Preface
Introduction
The Salespoint Framework is intended to minimize developing effort of point-of-sale applications. Salespoint 2010 users complainted about complexity, missing features and bugs. Thus, the decision was made to re-design and re-implement the framework from scratch. In 2013 some parts again were re-implemented with components of the Spring framework. Our development goal was an easy-to-use framework primarily targeted for educational purposes. As such Salespoint is not tailored to any specific application, but designed with a wide area of applications in mind.
Models and design patterns employed in Salespoint are inspired by Enterprise Patterns and MDA [epam]. An overview of the functionality of the new features in Salespoint are detailed in this document. We would like to thank all Salespoint users who submitted their feedback and encourage future users of Salespoint to do the same.
Prerequisites
Working with Salespoint requires some basic knowledge of the underlying technologies such as the Spring Framework as well as JPA to implement the persistence layer. This script gives you an introduction into these technologies. Also the Guestbook sample project is a good starting point to get familiar with the technology stack.
The Videoshop sample project
To give you an example of what applications built on top of Salespoint can actually look like, we provide the Videoshop sample project that covers a few key areas:
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Using and extending the business modules provided by Salespoint to abstract a concrete problem domain.
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Implementing a web layer (controllers, views) on top of Java based business services of that business domain.
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Adding the necessary bits and pieces for technical aspects such as security and transactions.
| The following sections will refer to code of the sample project here and there so it might be useful to have the repository cloned and the project imported into your IDE of choice to quickly be able to navigate these. |
Technical Architecture
Technically Salespoint is a library to build Java based Point of Sales (PoS) applications. Whiles these applications are usually implemented as web applications, large parts of Salespoint are also usable if the application architecture of choice is a rich client implemented in Java.
The technical foundation for Salespoint is the Spring Framework[1] and Spring Boot (as some kind of opinionated layer on top of Spring Framework to ease its applicance to a large degree. Salespoint is built using the Maven build system[2]. The reference documentation and static website are built with Asciidoctor[3].
Here you can find the salespoint class diagram about the basic structure.
1. Core concepts
Before we dive into the technical details of Salespoint, let’s make sure we understand some core concepts that will help us understand the different modules of the framework a bit better. Each of the modules consists of a bit of code and the classes can usually be grouped into 3 different buckets: entities and value objects, summarized to newables, repositories and services, summarized to injectables as well as Spring container configuration and extension code. The following sections will cast some light on these kinds of types and their special traits.
1.1. Repositories, services and controllers
Repositories and services are used ot interact with entities and value objects. Repositories usually abstract data access into an interface that simulates a collection of entities that instances can be added to, or looked up using a parameterized criteria like "all users with a certain lastname like…". As Salespoint is using a relational database as persistence backend it leverages the Java Persistence API (JPA) and Spring Data JPA to implement repositories.
As repositories abstract a collection of an aggregate root (e.g. an order with it’s charge and line items), we usually need a higher level abstraction that exposes a more use-case oriented API: a service. It fulfils a more coarse grained set of operations that might include the interaction with multiple repositories, trigger other services in the system etc.
Repositories, services and controllers are types whose instances are managed by the Spring container. Thus they’re never instantiated manually in the application code, except for unit testing purposes. Application code uses dependency injection to access
2. Maven
Maven simplifies the management of project dependencies, furthermore it downloads and organizes all dependencies. Another focus of Maven is the management of a project’s build. All settings are placed in the pom.xml file (http://maven.apache.org/pom.html).
3. Spring
In contrast to earlier versions of the Salespoint Framework, Salespoint obeys the MVC pattern. Salespoint mostly provides the application services to interact with the the model of an MVC application as no parts of the view or the controller are implemented in the framework.
Salespoint is designed as foundation for development of web applications, using the Spring Framework to implement views, controllers and business services abstracting the domain your application-to-be-created is applied to. To ease this development, Salespoint includes a few extensions to the Spring Framework that will avoid you having to implement very tedious tasks over and over again. Read more about this in Spring 5.
As a big new approach in development with JPA, the Spring Framework with its repository interfaces can make the work very fast, clean and easy. The Crudrepository provides the basic methods for working with the database (CRUD stands for Create, Read, Update and Delete).
Business modules
Salespoint ships with a set of business modules that interact with each other. The following component diagram shows the overall structure of the setup. The relationships are used as follows:
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uses — The module has a Spring component dependency and actively interacts with the target module. Implies a type dependency into the target module as well.
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listens to — The module contains an event listener for events the target modules publishes. Implies a type dependency into the target module as well.
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depends on — The module has a general type dependency to the target module, i.e. it uses the target module as library.
4. User accounts
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While Spring Security is used as primary technology to actually implement application security, there’s also a domain abstraction of a user acoount in the form of a UserAccount entity.
That means that UserAccount instances created within the system are automatically targets for authentication.
The primary purpose of a UserAccount besides the security integration is the ability to assign other business entities like an order or a reservation to an account.
A UserAccount primarily consists of a UserAccountIdentifier whose String representation is used as the login name and a Password (representing an encrypted or non-encrypted password) value object.
To create UserAccounts, the UserAccountManager's method create(String userName, String password, Role… role) can be used.
The String given as password will be encrypted using Spring Security.
Also, it makes sure that the login names are unique and a password is always set.
4.1. Roles
As the name suggests, a Role represents the role a UserAccount has. It is usually used in security expressions with either @Secured or in a Thymeleaf template to restrict access to functionality or change the appearance of a view depending on a user’s status.
A view for a user having an administrator role may display different content — e.g. delete buttons — than for a user not having that role.
Role is a value type[4] and thus instances are created using a factory method: Role.of("ROLE_ADMIN");.
Up until Salespoint 7.2.0, roles' names had to start with ROLE_, so that they could be used for verifications in Spring Security expressions in Thymeleaf templates (via sec:authorize attributes) or on methods (using the @PreAuthorize annotation) with their plain name.
I.e. users had to be created with Role.of("ROLE_CUSTOMER") but the verifications would have used hasRole("CUSTOMER") expressions.
This asymmetry has been fixed with Salespoint 7.2.1 in which the role itself can also be set up using an unprefixed name like CUSTOMER.
4.2. Login
To reduce code repetition, Salespoint contains code to automate the user log in. Using a Thymeleaf template, a special login form is generated, which is handled by an interceptor. The interceptor verifies the user password and associates the current session with the user using <login> and <logoff>. The session is required, because multiple users can be logged on at the same time.
As of version 7.1, Salespoint allows the login via a user’s registered email address.
To enable that, set salespoint.authentication.login-via-email to true.
application.properties)salespoint.authentication.login-via-email=trueIf the option is activated, UserAccountManager.save(…) will reject UserAccount instances that don’t have an email address set.
That means you need to provide an email when initially registering, i.e. you’ll need to use the overload of UserAccountManager.create(…) that takes an email address as parameter.
4.3. @LoggedIn annotation
The @LoggedIn annotation can be used to inject the currently logged in UserAccount into controller method parameters. The type of the annotated paramater can either be
Optional<UserAccount> for methods that should support non-authenticated access, or UserAccount. With the former variant, Optional.map() can be conveniently used to react
to the presence of the user account in a functional programming style:
@Controller
public class ExampleController {
@GetMapping("/example")
String example(@LoggedIn Optional<UserAccount> userAccount) {
// functional style using map and lambda expression:
return userAccount.map(account -> {
// things to be done if the user account is present
return "...";
}).orElse("redirect:/"); // if the user account is *not* present
}
}4.4. Limitation
The org.salespointframework.useraccount.UserAccount is limited to the given attributes and methods.
Due to the fact, that Salespoint use the SecurityContext for authentication, the UserAccount cannot be extended. In the background the org.salespointframework.useraccount.UserAccount is converted to an instance of org.springframework.security.core.userdetails.UserDetails.
If these properties don’t meet all requirements, wrap the UserAccount in a new entity. Put all the new features in this new entity and connect this information via a @OneToOne relationship with the UserAccount. An example can be found in the Videoshop project.
@Entity
public class UserAccountExtension {
private String string;
private Integer integer;
@OneToOne
private UserAccount userAccount;
…
}5. Quantity
Quantity is used to represent amounts of anything. Furthermore a Quantity can be used to calc with (plus, minus).
But only Quantities with the same Metric can be combined or compared, so every Quantity has an Metric attribute.
And of course, every Quantity has an amount value, represented as a java.math.BigDecimal.
5.1. Metric
The composite type Metric contains all information pertaining to the unit or metric of the represented object.
Furthermore, an object of type Metric has a description field, to explain the meaning of the metric in detail. For the example of a meter a possible description could be "The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.".
6. Catalog
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Salespoint is intended as framework for point-of-sale applications. Items for sale are called products and represented by instances of the class Product. To represent different kinds of products, Product can be sub-classed. Products are managed by a Catalog implementation (see below). Products are identified using a ProductIndentifier.
