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[[servlet-rememberme]]
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= Remember-Me Authentication
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[[remember-me-overview]]
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Remember-me or persistent-login authentication refers to web sites being able to remember the identity of a principal between sessions.
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This is typically accomplished by sending a cookie to the browser, with the cookie being detected during future sessions and causing automated login to take place.
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Spring Security provides the necessary hooks for these operations to take place and has two concrete remember-me implementations.
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One uses hashing to preserve the security of cookie-based tokens and the other uses a database or other persistent storage mechanism to store the generated tokens.
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Note that both implementations require a `UserDetailsService`.
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If you use an authentication provider that does not use a `UserDetailsService` (for example, the LDAP provider), it does not work unless you also have a `UserDetailsService` bean in your application context.
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[[remember-me-hash-token]]
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== Simple Hash-Based Token Approach
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This approach uses hashing to achieve a useful remember-me strategy.
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In essence, a cookie is sent to the browser upon successful interactive authentication, with the cookie being composed as follows:
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[source,txt]
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----
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base64(username + ":" + expirationTime + ":" + algorithmName + ":"
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algorithmHex(username + ":" + expirationTime + ":" password + ":" + key))
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username:          As identifiable to the UserDetailsService
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password:          That matches the one in the retrieved UserDetails
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expirationTime:    The date and time when the remember-me token expires, expressed in milliseconds
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key:               A private key to prevent modification of the remember-me token
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algorithmName:     The algorithm used to generate and to verify the remember-me token signature
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----
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The remember-me token is valid only for the period specified and only if the username, password, and key do not change.
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Notably, this has a potential security issue, in that a captured remember-me token is usable from any user agent until such time as the token expires.
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This is the same issue as with digest authentication.
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If a principal is aware that a token has been captured, they can easily change their password and immediately invalidate all remember-me tokens on issue.
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If more significant security is needed, you should use the approach described in the next section.
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Alternatively, remember-me services should not be used at all.
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If you are familiar with the topics discussed in the chapter on xref:servlet/configuration/xml-namespace.adoc#ns-config[namespace configuration], you can enable remember-me authentication by adding the `<remember-me>` element:
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[source,xml]
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----
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<http>
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...
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<remember-me key="myAppKey"/>
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</http>
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----
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The `UserDetailsService` is normally selected automatically.
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If you have more than one in your application context, you need to specify which one should be used with the `user-service-ref` attribute, where the value is the name of your `UserDetailsService` bean.
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[[remember-me-persistent-token]]
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== Persistent Token Approach
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This approach is based on the article titled http://jaspan.com/improved_persistent_login_cookie_best_practice[http://jaspan.com/improved_persistent_login_cookie_best_practice], with some minor modifications. (Essentially, the username is not included in the cookie, to prevent exposing a valid login name unecessarily.
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There is a discussion on this in the comments section of this article.)
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To use the this approach with namespace configuration, supply a datasource reference:
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[source,xml]
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----
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<http>
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...
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<remember-me data-source-ref="someDataSource"/>
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</http>
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----
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The database should contain a `persistent_logins` table, created by using the following SQL (or equivalent):
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[source,ddl]
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----
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create table persistent_logins (username varchar(64) not null,
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								series varchar(64) primary key,
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								token varchar(64) not null,
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								last_used timestamp not null)
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----
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[[remember-me-impls]]
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== Remember-Me Interfaces and Implementations
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Remember-me is used with `UsernamePasswordAuthenticationFilter` and is implemented through hooks in the `AbstractAuthenticationProcessingFilter` superclass.
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It is also used within `BasicAuthenticationFilter`.
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The hooks invoke a concrete `RememberMeServices` at the appropriate times.
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The following listing shows the interface:
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[source,java]
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----
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Authentication autoLogin(HttpServletRequest request, HttpServletResponse response);
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void loginFail(HttpServletRequest request, HttpServletResponse response);
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void loginSuccess(HttpServletRequest request, HttpServletResponse response,
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	Authentication successfulAuthentication);
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----
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See the Javadoc for {security-api-url}org/springframework/security/web/authentication/RememberMeServices.html[`RememberMeServices`] for a fuller discussion on what the methods do, although note that, at this stage, `AbstractAuthenticationProcessingFilter` calls only the `loginFail()` and `loginSuccess()` methods.
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The `autoLogin()` method is called by `RememberMeAuthenticationFilter` whenever the `SecurityContextHolder` does not contain an `Authentication`.
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This interface, therefore, provides the underlying remember-me implementation with sufficient notification of authentication-related events and delegates to the implementation whenever a candidate web request might contain a cookie and wish to be remembered.
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This design allows any number of remember-me implementation strategies.
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We have seen earlier that Spring Security provides two implementations.
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We look at each of these in turn.
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=== TokenBasedRememberMeServices
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This implementation supports the simpler approach described in <<remember-me-hash-token>>.
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`TokenBasedRememberMeServices` generates a `RememberMeAuthenticationToken`, which is processed by `RememberMeAuthenticationProvider`.
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A `key` is shared between this authentication provider and the `TokenBasedRememberMeServices`.
