Spring IOC (Inversion of Control, inversion of control) is the core of the Spring framework, and its implementation principle is based on reflection and configuration files.
In Spring IOC, control is inverted. In traditional application development, the creation, management, and destruction of objects are all controlled by the application itself. This process is called Active Management. In Spring IOC, the creation, management, and destruction of objects are all managed by the Spring container. The application only needs to declare which objects are needed. This process is called Passive Management.
The basic principle of Spring IOC implementation is to create and manage dependencies between objects by reading configuration files. When an application needs an object, it does not directly create the object, but obtains the object through the Spring container. According to the information in the configuration file, the Spring container instantiates the corresponding objects and injects them where they are needed.
The underlying principles of Spring IOC can be divided into the following steps:
Configuration file parsing
Spring IOC obtains application-related information by reading configuration files such as XML, annotations, or JavaConfig. The configuration file contains various objects in the application and the dependencies between them.
Create BeanDefinition
When the Spring container reads the configuration file, it will create a BeanDefinition object according to the information in the configuration file. The BeanDefinition object is the core data structure of Spring IOC, which encapsulates the definition information of a Bean, including class name, attribute, dependency and other information.
Instantiate Bean
Spring IOC will use the reflection mechanism to instantiate the Bean object according to the information in the BeanDefinition object. When instantiating a Bean object, Spring will inject other Bean objects that the Bean object depends on according to the information in the configuration file.
Bean life cycle management
After instantiating the Bean object, Spring IOC also manages the life cycle of the Bean. In the life cycle of the bean, Spring IOC will call specific methods at specific points in time in order to complete some necessary operations. For example, when the Bean is instantiated, Spring IOC will call the Bean's initialization method (init-method), and when the Bean is destroyed, Spring IOC will call the Bean's destruction method (destroy-method).
Dependency injection
Dependency injection is one of the core functions of Spring IOC. When instantiating a Bean object, Spring IOC will automatically inject other Bean objects that the Bean depends on into its properties, thus completing the establishment of the dependency relationship.
AOP supports
Spring IOC also supports AOP (Aspect-Oriented Programming, aspect-oriented programming), which uses dynamic proxy technology to intercept and enhance the specified method when the application is running.
The implementation process of Spring IOC can be roughly divided into three stages: configuration file parsing, Bean instantiation and dependency injection. Their procedure is as follows:
Configuration file parsing stage
In the configuration file parsing stage, Spring IOC will read the configuration file and parse out the Bean definition information. This information includes the Bean's name, class name, attributes, dependencies, and so on. Spring IOC can read many types of configuration files, including XML, annotations or JavaConfig, etc.
Bean instantiation phase
In the Bean instantiation phase, Spring IOC will instantiate the Bean object according to the information in the configuration file. This process is usually implemented using the reflection mechanism. Spring IOC can instantiate the Bean object by calling the Bean's constructor, and save it in memory for subsequent use.
Dependency injection phase
In the dependency injection phase, Spring IOC will automatically inject other Bean objects that the Bean depends on into its properties, thus completing the establishment of the dependency relationship. Spring IOC determines the dependencies between Beans by reading the information in the configuration file, and completes the dependency injection operation when instantiating Bean objects.
Spring IOC is not the only framework that implements inversion of control, there are other frameworks, such as Google Guice, etc. The implementation principles of these frameworks are similar, but the details may differ.