Why dependency injection is used

When a new object is required, its dependencies need to be assigned to concrete classes. This task can be delegated to a container. When an instance of a particular type is requested to a container, it will inject the implementations required by that type. Those implementations are defined in a set of mappings that can easily be changed.

The Container creates an object of the specified class and also injects all the dependency objects through a constructor, a property, or a method at run time and disposes it at the appropriate time.

This is done so that we don't have to create and manage objects manually. Containers provide support for the following DI life cycle. The service life-time means how long the service will live before it's disposed of. There are currently three different lifetimes:.

Net Core Container. This is a simple example to show how you register and resolve dependencies using the Dependency Injection technique with. NET Core. The objective is to provide a web API that gets the list of albums consuming a Service. Create a new class library named Services and a new Class to represent an Album. We will register a dependency on the ConfigureService:. Add a new Controller Album with a method Get to return the list of albums.

The dependency will be injected by the constructor. As I demonstrated above, with Pure Dependency Injection all components are created explicitly by an application code.

But Pure DI is rare. Most applications and frameworks include a helper module referred to as DI Container that helps automate the wiring and instantiation. Angular and Ember have DI Containers. The container knows how to create all of your objects and their dependencies, so you can easily get an object with all of its dependencies with one simple call. It automatically creates an object of the specified class and also injects all the dependency objects through a constructor at run time and disposes it at the appropriate time.

Because doing it right is hard. Each service might be slightly different. For some of them we need to have just a single instance, that is reused everywhere, for others, we want to provide a new instance each time one is needed, or may be reuse instances within the scope of a single cell, or any other arbitrary scope. To satisfy the above-mentioned requirements and maintain loose coupling we needed to write a lot of code that grew both in size and complexity with every change.

DI container was meant to replace that code. We modeled it after the IoC container in Spring framework. Because TypeScript that we use for development supports decorators annotations we could carry over some design decisions from Spring. Particularly, we the use the Bean decorator to denote a service that should be managed by the DI container and Autowired to identify dependencies that should be injected by the container.

This is how the code looks today:. In previous implementations we also had a function called setupComponents responsible for instantiating and wiring the dependencies together. In the current version the code migrated inside the DI container implemented as the Context class. All we need to do in the Grid is to define our services beans and create the container instance:.

With the introduction of DI container the code has become much more readable, easier to reason about and test. Make your own mind about terminology. Inside ag-Grid stack the Context class represents the IoC container and is responsible for instantiating, configuring, and assembling the services beans.

The container gets its instructions on what objects to instantiate, configure, and assemble by reading configuration metadata. The configuration metadata is provided through TypeScript annotations and JavaScript configuration object available to the context during initialization. The container is created inside the constructor of the main Grid service when the datagrid is initialized:.

As you can see, the configuration object contextParams that describes all beans to managed by the container is provided as a constructor parameter. Here, we are going to implement Dependency Injection and strategy pattern together to move the dependency object creation completely out of the class. This is our third step in making the classes completely loose coupled.

It allows the creation of dependent objects outside of a class and provides those objects to a class through different ways. Using DI, we move the creation and binding of the dependent objects outside of the class that depends on them.

As you can see, the injector class creates an object of the service class, and injects that object to a client object. In this way, the DI pattern separates the responsibility of creating an object of the service class out of the client class.

As you have seen above, the injector class injects the service dependency to the client dependent. The injector class injects dependencies broadly in three ways: through a constructor, through a property, or through a method. Constructor Injection: In the constructor injection, the injector supplies the service dependency through the client class constructor.

Property Injection: In the property injection aka the Setter Injection , the injector supplies the dependency through a public property of the client class.

Dependency Injection is a software design approach that allows avoiding hard-coding dependencies and makes it possible to change the dependencies both at runtime and compile time.

Injected component can be used anywhere within the class. Recommended to use when the injected dependency, you are using across the class methods. Simply put, this allows for loose coupling of components and moves the responsibility of managing components onto the container. Autowiring feature of spring framework enables you to inject the object dependency implicitly.

It internally uses setter or constructor injection. Bean life cycle is managed by the spring container. When we run the program then, first of all, the spring container gets started.

After that, the container creates the instance of a bean as per the request and then dependencies are injected. And finally, the bean is destroyed when the spring container is closed. The main goal of the Spring Boot framework is to reduce overall development time and increase efficiency by having a default setup for unit and integration tests. If you want to get started quickly with your Java application, you can easily accept all defaults and avoid the XML configuration completely.

Spring is the most popular application development framework for enterprise Java. Spring framework targets to make J2EE development easier to use and promotes good programming practices by enabling a POJO-based programming model. By default, Spring Boot uses Tomcat 7. If you want to use Tomcat 8, just say so! You can view this fact yourself at the class EmbeddedServletContainerAutoConfiguration whose source you can find here.

JVM hot swapping is somewhat limited with the bytecode that it can replace, for a more complete solution JRebel or the Spring Loaded project can be used.

While the Spring framework focuses on providing flexibility to you, Spring Boot aims to shorten the code length and provide you with the easiest way to develop a web application. With annotation configuration and default codes, Spring Boot shortens the time involved in developing an application.