Software as a Service or SaaS has been around for quite some time now.  But most of the time, developers are building single tenant applications as per requirements.  These applications have just one database and one web server at their core.

Many e-commerce application want a solution where multi-tenancy is achieved by having a database per tenant and all user information (username, password, etc) for authentication and authorization stored in a user table in the respective tenant databases. It meant that not only do they need a multi-tenant application, but also a secure application like any other web application secured by Spring Security.

Let’s see how to use Spring Security to secure a web application and how to use Hibernate to connect to a database. The requirement further dictates that all users belonging to a tenant be stored in the tenant database and not a separate or central database. This would allow for complete data isolation for each tenant.

For this example we are going to built a SaaS style web app using the latest Spring Boot 2, Spring JPA with Hibernate as the JPA provider to connect to MySQL and Spring Security 5 to secure the web application.

What is Multi-Tenancy?

A multi-tenant application is where a tenant (i.e. users in a company) feels that the application has been created and deployed for them. In reality, there are many such tenants/business users and they too are using the same application but they get a feel that it is built just for them. Typical examples are online e-commerce marketplaces, storefronts, white labelled shopping mobile apps etc.

A more technical explanation of multi-tenancy

SaaS applications are multi-tenant applications which require tenant data isolation in the database layer. There are different approaches for achieving this data isolation.
The most widely used approaches are

• Schema per tenant – all tenants have their own schema but these schema live in the same database 
• Discriminator per tenant – data for all tenants is in the same schema in the database but is distinguished by a tenant discriminator column
• Database per tenant – each tenant has a separate database

Which multi-tenancy approach is better?

There are many factors to consider before choosing a data isolation approach as mentioned above. We will not go into depth of each but focus mainly on building a multi-tenant application using the latest Spring Boot 2 framework along with JPA, Hibernate as the JPA provider and Spring Security 5.

Multi-tenancy using database per tenant approach

In many applications, especially in financial and medical domains, there is a strict need for absolute data isolation, security and privacy in multi-tenant applications. Multi-tenancy using database per tenant one of the best ways for achieving data isolation and security.

How does the SaaS application work?

We will build a database per tenant multi-tenant application secured by Spring Security.
In most SaaS applications, there is an entry point where a user belonging to a tenant enters the

• tenant name (company name in most cases or some other identifier)
• username
• password

Step 1 – Process the login form with the extra ‘tenant’ field

• The login form will present the tenant name, username and password to Spring Security for authentication.
• The Spring Security UsernamePasswordAuthenticationFilter filter intercepts the login form’s request to the server. This filter needs to be extended so that the extra field ‘tenant’ can be extracted.
• This tenant field is the tenant database identifier and is required by Hibernate to identify the tenant database to connect to.
• Once the tenant field is extracted by the security filter, it needs to be set in memory so that after the security filter is finished doing its job, this tenant identifier is available to the Hibernate code to connect to the correct tenant database.
• To make this happen, the tenant field is stored in a ThreadLocal

Step 2 – Compare the login form values with existing user in the database

• Now that the username, password and tenant values are available to Spring Security, the AbstractUserDetailsAuthenticationProvider will use the retrieveUser() method to call upon the UserDetailsService to get the user details from the database. 
• If the credentials from the login form match those of the result returned by the tenant database, then the authentication is successful and Spring Security will now log the user into the application and create a secure session for the user for further access and interaction with the protected resources in the application. 

Step 3 – Select the correct tenant database

• Step 3 happens along side Step 2. 
• It means that Spring Security does its job of preparing user credentials for comparison with existing users in the database. 
• And Hibernate does its job of selecting the correct tenant database to provide such a user record to Spring Security for comparison. 

How does Step 3 work?

• In Step 2 above, Spring Security queries the database for a user based on the login form username and tenant field. So how does Hibernate know to get the user from the correct tenant database. In Step 1 note that the tenant field is stored in a ThreadLocal for later access. 
• Hibernate provides CurrentTenantIdentifierResolver for resolving or identifying the correct tenant id. The tenant id is extracted from the ThreadLocal storage.
• Hibernate also provides AbstractDataSourceBasedMultiTenantConnectionProviderImpl (also see DataSourceBasedMultiTenantConnectionProviderImpl) whose job is to provide the datasource corresponding to the tenant identifier.

