It is designed to be very simple to use, and to make it very easy for any developer to integrate other components with **FastAPI**.

## What is "Dependency Injection" { #what-is-dependency-injection }

## Code size { #code-size }

This example might seem verbose. Keep in mind that we are mixing security, data models, utility functions and *path operations* in the same file.

But here's the key point.

The security and dependency injection stuff is written once.

* Any keyword arguments that should be passed to the task function (`message="some notification"`).

## Dependency Injection { #dependency-injection }

Using `BackgroundTasks` also works with the dependency injection system, you can declare a parameter of type `BackgroundTasks` at multiple levels: in a *path operation function*, in a dependency (dependable), in a sub-dependency, etc.

It achieves something somewhat similar to what can be done with Flask-apispec.

It has an integrated dependency injection system, inspired by Angular 2. It requires pre-registering the "injectables" (like all the other dependency injection systems I know), so, it adds to the verbosity and code repetition.

### Dependency Injection { #dependency-injection }

FastAPI includes an extremely easy to use, but extremely powerful <abbr title='also known as "components", "resources", "services", "providers"'><strong>Dependency Injection</strong></abbr> system.

* Even dependencies can have dependencies, creating a hierarchy or **"graph" of dependencies**.

 * either express or implied. See the License for the specific language
 * governing permissions and limitations under the License.
 */
package org.codelibs.fess.exception;

/**
 * Exception thrown when a dependency injection container or component is not available.
 * This exception indicates that the required container or a specific component within it cannot be accessed.
 */
public class ContainerNotAvailableException extends FessSystemException {

async def needy_dependency(fresh_value: str = Depends(get_value, use_cache=False)):
    return {"fresh_value": fresh_value}
```

////

## Recap { #recap }

Apart from all the fancy words used here, the **Dependency Injection** system is quite simple.

Just functions that look the same as the *path operation functions*.

But still, it is very powerful, and allows you to declare arbitrarily deeply nested dependency "graphs" (trees).

/// tip

        }

        public WitnessNotification getLastNotification() {
            return lastNotification;
        }
    }

    /**
     * Test witness client that allows injection of mock RPC client
     */
    private static class TestWitnessClient extends WitnessClient {

        public TestWitnessClient(InetAddress witnessServer, CIFSContext context, WitnessRpcClient rpcClient) {

        }
    }

    public void test_searchEngineClient_injection() {
        // Test SearchEngineClient field injection
        ThumbnailGenerator generator = new ThumbnailGenerator();
        assertNull(generator.searchEngineClient);

        // After proper initialization, it should be injected

# Classes as Dependencies { #classes-as-dependencies }

Before diving deeper into the **Dependency Injection** system, let's upgrade the previous example.

## A `dict` from the previous example { #a-dict-from-the-previous-example }

In the previous example, we were returning a `dict` from our dependency ("dependable"):

{* ../../docs_src/dependencies/tutorial001_an_py310.py hl[9] *}