一、前言
数据库认证（Database-provided authentication）：应用通过数据库获取用户认证信息。
认证提供者（Authentication Provider）：Spring提供了一系列认证方式，如LDAP、OpenID等，对应的认证服务都需要实现AuthenticationProvider接口，开发人员可以自定义AuthenticationProvider实现。
二、Spring配置
数据库认证本质上与其它认证方式没有区别，区别就是获取用户及其权限信息的方式不同，数据库认证需要从数据库读取用户信息，Spring已经预定义了用户安全的相关表结构和操作实现，请参考下面两个类的源码，这些默认实现能满足大部分应用需求，当然也很容易自己定制。
org.springframework.security.core.userdetails.jdbc.JdbcDaoImpl
org.springframework.security.provisioning.JdbcUserDetailsManager
对于获取用户、用户组、权限的查询都提供了默认的SQL，所以这也不需要开发者定义，但需要了解Spring的默认实现，如：
public class JdbcDaoImpl extends JdbcDaoSupport implements UserDetailsService {
//~ Static fields/initializers =====================================================================================
public static final String DEF_USERS_BY_USERNAME_QUERY =
"select username,password,enabled " +
"from users " +
"where username = ?";
public static final String DEF_AUTHORITIES_BY_USERNAME_QUERY =
"select username,authority " +
"from authorities " +
"where username = ?";
public static final String DEF_GROUP_AUTHORITIES_BY_USERNAME_QUERY =
"select g.id, g.group_name, ga.authority " +
"from groups g, group_members gm, group_authorities ga " +
"where gm.username = ? " +
"and g.id = ga.group_id " +
"and g.id = gm.group_id";我们可以看到Spring尽一切可能减少开发者的工作，但如果需求不能满足，开发者需要自定义表结构并覆盖查询SQL定义。
public class JdbcUserDetailsManager extends JdbcDaoImpl implements UserDetailsManager, GroupManager {
//~ Static fields/initializers =====================================================================================
// UserDetailsManager SQL
public static final String DEF_CREATE_USER_SQL =
"insert into users (username, password, enabled) values (?,?,?)";
public static final String DEF_DELETE_USER_SQL =
"delete from users where username = ?";
public static final String DEF_UPDATE_USER_SQL =
"update users set password = ?, enabled = ? where username = ?";
public static final String DEF_INSERT_AUTHORITY_SQL =
"insert into authorities (username, authority) values (?,?)";
public static final String DEF_DELETE_USER_AUTHORITIES_SQL =
"delete from authorities where username = ?";
public static final String DEF_USER_EXISTS_SQL =
"select username from users where username = ?";
public static final String DEF_CHANGE_PASSWORD_SQL =
"update users set password = ? where username = ?";
// GroupManager SQL
public static final String DEF_FIND_GROUPS_SQL =
"select group_name from groups";
public static final String DEF_FIND_USERS_IN_GROUP_SQL =
"select username from group_members gm, groups g " +
"where gm.group_id = g.id" +
" and g.group_name = ?";
public static final String DEF_INSERT_GROUP_SQL =
"insert into groups (group_name) values (?)";
public static final String DEF_FIND_GROUP_ID_SQL =
"select id from groups where group_name = ?";
public static final String DEF_INSERT_GROUP_AUTHORITY_SQL =
"insert into group_authorities (group_id, authority) values (?,?)";
public static final String DEF_DELETE_GROUP_SQL =
"delete from groups where id = ?";
public static final String DEF_DELETE_GROUP_AUTHORITIES_SQL =
"delete from group_authorities where group_id = ?";
public static final String DEF_DELETE_GROUP_MEMBERS_SQL =
"delete from group_members where group_id = ?";
public static final String DEF_RENAME_GROUP_SQL =
"update groups set group_name = ? where group_name = ?";
public static final String DEF_INSERT_GROUP_MEMBER_SQL =
"insert into group_members (group_id, username) values (?,?)";
public static final String DEF_DELETE_GROUP_MEMBER_SQL =
"delete from group_members where group_id = ? and username = ?";
public static final String DEF_GROUP_AUTHORITIES_QUERY_SQL =
"select g.id, g.group_name, ga.authority " +
"from groups g, group_authorities ga " +
"where g.group_name = ? " +
"and g.id = ga.group_id ";
public static final String DEF_DELETE_GROUP_AUTHORITY_SQL =
"delete from group_authorities where group_id = ? and authority = ?";