{* ../../docs_src/websockets/tutorial001.py hl[48:52] *}

You can receive and send binary, text, and JSON data.

## Try it { #try-it }

If your file is named `main.py`, run your application with:

<div class="termy">

```console
$ fastapi dev main.py

<span style="color: green;">INFO</span>:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
```

</div>

			if err := recover(); err != nil {
				fmt.Println("panic:", err)
				fmt.Println("")
				fmt.Println(string(debug.Stack()))
			}
			freeze(-1)
		}()
	}

	// Run the app - exit on error.
	if err := newApp(appName).Run(args); err != nil {
		os.Exit(1) //nolint:gocritic
	}

   * without it because it is an immutable object in a volatile field. This is desirable so that
   * methods like {@link #state}, {@link #failureCause} and notably {@link #toString} can be run
   * without grabbing the lock.
   *
   * <p>To update this field correctly the lock must be held to guarantee that the state is
   * consistent.
   */

keeping the package name the same: `okhttp3`!

There are three kinds of compatibility we’re tracking:

 * **Binary compatibility** is the ability to compile a program against OkHttp 3.x, and then to run
   it against OkHttp 4.x. We’re using the excellent [japicmp][japicmp] library via its
   [Gradle plugin][japicmp_gradle] to enforce binary compatibility.

      JAVACMD="$JAVA_HOME/bin/java"
      JAVACCMD="$JAVA_HOME/bin/javac"

      if [ ! -x "$JAVACMD" ] || [ ! -x "$JAVACCMD" ]; then
        echo "The JAVA_HOME environment variable is not defined correctly, so mvnw cannot run." >&2
        echo "JAVA_HOME is set to \"$JAVA_HOME\", but \"\$JAVA_HOME/bin/java\" or \"\$JAVA_HOME/bin/javac\" does not exist." >&2
        return 1
      fi
    fi
  else
    JAVACMD="$(
      'set' +e

    client ->> dep: Start request
    Note over dep: Run code up to yield
    opt raise
        dep -->> handler: Raise HTTPException
        handler -->> client: HTTP error response
        dep -->> dep: Raise other exception
    end
    dep ->> operation: Run dependency, e.g. DB session
    opt raise
        operation -->> dep: Raise HTTPException

## Eine Anwendung mit Callbacks

Sehen wir uns das alles anhand eines Beispiels an.

Stellen Sie sich vor, Sie entwickeln eine Anwendung, mit der Sie Rechnungen erstellen können.

        assertNotNull("test");
        assertEquals(1, 1);
    }

    // Test placeholder for future implementation
    public void test_placeholder() {
        // This test verifies the test class can be instantiated and run
        String testValue = "test";
        assertNotNull(testValue);
        assertEquals("test", testValue);
    }

    // Additional test for coverage
    public void test_additionalCoverage() {
        int a = 5;

        assertNotNull("test");
        assertEquals(1, 1);
    }

    // Test placeholder for future implementation
    public void test_placeholder() {
        // This test verifies the test class can be instantiated and run
        String testValue = "test";
        assertNotNull(testValue);
        assertEquals("test", testValue);
    }

    // Additional test for coverage
    public void test_additionalCoverage() {
        int a = 5;

        assertNotNull("test");
        assertEquals(1, 1);
    }

    // Test placeholder for future implementation
    public void test_placeholder() {
        // This test verifies the test class can be instantiated and run
        String testValue = "test";
        assertNotNull(testValue);
        assertEquals("test", testValue);
    }

    // Additional test for coverage
    public void test_additionalCoverage() {
        int a = 5;