Mikel
[转载]没有eclipse截图的OSGi入门教程 – chenjianjx – 博客园.
网上能找到的OSGi教程，都喜欢在开篇时就教你如何在eclipse里做一个OSGi的东西，就算你跟着做了（如果有足够的耐心，还是不知道OSGi是怎么一回事）；更可气的是，这些教程还会配上大段大段的eclipse截图，以达到浪费篇幅的目的。
鉴于这种情况，我只好找本英文OSGi书看了看。这本书上没有eclipse截图，我看时觉得很满意，所以顺便把入门部分微缩一下，给大家做个简短的教程。
Why OSGi?
OSGi用于在同一个JVM内部实现“模块化” (“组件化”)。
首先，它用来实现模块化，提供模块化的一些常见特性：
1. 模块向外只暴露特定的接口，内部实现对外不可见
2. 模块可独立部署
3. 像服务治理一样管理自己模块暴露的接口，包括服务发布、寻找和版本管理
其次，和SOA那种分布式模块化方案不同，OSGi的模块并不会分布在多台机器上，而是部署在同一个jvm进程里的：
1. 每个模块打包成一个JAR
2. 整个应用只需要一个进程，开发、部署会比SOA更高效
OSGi的具体功能有哪些？
官方文档把OSGi的功能分成三大块：
1. Module: 将程序组装成一个一个的“模块”，模块里除了业务逻辑，还有各种元数据
2. Lifecycle：管理模块的生命周期，比如启动和停止模块，并在运行时让模块跟OSGi框架发生关系（比如“热部署”）
3. Service层：把模块暴露的接口当作SOA服务来管理，比如服务的发布和寻找。
实际使用时，你并不需要用上所有的功能。如果你只对模块组织感兴趣，“热部署”、“服务治理”等等你都可以不用。
顺便提一下OSGi的规范与实现。”OSGi Alliance”组织只提供了规范(specification)，具体的java实现则由第三方提供。目前主流的实现有：
1. Apache Felix
2. Eclipse Euinox — Eclipse组织提供的实现。Eclipse还为它提供了相应的IDE Plugin，以快速搭建OSGi应用。估计这也是为什么很多OSGi教程喜欢从Eclipse截图开始的原因。
3. Knopflerfish
OSGi应用举例
闲话少叙，接下来我们直接看代码，看看基于OSGi的模块化是怎么实现的。在下面的例子里，我们会涉及到OSGi模块的服务化、打包和生命周期管理。
Service/Client
在模块化的语境中，一个模块总是在充当服务提供者(Service)或者服务消费者(Client)。所以我们可以先把Service/Client建好：
Service:
<span class="kwd">public</span><span class="pln"> </span><span class="kwd">interface</span><span class="pln"> </span><span class="typ">HelloService</span><span class="pln"> </span><span class="pun">{</span><span class="pln"></span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> sayHelloTo</span><span class="pun">(</span><span class="typ">String</span><span class="pln"> whom</span><span class="pun">);</span><span class="pln"></span><span class="pun">}</span><span class="pln"></span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">HelloServiceImpl</span><span class="pln"> </span><span class="kwd">implements</span><span class="pln"> </span><span class="typ">HelloService</span><span class="pln"> </span><span class="pun">{</span><span class="pln"></span><span class="lit">@Override</span><span class="pln"></span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> sayHelloTo</span><span class="pun">(</span><span class="typ">String</span><span class="pln"> whom</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln"></span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Saying hello to "</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> whom</span><span class="pun">);</span><span class="pln"></span><span class="pun">}</span><span class="pln"></span><span class="pun">}</span> |
Client:
<span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">HelloClient</span><span class="pln"> </span><span class="pun">{</span><span class="pln"></span><span class="kwd">private</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="typ">HelloService</span><span class="pln"> helloService</span><span class="pun">;</span><span class="pln"></span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> callHelloService</span><span class="pun">()</span><span class="pln"> </span><span class="pun">{</span><span class="pln">helloService</span><span class="pun">.</span><span class="pln">sayHelloTo</span><span class="pun">(</span><span class="str">"beckham"</span><span class="pun">);</span><span class="pln"> </span><span class="com">//向Beckham问好</span><span class="pln"></span><span class="pun">}</span><span class="pln"></span><span class="pun">…</span><span class="pln"></span><span class="pun">}</span> |
将Service/Client都打包成模块
接下来我们把Service和Client分别打成jar包，形成OSGi模块。当然，OSGi模块不仅仅是一个jar包，它还要包含一些必要的元数据和辅助类，以被OSGi框架识别和管理。
我们先从Service模块开始。
Service:
1. 先写一个“服务激活者”，用于指定本模块的服务的实现者并发布服务。这个类会在模块启动时被执行
<span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">ServiceActivator</span><span class="pln"> </span><span class="kwd">implements</span><span class="pln"> org</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">framework</span><span class="pun">.</span><span class="typ">BundleActivator</span><span class="pun">{</span><span class="pln"></span><span class="lit">@Override</span><span class="pln"></span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> start</span><span class="pun">(</span><span class="typ">BundleContext</span><span class="pln"> context</span><span class="pun">)</span><span class="pln"> </span><span class="kwd">throws</span><span class="pln"> </span><span class="typ">Exception</span><span class="pln"> </span><span class="pun">{</span><span class="pln">context</span><span class="pun">.</span><span class="pln">registerService</span><span class="pun">(</span><span class="typ">HelloService</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">.</span><span class="pln">getName</span><span class="pun">(),</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">HelloServiceImpl</span><span class="pun">(),</span><span class="pln"> </span><span class="kwd">null</span><span class="pun">);</span><span class="pln"></span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="typ">HelloService</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">.</span><span class="pln">getName</span><span class="pun">()</span><span class="pln"> </span><span class="pun">+</span><span class="pln"> </span><span class="str">" has been registred as a service"</span><span class="pun">);</span><span class="pln"></span><span class="pun">}</span><span class="pln"></span><span class="pun">…….</span><span class="pln"></span><span class="pun">}</span> |
你可能注意到这里出现了”Bundle”字样。这是因为在OSGi术语中，模块就是Bundle.