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In addition, `TokenBasedRememberMeServices` requires a `UserDetailsService`, from which it can retrieve the username and password for signature comparison purposes and generate the `RememberMeAuthenticationToken` to contain the correct `GrantedAuthority` instances.
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`TokenBasedRememberMeServices` also implements Spring Security's `LogoutHandler` interface so that it can be used with `LogoutFilter` to have the cookie cleared automatically.
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By default, this implementation uses the SHA-256 algorithm to encode the token signature.
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To verify the token signature, the algorithm retrieved from `algorithmName` is parsed and used.
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If no `algorithmName` is present, the default matching algorithm will be used, which is SHA-256.
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You can specify different algorithms for signature encoding and for signature matching, this allows users to safely upgrade to a different encoding algorithm while still able to verify old ones if there is no `algorithmName` present.
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To do that you can specify your customized `TokenBasedRememberMeServices` as a Bean and use it in the configuration.
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[tabs]
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======
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Java::
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+
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[source,java,role="primary"]
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----
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@Bean
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SecurityFilterChain securityFilterChain(HttpSecurity http, RememberMeServices rememberMeServices) throws Exception {
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	http
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			.authorizeHttpRequests((authorize) -> authorize
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					.anyRequest().authenticated()
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			)
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			.rememberMe((remember) -> remember
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				.rememberMeServices(rememberMeServices)
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			);
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	return http.build();
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}
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@Bean
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RememberMeServices rememberMeServices(UserDetailsService userDetailsService) {
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	RememberMeTokenAlgorithm encodingAlgorithm = RememberMeTokenAlgorithm.SHA256;
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	TokenBasedRememberMeServices rememberMe = new TokenBasedRememberMeServices(myKey, userDetailsService, encodingAlgorithm);
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	rememberMe.setMatchingAlgorithm(RememberMeTokenAlgorithm.MD5);
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	return rememberMe;
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}
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----
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XML::
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+
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[source,xml,role="secondary"]
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----
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<http>
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  <remember-me services-ref="rememberMeServices"/>
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</http>
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<bean id="rememberMeServices" class=
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"org.springframework.security.web.authentication.rememberme.TokenBasedRememberMeServices">
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    <property name="userDetailsService" ref="myUserDetailsService"/>
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    <property name="key" value="springRocks"/>
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    <property name="matchingAlgorithm" value="MD5"/>
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    <property name="encodingAlgorithm" value="SHA256"/>
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</bean>
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----
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======
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The following beans are required in an application context to enable remember-me services:
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[tabs]
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======
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Java::
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+
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[source,java,role="primary"]
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----
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@Bean
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RememberMeAuthenticationFilter rememberMeFilter() {
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    RememberMeAuthenticationFilter rememberMeFilter = new RememberMeAuthenticationFilter();
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    rememberMeFilter.setRememberMeServices(rememberMeServices());
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    rememberMeFilter.setAuthenticationManager(theAuthenticationManager);
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    return rememberMeFilter;
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}
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@Bean
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TokenBasedRememberMeServices rememberMeServices() {
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    TokenBasedRememberMeServices rememberMeServices = new TokenBasedRememberMeServices();
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    rememberMeServices.setUserDetailsService(myUserDetailsService);
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    rememberMeServices.setKey("springRocks");
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    return rememberMeServices;
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}
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@Bean
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RememberMeAuthenticationProvider rememberMeAuthenticationProvider() {
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    RememberMeAuthenticationProvider rememberMeAuthenticationProvider = new RememberMeAuthenticationProvider();
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    rememberMeAuthenticationProvider.setKey("springRocks");
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    return rememberMeAuthenticationProvider;
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}
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----
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XML::
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+
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[source,xml,role="secondary"]
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----
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<bean id="rememberMeFilter" class=
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"org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter">
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<property name="rememberMeServices" ref="rememberMeServices"/>
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<property name="authenticationManager" ref="theAuthenticationManager" />
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</bean>
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<bean id="rememberMeServices" class=
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"org.springframework.security.web.authentication.rememberme.TokenBasedRememberMeServices">
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<property name="userDetailsService" ref="myUserDetailsService"/>
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<property name="key" value="springRocks"/>
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</bean>
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<bean id="rememberMeAuthenticationProvider" class=
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"org.springframework.security.authentication.RememberMeAuthenticationProvider">
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<property name="key" value="springRocks"/>
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</bean>
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----
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======
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Remember to add your `RememberMeServices` implementation to your `UsernamePasswordAuthenticationFilter.setRememberMeServices()` property, include the `RememberMeAuthenticationProvider` in your `AuthenticationManager.setProviders()` list, and add `RememberMeAuthenticationFilter` into your `FilterChainProxy` (typically immediately after your `UsernamePasswordAuthenticationFilter`).
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=== PersistentTokenBasedRememberMeServices
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You can use this class in the same way as `TokenBasedRememberMeServices`, but it additionally needs to be configured with a `PersistentTokenRepository` to store the tokens.
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* `InMemoryTokenRepositoryImpl` which is intended for testing only.
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* `JdbcTokenRepositoryImpl` which stores the tokens in a database.
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See <<remember-me-persistent-token>> for the database schema.
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