is, the name is surrounded by ${ and } characters:

The value on the inside is the symbol name. By convention, the symbol name is segmented with periods (for example, "tapestry.production-mode").

Built-in Symbols

The Configuration page lists the symbol names used by Tapestry's built-in services.

Using Symbols in your Services

Symbols are used inside the @Value and @InjectService annotations.

For example:

Here, the symbol name, some-service-id is a service id, such as WackyCollaborator.

Although not shown here, it is possible to use multiple symbols inside the string, or mix literal text with symbols.

Injecting Values from Symbols

You may also inject symbol values. For example, if you are interested in whether the application is in production mode or developer mode:

Here Tapestry has coerced the "tapestry.production-mode" symbol to a boolean to be injected.

As an alternative, the @Symbol annotation, may be used:

This is very useful when a constant value is defined for the symbol; it means that the compiler can catch a typo, rather than detecting it a runtime.

Note: When injecting a symbol as a string into a service, you must use the @Inject annotation as well as @Value or @Symbol; otherwise Tapestry will inject the service's service id.

Symbol Resolution

Symbols are resolved by the SymbolSource service. The SymbolSource service checks against an ordered list of SymbolProvider objects.

You may employ additional symbol providers by contributing to the SymbolSource service configuration, which is an ordered list of SymbolProviders.

By default, there are three providers:

SystemProperties Provider

The first provider allows JVM System Properties to provide symbol values. This allows the use of the java command's -D option to provide runtime overrides. This is most often used when testing code, rather than in production. SystemProperties is always checked first.

ApplicationDefaults Provider

Values not found as System Properties are searched for in the ApplicationDefaults. This service, ApplicationDefaults, may be configured using a mapped configuration to provide values.

From the previous example:

FactoryDefaults Provider

This is the same as ApplicationDefaults, but checked only if a value is not satisfied by SystemProperties or ApplicationDefaults.

Libraries will typically set reasonable defaults as contributions to the FactoryDefaults service configuration. Individual applications may hard code overrides of those defaults using ApplicationDefaults. Individual developers may override even those defaults by setting JVM System Properties.

FactoryDefaults is always checked last when resolving symbol names to symbol values.

Recursive Symbols

It is possible and valid to define one symbol in terms of one or more other symbols.

The ordinary default for report.url will be http://www.myreportsite.com:80/report.cgi.

However, this can be changed by making an overriding contribution to the ApplicationDefaults service configuration.

Tapestry checks that no symbol is directly or indirectly dependent on itself. For example, the following contribution is illegal:

When the report.url is referenced, an exception will be thrown with the message: Symbol 'report.path' is defined in terms of itself (report.path --> report.url --> report.path).