关于OSGI模块化编程中MENIFEST.MF文件中的Required Plug-ins和Imported Packages的区别

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最近在做一个“把OSIG上的bundle结构的工程打成标准的war包来在各中间件上发布”的工作，给了自己一个了解OSGI底层源码的机会。过程中也发现了许多之前开发时没注意的小问题，譬如这个Dependencise中的Required Plug-ins和Imported Packages的区别，网上搜集了些资料，总结到这里吧：

参考自：http://blog.sina.com.cn/s/blog_69d0d3310100mehz.html

Required-Bundle： Specifythe list of plug-ins required for the operation of this plug-in.

Imported-Package：Specifypackages on which this plug-in depends without explicitly identifying theiroriginating plug-in.

Eclipse allows to define dependencies between plug-ins based on bundle or on package level. In theMANIFEST.MFof the plugin.xml you can tell that your plug-in should either depend onanother plug-in (and all its exported packages) or if your plug-in should onlydepend on the availability of certain packages no matter which plug-in isexporting this package.

The corresponding statements inMANIFEST.MF are Require-Bundle: or Import-Package:

Today I got the question what approach should bepreferred.

I believe the preferred way should be to useImport-Package because the coupling between the plug-ins is not that tight. Asonly as one plug-in exports this package the other plug-in will be fine. Withthis you can replaced a plug-in without changing the other plug-ins.

Of course if you have the same package in different plug-ins then you have to use the plug-in dependency.

The other advantage of plug-in dependencies isthat the dependency is very clearly explicit, with the import-package it is notimmediately obvious which plug-in is contributing this package.

But in total I would argue that loose coupling isalways better so in general import-package should be preferred.

其实这段E文已经说的很清晰了。

再摘抄一段中文注解吧：

参考自：http://www.xeclipse.com/?p=1195（其中部分观点我认为有误）

理解OSGI的Package import-export机制的关键在于明白每个bundle都有自己的Class Loader。

对于OSGI环境来说，它们是没有区别的。无论是Require-Bundle还是Imported-Package，前提都是需要把相关的packages先Export出。

它们之间，仅仅只是依赖的颗粒度的区别。Required-Bundle依赖的是精确指定版本的bundle，而Import-Package依赖的是package，它会根据class loader的顺序选择最先找到的class，而不关心是哪一个bundle export出来的。

对于同一个package被许多不同的bundle同时export的时候，优先考虑使用required bundle（这跟网上流传的优先使用imported package不同，我个人认为使用imported package有误）。当依赖的多个piug-in同时export了相同的package时，根据依赖中的顺序，优先使用顺序靠前的bundle中的package。这里的顺序是指如下图：

2.0.0先与1.0.0被依赖甚至org.eclipse.jetty中也有相同的package时，因为org.eclipse.ui.thr.ole(2.0.0)最先被依赖，所以优先找它中的package

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