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itemtype=http://schema.org/Person><meta content=/uploads/avatar.jpg itemprop=image><meta content=Nicksxs itemprop=name></span><span hidden itemprop=publisher itemscope itemtype=http://schema.org/Organization><meta content="Nicksxs's Blog" itemprop=name><meta content="Nicksxs 的技术博客,深入 Java 运行时与生产诊断、低成本大模型推理和 Agent 工程实践。" itemprop=description></span><span hidden itemprop=post itemscope itemtype=http://schema.org/CreativeWork><meta content="聊聊 Dubbo 的 SPI | Nicksxs's Blog" itemprop=name><meta content="SPI全称是Service Provider Interface,咋眼看跟api是不是有点相似,api是application interface,这两个其实在某些方面有类似的地方,也有蛮大的区别,比如我们基于 dubbo 的微服务,一般我们可以提供服务,然后非泛化调用的话,我…" itemprop=description></span><header class=post-header><h1 itemprop="name headline" class=post-title>聊聊 Dubbo 的 SPI</h1><div class=post-meta-container><div class=post-meta><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-calendar"></i> </span><span class=post-meta-item-text>发表于</span> <time itemprop="dateCreated datePublished" title="创建时间:2020-05-31 21:06:13" datetime=2020-05-31T21:06:13+08:00>2020-05-31</time> </span><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-folder"></i> </span><span class=post-meta-item-text>分类于</span> <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/ itemprop=url rel=index><span itemprop=name>Java</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/Dubbo/ itemprop=url rel=index><span itemprop=name>Dubbo</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/Dubbo/RPC/ itemprop=url rel=index><span itemprop=name>RPC</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Java/Dubbo/RPC/SPI/ itemprop=url rel=index><span itemprop=name>SPI</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Dubbo/ itemprop=url rel=index><span itemprop=name>Dubbo</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Dubbo/SPI/ itemprop=url rel=index><span itemprop=name>SPI</span></a> </span></span><span class="post-meta-item cloudflare-page-counter" data-title="聊聊 Dubbo 的 SPI" data-increment=true data-path=/2020/05/31/%E8%81%8A%E8%81%8A-Dubbo-%E7%9A%84-SPI/ title=阅读次数><span class=post-meta-item-icon><i class="far fa-eye"></i> </span><span class=post-meta-item-text>阅读次数:</span> <span class=cloudflare-page-counter-count></span></span></div><div class=post-description>SPI全称是Service Provider Interface,咋眼看跟api是不是有点相似,api是application interface,这两个其实在某些方面有类似的地方,也有蛮大的区别,比如我们基于 dubbo 的微服务,一般我们可以提供服务,然后非泛化调用的话,我…</div></div></header><div class=post-body itemprop=articleBody><p>SPI全称是Service Provider Interface,咋眼看跟api是不是有点相似,api是application interface,这两个其实在某些方面有类似的地方,也有蛮大的区别,比如我们基于 dubbo 的微服务,一般我们可以提供服务,然后非泛化调用的话,我们可以把 api 包提供给应用调用方,他们根据接口签名传对应参数并配置好对应的服务发现如 zk 等就可以调用我们的服务了,然后 spi 会有点类似但是是反过来的关系,相当于是一种规范,比如我约定完成这个功能需要两个有两个接口,一个是连接的,一个是断开的,其实就可以用 jdbc 的驱动举例,比较老套了,然后各个厂家去做具体的实现吧,到时候根据我接口的全限定名的文件来加载实际的实现类,然后运行的时候调用对应实现类的方法就完了<p><img alt=3sKdpg loading=lazy src=https://mystore-1255223546.cos.ap-chengdu.myqcloud.com/uPic/3sKdpg.png><p>看上面的图,<code>java.sql.Driver</code>就是 spi,对应在classpath 的 META-INF/services 目录下的这个文件,里边的内容就是具体的实现类<p><img alt=1590735097909 loading=lazy src=https://mystore-1255223546.cos.ap-chengdu.myqcloud.com/uPic/1590735097909.jpg><p>简单介绍了 Java的 SPI,再来说说 dubbo 的,dubbo 中为啥要用 SPI 呢,主要是为了框架的可扩展性和性能方面的考虑,比如协议层 dubbo 默认使用 dubbo 协议,同时也支持很多其他协议,也支持用户自己实现协议,那么跟 Java 的 SPI 会有什么区别呢,我们也来看个文件<p><img alt=bqxWMp loading=lazy src=https://mystore-1255223546.cos.ap-chengdu.myqcloud.com/uPic/bqxWMp.png><p>是不是看着很想,又有点不一样,在 Java 的 SPI 配置文件里每一行只有一个实现类的全限定名,在 Dubbo的 SPI配置文件中是 key=value 的形式,我们只需要对应的 key 就能加载对应的实现,<figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br><span class=line>12</span><br><span class=line>13</span><br><span class=line>14</span><br><span class=line>15</span><br><span class=line>16</span><br><span class=line>17</span><br><span class=line>18</span><br><span class=line>19</span><br><span class=line>20</span><br><span class=line>21</span><br><span class=line>22</span><br><span class=line>23</span><br><span class=line>24</span><br><span class=line>25</span><br><span class=line>26</span><br><span class=line>27</span><br><span class=line>28</span><br><span class=line>29</span><br><span class=line>30</span><br></pre><td class=code><pre><span class=line><span class=comment>/**</span></span><br><span class=line><span class=comment> * 返回指定名字的扩展。