*
 * @since 4.0.0
 */
@Experimental
@Immutable
public interface Dependency extends Artifact {
    /**
     * {@return the type of the dependency}.
     * A dependency can be a <abbr>JAR</abbr> file,
     * a modular-<abbr>JAR</abbr> if it is intended to be placed on the module path,
     * a <abbr>JAR</abbr> containing test classes, <i>etc.</i>

**FastAPI** will make sure that the "exit code" in each dependency with `yield` is run in the correct order.

For example, `dependency_c` can have a dependency on `dependency_b`, and `dependency_b` on `dependency_a`:

{* ../../docs_src/dependencies/tutorial008_an_py39.py hl[6,14,22] *}

And all of them can use `yield`.

In this case `dependency_c`, to execute its exit code, needs the value from `dependency_b` (here named `dep_b`) to still be available.

     */
    String TEST_JAVA_SOURCE = "test-java-source";

    /**
     * Returns the dependency type id.
     * The id uniquely identifies this <i>dependency type</i>.
     *
     * @return the id of this type, never {@code null}.
     */
    @Nonnull
    @Override
    String id();

    /**
     * Returns the dependency type language.
     *
     * @return the language of this type, never {@code null}.

		</dependency>
		<dependency>
			<groupId>org.apache.logging.log4j</groupId>
			<artifactId>log4j-api</artifactId>
			<version>${log4j.version}</version>
		</dependency>
		<dependency>
			<groupId>org.apache.logging.log4j</groupId>
			<artifactId>log4j-core</artifactId>
			<version>${log4j.version}</version>
		</dependency>
		<dependency>
			<groupId>org.apache.logging.log4j</groupId>

We can do better...

## What makes a dependency { #what-makes-a-dependency }

Up to now you have seen dependencies declared as functions.

But that's not the only way to declare dependencies (although it would probably be the more common).

The key factor is that a dependency should be a "callable".

A "**callable**" in Python is anything that Python can "call" like a function.

    </dependency>
    <dependency>
      <groupId>org.jline</groupId>
      <artifactId>jline-terminal-jni</artifactId>
    </dependency>
    <dependency>
      <groupId>org.jline</groupId>
      <artifactId>jline-terminal-ffm</artifactId>
    </dependency>

    <!-- (legacy) DI annotations -->
    <dependency>
      <groupId>javax.inject</groupId>
      <artifactId>javax.inject</artifactId>
    </dependency>

import org.apache.maven.api.annotations.Nonnull;

/**
 * Indicates when the dependency will be used.
 * For example, it may be at compile time only, at runtime, or at test time.
 * For a given dependency, the scope is directly derived from the
 * {@link org.apache.maven.api.model.Dependency#getScope()} and will be used when using {@link PathScope}
 * and the {@link org.apache.maven.api.services.DependencyResolver}.

///

## Dependencies errors and return values { #dependencies-errors-and-return-values }

You can use the same dependency *functions* you use normally.

### Dependency requirements { #dependency-requirements }

They can declare request requirements (like headers) or other sub-dependencies:

{* ../../docs_src/dependencies/tutorial006_an_py39.py hl[8,13] *}

    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-api-settings</artifactId>
    </dependency>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-api-toolchain</artifactId>
    </dependency>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-api-plugin</artifactId>
    </dependency>
    <dependency>

This is quite simple (not very useful), but will help us focus on how the sub-dependencies work.

## Second dependency, "dependable" and "dependant" { #second-dependency-dependable-and-dependant }

Then you can create another dependency function (a "dependable") that at the same time declares a dependency of its own (so it is a "dependant" too):

{* ../../docs_src/dependencies/tutorial005_an_py310.py hl[13] *}

Let's focus on the parameters declared: