Java Web应用集成OSGI

对OSGI的简单理解

就像Java Web应用程序需要运行在Tomcat、Weblogic这样的容器中一样。程序员开发的OSGI程序包也需要运行在OSGI容器中。目前主流的OSGI容器包括：Apache Felix以及Eclipse Equinox。OSGI程序包在OSGI中称作Bundle。
Bundle的整个生命周期都交与OSGI容器进行管理。可以在不停止服务的情况下，对Bundle进行加载和卸载，实现热部署。
Bundle对于外部程序来说就是一个黑盒。他只是向OSGI容器中注册了供外部调用的服务接口，至于实现则对外部不可见。不同的Bundle之间的调用，也需要通过OSGI容器来实现。

Bundle如何引入jar

刚才说到Bundle是一个黑盒，他所有实现都包装到了自己这个“盒子”中。在开发Bundle时，避免不了引用一些比如Spring、Apache commons等开源包。在为Bundle打包时，可以将当前Bundle依赖jar与Bundle的源码都打包成一个包（all-in-one）。这种打包结果就是打出的包过大，经常要几兆或者十几兆，这样当然我们是不可接受的。下面就介绍一种更优的做法。

Bundle与OSGI容器的契约

Bundle可以在MANIFEST.MF配置文件中声明他要想运行起来所要的包以及这些包的版本 !!!而OSGI容器在加载Bundle时会为Bundle提供Bundle所需要的包 !!!在启动OSGI容器时，需要在OSGI配置文件中定义org.osgi.framework.system.packages.extra，属性。这个属性定义了 OSGI容器能提供的包以及包的版本。OSGI在加载Bundle时，会将他自己能提供的包以及版本与Bundle所需要的包以及版本列表进行匹配。如果匹配不成功则直接抛出异常:

Unable to execute command on bundle 248: Unresolved constraint in bundle
com.osgi.demo2 [248]: Unable to resolve 248.0: missing requirement [248.0] osgi
.wiring.package; (&(osgi.wiring.package=org.osgi.framework)(version>=1.8.0)(!(version>=2.0.0)))

也可能加载Bundle通过，但是运行Bundle时报ClassNotFoundException。这些异常都由于配置文件没配置造成的。理解了配置文件的配置方法，就能解决60%的异常。

Import-Package

在Bundle的Import-Package属性中通过以下格式配置：

<!--pom.xml-->
 <Import-Package>
javax.servlet,
javax.servlet.http,
org.xml.sax.*,
org.springframework.beans.factory.xml;org.springframework.beans.factory.config;version=4.1.1.RELEASE,
org.springframework.util.*;version="[2.5,5.0]"
</Import-Package>

• 包与包之间通过逗号分隔
• 可以使用*这类的通配符，表示这个包下的所有包。如果不想使用通配符，则同一个包下的其他包彼此之间可以使用;分隔。
• 如果需要指定包的版本则在包后面增加;version="[最低版本,最高版本]"。其中[表示大于等于、]表示小于等于、)表示小于。

org.osgi.framework.system.packages.extra

语法与Impirt-Package基本一致，只是org.osgi.framework.system.packages.extra不支持通配符。

• 错误的方式

  org.springframework.beans.factory.*;version=4.1.1.RELEASE
  

• 正确的方式：

  org.springframework.beans.factory.xml;org.springframework.beans.factory.config;version=4.1.1.RELEASE,
  

Class文件加载

在我们平时开发中有些情况下加载一个Class会使用this.getClassLoader().loadClass。但是通过这种方法加载Bundle中所书写的类的class会失败，会报ClassNotFoundException。在Bundle需要使用下面的方式来替换classLoader.loadClass方法

 public void start(BundleContext context) throws Exception {
     Class classType = context.loadClass(name);
 }

Bundle中加载Spring配置文件时的问题

由于Bundle加载Class的特性,会导致在加载Spring配置文件时报错。所以需要将Spring启动所需要的ClassLoader进行更改，使其调用BundleContext.loadClass来加载Class。

String xmlPath = "";
ClassLoader classLoader = new ClassLoader(ClassUtils.getDefaultClassLoader()) {

    @Override
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        try {
            return currentBundle.loadClass(name);
        } catch (ClassNotFoundException e) {
            return super.loadClass(name);
        }
    }
    };
    DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
    beanFactory.setBeanClassLoader(classLoader);
    GenericApplicationContext ctx = new GenericApplicationContext(beanFactory);
    ctx.setClassLoader(classLoader);
    DefaultResourceLoader resourceLoader = new DefaultResourceLoader(classLoader) {
        @Override
        public void setClassLoader(ClassLoader classLoader) {
            if (this.getClassLoader() == null) {
                super.setClassLoader(classLoader);
            }
        }
    };
    ctx.setResourceLoader(resourceLoader);
    XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(ctx);
    reader.loadBeanDefinitions(xmlPath);
    ctx.refresh();

