Change ClientAuthenticationMethod from enum to class

Fixes gh-4313

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/authentication/nimbus/NimbusAuthorizationCodeTokenExchanger.java

@@ -78,7 +78,7 @@ public class NimbusAuthorizationCodeTokenExchanger implements AuthorizationGrant
 		ClientID clientId = new ClientID(clientRegistration.getClientId());
 		Secret clientSecret = new Secret(clientRegistration.getClientSecret());
 		ClientAuthentication clientAuthentication;
-		if (ClientAuthenticationMethod.FORM.equals(clientRegistration.getClientAuthenticationMethod())) {
+		if (ClientAuthenticationMethod.POST.equals(clientRegistration.getClientAuthenticationMethod())) {
 			clientAuthentication = new ClientSecretPost(clientId, clientSecret);
 		} else {
 			clientAuthentication = new ClientSecretBasic(clientId, clientSecret);

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/ClientRegistration.java

@@ -35,7 +35,7 @@ import java.util.Set;
 public class ClientRegistration {
 	private String clientId;
 	private String clientSecret;
-	private ClientAuthenticationMethod clientAuthenticationMethod = ClientAuthenticationMethod.HEADER;
+	private ClientAuthenticationMethod clientAuthenticationMethod = ClientAuthenticationMethod.BASIC;
 	private AuthorizationGrantType authorizedGrantType;
 	private String redirectUri;
 	private Set<String> scopes = Collections.emptySet();
@@ -154,7 +154,7 @@ public class ClientRegistration {
 	public static class Builder {
 		protected String clientId;
 		protected String clientSecret;
-		protected ClientAuthenticationMethod clientAuthenticationMethod = ClientAuthenticationMethod.HEADER;
+		protected ClientAuthenticationMethod clientAuthenticationMethod = ClientAuthenticationMethod.BASIC;
 		protected AuthorizationGrantType authorizedGrantType;
 		protected String redirectUri;
 		protected Set<String> scopes;

oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/registration/ClientRegistrationProperties.java

@@ -35,7 +35,7 @@ import java.util.Set;
 public class ClientRegistrationProperties {
 	private String clientId;
 	private String clientSecret;
-	private ClientAuthenticationMethod clientAuthenticationMethod = ClientAuthenticationMethod.HEADER;
+	private ClientAuthenticationMethod clientAuthenticationMethod = ClientAuthenticationMethod.BASIC;
 	private AuthorizationGrantType authorizedGrantType;
 	private String redirectUri;
 	private Set<String> scopes;

oauth2/oauth2-core/src/main/java/org/springframework/security/oauth2/core/ClientAuthenticationMethod.java

@@ -15,6 +15,8 @@
  */
 package org.springframework.security.oauth2.core;
 
+import org.springframework.util.Assert;
+
 /**
  * The available authentication methods used when authenticating the client with the authorization server.
  *
@@ -22,17 +24,34 @@ package org.springframework.security.oauth2.core;
  * @since 5.0
  * @see <a target="_blank" href="https://tools.ietf.org/html/rfc6749#section-2.3">Section 2.3 Client Authentication</a>
  */
-public enum ClientAuthenticationMethod {
-	HEADER("header"),
-	FORM("form");
-
+public final class ClientAuthenticationMethod {
+	public static final ClientAuthenticationMethod BASIC = new ClientAuthenticationMethod("basic");
+	public static final ClientAuthenticationMethod POST = new ClientAuthenticationMethod("post");
 	private final String value;
 
-	ClientAuthenticationMethod(String value) {
+	public ClientAuthenticationMethod(String value) {
+		Assert.hasText(value, "value cannot be empty");
 		this.value = value;
 	}
 
-	public String value() {
+	public String getValue() {
 		return this.value;
 	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj) {
+			return true;
+		}
+		if (obj == null || this.getClass() != obj.getClass()) {
+			return false;
+		}
+		ClientAuthenticationMethod that = (ClientAuthenticationMethod) obj;
+		return this.getValue().equalsIgnoreCase(that.getValue());
+	}
+
+	@Override
+	public int hashCode() {
+		return this.getValue().hashCode();
+	}
 }