Package org.gradle.api.provider

Interface ValueSource<T,P extends ValueSourceParameters>

Type Parameters:

T - The type of value obtained from this source.

P - The source specific parameter type.

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 the latter 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 abstract class MyValueSource implements ValueSource<String, ValueSourceParameters.None> {
     private final ExecOperations execOperations;

     @Inject
     public MyValueSource(ExecOperations execOperations) {
         this.execOperations = execOperations;
     }

     @Override
     @Nullable
     public String obtain() {
         // your custom implementation
     }
 }
 

Currently, only a small subset of services is supported:

• ExecOperations provides means to execute external processes. It is possible to use this service even at 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. Starting an external process with a standard API (for example, java.lang.ProcessBuilder) is also allowed.

Implementations of ValueSource are subject to the following constraint:

• Do not implement getParameters() in your class, the method will be implemented by Gradle.

Since:

6.1

See Also:

• ProviderFactory.environmentVariable(String)
• ProviderFactory.systemProperty(String)
• ProviderFactory.fileContents(RegularFile)
• ProviderFactory.of(Class, Action)
• Configuration Cache

Method Summary

Modifier and Type	Method	Description
P	getParameters()	The object provided by ValueSourceSpec.getParameters() when creating a provider from the value source.
T	obtain()	Obtains the value from the source.

Method Details

getParameters

@Inject
P getParameters()

The object provided by ValueSourceSpec.getParameters() when creating a provider from the value source.

Do not implement this method in your subclass. Gradle provides the implementation when creating a provider from the value source via ProviderFactory.of(Class, Action).

obtain

@Nullable
T obtain()

Obtains the value from the source. The returned value must be effectively immutable. The implementation is exempt from the automatic detection of work graph cache inputs.

This method must be implemented in the subclass.

This method is only called if the provider value is requested and only once in that case.

Returns:

the value obtained or null if the value is not present.