@Incubating
public interface ValueSource<T,​P extends ValueSourceParameters>

Represents an external source of information used by a Gradle build. Examples of external sources include client environment variables, system properties, configuration files, shell commands, network services, among others.

Representing external sources as ValueSources allows Gradle to transparently manage the configuration cache as values obtained from those sources change. For example, a build might run a different set of tasks depending on whether the CI environment variable is set or not.

To integrate a new type of value source, create an abstract subclass of this interface and use ProviderFactory.of(Class, Action) to get a provider to a configured source. The returned Provider can be passed to tasks or queried by build logic during the configuration phase, in which case the source would be automatically considered as an input to the work graph cache.

It is possible to have some Gradle services to be injected into the implementation, similar to tasks and plugins. It can be done by adding a parameter to the constructor and annotating the constructor with the @Inject annotation:

 public class MyValueSource implements ValueSource<...> {
     @Inject
     public MyValueSource(ExecOperations execOperations) {
         ...
     }
 }
 

Currently, only a small subset of services is supported:

• ExecOperations provides means to execute external processes. It is possible to use this service even during the configuration time. However, as the returned value is used to check the configuration cache, the obtain() method will be called during each build. Calling slow commands here will slow things down.

A value source implementation will most likely take parameters. To do this create a subtype of ValueSourceParameters and declare this type as the type parameter to the value source implementation.

A value source implementation doesn't have to be thread-safe, as the single call to obtain() is synchronized.

A value source implementation is exempt from the automatic detection of work graph cache inputs. For example, if the obtain() method calls System.getenv("FOO") then changes to the FOO environment variable only invalidate the cache if the value returned by the obtain() method itself changes. The same applies to reading files or system properties.