2. 在即将打出的jar包的MANIFET.MF中，标识一下本模块(Bundle)，指定模块中的哪些类允许被外界使用，并指定本模块的“激活者”：
<span class="typ">Bundle</span><span class="pun">-</span><span class="typ">SymbolicName</span><span class="pun">:</span><span class="pln"> player</span><span class="pun">.</span><span class="pln">kent</span><span class="pun">.</span><span class="pln">chen</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">service</span><span class="typ">Bundle</span><span class="pun">-</span><span class="typ">Version</span><span class="pun">:</span><span class="pln"> </span><span class="lit">1.0</span><span class="pun">.</span><span class="lit">0</span><span class="pln"></span><span class="typ">Export</span><span class="pun">-</span><span class="typ">Package</span><span class="pun">:</span><span class="pln"> player</span><span class="pun">.</span><span class="pln">kent</span><span class="pun">.</span><span class="pln">chen</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">service</span><span class="pun">.</span><span class="pln">interfaces</span><span class="typ">Bundle</span><span class="pun">-</span><span class="typ">Activator</span><span class="pun">:</span><span class="pln"> player</span><span class="pun">.</span><span class="pln">kent</span><span class="pun">.</span><span class="pln">chen</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">service</span><span class="pun">.</span><span class="pln">impl</span><span class="pun">.</span><span class="typ">ServiceActivator</span> |
还有一个条目：
<span class="typ">Import</span><span class="pun">-</span><span class="typ">Package</span><span class="pun">:</span><span class="pln"> org</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">framework</span> |
这指明了本模块可以依赖哪些外部java package；由于“激活者”在运行时依赖了org.osgi.framework，如果没有上面这一句，激活者的运行就会失败。
3. 打包成jar
把Service的接口、实现、激活者及MANIFEST.MF打成jar包，并命名为play-osgi-service.jar.bundle
Client:
1. 先写一个“激活者”，从OSGi Context中寻找Service
<span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">ClientActivator</span><span class="pln"> </span><span class="kwd">implements</span><span class="pln"> </span><span class="typ">BundleActivator</span><span class="pun">{</span><span class="pln"></span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="typ">HelloService</span><span class="pln"> helloService</span><span class="pun">;</span><span class="pln"></span><span class="lit">@Override</span><span class="pln"></span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> start</span><span class="pun">(</span><span class="typ">BundleContext</span><span class="pln"> context</span><span class="pun">)</span><span class="pln"> </span><span class="kwd">throws</span><span class="pln"> </span><span class="typ">Exception</span><span class="pln"> </span><span class="pun">{</span><span class="pln"></span><span class="typ">ServiceReference</span><span class="pln"> </span><span class="kwd">ref</span><span class="pln"> </span><span class="pun">=</span><span class="pln"> context</span><span class="pun">.</span><span class="pln">getServiceReference</span><span class="pun">(</span><span class="typ">HelloService</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">.</span><span class="pln">getName</span><span class="pun">());</span><span class="pln">helloService </span><span class="pun">=</span><span class="pln"> </span><span class="pun">(</span><span class="typ">HelloService</span><span class="pun">)</span><span class="pln"> context</span><span class="pun">.</span><span class="pln">getService</span><span class="pun">(</span><span class="kwd">ref</span><span class="pun">);</span><span class="pln"></span><span class="typ">HelloClient</span><span class="pun">.</span><span class="pln">setHelloService</span><span class="pun">(</span><span class="pln">helloService</span><span class="pun">);</span><span class="pln"></span><span class="pun">}</span><span class="pln"></span><span class="pun">……</span><span class="pln"></span><span class="pun">}</span> |
2. 在MANIFET.MF中标识一下本模块，指定欲依赖的类，并指定激活者