如果指定名字的扩展不存在,则抛异常 {<span class=doctag>@link</span> IllegalStateException}.</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> * <span class=doctag>@param</span> name</span></span><br><span class=line><span class=comment> * <span class=doctag>@return</span></span></span><br><span class=line><span class=comment> */</span></span><br><span class=line> <span class=meta>@SuppressWarnings("unchecked")</span></span><br><span class=line> <span class=keyword>public</span> T <span class="title function_">getExtension</span><span class=params>(String name)</span> {</span><br><span class=line> <span class=keyword>if</span> (name == <span class=literal>null</span> || name.length() == <span class=number>0</span>)</span><br><span class=line> <span class=keyword>throw</span> <span class=keyword>new</span> <span class="title class_">IllegalArgumentException</span>(<span class=string>"Extension name == null"</span>);</span><br><span class=line> <span class=keyword>if</span> (<span class=string>"true"</span>.equals(name)) {</span><br><span class=line> <span class=keyword>return</span> getDefaultExtension();</span><br><span class=line> }</span><br><span class=line> Holder<Object> holder = cachedInstances.get(name);</span><br><span class=line> <span class=keyword>if</span> (holder == <span class=literal>null</span>) {</span><br><span class=line> cachedInstances.putIfAbsent(name, <span class=keyword>new</span> <span class="title class_">Holder</span><Object>());</span><br><span class=line> holder = cachedInstances.get(name);</span><br><span class=line> }</span><br><span class=line> <span class=type>Object</span> <span class=variable>instance</span> <span class=operator>=</span> holder.get();</span><br><span class=line> <span class=keyword>if</span> (instance == <span class=literal>null</span>) {</span><br><span class=line> <span class=keyword>synchronized</span> (holder) {</span><br><span class=line> instance = holder.get();</span><br><span class=line> <span class=keyword>if</span> (instance == <span class=literal>null</span>) {</span><br><span class=line> instance = createExtension(name);</span><br><span class=line> holder.set(instance);</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> <span class=keyword>return</span> (T) instance;</span><br><span class=line> }</span><br></pre></table></figure><p>这里其实就可以看出来第二个不同点了,就是这个<code>cachedInstances</code>,第一个是不用像 Java 原生的 SPI 那样去遍历加载对应的服务类,只需要通过 key 去寻找,并且寻找的时候会先从缓存的对象里去取,还有就是注意下这里的 DCL(double check lock)<figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br><span class=line>12</span><br><span class=line>13</span><br><span class=line>14</span><br><span class=line>15</span><br><span class=line>16</span><br><span class=line>17</span><br><span class=line>18</span><br><span class=line>19</span><br><span class=line>20</span><br><span class=line>21</span><br><span class=line>22</span><br><span class=line>23</span><br><span class=line>24</span><br><span class=line>25</span><br></pre><td class=code><pre><span class=line><span class=meta>@SuppressWarnings("unchecked")</span></span><br><span class=line> <span class=keyword>private</span> T <span class="title function_">createExtension</span><span class=params>(String name)</span> {</span><br><span class=line> Class<?