The Catalog interface was designed to manage Product instances in the system. It provides functionality to add, remove, and find products. Products can be searched by their name or category. The PersistentCatalog is an implementation of the Catalog interface.
7. Inventory
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Event listeners
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The inventory package contains functionality to keep track of the amount of items we have for a given Product (see Catalog for details).
Inventory is a simple Spring Data repository, that allows to access InventoryItem instances and acts as base type for the actual concrete provided inventory interfaces.
A Salespoint application usually decides for one of the following models and uses one or the other inventory repositories throughout the entire application.
7.1. UniqueInventory — one inventory item per product
This model assumes that there is a single InventoryItem created per Product instance.
This approach acts like there is a single warehouse keeping track of the stock for a given Product.
It’s the easier mode as the Quantity provided in the InventoryItem is logically equivalent to the number of items available for a given Product.
UniqueInventoryItems will also automatically be deducted for order completions (see Handling order completion events).
7.2. MultiInventory — multiple inventory items per product
In case you need to model multiple warehouses that store products, MultiInventory relaxes the relationship between Product and InventoryItem to a 1:n one.
This of course results in …findByProductIdentifier(…) returning a InventoryItems (note the plural) instance which is a collection of all InventoryItem instances available.
InventoryItems exposes a ….getTotalQuantity() to easily obtain the overall number of items available across all InventoryItems.
MultiInventoryItems are not automatically deduced upon order completions as it’s not clear, which of the MultiInventoryItem instances to reduce the quantity for.
7.3. Handling order completion events
The inventory ships with a listener for OrderCompleted events (see The Order lifecycle for details) to update the stock for each line item contained in an Order that is managed by a UniqueInventoryItem.
By default, an OrderCompletionFailure is thrown in case any of the OrderLine items aren’t available in the required amount.
The exception contains an OrderCompletionReport that can be inspected for the item lacking enough stock.
By default, all OrderLine instances are processed.
If you want to disable order updates completely, configure the property salespoint.inventory.disable-updates to true.
However, you might want to only exclude some of them as the Product they point to don’t represent a product to keep track of (e.g. some service like "gift wrapping" or the like.
The LineItemFilter interface allows defining a strategy to decide which of the OrderLine instances are supposed to be processed by the inventory.
To customize the strategy, simply declare an implementation of LineItemFilter as Spring bean to your application configuration:
class Config {
@Bean
LineItemFilter filter() {
return item -> !item.getProductName().startsWith("to filter:");
}
}A declaration like this will cause only OrderLine instances pointing to Product instance with a matching name to be subject for reduction in the inventory.
On OrderCancelled the Inventory will restock if the Order has already been completed.
See the Javadoc of the event listener for details.
8. Accountancy
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The accountancy package contains functionality supporting book keeping. AccountancyEntry is a representation of an accounting entry. Accountancy aggregates AccountancyEntrys. Every AccountancyEntry is uniquely identified by an AccountancyEntryIdentifier. AccountancyEntry extends AbstractEntity and serves as persistence entity, while PersistentAccountancy implements Accountancy and provides transparent access to the JPA layer. AccountancyEntryIdentifier is used as primary key attribute, when entities are stored in the database.
By implementing and sub-classing Accountancy, the notion of different accounts, as known from double-entry bookkeeping, can be realized.
To create a new account, AccountancyEntry has to be sub-classed. Every object of such a class belongs to the same account. Accessing per-account entries is facilitated by specifiying the desired class type when calling get() or find() methods of Accountancy.
8.1. Handling OrderPaid events
The accountancy subsystem handles OrderPaid events by creating a ProductPaymentEntry for the order.
Reversely, it will create a compensating ProductPaymentEntry on OrderCancelled if theres a revenue ProductPaymentEntry available for Order.
See the Javadoc of the event listener for details.
9. Payments
For the paying transaction, Salespoint provides some payment strategies.
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DebitCard— representation of e.g. EC-card or MaestroCard -
CreditCard— equal toDebitCardbut with creditLimit-Attribute -
Cheque— represents a written order of payment in a digital form -
Cash— represents a payment, made direct in cash
10. Order management
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The order management is centered around the Order aggregate that aggregates OrderLines and ChargeLines.
An OrderLine refers to a Product from the Catalog in combination with a Quantity.
Note, that the OrderLine creates a copy of the Products name and price, so that changes to those attributes of a Product don’t affect existing Orders.