<span class="typ">Bundle</span><span class="pun">-</span><span class="typ">SymbolicName</span><span class="pun">:</span><span class="pln"> player</span><span class="pun">.</span><span class="pln">kent</span><span class="pun">.</span><span class="pln">chen</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">client</span><span class="typ">Bundle</span><span class="pun">-</span><span class="typ">Version</span><span class="pun">:</span><span class="pln"> </span><span class="lit">1.0</span><span class="pun">.</span><span class="lit">0</span><span class="pln"></span><span class="typ">Import</span><span class="pun">-</span><span class="typ">Package</span><span class="pun">:</span><span class="pln"> player</span><span class="pun">.</span><span class="pln">kent</span><span class="pun">.</span><span class="pln">chen</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">service</span><span class="pun">.</span><span class="pln">interfaces</span><span class="pun">,</span><span class="pln"> org</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">framework</span><span class="typ">Bundle</span><span class="pun">-</span><span class="typ">Activator</span><span class="pun">:</span><span class="pln"> player</span><span class="pun">.</span><span class="pln">kent</span><span class="pun">.</span><span class="pln">chen</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">client</span><span class="pun">.</span><span class="typ">ClientActivator</span> |
3. 打包成jar
把Client类、激活者及MANIFEST.MF打成jar包，并命名为play-osgi-client.jar.bundle
创建launch程序
Service和Client模块都构建好了，最后写一个Main程序把它们集成在一起跑起来(launch it)。
<span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Main</span><span class="pln"> </span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> main</span><span class="pun">(</span><span class="typ">String</span><span class="pun">[]</span><span class="pln"> args</span><span class="pun">)</span><span class="pln"> </span><span class="kwd">throws</span><span class="pln"> </span><span class="typ">Exception</span><span class="pln"> </span><span class="pun">{</span><span class="pln"> </span><span class="com">// 创建launch framework，它是OSGi launch的入口. (getFramework()方法的实现请参见附件里的代码文件)</span><span class="pln"> org</span><span class="pun">.</span><span class="pln">osgi</span><span class="pun">.</span><span class="pln">framework</span><span class="pun">.</span><span class="pln">launch</span><span class="pun">.</span><span class="typ">Framework</span><span class="pln"> framework </span><span class="pun">=</span><span class="pln"> getFramework</span><span class="pun">();</span><span class="pln"> </span><span class="com">// 安装service bundle</span><span class="pln"> </span><span class="typ">Bundle</span><span class="pln"> serviceBundle </span><span class="pun">=</span><span class="pln"> framework</span><span class="pun">.</span><span class="pln">getBundleContext</span><span class="pun">().</span><span class="pln">installBundle</span><span class="pun">(</span><span class="pln">fileToUri</span><span class="pun">(</span><span class="str">"../play-osgi-service-and-client/service/dist/play-osgi-service.jar.bundle"</span><span class="pun">));</span><span class="pln"> </span><span class="com">// 安装client bundle</span><span class="pln"> </span><span class="typ">Bundle</span><span class="pln"> clientBundle </span><span class="pun">=</span><span class="pln"> framework</span><span class="pun">.</span><span class="pln">getBundleContext</span><span class="pun">().</span><span class="pln">installBundle</span><span class="pun">(</span><span class="pln">fileToUri</span><span class="pun">(</span><span class="str">"../play-osgi-service-and-client/client/dist/play-osgi-client.jar.bundle"</span><span class="pun">));</span><span class="pln"> </span><span class="com">// 启动framework和bundles</span><span class="pln"> framework</span><span class="pun">.</span><span class="pln">start</span><span class="pun">();</span><span class="pln"> serviceBundle</span><span class="pun">.</span><span class="pln">start</span><span class="pun">();</span><span class="pln"> clientBundle</span><span class="pun">.</span><span class="pln">start</span><span class="pun">();</span><span class="pln"> </span><span class="com">// 启动client</span><span class="pln"> </span><span class="typ">Class</span><span class="pun"><?></span><span class="pln"> clientClass </span><span class="pun">=</span><span class="pln"> clientBundle</span><span class="pun">.</span><span class="pln">loadClass</span><span class="pun">(</span><span class="str">"player.kent.chen.osgi.client.HelloClient"</span><span class="pun">);</span><span class="pln"> </span><span class="typ">Method</span><span class="pln"> method </span><span class="pun">=</span><span class="pln"> clientClass</span><span class="pun">.