Web应用集成OSGI

这里选用了Apache Felix来开发，主要是因为Apache Felix是Apache的顶级项目。社区活跃，对OSGI功能支持比较完备，并且文档例子比较全面。
其实OSGI支持两种方式来部署Bundle。

• 单独部署OSGI容器，通过OSGI自带的Web中间件（目前只有jetty）来对外提供Web服务
• 将OSGI容器嵌入到Web应用中，然后就可以使用Weblogic等中间件来运行Web应用

从项目的整体考虑，我们选用了第二种方案。

BundleActivator开发

开发Bundle时，首先需要开发一个BundleActivator。OSGI在加载Bundle时，首先调用BundleActivator的start方法，对Bundle进行初始化。在卸载Bundle时，会调用stop方法来对资源进行释放。

public void start(BundleContext context) throws Exception;
public void stop(BundleContext context) throws Exception;

在start方法中调用context.registerService来完成对外服务的注册。

Hashtable props = new Hashtable();
props.put("servlet-pattern", new String[]{"/login","/logout"})
ServiceRegistration servlet = context.registerService(Servlet.class, new DispatcherServlet(), props);

• context.registerService方法的第一个参数表示服务的类型，由于我们提供的是Web请求服务，所以这里的服务类型是一个javax.servlet.Servlet，所以需要将javax.servlet.Servlet传入到方法中
• 第二个参数为服务处理类，这里配置了一个路由Servlet，其后会有相应的程序来处理具体的请求。
• 第三个参数为Bundle对外提供服务的属性。在例子中，在Hashtable中定义了Bundle所支持的servlet-pattern。OSGI容器所在Web应用通过Bundle定义的servlet-pattern判断是否将客户请求分发到这个Bundle。servlet-pattern这个名称是随意起的，并不是OSGI框架要求的名称。

应用服务集成OSGI容器

• 首先工程需要添加如下依赖

   <dependency>
            <groupId>org.apache.felix</groupId>
            <artifactId>org.apache.felix.framework</artifactId>
            <version>5.6.10</version>
        </dependency>

        <dependency>
            <groupId>org.apache.felix</groupId>
            <artifactId>org.apache.felix.http.bundle</artifactId>
            <version>3.0.0</version>
        </dependency>

        <dependency>
            <groupId>org.apache.felix</groupId>
            <artifactId>org.apache.felix.http.bridge</artifactId>
            <version>3.0.18</version>
        </dependency>
        <dependency>
            <groupId>org.apache.felix</groupId>
            <artifactId>org.apache.felix.http.proxy</artifactId>
            <version>3.0.0</version>
        </dependency>

• 然后在web.xml中添加

    <listener>
        <listener-class>org.apache.felix.http.proxy.ProxyListener</listener-class>
    </listener>

• 开发ServletContextListener用以初始化并启动OSGI容器
  请参考Apache Felix提供的例子程序。例子中提供的ProvisionActivator会扫描/WEB-INF/bundles/，加载其中的Bundle包。(当然例子中提供的ProvisionActivator并不带有Bundle自动发现注册等机制，这些逻辑需要自行增加。请参照后续的Bundle自动加载章节)

路由开发

通过上面的配置，只是将OSGI容器加载到了Web应用中。还需要修改Web应用程序路由的代码。

• 在Bundle加载到OSGI容器中后，可以通过bundleContext.getBundles()方法获取到OSGI容器中的所有已经加载的Bundle。
• 可以调用Bundle的bundle.getRegisteredServices()方法获取到该Bundle对外提供的所有服务服务。getRegisteredServices方法返回ServiceReference的数组。前文中我们调用context.registerService(Servlet.class, new DispatcherServlet(), props)我们已经注册了一个服务，getRegisteredServices返回的数据只有一个ServiceReference对象。
• 获取Bundle所能提供的服务
  可以通过ServiceReference对象的getProperty方法获取context.registerService中传入的props中的值。这样我们就能通过调用ServiceReference.getProperty方法获取到该Bundle所能提供的服务。
• 通过上面提供的接口，我们可以将Bundle对应ServiceReference以及Bundle对应的servlet-pattern进行缓存。当用户请求进入到应用服务器后，通过缓存的servlet-pattern可以判断Bundle是否能提供用户所请求的服务，如果可以提供通过下面的方式，来调用Bundle所提供的服务。

 ServiceReference sr = cache.get(bundleName);
 HttpServlet servlet = (HttpServlet) this.bundleContext.getService(sr);
 servlet.service(request, response);

