Spring Basic Notes 1

To talk about the history of Spring, we need to talk about J2EE. The widespread implementation of J2EE applications started in 1999 and 2000, bringing standardization of core middle-tier concepts such as transaction management, but it was not an absolute success in practice, as development efficiency, development difficulty and actual performance were disappointing.

Anyone who has ever used EJB to develop a JAVA EE application like me must know that learning and applying EJB is very hard at the beginning and many things are not easy to understand at once. The configuration is also complex and monotonous, and the code for object lookup using JNDI is also monotonous and boring. The high cost of learning EJB, and very low development efficiency, very high resource consumption, have caused the use of EJB difficulties. And Spring emerged initially to solve these problems like these.

One of the biggest purposes of Spring is to make Java EE development easier. Much more easier.

Core Container

Spring’s core container is the foundation on which other modules are built, consisting of

• Beans module,
• Core module
• Context context module
• The SpEL expression language module

Without these core containers, it is also impossible to have AOP, Web, and other upper layer features.

AOP module

It provides a aspect-oriented programming implementation, which provides functions such as logging, permission control, performance statistics, and other general functions that separated from business logic , and can dynamically add these functions to the required code, so that each has its own role to reduce the coupling of business logic and general functions

很多开发者入门都是从Spring Boot开始的，他对Spring整体框架底层，以及发展历史不是很了解； 特别是对于一些老旧项目维护和底层bug分析没有全局观。
Many developers start with Spring Boot, he does not have a good understanding of the overall framework underlying Spring, and the development history; especially for some older projects maintenance and underlying bug analysis does not have a global view.
Spring represents a framework design philosophy that requires a global understanding of how Spring Framework components work together and the need to understand the original intent of its design and future trends.

Spring框架管理这些Bean的创建工作，即由用户管理Bean转变为框架管理Bean，这个就叫控制反转 - Inversion of Control (IoC)Spring
框架托管创建的Bean放在哪里呢？ 这便是IoC Container;Spring 框架为了更好让用户配置Bean，必然会引入不同方式来配置Bean？ 这便是xml配置，Java配置，注解配置等支持Spring
框架既然接管了Bean的生成，必然需要管理整个Bean的生命周期等；
应用程序代码从Ioc Container中获取依赖的Bean，注入到应用程序中，这个过程叫 依赖注入(Dependency Injection，DI) ； 所以说控制反转是通过依赖注入实现的，其实它们是同一个概念的不同角度描述。通俗来说就是IoC是设计思想，DI是实现方式在依赖注入时，有哪些方式呢？这就是构造器方式，@Autowired, @Resource, @Qualifier… 同时Bean之间存在依赖（可能存在先后顺序问题，以及循环依赖问题等）