Provide configuration for authorization code generator

Closes gh-376

oauth2-authorization-server/src/test/java/org/springframework/security/oauth2/server/authorization/authentication/OAuth2AuthorizationCodeRequestAuthenticationProviderTests.java

@@ -22,6 +22,7 @@ import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 import java.util.function.Function;
+import java.util.function.Supplier;
 
 import org.junit.Before;
 import org.junit.Test;
@@ -56,6 +57,7 @@ import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.spy;
 import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
@@ -113,6 +115,13 @@ public class OAuth2AuthorizationCodeRequestAuthenticationProviderTests {
 		assertThat(this.authenticationProvider.supports(OAuth2AuthorizationCodeRequestAuthenticationToken.class)).isTrue();
 	}
 
+	@Test
+	public void setAuthorizationCodeGeneratorWhenNullThenThrowIllegalArgumentException() {
+		assertThatThrownBy(() -> this.authenticationProvider.setAuthorizationCodeGenerator(null))
+				.isInstanceOf(IllegalArgumentException.class)
+				.hasMessage("authorizationCodeGenerator cannot be null");
+	}
+
 	@Test
 	public void setAuthenticationValidatorResolverWhenNullThenThrowIllegalArgumentException() {
 		assertThatThrownBy(() -> this.authenticationProvider.setAuthenticationValidatorResolver(null))
@@ -487,6 +496,34 @@ public class OAuth2AuthorizationCodeRequestAuthenticationProviderTests {
 		assertAuthorizationCodeRequestWithAuthorizationCodeResult(registeredClient, authentication, authenticationResult);
 	}
 
+	@Test
+	public void authenticateWhenCustomAuthorizationCodeGeneratorThenUsed() {
+		RegisteredClient registeredClient = TestRegisteredClients.registeredClient().build();
+		when(this.registeredClientRepository.findByClientId(eq(registeredClient.getClientId())))
+				.thenReturn(registeredClient);
+
+		@SuppressWarnings("unchecked")
+		Supplier<String> authorizationCodeGenerator = spy(new Supplier<String>() {
+			@Override
+			public String get() {
+				return "custom-code";
+			}
+		});
+		this.authenticationProvider.setAuthorizationCodeGenerator(authorizationCodeGenerator);
+
+		OAuth2AuthorizationCodeRequestAuthenticationToken authentication =
+				authorizationCodeRequestAuthentication(registeredClient, this.principal)
+						.build();
+
+		OAuth2AuthorizationCodeRequestAuthenticationToken authenticationResult =
+				(OAuth2AuthorizationCodeRequestAuthenticationToken) this.authenticationProvider.authenticate(authentication);
+
+		assertAuthorizationCodeRequestWithAuthorizationCodeResult(registeredClient, authentication, authenticationResult);
+
+		verify(authorizationCodeGenerator).get();
+		assertThat(authenticationResult.getAuthorizationCode().getTokenValue()).isEqualTo(authorizationCodeGenerator.get());
+	}
+
 	@Test
 	public void authenticateWhenCustomAuthenticationValidatorResolverThenUsed() {
 		RegisteredClient registeredClient = TestRegisteredClients.registeredClient().build();