* Compares strings in natural order except that null comes immediately before a given value. This
   * works better than Ordering.natural().nullsFirst() because, if null comes before all other
   * values, it lies outside the submap/submultiset ranges we test, and the variety of tests that
   * exercise null handling fail on those subcollections.
   */
  public abstract static class NullsBefore implements Comparator<@Nullable String>, Serializable {

And then, open the docs for the sub-application, at <a href="http://127.0.0.1:8000/subapi/docs" class="external-link" target="_blank">http://127.0.0.1:8000/subapi/docs</a>.

You will see the automatic API docs for the sub-application, including only its own _path operations_, all under the correct sub-path prefix `/subapi`:

<img src="/img/tutorial/sub-applications/image02.png">

<img src="/img/tutorial/sub-applications/image01.png">

&amp; ⤴️, 📂 🩺 🎧-🈸, <a href="http://127.0.0.1:8000/subapi/docs" class="external-link" target="_blank">http://127.0.0.1:8000/subapi/docs</a>.

👆 🔜 👀 🏧 🛠️ 🩺 🎧-🈸, ✅ 🕴 🚮 👍 _➡ 🛠️_, 🌐 🔽 ☑ 🎧-➡ 🔡 `/subapi`:

<img src="/img/tutorial/sub-applications/image02.png">

🚥 👆 🔄 🔗 ⏮️ 🙆 2️⃣ 👩‍💻 🔢, 👫 🔜 👷 ☑, ↩️ 🖥 🔜 💪 💬 🔠 🎯 📱 ⚖️ 🎧-📱.

E então, abra a documentação para a sub-aplicação, em <a href="http://127.0.0.1:8000/subapi/docs" class="external-link" target="_blank">http://127.0.0.1:8000/subapi/docs</a>.

Você verá a documentação automática da API para a sub-aplicação, incluindo apenas suas próprias _operações de rota_, todas sob o prefixo de sub-caminho correto `/subapi`:

<img src="/img/tutorial/sub-applications/image02.png">

    assert response.json() == {"message": "Hello World from main app"}


def test_openapi_schema_sub():
    response = client.get("/subapi/openapi.json")
    assert response.status_code == 200, response.text
    assert response.json() == openapi_schema_sub


def test_sub():
    response = client.get("/subapi/sub")
    assert response.status_code == 200, response.text

<img src="/img/tutorial/sub-applications/image01.png">

و سپس اسناد زیر برنامه را در آدرس <a href="http://127.0.0.1:8000/subapi/docs" class="external-link" target="_blank">http://127.0.0.1:8000/subapi/docs</a>. باز کنید.

اسناد API خودکار برای زیر برنامه را خواهید دید، که فقط شامل path operations خود می شود، همه در زیر مسیر `/subapi` قرار دارند:

<img src="/img/tutorial/sub-applications/image02.png">

Öffnen Sie dann die Dokumentation für die Unteranwendung unter <a href="http://127.0.0.1:8000/subapi/docs" class="external-link" target="_blank">http://127.0.0.1:8000/subapi/docs</a>.

Sie sehen die automatische API-Dokumentation für die Unteranwendung, welche nur deren eigene _Pfadoperationen_ anzeigt, alle unter dem korrekten Unterpfad-Präfix `/subapi`:

<img src="/img/tutorial/sub-applications/image02.png">

from fastapi import FastAPI

app = FastAPI()


@app.get("/app")
def read_main():
    return {"message": "Hello World from main app"}


subapi = FastAPI()


@subapi.get("/sub")
def read_sub():
    return {"message": "Hello World from sub API"}

下图显示的是主应用 API 文档，只包括其自有的*路径操作*。

<img src="/img/tutorial/sub-applications/image01.png">

然后查看子应用文档 <a href="http://127.0.0.1:8000/subapi/docs" class="external-link" target="_blank">http://127.0.0.1:8000/subapi/docs。</a>

下图显示的是子应用的 API 文档，也是只包括其自有的*路径操作*，所有这些路径操作都在 `/subapi` 子路径前缀下。

<img src="/img/tutorial/sub-applications/image02.png">

两个用户界面都可以正常运行，因为浏览器能够与每个指定的应用或子应用会话。

### 技术细节：`root_path`

    padding:0;
    width:100%;
    clear:right;
    height:2.8em;
    padding-top:10px;
    overflow:hidden;
}
.subNav {
    background-color:#dee3e9;
    border-bottom:1px solid #9eadc0;
    float:left;
    width:100%;
    overflow:hidden;
}
.subNav div {
    clear:left;
    float:left;
    padding:0 0 5px 6px;
}
ul.navList, ul.subNavList {
    float:left;