### Dependency Injection { #dependency-injection }

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

* Even dependencies can have dependencies, creating a hierarchy or **"graph" of dependencies**.
* All **automatically handled** by the framework.

    /**
     * @throws Exception
     */
    @Test
    public void test() throws Exception {
        final ConverterRuntimeException e = new ConverterRuntimeException("hoge", "xxx", new RuntimeException("cause"));
        System.out.println(e.getMessage());
        assertThat(e.getPropertyName(), is("hoge"));
        assertThat(e.getValue(), is((Object) "xxx"));
        assertThat(e.getCause().getMessage(), is("cause"));
    }

  Documentation
  -------------

  The most up-to-date documentation can be found at https://maven.apache.org/.

  Release Notes
  -------------

  The full list of changes, system requirements and related can be found at https://maven.apache.org/docs/history.html.

  Installing Maven
  ----------------

  For complete documentation see https://maven.apache.org/download.html#Installation

            }
            item = newItem;
        }

        /**
         * Writes the old item to the new file.
         *
         * @param oldItem The old item.
         * @return The new item if it was updated, otherwise the old item.
         */
        public KuromojiItem write(final KuromojiItem oldItem) {
            try {

Supongamos que solo tienes un único parámetro de cuerpo `item` de un modelo Pydantic `Item`.

Por defecto, **FastAPI** esperará su cuerpo directamente.

Pero si deseas que espere un JSON con una clave `item` y dentro de ella los contenidos del modelo, como lo hace cuando declaras parámetros de cuerpo extra, puedes usar el parámetro especial `Body` `embed`:

```Python
item: Item = Body(embed=True)
```

como en:

<div class="announce-wrapper">
  <div id="announce-left">
    <div class="item">
      <a class="announce-link" href="https://fastapicloud.com" target="_blank">
        <span class="twemoji">
          {% include ".icons/material/cloud-arrow-up.svg" %}
        </span> Join the <strong>FastAPI Cloud</strong> waiting list 🚀
      </a>
    </div>
    <div class="item">
      <a class="announce-link" href="https://x.com/fastapi" target="_blank">

async def sse_items() -> AsyncIterable[Item]:
    for item in items:
        yield item


@app.get("/items/stream-no-async", response_class=EventSourceResponse)
def sse_items_no_async() -> Iterable[Item]:
    for item in items:
        yield item


@app.get("/items/stream-no-annotation", response_class=EventSourceResponse)
async def sse_items_no_annotation():
    for item in items:
        yield item

        final TimestampConverter converter = new TimestampConverter("yyyy/MM/dd HH:mm:ss");
        final java.sql.Timestamp result = (java.sql.Timestamp) converter.getAsObject("2008/12/31 12:34:56");
        System.out.println(result);
        assertThat(converter.getAsString(result), is("2008/12/31 12:34:56"));
    }

    /**
     * @throws Exception
     */
    @Test
    public void testIsTarget() throws Exception {

        assertNotNull(builder);

        DefaultSettingsBuildingRequest request = new DefaultSettingsBuildingRequest();
        request.setSystemProperties(System.getProperties());
        request.setUserSettingsFile(getSettings("simple"));

        SettingsBuildingResult result = builder.build(request);
        assertNotNull(result);

sequenceDiagram
    participant App
    participant Client

    App->>App: Produce Item 1
    App->>Client: Send Item 1
    App->>App: Produce Item 2
    Client->>Client: Process Item 1
    App->>Client: Send Item 2
    App->>App: Produce Item 3
    Client->>Client: Process Item 2
    App->>Client: Send Item 3
    Client->>Client: Process Item 3
    Note over App: Keeps producing...
    Note over Client: Keeps consuming...
```