Technology Stack

The example application was built using:

• Spring Boot 2 (2.0.1.RELEASE)
• Spring Security 5 
• MySQL 
• Spring JPA (Spring Boot 2 provides latest Hibernate 5 as the JPA provider)
• Java 8
• Spring STS IDE

Understanding the code for the application

Let’s take a look at the code for building multi tenancy SaaS application:

Project Layout

This is how a typical project layout will look in Spring STS

File: pom.xml

Using the Spring Initialzr create a project with Spring Web, Security, Thymeleaf, MySQL and JPA. It should generate a pom.xml as follows:

Note that there are a few extra dependencies added here: 

• Cyber NekoHTML for setting ThymeLeaf in legacy mode. Why? Thymeleaf complains if there are no proper closing tags but HTML5 allows some attributes to not have closing tags. So the fix is to use this dependency. A detailed problem statement and its solution is on this stackoverflow thread. 
• Apache commons-lang-3 for StringUtils
• spring-boot-configuration-processor for helping with reading config properties. When you have annotations like @ConfigurationProperties, the STS IDE itself suggests that this dependency should be added.

This is the final pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.example</groupId>
 <artifactId>multitenancy-mysql</artifactId>
 <version>1.0.1</version>
 <packaging>jar</packaging>
 <name>multitenancy-mysql</name>
 <description>Spring Boot JPA Hibernate with Per Database Multi-Tenancy with Spring Security</description>
 <contributors>
  <contributor>
   <name>Sunit Katkar</name>
   <email>sunitkatkar@gmail.com</email>
   <url>https://sunitkatkar.blogspot.com/</url>
  </contributor>
 </contributors>

 <parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-parent</artifactId>
  <version>2.0.1.RELEASE</version>
  <relativePath /> <!-- lookup parent from repository -->
 </parent>

 <properties>
  <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
  <java.version>1.8</java.version>
 </properties>

 <dependencies>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-data-jpa</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-security</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-thymeleaf</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-web</artifactId>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-configuration-processor</artifactId>
   <optional>true</optional>
  </dependency>
  <dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <scope>runtime</scope>
  </dependency>
  <dependency>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-test</artifactId>
   <scope>test</scope>
  </dependency>
  <dependency>
   <groupId>org.springframework.security</groupId>
   <artifactId>spring-security-test</artifactId>
   <scope>test</scope>
  </dependency>
  <dependency>
   <groupId>net.sourceforge.nekohtml</groupId>
   <artifactId>nekohtml</artifactId>
   <version>1.9.21</version>
  </dependency>
  <!-- https://mvnrepository.com/artifact/org.apache.commons/commons-lang3 -->
  <dependency>
   <groupId>org.apache.commons</groupId>
   <artifactId>commons-lang3</artifactId>
   <version>3.7</version>
  </dependency>
 </dependencies>

 <build>
  <plugins>
   <plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
   </plugin>
  </plugins>
 </build>
</project>

File: application.yml

I have worked mostly with application.properties files, but found that the YAML format is easier to use when defining multiple data sources. In this example application, I have defined two data sources multitenancy.mtapp.dataSources which are pointing to 2 MySQL databases running on a MySQL instance.

spring:
  thymeleaf:
    cache: false
    mode: LEGACYHTML5
  jpa:
    database: mysql
    show-sql: true 
    generate-ddl: false
    hibernate: 
      ddl-auto: none  

multitenancy: 
  mtapp: 
    dataSources: 
      -
        tenantId: tenant_1 
        url: jdbc:mysql://localhost:3306/dbtenant1?useSSL=false 
        username: tenant1 
        password: admin123 
        driverClassName: com.mysql.jdbc.Driver 
      -
        tenantId: tenant_2
        url: jdbc:mysql://localhost:3306/dbtenant2?useSSL=false
        username: tenant1
        password: admin123
        driverClassName: com.mysql.jdbc.Driver   

Note that spring.thymeleaf.mode is set to LEGACYHTML5. The Cyber NekoHTML dependency will then take care of not being so strict about parsing HTML5 and ThymeLeaf will not complain.

