The Observer Pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
It's a very useful pattern for event-driven programming, and it's one of the foundations of the Model-View-Controller (MVC) architectural pattern.
In this article, we'll take a look at how to use the Observer Pattern in a Spring Boot application. We'll start by looking at the basic concepts of the pattern and how it's typically used. Then we'll see how Spring Boot makes it easy to implement the pattern with its built-in support for the JavaBeans specification.
As we mentioned, the Observer Pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
There are a few things to note about this definition:
The subject is an object, not a class. This means that a subject can be any type of object, including a primitive type like an int or a String.
The observers are also objects, not classes. This means that an observer can be any type of object, including a primitive type like an int or a String.
The subject maintains a list of its observers. This list is typically implemented as a collection, such as an ArrayList or a LinkedList.
The subject notifies its observers by calling one of their methods. The method that is called depends on the implementation, but it is typically a method that is specific to the Observer Pattern, such as update() or handleEvent().
The Observer Pattern is often used in event-driven programming. This means that the observers are notified of state changes in the subject as they happen, rather than being polled for changes.
Now that we've seen the basic concepts of the Observer Pattern, let's take a look at how it's typically used.
There are a few different ways that the Observer Pattern can be used, but the most common is in event-driven programming.
In event-driven programming, the application is typically organized around a central event loop. Events are generated by the user, by the hardware, or by the application itself, and they are handled by event handlers.
The Observer Pattern can be used to decouple the event handlers from the event loop. The event handlers can be implemented as observers, and they can be registered with the event loop. When an event occurs, the event loop notifies the observers, and they can handle the event.
This has a few advantages:
The event handlers can be implemented as reusable components.
The event handlers can be registered and unregistered at runtime.
The event handlers can be called in any order.
The event handlers can be called concurrently.
Let's see how to use the Observer Pattern in a Spring Boot application.
Spring Boot makes it easy to use the Observer Pattern with its built-in support for the JavaBeans specification.
JavaBeans is a specification for Java components that defines a set of conventions for writing Java code. One of the conventions is that a JavaBeans component must have a JavaBeans-style setter method for each of its properties.
For example, if a component has a property called foo, the component must have a setFoo() method. When the setFoo() method is called, the component must fire a FooPropertyChangeEvent.
Spring Boot uses this convention to support the Observer Pattern. When a Spring Boot application is started, it scans the classpath for classes that have JavaBeans-style setter methods. For each setter method, it registers an observer.
When the setter method is called, the observer is notified. The observer can then take any action that is appropriate, such as calling a method on the subject or logging a message.
Let's see how this works with a simple example.
Consider a Spring Boot application that has a property called foo. The application sets the foo property in the application.properties file:
foo=bar
The application also has a class called FooObserver that implements the Observer interface:
public class FooObserver implements Observer {
@Override
public void update(Observable o, Object arg) {
// do something
}
}
When the application is started, the Spring Boot framework scans the classpath for classes that have JavaBeans-style setter methods. It finds the FooObserver class and registers it as an observer for the foo property.
When the application sets the foo property, the Spring Boot framework calls the FooObserver's update() method. The update() method can then take any action that is appropriate, such as logging a message or calling a method on the subject.
This example shows how the Observer Pattern can be used to decouple the components of a Spring Boot application. The observers can be implemented as reusable components, and they can be registered and unregistered at runtime.
In this article, we've seen how to use the Observer Pattern in a Spring Boot application. We've looked at the basic concepts of the pattern and how it's typically used. Then we've seen how Spring Boot makes it easy to implement the pattern with its built-in support for the JavaBeans specification.