</span><span class="pln">getMethod</span><span class="pun">(</span><span class="str">"callHelloService"</span><span class="pun">,</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">Class</span><span class="pun"><?>[]{});</span><span class="pln"> method</span><span class="pun">.</span><span class="pln">invoke</span><span class="pun">(</span><span class="pln">clientClass</span><span class="pun">.</span><span class="pln">newInstance</span><span class="pun">());</span><span class="pln"> </span><span class="com">// 最后停止framework</span><span class="pln"> framework</span><span class="pun">.</span><span class="pln">stop</span><span class="pun">();</span><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="pun">…</span><span class="pln"></span><span class="pun">}</span> |
运行一下这个Main程序，控制台将打印” Saying hello to Beckham”，表明HelloService调用成功。
你的第一个OSGi应用写好了！它组建了模块(Bundle)，指定了依赖关系；但这个例子本身还不能充分地展示“模块化”，我们要再看一个例子：
体验一个反例：越过Service的接口直接调用内部实现
OSGi的模块中未显式暴露的类对其他模块来说是不可见的，我们来测试一下。
把client的激活者的代码改一下，让它直接依赖HelloServiceImpl对象，看看会怎么样。
<span class="kwd">public</span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">ClientActivator</span><span class="pln"> </span><span class="kwd">implements</span><span class="pln"> </span><span class="typ">BundleActivator</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">static</span><span class="pln"> </span><span class="typ">HelloServiceImpl</span><span class="pln"> helloServiceImpl</span><span class="pun">;</span><span class="pln"> </span><span class="lit">@Override</span><span class="pln"> </span><span class="kwd">public</span><span class="pln"> </span><span class="kwd">void</span><span class="pln"> start</span><span class="pun">(</span><span class="typ">BundleContext</span><span class="pln"> context</span><span class="pun">)</span><span class="pln"> </span><span class="kwd">throws</span><span class="pln"> </span><span class="typ">Exception</span><span class="pln"> </span><span class="pun">{</span><span class="pln"> </span><span class="typ">ServiceReference</span><span class="pln"> </span><span class="kwd">ref</span><span class="pln"> </span><span class="pun">=</span><span class="pln"> context</span><span class="pun">.</span><span class="pln">getServiceReference</span><span class="pun">(</span><span class="typ">HelloService</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">.</span><span class="pln">getName</span><span class="pun">());</span><span class="pln"> helloServiceImpl </span><span class="pun">=</span><span class="pln"> </span><span class="pun">(</span><span class="typ">HelloServiceImpl</span><span class="pun">)</span><span class="pln"> context</span><span class="pun">.</span><span class="pln">getService</span><span class="pun">(</span><span class="kwd">ref</span><span class="pun">);</span><span class="pln"> </span><span class="typ">HelloClient</span><span class="pun">.</span><span class="pln">setHelloService</span><span class="pun">(</span><span class="pln">helloServiceImpl</span><span class="pun">);</span><span class="pln"> </span><span class="pun">}</span><span class="pln"> </span><span class="pun">…</span><span class="pln"></span><span class="pun">}</span> |
运行程序，结果是： java.lang.NoClassDefFoundError: player/kent/chen/osgi/service/impl/HelloServiceImpl . 也就是说， Client在执行时，相关的类装载器装载不到HelloServiceImpl，因为HelloServiceImpl不属于client模块的Import-Package (player.kent.chen.osgi.service.interfaces)
那如果client模块引入了HelloServiceImpl呢？ 在MANIFEST.MF中加入一个Import-Package试试：
Import-Package: player.kent.chen.osgi.service.interfaces, org.osgi.framework, player.kent.chen.osgi.service.impl
运行结果是：
Unresolved constraint in bundle player.kent.chen.osgi.client: missing requirement package; (package=player.kent.chen.osgi.service.impl)
也就是说，虽然client模块中引入了对player.kent.chen.osgi.service.impl的依赖，但并没有任何模块提供了这个package，client的调用依然以异常告终。
试想，如果没有OSGi，仅把client和service两个jar文件放在一起，除非自写Class Loader，否则你无法阻止client里的代码直接调用service的内部实现，继而导致弱封装、强耦合了。这下你可以看出OSGi的模块化的功效了吧？
总结
通过上面的介绍，估计你对OSGi的基本作用已经有了一定的了解，对它如何使用也有了感性的认识。你可以下载附件所含的代码示例，自己体验一下。(这些代码使用Apache Felix作为OSGi的实现，由于Felix对bundle的装载使用了缓存，在每次运行之前，你最好都清除一下前一次运行时产生的felix-cache文件夹)
另外，本文内容比较少，只是“入门的入门”，它只能帮你消除对OSGi的神秘感。要想“完整入门”、或者想全面学习OSGi的强大功能，建议阅读专门的书籍。个人推荐的书是： ‘OSGi in Action’
