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

Syntax

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Symbols

Tapestry IOC makes use of runtime-evaluated symbols to handle certain types of configuration tasks.

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

${some.symbol.name}

that is, a leading ${ before the symbol name and a trailing } after. 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 most of the symbol names used by Tapestry's built-in services.

Using Symbols in your Services

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

For example:

  public static MyService build(
      @InjectService("${some-service-id}") Collaborator collab)
  {
    return . . . ;
  }

...

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:

public class MyService implements MyServiceInterface
{
  private boolean productionMode;

  public MyService(@Value("${tapestry.production-mode}") boolean productionMode, ...)
  {
    this.productionMode = productionMode;
if (productionMode) {
      . . .

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

An alternate annotation, @SymbolAs an alternative, the @Symbol annotation, may be used:

public class MyService implements MyServiceInterface
{
  private boolean productionMode;

  public MyService(@Symbol(SymbolConstants.PRODUCTION_MODE) boolean productionMode, ...)
  {
    if this.productionMode = productionMode;
(productionMode) {
      . . .

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.

Symbols in Component Classes and Templates

It's easy to use a symbol in a component class:

  @Inject
  @Symbol(SymbolConstants.PRODUCTION_MODE)
  private boolean productionMode;
  . . .
  void setupRender() {
    if (productionMode) {
        . . .
    }
  }

You can even use them directly in a component template, using the "symbol" binding prefix:

<t:if test="!symbol:tapestry.production-mode">
  <p>WARNING: We're running in development mode (slower, and less secure)</p>
</t:if>

Symbol Resolution

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

Additional You may employ additional symbol providers may be employed by contributing to the tapestry.ioc. 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:

  public void contributeApplicationDefaults(MappedConfiguration<String, String> configuration)
  {
    configuration.add("some-service-id", "WackyCollaborator");
  }

FactoryDefaults Provider

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

...

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.

  public void contributeFactoryDefaults(MappedConfiguration<String, String> configuration)
  {
      configuration.add("report.url", "http://${report.host}:${report.port}/${report.path}");
      configuration.add("report.host", "www.myreportsite.com");
      configuration.add("report.port", "80");
      configuration.add("report.path", "/report.cgi");
  }

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

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:

  public void contributeApplicationDefaults(MappedConfiguration<String, String> configuration)
  {
      configuration.add("report.path", "${report.url}/report.cgi");
  }

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).

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