A ChargeLine describes any additional charge that can be associated with the Order, e.g. discounts, any general service charges and so on.
AttachedChargeLine are special ChargeLines that are attached to an OrderLine.
Just as a standard ChargeLine they can be used to add charged services or discounts but attached to the particular OrderLine instead of the overall Order.
Order exposes a variety of methods to lookup
10.1. The Order lifecycle
An Order is an aggregate root[5], which means that state changes to it that have important business meaning trigger events.
These events are then used by other business modules to act on them. Currently, the following events are exposed:
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OrderPaid— the event being thrown if the order gets paid, usually through a call toOrderManagement.pay(…). The Accountancy module makes use of those events as described in Handling OrderPaid events. -
OrderCompleted— the event being thrown when theOrderis about to be completed, usually through a call toOrderManagement.completeOrder(…). The Inventory module ships with a listener for theOrderCompletedevent to update the inventory and reduce the stock of ordered items. See Handling order completion events for details. -
OrderCanceled— the event being thrown when anOrderwas canceled, usually throughOrderManagement.cancelOrder(…). Accountancy will react by creating a newAccountancyEntrythat compensates for the revenue entry created for theOrderPaidevent. Inventory will restore the previously subtracted stock if the order had been cancelled already.
10.1.1. Handling events
In case you want to write custom code to react to those events you need to implement an event handler.
An event handler is a simple Spring component that has methods annotated with either @EventListener or @TransactionalEventListener.
@Component
class MyEventListener {
@EventListener
public void handleEvent(OrderCompleted event) {
// Your code goes here
}
// alternatively
@TransactionalEventListener
public void handleEventAfterTransaction(OrderCompleted event) {
// Your code goes here
}
}The difference between the two methods is when they’re actually invoked by the framework.
The former method is called before the changes to the database are eventually persisted to the database.
That means, the event listener can throw an exception and the action that caused the event to be published in the first place will be canceled, too.
On the other hand an @EventListener will not be guaranteed that the original action will succeed as it might be rejected if it violates e.g. database constraints.
In contrast, a @TransactionalEventListener will be called after the original action has completed successfully.
That in turn means, that the listener has no chance to abort that action.
11. Business time
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To be able to simulate the state of the system in a given point in time all components in the system that set a point in time on entities or lookup the current time use the BusinessTime interface.
By default, the implementation will just return the current system date. However, it also allows to "forward" the current business time by calling the BusinessTime.forward(…) method. This will augment the current business time by the duration handed into the method. E.g. forwading the business time by 2 months will return the very same time but two months ahead for each call to BusinessTime.getTime().
This is particularly useful for demoing purposes if e.g. orders can be timed to a certain date, credit cards become invalid after a certain point in time etc.
12. Technical APIs
Besides the business modules, Salespoint containes a few SPIs that can be used to extend the functionality of the core framework. These SPIs usually exist to make repeated use-cases easier to implement and free you from writing too much boilerplate code.
12.1. DataInitializer
Every application should be tested, so an easy way is, to use test data. Salespoint provides an SPI called DataInitializer for you to implement. If your implementation is registered as Spring bean (e.g. by annotating it with @Component), it will automatically be found and initialize() will be called at application startup. As shown in the Videoshop project, a DataInitializer class is registered and creates a lot of data and finally adds this data to the application.
12.2. Entity binding in WebMVC controller methods
Controller methods can bind request parameters and path variables to entity instances directly if the value to be bound is an identifier of the entity to bind. Assuming a controller method displaying a form registers an entity with the model:
@Controller
class MyController {
@GetMapping("/myform")
String displayForm(Model model) {
Order order = … // Order with id 4711.
model.addAttribute("order", order);
return "myform";
}
}The identifier of the order can be used in anchors and form values like this.
<!-- Expands to /orders/4711 -->
<a th:href="@{/orders/{id}(id=${order.id})}" …>…</a>
<form th:object="${myForm}" …>
<input type="hidden" name="order" th:value="${order.id}" />
…
</form>When following the link or submitting the form the identifier can be bound to the backing entity type directly:
@Controller
class MyController {
@GetMapping("/order/{order}")
String displayForm(Order order) { … }
}Note how we do not use a String or e.g. Long parameter type as controller method parameter but the entity type directly.
This is achieved by Spring MVC trying to convert the source String value submitted through the request into the identifier type of the entity in the first place as well as a subsequent repository lookup using the identifier value just created.