> clazz = getExtensionClasses().get(name);</span><br><span class=line> <span class=keyword>if</span> (clazz == <span class=literal>null</span>) {</span><br><span class=line> <span class=keyword>throw</span> findException(name);</span><br><span class=line> }</span><br><span class=line> <span class=keyword>try</span> {</span><br><span class=line> <span class=type>T</span> <span class=variable>instance</span> <span class=operator>=</span> (T) EXTENSION_INSTANCES.get(clazz);</span><br><span class=line> <span class=keyword>if</span> (instance == <span class=literal>null</span>) {</span><br><span class=line> EXTENSION_INSTANCES.putIfAbsent(clazz, (T) clazz.newInstance());</span><br><span class=line> instance = (T) EXTENSION_INSTANCES.get(clazz);</span><br><span class=line> }</span><br><span class=line> injectExtension(instance);</span><br><span class=line> Set<Class<?>> wrapperClasses = cachedWrapperClasses;</span><br><span class=line> <span class=keyword>if</span> (wrapperClasses != <span class=literal>null</span> && wrapperClasses.size() > <span class=number>0</span>) {</span><br><span class=line> <span class=keyword>for</span> (Class<?> wrapperClass : wrapperClasses) {</span><br><span class=line> instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> <span class=keyword>return</span> instance;</span><br><span class=line> } <span class=keyword>catch</span> (Throwable t) {</span><br><span class=line> <span class=keyword>throw</span> <span class=keyword>new</span> <span class="title class_">IllegalStateException</span>(<span class=string>"Extension instance(name: "</span> + name + <span class=string>", class: "</span> +</span><br><span class=line> type + <span class=string>") could not be instantiated: "</span> + t.getMessage(), t);</span><br><span class=line> }</span><br><span class=line> }</span><br></pre></table></figure><p>然后就是创建扩展了,这里如果 wrapperClasses 就会遍历生成wrapper实例,并做 setter 依赖注入,但是这里cachedWrapperClasses的来源还是有点搞不清楚,得再看下 com.alibaba.dubbo.common.extension.ExtensionLoader#loadFile的具体逻辑<br>又看了遍新的代码,这个函数被抽出来了<figure class="highlight java"><table><tr><td class=gutter><pre><span class=line>1</span><br><span class=line>2</span><br><span class=line>3</span><br><span class=line>4</span><br><span class=line>5</span><br><span class=line>6</span><br><span class=line>7</span><br><span class=line>8</span><br><span class=line>9</span><br><span class=line>10</span><br><span class=line>11</span><br><span class=line>12</span><br><span class=line>13</span><br></pre><td class=code><pre><span class=line><span class=comment>/**</span></span><br><span class=line><span class=comment> * test if clazz is a wrapper class</span></span><br><span class=line><span class=comment> * <p></span></span><br><span class=line><span class=comment> * which has Constructor with given class type as its only argument</span></span><br><span class=line><span class=comment> */</span></span><br><span class=line> <span class=keyword>private</span> <span class=type>boolean</span> <span class="title function_">isWrapperClass</span><span class=params>(Class<?> clazz)</span> {</span><br><span class=line> <span class=keyword>try</span> {</span><br><span class=line> clazz.getConstructor(type);</span><br><span class=line> <span class=keyword>return</span> <span class=literal>true</span>;</span><br><span class=line> } <span class=keyword>catch</span> (NoSuchMethodException e) {</span><br><span class=line> <span class=keyword>return</span> <span class=literal>false</span>;</span><br><span class=line> }</span><br><span class=line> }</span><br></pre></table></figure><p>是否是 wrapperClass 其实就看构造函数的。</div><footer class=post-footer><div class="affiliate-creative affiliate-creative--post-end" data-affiliate-id=tencent-cloud-general><a rel="sponsored noopener" aria-label=腾讯云产品优惠 href=https://cloud.tencent.com/act/cps/redirect?redirect=2446&cps_key=2e33a1eedb380997d03618b4a5ba6ca7&from=console target=_blank><img alt=腾讯云服务器、数据库和云产品优惠 decoding=async height=419 loading=lazy src=/uploads/affiliate/tencent-cloud-general.webp width=690> </a><span class=affiliate-disclosure>推广</span></div><div 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