Bundle自动加载

在Apache Felix例子中提供的ProvisionActivator,只会在系统启动时加载/WEB-INF/bundles/目录下的Bundle。当文件夹下的Bundle文件有更新时，并不会自动更新OSGI容器中的Bundle。所以Bundle自动加载的逻辑，需要我们自己增加。下面提供实现的思路：

• 在第一次加载文件夹下的Bundle时，记录Bundle包所对应的最后的更新时间。
• 在程序中创建一个独立线程，用以扫描/WEB-INF/bundles/目录，逐个的比较Bundle的更新时间。如果与内存中的不相符合，则从OSGI中获取Bundle对象然后调用其stop以及uninstall方法，将其从OSGI容器中卸载。
• 卸载后，再调用bundleContext.installBundle以及bundle.start将最新的Bundle加载到OSGI容器中

BundleListener

最后一个问题，通过上面的方式，可以实现Bundle的自动加载。但是刚才我们介绍了，在路由程序中，我们会缓存OSGI容器中所有的Bundle所对应的ServiceReference以及所有Bundle所对应的servlet-pattern。所以Bundle自动更新后，我们还需要将路由程序中的缓存同步的进行更新。
可以通过向bundleContext中注册BundleListener，当OSGI容器中的Bundle状态更新后，会调用BundleListener的bundleChanged回调方法。然后我们可以在bundleChanged回调方法中书写更新路由缓存的逻辑

this.bundleContext.addBundleListener(new BundleListener() {
    @Override
    public void bundleChanged(BundleEvent event) {
        if (event.getType() == BundleEvent.STARTED) {
            initBundle(event.getBundle());
        } else if (event.getType() == BundleEvent.UNINSTALLED) {
            String name = event.getBundle().getSymbolicName();
            indexes.remove(name);
        }
     }
 });

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transparent
);
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transparent
);
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135deg,
rgba(255, 255, 255, .4) 27%, transparent 27%,
transparent 52%, rgba(255, 255, 255, .4) 52%,
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100% {
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}

.markdown-body .critic_insert {
color: #718c00 ;
text-decoration: underline;
}

.markdown-body .critic_comment {
color: #8e908c;
font-style: italic;
}

.markdown-body .headeranchor {
font: normal normal 16px octicons-anchor;
line-height: 1;
display: inline-block;
text-decoration: none;
-webkit-font-smoothing: antialiased;
-moz-osx-font-smoothing: grayscale;
-webkit-user-select: none;
-moz-user-select: none;
-ms-user-select: none;
user-select: none;
}

.headeranchor:before {
content: '\f05c';
}

.markdown-body .task-list-item {
list-style-type: none;
}

.markdown-body .task-list-item+.task-list-item {
margin-top: 3px;
}

.markdown-body .task-list-item input {
margin: 0 4px 0.25em -20px;
vertical-align: middle;
}

/* Media */
@media only screen and (min-width: 480px) {
.markdown-body {
font-size:14px;
}
}

@media only screen and (min-width: 768px) {
.markdown-body {
font-size:16px;
}
}

@media print {
.markdown-body * {
background: transparent !important;
color: black !important;
filter:none !important;
-ms-filter: none !important;
}

.markdown-body {
font-size:12pt;
max-width:100%;
outline:none;
border: 0;
}

.markdown-body a,
.markdown-body a:visited {
text-decoration: underline;
}

.markdown-body .headeranchor-link {
display: none;
}

.markdown-body a[href]:after {
content: " (" attr(href) ")";
}

.markdown-body abbr[title]:after {
content: " (" attr(title) ")";
}

.markdown-body .ir a:after,
.markdown-body a[href^="javascript:"]:after,
.markdown-body a[href^="#"]:after {
content: "";
}

.markdown-body pre {
white-space: pre;
white-space: pre-wrap;
word-wrap: break-word;
}

.markdown-body pre,
.markdown-body blockquote {
border: 1px solid #999;
padding-right: 1em;
page-break-inside: avoid;
}

.markdown-body .progress,
.markdown-body .progress-bar {
-moz-box-shadow: none;
-webkit-box-shadow: none;
box-shadow: none;
}

.markdown-body .progress {
border: 1px solid #ddd;
}

.markdown-body .progress-bar {
height: 22px;
border-right: 1px solid #ddd;
}

.markdown-body tr,
.markdown-body img {
page-break-inside: avoid;
}

.markdown-body img {
max-width: 100% !important;
}

.markdown-body p,
.markdown-body h2,
.markdown-body h3 {
orphans: 3;
widows: 3;
}

.markdown-body h2,
.markdown-body h3 {
page-break-after: avoid;
}
}
-->