In the above example we’d convert "4711" to a Long 4711 (assuming the identifier of Order is declared as Long).
That in turn gets converted into an Order instance by calling OrderRepository.findById(Long) with the just converted value.
Technical Stack
15. Spring Data JPA
Spring module to easily build data acess layers using JPA 2.1 (Java Persistence API).
17. Java Money
With Salespoint Money class and its related components were replaced by the Java Money library. Prices and other monetary values are represented as org.javamoney.moneta.Money objects. Due to the fact that all representing objects are immutable, all arithmetic functions produce a new instance.
Money first = Money.parse(“USD 23.07”);
Money second = Money.parse(“USD 18.07”);
Money sum = first.plus(second);When used in domain objects, Money instances need to be mapped using @Lob.
Appendix
Appendix A: New and Noteworthy
A.1. What’s new in Salespoint 7.1?
A.1.1. Upgrade to Spring Boot 2.1 GA
#225 — Simple maintenance upgrade as 7.0 still referred to a milestone release of Spring Boot 2.0.
A.1.3. Allow login via email address
#222 — Added the ability to use a user’s email address for login purposes. For more details, see this section.
A.1.4. Improvements in the order aggregate and event handling
#226 — We added the ability to register ChargeLine instances for a dedicated OrderLine.
Read more about that in the updated section of Order management.
A.1.5. New order cancelled event and fixes in event handling in general
#230 — The creation of a PaymentAccountancyEntry has been properly attached to the OrderPaid event now.
An OrderCancelled event is now published on order cancellation which triggers a rollback of the inventory updates as well as a compensating ProductPaymentEntry.
Read more on that in the updated section The Order lifecycle.
A.2. What’s new in Salespoint 6.1?
A.2.1. Upgrade to Spring Boot 1.2 and Spring 4.1
For Salespoint 6.1 we upgraded to Spring Boot 1.2, which transitively updates a few third party dependencies, most notably Spring 4.1. Read more on the new features of Spring Boot in its reference documentation and skim through the upgrade notes as well. The new features of Spring 4.1 are described here.
A.2.2. New @EnableSalespoint annotation to simplify configuration
Activating Salespoint in a web application has been quite cumbersome so far and required a dedicated set of annotations and imports of configuration classes. To simplify this, Salespoint 6.1 introduces @EnableSalespoint to take care of the previously manual steps. For more information on what gets acivated and imported, see the JavaDoc of the annotation.
A.2.3. Usage of the mail autoconfiguration shipping with Spring Boot 1.2
As Salespoint 6.1 upgrades to Spring Boot 1.2 we now use the email auto-configuration settings shipping with that release. This most notably means that you slightly have to alter your properties to configure the mail user, password and host. See Boot’s reference documentation and the list of current application properties for details.
Appendix B: FAQ
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Why do I see warning log statements about database tables not being accessible at startup?
This is due to a bug in Hibernate in combination with HSQLDB and constraints in databases. Hibernate tries to drop those on application startup but fails to as the (in-memory database) is empty. See this bug report for reference.
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How do I send emails from a Salespoint application?
Salespoint has everything prepared for you in terms of necessary dependencies. All you need to do is configure the following properties in
application.properties:spring.mail.host= spring.mail.username= spring.mail.password=If these properties get real values assigned, Salespoint will automatically create a Spring component of type
JavaMailSenderfor you that you can inject into your clients and use to send emails. See the Spring reference documentation on details for that.Never push your email credentials into the version control system. Alternatively for testing purposes, configure the
RecordingMailSenderas a Spring bean to simply write emails to be sent to the console instead of sending them to a real SMTP server. To automatically switch between the two setups have a look at how to use Spring’s profile support with Boot.
Appendix C: Glossary
- Dependency injection
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Dependency injection is a software design pattern that implements inversion of control and allows a program design to follow the dependency inversion principle. The term was coined by Martin Fowler. See Wikipedia.
- SPI
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Service Provider Interface — interfaces to be implemented to extend the functionality of the core framework. See Wikipedia for details.
Appendix D: Bibliography
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[] Eric Evans. Domain-Driven Design: Tackling Complexity in the Heart of Software. Prentice Hall. 2003
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[] Jim Arlow, Ila Neustadt. Enterprise Patterns and MDA: Building Better Software with Archetype Patterns and UML. Addison-Wesley. 2004.
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[] Martin Fowler. Inversion of Control Containers and the Dependency Injection pattern. 2004. http://www.martinfowler.com/articles/injection.html