Add public getAccessToken() methods to expose OAuth2 tokens from client credentials

src/main/java/dev/openfga/sdk/api/OpenFgaApi.java

@@ -1248,6 +1248,29 @@ private String pathWithParams(String basePath, Object... params) {
         return path.toString();
     }
 
+    /**
+     * Get an access token asynchronously. This method is only available when using CLIENT_CREDENTIALS 
+     * authentication. The token can be used for making API calls to other services that accept the same token.
+     * 
+     * @return A CompletableFuture containing the access token
+     * @throws IllegalStateException when the credentials method is not CLIENT_CREDENTIALS
+     * @throws FgaInvalidParameterException when the configuration is invalid
+     * @throws ApiException when token retrieval fails
+     */
+    public CompletableFuture<String> getAccessToken() throws IllegalStateException, FgaInvalidParameterException, ApiException {
+        CredentialsMethod credentialsMethod = this.configuration.getCredentials().getCredentialsMethod();
+
+        if (credentialsMethod != CredentialsMethod.CLIENT_CREDENTIALS) {
+            throw new IllegalStateException("getAccessToken() is only available when using CLIENT_CREDENTIALS authentication method");
+        }
+
+        if (oAuth2Client == null) {
+            throw new IllegalStateException("OAuth2Client is not initialized");
+        }
+
+        return oAuth2Client.getAccessToken();
+    }
+
     /**
      * Get an access token. Expects that configuration is valid (meaning it can
      * pass {@link Configuration#assertValid()}) and expects that if the

src/main/java/dev/openfga/sdk/api/client/OpenFgaClient.java

@@ -75,6 +75,20 @@ public void setConfiguration(ClientConfiguration configuration) throws FgaInvali
         this.api = new OpenFgaApi(configuration, apiClient);
     }
 
+    /**
+     * Get an access token asynchronously. This method is only available when using CLIENT_CREDENTIALS 
+     * authentication. The token can be used for making API calls to other services that accept the same token.
+     * 
+     * @return A CompletableFuture containing the access token
+     * @throws IllegalStateException when the credentials method is not CLIENT_CREDENTIALS
+     * @throws FgaInvalidParameterException when the configuration is invalid
+     * @throws ApiException when token retrieval fails
+     */
+    public CompletableFuture<String> getAccessToken() throws IllegalStateException, FgaInvalidParameterException, ApiException {
+        configuration.assertValid();
+        return api.getAccessToken();
+    }
+
     /* ********
      * Stores *
      **********/

src/test/java/dev/openfga/sdk/api/OpenFgaApiTest.java

@@ -1928,4 +1928,68 @@ DEFAULT_STORE_ID, DEFAULT_AUTH_MODEL_ID, new WriteAssertionsRequest())
         assertEquals(
                 "{\"code\":\"internal_error\",\"message\":\"Internal Server Error\"}", exception.getResponseData());
     }
+
+    @Test
+    public void getAccessToken_withClientCredentials() throws Exception {
+        // Given
+        String accessToken = "test-access-token-12345";
+        String tokenResponse = String.format("{\"access_token\":\"%s\",\"token_type\":\"Bearer\",\"expires_in\":3600}", accessToken);
+
+        // Mock the token endpoint
+        mockHttpClient.onPost("https://auth.fga.example/oauth/token").doReturn(200, tokenResponse);
+
+        ClientCredentials clientCredentials = new ClientCredentials()
+                .clientId("test-client-id")
+                .clientSecret("test-client-secret")
+                .apiTokenIssuer("https://auth.fga.example")
+                .apiAudience("https://api.fga.example");
+
+        Configuration configuration = new Configuration()
+                .apiUrl("https://api.fga.example")
+                .credentials(new Credentials(clientCredentials));
+
+        OpenFgaApi api = new OpenFgaApi(configuration, mockApiClient);
+
+        // When
+        String result = api.getAccessToken().get();
+
+        // Then
+        assertEquals(accessToken, result);
+        mockHttpClient.verify().post("https://auth.fga.example/oauth/token").called();
+    }
+
+    @Test
+    public void getAccessToken_withApiToken() throws Exception {
+        // Given - API token configuration
+        ApiToken apiToken = new ApiToken("static-api-token");
+        Configuration configuration = new Configuration()
+                .apiUrl("https://api.fga.example")
+                .credentials(new Credentials(apiToken));
+
+        OpenFgaApi api = new OpenFgaApi(configuration, mockApiClient);
+
+        // When & Then
+        IllegalStateException exception = assertThrows(IllegalStateException.class, () -> {
+            api.getAccessToken().get();
+        });
+
+        assertEquals("getAccessToken() is only available when using CLIENT_CREDENTIALS authentication method", exception.getMessage());
+    }
+
+    @Test
+    public void getAccessToken_withNoCredentials() throws Exception {
+        // Given - No credentials configuration
+        Configuration configuration = new Configuration()
+                .apiUrl("https://api.fga.example")
+                .credentials(new Credentials());
+
+        OpenFgaApi api = new OpenFgaApi(configuration, mockApiClient);
+
+        // When & Then  
+        IllegalStateException exception = assertThrows(IllegalStateException.class, () -> {
+            api.getAccessToken().get();
+        });
+
+        assertEquals("getAccessToken() is only available when using CLIENT_CREDENTIALS authentication method", exception.getMessage());
+    }
 }

