Symbols

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

The syntax of symbols is based on Ant expression, 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.

These symbols are used inside the Value and InjectService annotations.

For example:

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

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

Although not shown here, it is possible to use multple 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;
    . . .

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

An alternate annotation, @Symbol, may be used:

public class MyService implements MyServiceInterface
{
  private boolean productionMode;

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

Symbol Resolution

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

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

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

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

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 ore 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.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:

  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).*