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1.前言
时代的眼泪-JNDI，SPI？
2.JNDI
首先我们看下Oracle官方的描述
[success]The Java Naming and Directory Interface™ (JNDI) is an application programming interface (API) that provides naming and directory functionality to applications written using the Java™ programming language. It is defined to be independent of any specific directory service implementation. Thus a variety of directories -new, emerging, and already deployed can be accessed in a common way.[/success]
读完这句话，有点云里雾里的，好像和代码的解耦有点关系。毕竟是个提案，最终还是要结合实现来理解，看看它的架构图
[success]The JNDI architecture consists of an API and a service provider interface (SPI). Java applications use the JNDI API to access a variety of naming and directory services. The SPI enables a variety of naming and directory services to be plugged in transparently, thereby allowing the Java application using the JNDI API to access their services. [/success]
到这里大致可以知道JNDI主要由API和SPI两部分构成，java应用调用JNDI API，JNDI API通过Naming Manager（绑定关系的载体）去调用SPI，SPI的具体实现通常是由第三方去实现的。在这个过程中API调用者其实是感知不到SPI的具体实现的，换言之，调用者只关心我能得到什么，而不在意结果如何而来的。The Java Naming and Directory Interface™ (JNDI)，从名称我们也可以发现是一种基于命名和目录的抽象。举个例子，访问网站时，在没有DNS时，我们需要输入网站的IP地址才可以访问，有了DNS，我们可能只需要输入***.com之类的域名就可以访问了，这种现象我们可以称为Naming作用的结果，将IP和域名联系到一起称为绑定，DNS就是一种Naming Service。Directory就比较玄学了 
，它比命名多了些许属性，directory service = naming service + objects containing attributes
可以理解为Naming的目录，可能会包含更多的属性，比如大小，时间等，当然笔者水平有限，理解可能偏差，这里附上维基百科
In computing, a directory service or name service maps the names of network resources to their respective network addresses. It is a shared information infrastructure for locating, managing, administering and organizing everyday items and network resources, which can include volumes, folders, files, printers, users, groups, devices, telephone numbers and other objects. A directory service is a critical component of a network operating system. A directory server or name server is a server which provides such a service. Each resource on the network is considered an object by the directory server. Information about a particular resource is stored as a collection of attributes associated with that resource or object.
A directory service defines a namespace for the network. The namespace is used to assign a name (unique identifier) to each of the objects. Directories typically have a set of rules determining how network resources are named and identified, which usually includes a requirement that the identifiers be unique and unambiguous. When using a directory service, a user does not have to remember the physical address of a network resource; providing a name locates the resource. Some directory services include access control provisions, limiting the availability of directory information to authorized users.
3.SPI
SPI全称Service Provider Interface，是一种动态加载服务的功能。虽然JNDI使用场景很少了，但是SPI的使用还是很多的。这里不得不提ServiceLoader这个工具类
通过ServiceLoader可以很方便的加载服务实现，可以有效的降低代码的耦合性，提高灵活性，比如java.sql.Driver的实现，javax.validation.spi.ValidationProvider的实现等。阅读过springboot源码的小伙伴，会发现spring.factories也在类似的目录下 :biggrin:
4.小结
本文简单介绍JNDI和SPI，JNDI虽然是上个时代的产物，但是还是要有所了解的；目前对轻量化容器化的Java应用其实扮演的角色很少了，在JDBC，EJB，JMS方面仍然使用着。