src/test/java/dev/openfga/sdk/api/client/OpenFgaClientTest.java

@@ -2993,6 +2993,65 @@ public void setAuthorizationModelId() throws Exception {
                         + "If this behavior ever changes, it could be a subtle breaking change.");
     }
 
+    @Test
+    public void getAccessToken_withClientCredentials() throws Exception {
+        // Given
+        String accessToken = "client-test-token-67890";
+        String tokenResponse = String.format("{\"access_token\":\"%s\",\"token_type\":\"Bearer\",\"expires_in\":3600}", accessToken);
+
+        // Mock the token endpoint
+        mockHttpClient.onPost("https://auth.fga.example/oauth/token").doReturn(200, tokenResponse);
+
+        ClientCredentials clientCredentials = new ClientCredentials()
+                .clientId("test-client-id")
+                .clientSecret("test-client-secret")
+                .apiTokenIssuer("https://auth.fga.example")
+                .apiAudience("https://api.fga.example");
+
+        ClientConfiguration clientConfiguration = new ClientConfiguration()
+                .apiUrl("https://api.fga.example")
+                .storeId(DEFAULT_STORE_ID)
+                .credentials(new Credentials(clientCredentials));
+
+        var mockApiClient = mock(ApiClient.class);
+        when(mockApiClient.getHttpClient()).thenReturn(mockHttpClient);
+        when(mockApiClient.getObjectMapper()).thenReturn(new ObjectMapper());
+        when(mockApiClient.getHttpClientBuilder()).thenReturn(mock(HttpClient.Builder.class));
+
+        OpenFgaClient client = new OpenFgaClient(clientConfiguration, mockApiClient);
+
+        // When
+        String result = client.getAccessToken().get();
+
+        // Then
+        assertEquals(accessToken, result);
+        mockHttpClient.verify().post("https://auth.fga.example/oauth/token").called();
+    }
+
+    @Test
+    public void getAccessToken_withApiToken() throws Exception {
+        // Given - API token configuration
+        ApiToken apiToken = new ApiToken("static-api-token-client");
+        ClientConfiguration clientConfiguration = new ClientConfiguration()
+                .apiUrl("https://api.fga.example")
+                .storeId(DEFAULT_STORE_ID)
+                .credentials(new Credentials(apiToken));
+
+        var mockApiClient = mock(ApiClient.class);
+        when(mockApiClient.getHttpClient()).thenReturn(mockHttpClient);
+        when(mockApiClient.getObjectMapper()).thenReturn(new ObjectMapper());
+        when(mockApiClient.getHttpClientBuilder()).thenReturn(mock(HttpClient.Builder.class));
+
+        OpenFgaClient client = new OpenFgaClient(clientConfiguration, mockApiClient);
+
+        // When & Then - The exception is thrown directly, not wrapped in ExecutionException
+        IllegalStateException exception = assertThrows(IllegalStateException.class, () -> {
+            client.getAccessToken();
+        });
+
+        assertEquals("getAccessToken() is only available when using CLIENT_CREDENTIALS authentication method", exception.getMessage());
+    }
+
     private Matcher<String> anyValidUUID() {
         return new UUIDMatcher();
     }