# Core Services

Now that we have a high-level overview of the Spring Security architecture and its core classes, let's take a closer look at one or two of the core interfaces and their implementations, in particular the AuthenticationManager, UserDetailsService and the AccessDecisionManager. These crop up regularly throughout the remainder of this document so it's important you know how they are configured and how they operate.

## The AuthenticationManager, `ProviderManager` and `AuthenticationProvider`s

The AuthenticationManager is just an interface, so the implementation can be anything we choose, but how does it work in practice? What if we need to check multiple authentication databases or a combination of different authentication services such as a database and an LDAP server?

The default implementation in Spring Security is called `ProviderManager` and rather than handling the authentication request itself, it delegates to a list of configured `AuthenticationProvider`s, each of which is queried in turn to see if it can perform the authentication. Each provider will either throw an exception or return a fully populated Authentication object. Remember our good friends, UserDetails and UserDetailsService? If not, head back to the previous chapter and refresh your memory. The most common approach to verifying an authentication request is to load the corresponding UserDetails and check the loaded password against the one that has been entered by the user. This is the approach used by the `DaoAuthenticationProvider` (see below). The loaded UserDetails object - and particularly the `GrantedAuthority`s it contains - will be used when building the fully populated Authentication object which is returned from a successful authentication and stored in the `SecurityContext`.

If you are using the namespace, an instance of `ProviderMananger` is created and maintained internally, and you add providers to it either by using the namespace authentication provider elements, or by adding the `<custom-authentication-provider>` element to a bean (see the namespace chapter). In this case, you should not declare a `ProviderManager` bean in your application context. However, if you are not using the namespace then you would declare it like so:

    <bean id="authenticationManager"
         class="org.springframework.security.authentication.ProviderManager">
      <property name="providers">
        <list>
          <ref local="daoAuthenticationProvider"/>
          <ref local="anonymousAuthenticationProvider"/>
          <ref local="ldapAuthenticationProvider"/>
        </list>
      </property>
    </bean>

In the above example we have three providers. They are tried in the order shown (which is implied by the use of a `List`), with each provider able to attempt authentication, or skip authentication by simply returning `null`. If all implementations return null, the `ProviderManager` will throw a ProviderNotFoundException. If you're interested in learning more about chaining providers, please refer to the `ProviderManager` JavaDocs.

Authentication mechanisms such as a web form-login processing filter are injected with a reference to the ProviderManager and will call it to handle their authentication requests. The providers you require will sometimes be interchangeable with the authentication mechanisms, while at other times they will depend on a specific authentication mechanism. For example, `DaoAuthenticationProvider` and `LdapAuthenticationProvider` are compatible with any mechanism which submits a simple username/password authentication request and so will work with form-based logins or HTTP Basic authentication. On the other hand, some authentication mechanisms create an authentication request object which can only be interpreted by a single type of `AuthenticationProvider`. An example of this would be JA-SIG CAS, which uses the notion of a service ticket and so can therefore only be authenticated by a `CasAuthenticationProvider`. You needn't be too concerned about this, because if you forget to register a suitable provider, you'll simply receive a `ProviderNotFoundException` when an attempt to authenticate is made.

### `DaoAuthenticationProvider`

The simplest AuthenticationProvider implemented by Spring Security is `DaoAuthenticationProvider`, which is is also one of the earliest supported by the framework. It leverages a UserDetailsService (as a DAO) in order to lookup the username, password and GrantedAuthoritys. It authenticates the user simply by comparing the password submitted in a `UsernamePasswordAuthenticationToken` against the one loaded by the UserDetailsService. Configuring the provider is quite simple:

    <bean id="daoAuthenticationProvider"
        class="org.springframework.security.authentication.dao.DaoAuthenticationProvider">
      <property name="userDetailsService" ref="inMemoryDaoImpl"/>
      <property name="saltSource" ref bean="saltSource"/>
      <property name="passwordEncoder" ref="passwordEncoder"/>
    </bean>

The PasswordEncoder and SaltSource are optional. A PasswordEncoder provides encoding and decoding of passwords presented in the UserDetails object that is returned from the configured UserDetailsService. A SaltSource enables the passwords to be populated with a "salt", which enhances the security of the passwords in the authentication repository. These will be discussed in more detail in ???.

## UserDetailsService Implementations

As mentioned in the earlier in this reference guide, most authentication providers take advantage of the UserDetails and UserDetailsService interfaces. Recall that the contract for UserDetailsService is a single method:

      UserDetails loadUserByUsername(String username) throws UsernameNotFoundException;

The returned UserDetails is an interface that provides getters that guarantee non-null provision of authentication information such as the username, password, granted authorities and whether the user account is enabled or disabled. Most authentication providers will use a UserDetailsService, even if the username and password are not actually used as part of the authentication decision. They may use the returned UserDetails object just for its `GrantedAuthority` information, because some other system (like LDAP or X.509 or CAS etc) has undertaken the responsibility of actually validating the credentials.

Given UserDetailsService is so simple to implement, it should be easy for users to retrieve authentication information using a persistence strategy of their choice. Having said that, Spring Security does include a couple of useful base implementations, which we'll look at below.

### In-Memory Authentication

Is easy to use create a custom UserDetailsService implementation that extracts information from a persistence engine of choice, but many applications do not require such complexity. This is particularly true if you're building a prototype application or just starting integrating Spring Security, when you don't really want to spend time configuring databases or writing UserDetailsService implementations. For this sort of situation, a simple option is to use the `user-service` element from the security namespace:

      <user-service id="userDetailsService">
        <user name="jimi" password="jimispassword" authorities="ROLE_USER, ROLE_ADMIN" />
        <user name="bob" password="bobspassword" authorities="ROLE_USER" />
      </user-service>

This also supports the use of an external properties file:

      <user-service id="userDetailsService" properties="users.properties"/>

The properties file should contain entries in the form

    username=password,grantedAuthority[,grantedAuthority][,enabled|disabled]

For example

     jimi=jimispassword,ROLE_USER,ROLE_ADMIN,enabled
     bob=bobspassword,ROLE_USER,enabled

### `JdbcDaoImpl`

Spring Security also includes a UserDetailsService that can obtain authentication information from a JDBC data source. Internally Spring JDBC is used, so it avoids the complexity of a fully-featured object relational mapper (ORM) just to store user details. If your application does use an ORM tool, you might prefer to write a custom UserDetailsService to reuse the mapping files you've probably already created. Returning to `JdbcDaoImpl`, an example configuration is shown below:

    <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
      <property name="driverClassName" value="org.hsqldb.jdbcDriver"/>
      <property name="url" value="jdbc:hsqldb:hsql://localhost:9001"/>
      <property name="username" value="sa"/>
      <property name="password" value=""/>
    </bean>

    <bean id="userDetailsService" class="org.springframework.security.core.userdetails.jdbc.JdbcDaoImpl">
      <property name="dataSource" ref="dataSource"/>
    </bean>

You can use different relational database management systems by modifying the `DriverManagerDataSource` shown above. You can also use a global data source obtained from JNDI, as with any other Spring configuration.

#### Authority Groups

By default, `JdbcDaoImpl` loads the authorities for a single user with the assumption that the authorities are mapped directly to users (see the database schema appendix). An alternative approach is to partition the authorities into groups and assign groups to the user. Some people prefer this approach as a means of administering user rights. See the `JdbcDaoImpl` Javadoc for more information on how to enable the use of group authorities. The group schema is also included in the appendix.