File: CustomUserDetails.java

Note that Spring Security provides the interface UserDetails. An implementation of this interface requires details about the user to be authenticated. For this, Spring Security provides a User class which can be used as is or extended. The example application extends this org.springframework.security.core.userdetails.User class to add the tenant attribute to the User.

package com.example.model;

import java.util.Collection;

import org.springframework.security.core.GrantedAuthority;

/**
 * CustomUserDetails class extends the Spring Security provided
 * {@link org.springframework.security.core.userdetails.User} class for
 * authentication purpose. Do not confuse this with the {@link User} class which
 * is an entity for storing application specific user details like username,
 * password, tenant, etc in the database using the JPA {@literal @}Entity
 * annotation.
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 *
 */
public class CustomUserDetails 
    extends org.springframework.security.core.userdetails.User {

    private static final long serialVersionUID = 1L;

    /**
     * The extra field in the login form is for the tenant name
     */
    private String tenant;

    /**
     * Constructor based on the spring security User class but with an extra
     * argument <code>tenant</code> to store the tenant name submitted by the end
     * user.
     * 
     * @param username
     * @param password
     * @param authorities
     * @param tenant
     */
    public CustomUserDetails(String username, String password, 
                Collection<? extends GrantedAuthority> authorities,
                String tenant) {
        super(username, password, authorities);
        this.tenant = tenant;
    }

    // Getters and Setters
    public String getTenant() {
        return tenant;
    }

    public void setTenant(String tenant) {
        this.tenant = tenant;
    }

}

File: CustomUserDetailsServiceImpl.java

Spring Security provides a interface UserDetailsService which has just one method declared in it – loadUserByUsername which returns the UserDetails.  Now this UserDetails is used for authentication. In the example application code, the CustomUserDetails already defines the attributes for the User.

package com.example.security;

import java.util.HashSet;
import java.util.Set;

import org.apache.commons.lang3.StringUtils;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.security.core.GrantedAuthority;
import org.springframework.security.core.authority.SimpleGrantedAuthority;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import org.springframework.stereotype.Service;

import com.example.model.CustomUserDetails;
import com.example.model.Role;
import com.example.model.User;
import com.example.service.UserService;

/**
 * {@link CustomUserDetailsService} contract defines a single method called
 * loadUserByUsernameAndTenantname.
 * 
 * The {@link CustomUserDetailsServiceImpl} class simply implements the contract
 * and delegates to {@link UserService} to get the
 * {@link com.example.model.User} from the database so that it can be compared
 * with the {@link org.springframework.security.core.userdetails.User} for
 * authentication. Authentication occurs via the
 * {@link CustomUserDetailsAuthenticationProvider}.
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 *
 */
@Service("userDetailsService")
public class CustomUserDetailsServiceImpl implements CustomUserDetailsService {

    @Autowired
    private UserService userService;

    @Override
    public UserDetails loadUserByUsernameAndTenantname(String username, String tenant)
            throws UsernameNotFoundException {
        if (StringUtils.isAnyBlank(username, tenant)) {
            throw new UsernameNotFoundException("Username and domain must be provided");
        }
        // Look for the user based on the username and tenant by accessing the
        // UserRepository via the UserService
        User user = userService.findByUsernameAndTenantname(username, tenant);

        if (user == null) {
            throw new UsernameNotFoundException(
                    String.format("Username not found for domain, "
                            + "username=%s, tenant=%s", username, tenant));
        }

        Set<GrantedAuthority> grantedAuthorities = new HashSet<>();
        for (Role role : user.getRoles()) {
            grantedAuthorities.add(new SimpleGrantedAuthority(role.getRole()));
        }

        CustomUserDetails customUserDetails = 
                new CustomUserDetails(user.getUsername(), 
                        user.getPassword(), grantedAuthorities, tenant);
        
        return customUserDetails;
    }
}

File: CustomUserDetailsAuthenticationProvider.java

Spring Security provides class AbstractUserDetailsAuthenticationProvider which allows subclasses to override and work with UserDetails objects. The class is designed to respond to UsernamePasswordAuthenticationToken authentication requests.

The CustomUserDetailsAuthenticationProvider implemented for this application delegates to the CustomUserDetailsService (implemented by CustomUserDetailsServiceImpl) for retrieving the UserDetails for authentication

package com.example.security;

import org.springframework.security.authentication.BadCredentialsException;
import org.springframework.security.authentication.InternalAuthenticationServiceException;
import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
import org.springframework.security.authentication.dao.AbstractUserDetailsAuthenticationProvider;
import org.springframework.security.core.AuthenticationException;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.util.Assert;

/**
 * {@link CustomUserDetailsAuthenticationProvider} extends
 * {@link AbstractUserDetailsAuthenticationProvider} and delegates to the
 * {@link CustomUserDetailService} to retrieve the User. The most important
 * feature of this class is the implementation of the <code>retrieveUser</code>
 * method.
 * 
 * Note that the authentication token must be cast to CustomAuthenticationToken
 * to access the custom field - tenant
 * 
 * 
 * @author Sunit Katkar
 * @version 1.0
 * @since 1.0 (April 2018)
 */
public class CustomUserDetailsAuthenticationProvider 
            extends AbstractUserDetailsAuthenticationProvider {

