# ValueSource

interface ValueSource<T, P : ValueSourceParameters?>(source)

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 of to get a provider to a configured source. The returned org.gradle.api.provider.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
        }
    }

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Currently, only a small subset of services is supported:

- org.gradle.process.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

#### Parameters

<T>

The type of value obtained from this source.

<P>

The source specific parameter type.

#### See also

ProviderFactory
<a href="https://docs.gradle.org/current/userguide/configuration_cache.html">Configuration Cache</a>

## Functions

getParameters
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@Inject
abstract fun getParameters(): P
The object provided by getParameters when creating a provider from the value source.
obtain
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@Nullable
abstract fun obtain(): T
Obtains the value from the source.