<authentication-manager>
  <authentication-provider user-service-ref="userDetailService">
    <password-encoder ref="passwordEncoder" />
  </authentication-provider>
</authentication-manager>

<beans:bean class="com.sapphire.security.MyPasswordEncoder" id="passwordEncoder">
  <beans:constructor-arg value="md5"></beans:constructor-arg>
</beans:bean>

* preAuthenticationChecks : 主要进行的是对用户是否过期等信息的校验，调用的方法在userDetail中有定义的。
* additionalAuthenticationChecks : 这个就是用户名密码验证的过程了。

public class MyPasswordEncoder extends MessageDigestPasswordEncoder {
  public MyPasswordEncoder(String algorithm) {
   super(algorithm);
  }

  @Override
  public boolean isPasswordValid(String encPass, String rawPass, Object salt) {
   return encPass.equals(DigestUtils.md5DigestAsHex(rawPass.getBytes()));
  }
}