    /**
     * The plaintext password used to perform PasswordEncoder#matches(CharSequence,
     * String)} on when the user is not found to avoid SEC-2056
     * (https://github.com/spring-projects/spring-security/issues/2280).
     */
    private static final String USER_NOT_FOUND_PASSWORD = "userNotFoundPassword";

    /**
     * For encoding and/or matching the encrypted password stored in the database
     * with the user submitted password
     */
    private PasswordEncoder passwordEncoder;

    private CustomUserDetailsService userDetailsService;

    /**
     * The password used to perform
     * {@link PasswordEncoder#matches(CharSequence, String)} on when the user is not
     * found to avoid SEC-2056. This is necessary, because some
     * {@link PasswordEncoder} implementations will short circuit if the password is
     * not in a valid format.
     */
    private String userNotFoundEncodedPassword;

    public CustomUserDetailsAuthenticationProvider(PasswordEncoder passwordEncoder,
            CustomUserDetailsService userDetailsService) {
        this.passwordEncoder = passwordEncoder;
        this.userDetailsService = userDetailsService;
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.springframework.security.authentication.dao.
     * AbstractUserDetailsAuthenticationProvider#additionalAuthenticationChecks(org.
     * springframework.security.core.userdetails.UserDetails,
     * org.springframework.security.authentication.
     * UsernamePasswordAuthenticationToken)
     */
    @Override
    protected void additionalAuthenticationChecks(UserDetails userDetails,
            UsernamePasswordAuthenticationToken authentication) throws AuthenticationException {

        if (authentication.getCredentials() == null) {
            logger.debug("Authentication failed: no credentials provided");
            throw new BadCredentialsException(
                    messages.getMessage("AbstractUserDetailsAuthenticationProvider.badCredentials", 
                            "Bad credentials"));
        }
        // Get the password submitted by the end user
        String presentedPassword = authentication.getCredentials().toString();

        // If the password stored in the database and the user submitted password do not
        // match, then signal a login error
        if (!passwordEncoder.matches(presentedPassword, userDetails.getPassword())) {
            logger.debug("Authentication failed: password does not match stored value");
            throw new BadCredentialsException(
                    messages.getMessage("AbstractUserDetailsAuthenticationProvider.badCredentials", 
                            "Bad credentials"));
        }
    }

    @Override
    protected void doAfterPropertiesSet() throws Exception {
        Assert.notNull(this.userDetailsService, "A UserDetailsService must be set");
        this.userNotFoundEncodedPassword = this.passwordEncoder.encode(USER_NOT_FOUND_PASSWORD);
    }

    /*
     * (non-Javadoc)
     * 
     * @see org.springframework.security.authentication.dao.
     * AbstractUserDetailsAuthenticationProvider#retrieveUser(java.lang.String,
     * org.springframework.security.authentication.
     * UsernamePasswordAuthenticationToken)
     */
    @Override
    protected UserDetails retrieveUser(String username, UsernamePasswordAuthenticationToken authentication)
            throws AuthenticationException {
        CustomAuthenticationToken auth = (CustomAuthenticationToken) authentication;
        UserDetails loadedUser;

        try {
            loadedUser = this.userDetailsService
                    .loadUserByUsernameAndTenantname(auth.getPrincipal().toString(),
                            auth.getTenant());
        } catch (UsernameNotFoundException notFound) {
            if (authentication.getCredentials() != null) {
                String presentedPassword = authentication.getCredentials().toString();
                passwordEncoder.matches(presentedPassword, userNotFoundEncodedPassword);
            }
            throw notFound;
        } catch (Exception repositoryProblem) {
            throw new InternalAuthenticationServiceException(repositoryProblem.getMessage(), 
                    repositoryProblem);
        }

        if (loadedUser == null) {
            throw new InternalAuthenticationServiceException(
                    "UserDetailsService returned null, "
                    + "which is an interface contract violation");
        }
        return loadedUser;
    }
}

Conclusion of Part 1

In this part of the blog post, we have seen how this application works, how to set up your project, how Spring Security is set up.

In the next part, I will understand how the JPA and Hibernate part of the code is set up. Also we will see how Spring Security is tied in with the multi-tenancy Hibernate code.

Source