Hello Reza,
You can simply configure an LDAP Directory as a Data Source for the AS JAVA UME:
Configuring the UME to Use an LDAP Directory as Data Source - Identity Management - SAP Library
This way you will be able to configure the applications, running on AS JAVA Server (like the SAP Portal), to use the LDAP credentials. But this is simply authentication (no SSO).
SSO - single sign-on means the user to authenticate once to an identity provider (for example MS Domain or SAML IDP) and then to have SSO based on tokens (for example Kerberos/SAML) to all applications that trust this identity provider. This is SSO and if you need SSO for SAML 2.0 based applications (as you mentioned in your very first post), then you need a SAML Identity Provider.
If you don't need SAML 2.0 based SSO, you can simply go with configuring authentication based on the LDAP credentials.
Regards,
Donka Dimitrova