03. 扩展点背景以及JDK提供的扩展点

为什么需要扩展点

因为为了达成能扩展的目的。

更多细节可以阅读oracle官方文档Creating Extensible Applications With the Java Platform

重点：

Definitions

A service is a set of programming interfaces and classes that provide access to some specific application functionality or feature. The service may simply define the interfaces for the functionality and a way to retrieve an implementation. In the word-processor example, a dictionary service can define a way to retrieve a dictionary and the definition of a word, but it does not implement the underlying feature set. Instead, it relies on a service provider to implement that functionality.

A service provider interface (SPI) is the set of public interfaces and abstract classes that a service defines. The SPI defines the classes and methods available to your application.

A service provider implements the SPI. An application with extensible services will allow you, vendors, and perhaps even customers to add service providers without modifying the original application.

目前有什么扩展点机制实现

为了实现扩展，粗略的讲能支持bean容器的框架或机制均可以做扩展点

1. JDK自带的扩展点机制，ServiceLoader
2. 专门的bean容器：spring、guice等

JDK的扩展点

ServiceLoader：一个简单的服务提供者加载设施。

使用文档与demo可以参见这里官方doc

注意其使用时配置方式是：

If com.example.impl.StandardCodecs is an implementation of the CodecSet service then its jar file also contains a file named

META-INF/services/com.example.CodecSet
This file contains the single line:

com.example.impl.StandardCodecs # Standard codecs

简单地说：是将SPI接口的具体实现类的全限定名写入在 META-INF/services目录下以SPI接口全限定名命名的文件中。ServiceLoader能从此文件中知道这个SPI具体选用哪个实现。

自己实现一个扩展点机需要哪些基础能力

根据配置读取相应的实现类并创建实例。

自己实现一个扩展点可以做些什么高阶能力

自动装配（注入）字段、代理、适配、包装、激活(一次获取多个命中实例)等。