# Bigger Applications - Multiple Files { #bigger-applications-multiple-files }

If you are building an application or a web API, it's rarely the case that you can put everything in a single file.

**FastAPI** provides a convenience tool to structure your application while keeping all the flexibility.

/// info

If you come from Flask, this would be the equivalent of Flask's Blueprints.

///

## An example file structure { #an-example-file-structure }

You can mount WSGI applications as you saw with [Sub Applications - Mounts](sub-applications.md), [Behind a Proxy](behind-a-proxy.md).

For that, you can use the `WSGIMiddleware` and use it to wrap your WSGI application, for example, Flask, Django, etc.

## Using `WSGIMiddleware` { #using-wsgimiddleware }

/// info

This requires installing `a2wsgi` for example with `pip install a2wsgi`.

///

You can use any template engine you want with **FastAPI**.

A common choice is Jinja2, the same one used by Flask and other tools.

There are utilities to configure it easily that you can use directly in your **FastAPI** application (provided by Starlette).

## Install dependencies { #install-dependencies }

Make sure you create a [virtual environment](../virtual-environments.md), activate it, and install `jinja2`:

<div class="termy">

                                "content": {"application/json": {"schema": {}}},
                            },
                            "422": {
                                "description": "Validation Error",
                                "content": {
                                    "application/json": {
                                        "schema": {

import okhttp3.buildsupport.testJavaVersion

plugins {
  id("okhttp.base-conventions")
  id("java")
  id("application")
  alias(libs.plugins.jlink)
  alias(libs.plugins.extra.java.module.info)
}

dependencies {
  implementation(projects.okhttp)
  implementation(projects.loggingInterceptor)

  // Force version 26.0.2-1 which is a proper JPMS module, unlike transitive 13.0
  implementation(libs.jetbrains.annotations)

      Request
        .Builder()
        .url("https://api.github.com/repos/square/okhttp/issues")
        .header("User-Agent", "OkHttp Headers.java")
        .addHeader("Accept", "application/json; q=0.5")
        .addHeader("Accept", "application/vnd.github.v3+json")
        .build()

    client.newCall(request).execute().use { response ->
      if (!response.isSuccessful) throw IOException("Unexpected code $response")

            mimeType = "text/html";
        } else if (response instanceof JsonResponse) {
            mimeType = "application/json";
        } else if (response instanceof XmlResponse) {
            mimeType = "text/xml";
        } else if (response instanceof StreamResponse) {
            mimeType = "application/octet-stream";
        } else {
            logger.debug("Unknown response: response={}", response);
            return;

          "antlr",
          "base-diagnostics",
          "file-watching",
          "ide",
          "ide-native",
          "kotlin-dsl",
          "platform-base",
          "plugins-application",
          "plugins-java-library",
          "software-diagnostics",
          "test-kit"
        ],
        "parallelizationMethod": {
          "name": "TestDistribution"
        }
      },

import okio.BufferedSource
import okio.ByteString
import okio.use

/**
 * A one-shot stream from the origin server to the client application with the raw bytes of the
 * response body. Each response body is supported by an active connection to the webserver. This
 * imposes both obligations and limits on the client application.
 *
 * ### The response body must be closed.
 *

    assertThat(body!!.contentType().toString()).isEqualTo(
      "application/x-www-form-urlencoded; charset=utf-8",
    )
    assertThat(bodyAsString(body)).isEqualTo("foo")
  }

  @Test
  fun contentTypeHeader() {
    val request =
      fromArgs(
        "-d",
        "foo",
        "-H",
        "Content-Type: application/json",
        "http://example.com",
      ).createRequest()