Whenever you're working on existing code whether built by someone else or your past self , knowing the design patterns helps you begin to see things differently—you will discover problems and ways to improve the code. There are numerous design patterns, but three popular ones, which I'll present in this introductory article, are singleton pattern, factory pattern, and observer pattern. I want this tutorial to be as easy as possible for anyone to understand, whether you are an experienced programmer or a beginner to coding. If you just want to get the idea of design patterns in general, you do not need to clone the sample project or install any of the tools.
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However, to run the sample code, you need to have the following installed:. I am using Java, but you can implement the design pattern using any programming language that supports the abstraction principle. The singleton pattern is a very popular design pattern that is also relatively simple to implement because you need just one class. However, many developers debate whether the singleton design pattern's benefits outpace its problems because it lacks clear benefits and is easy to abuse.
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Few developers implement singleton directly; instead, programming frameworks like Spring Framework and Google Guice have built-in singleton design pattern features. But knowing about singleton is still tremendously useful. The singleton pattern makes sure that a class is created only once and provides a global point of access to it. Singleton pattern: Ensures that only one instantation is created and avoids creating multiple instances of the same object. The diagram below shows the typical process for creating a class object.
When the client asks to create an object, the constructor creates, or instantiates, an object and returns to the class with the caller method. However, this happens every single time an object is requested—the constructor is called, a new object is created, and it returns with a unique object. I guess the creators of the OOP language had a reason behind creating a new object every single time, but the proponents of the singleton process say this is redundant and a waste of resources.
The following diagram creates the object using the singleton pattern.
Here, the constructor is called only when the object is requested the first time through a designated getInstance method. This is usually done by checking the null value, and the object is saved inside the singleton class as a private field value. The next time the getInstance is called, the class returns the object that was created the first time. No new object is created; it just returns the old one. When multiple processes using threads access the resource simultaneously, a problem occurs.
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There is one solution to this, and it is called double-checked locking for multithread safety, which is shown here:. Just to emphasize the previous point, make sure to implement your singleton directly only when you believe is a safe option to do so. The best way is to leverage the singleton feature is by using a well-made programming framework. The factory pattern is another well-known design pattern, but it is a little more complex. The factory pattern defines an interface for creating an object but lets the subclasses decide which class to instantiate.
Factory pattern: Delegates object creation to the factory class so it hides the creation logic. Instead of the client directly calling the object creation, the client asks the factory class for a certain object, type x. Based on the type, the factory pattern decides which object to create and to return. When "RedHat" is passed as the type to the factory class, the WildFly server is created:. Finally, there is the observer pattern.
Like the singleton pattern, few professional programmers implement the observer pattern directly.
Java enterprise design patterns
However, many messaging queue and data service implementations borrow the observer pattern concept. The observer pattern defines one-to-many dependencies between objects so that when one object changes state, all of its dependents are notified and updated automatically. The easiest way to think about the observer pattern is to imagine a mailing list where you can subscribe to any topic, whether it is open source, technologies, celebrities, cooking, or anything else that interests you. Each topic maintains a list of its subscribers, which is equivalent to an "observer" in the observer pattern.
When a topic is updated, all of its subscribers observers are notified of the changes. And a subscriber can always unsubscribe from a topic. As the following diagram shows, the client can be subscribed to different topics and add the observer to be notified about new information. Because the observer listens continuously to the subject, the observer notifies the client about any change that occurs.
This code describes an interface for different topics to implement the defined methods. Notice how an observer can be added, removed, or notified. This class defines the implementation of a particular topic. With a design pattern, you can document a solution to a design problem. Design patterns were introduced by the architect Christopher Alexander and have been adapted for various other disciplines and are very popular among developers, as they provide solutions to general problems that software developers face.
It provides a template to solve common problems while designing a system or an application. Design patterns act as a common language between developers so that each developer easily understands the concept while working with common problems. Design patterns have evolved after many trials and captured solutions that have been applied to problems successfully. Even a new developer can learn a lot from these design patterns.
In the context of Java, design patterns are divided into three categories — creational , structural, and behavioral patterns. With a creational design pattern, you can instantiate an object in the best possible manner for specific problems. The object creation process can lead to unwanted complexity in the design problems.
This is where creational design patterns provide a solution by controlling the object creation process in different ways. There are five different creational design patterns:. The singleton pattern makes sure that only one instance of the class exists by restricting instantiation of a class. It is one of the Gangs of Four Design Patterns.
Additionally, it is one of the simplest design patterns in Java. Although it is the simplest design pattern, it has a lot of implementation concerns. The instance is created irrespective of whether the instance is required in runtime or not in the client application. Static blocks are executed during class loading before the constructor is called.
It is similar to Eager Initialization except for the fact that the class is created in a static block, which provides an option for exception handling.
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The drawback of this type of initialization is that if only two or three static fields are required out of a few static fields, we need to create an instance irrespective of whether it is required or not. Lazy initialization creates an instance in the global access method. By lazy initialization, you can actually delay the creation of an object until it is required. With this method, you can check if the instance is already created using an instance variable. It will create an instance if the instance is null, or else it will return the reference if the instance is already created. If there are two threads, both will create the instance and check if the instance is null.
This will result in two instances at the end. To resolve the issue, you can use double-checked locking method. It will recheck the instance variable in a synchronized block. Bill Pugh devised an approach to create Singleton class using inner static helper class.
Due to issues with Java memory model, if there are multiple threads, then each thread will create instance simultaneously. This is where Bill Pugh Singleton method works. This Singleton pattern was suggested by Joshua Bloch to use Enum as its value is instantiated only once in Java. With Enum, only one instance is guaranteed. It provides implicit support for thread safety and is a good way to have singleton with minimal effort. If the application is distributed, objects are serialized frequently into the file system so that they can be read later when required. While de-serializing, the objects always creates a new instance.
Since both variables have different values, there will be two instances in the class that will create the same problem of multiple instances in the application.