golang抽取接口，依赖注入(依赖倒置)解决包引用关系
发布时间：2022-01-05 编辑：jiaochengji.com
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本文记录了作者在golang开发中，通过抽取接口，依赖注入的方式，解决包与包之间的不合理引用关系。
总结来说：
<blockquote>面向接口编程，并且golang中接口函数的参数最好是标准库的类型
</blockquote> <h2>场景</h2>目前项目中有一个业务逻辑包<code>business_logic</code>，两个工具库包<code>pkg1</code>和<code>pkg2</code>，其中
<ul><li><code>pkg1</code>是旧库，API不宜改动，<code>pkg2</code>是新库，尚未正式使用</li><li><code>business_logic</code>会使用<code>pkg1</code>和<code>pkg2</code></li><li><code>pkg1</code>内部要添加使用<code>pkg2</code>的逻辑</li></ul><pre><code class="lang-go hljs"><span class="token comment">// pkg1/main.go</span> <span class="token keyword">package</span> pkg1 <span class="token keyword">import</span> <span class="token string">"pkg2"</span> <span class="token keyword">func</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> pkg2<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span>pkg2<span class="token punctuation">.</span>S<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> </code></pre> <pre><code class="lang-go hljs"><span class="token comment">// pkg2/main.go</span> <span class="token keyword">package</span> pkg2 <span class="token keyword">type</span> S <span class="token keyword">struct</span> <span class="token punctuation">{</span> param1 <span class="token builtin">int</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s S<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span> </code></pre> <pre><code class="lang-go hljs"><span class="token comment">// business_logic/main.go</span> <span class="token keyword">package</span> main <span class="token keyword">import</span> <span class="token punctuation">(</span> <span class="token string">"pkg1"</span> <span class="token string">"pkg2"</span> <span class="token punctuation">)</span> <span class="token keyword">func</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> pkg1<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token punctuation">)</span> pkg2<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span>pkg2<span class="token punctuation">.</span>S<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> </code></pre>这样就引起了一个问题：
<blockquote><code>business_logic</code>其实引用了两次<code>pkg2</code>，一次是直接引用，一次是通过<code>pkg1</code>间接引用，将来在版本更迭中，很有可能会出现直接引用的版本和间接引用的版本不一致的情况，从而引起未知bug
</blockquote> <h2>解决尝试</h2>如果不希望两次引用，那么最好的方式是消除<code>pkg1</code>对<code>pkg2</code>的引用，消除引用的方式是
<ul><li><code>pkg1</code>抽象出一个接口，</li><li>让<code>pkg2</code>提供结构体，实现<code>pkg1</code>抽象出的接口</li></ul>这样，<code>pkg2</code>实际上就变成了<code>pkg1</code>的一个插件，只要在<code>business_logic</code>初始化的时候，将<code>pkg2</code>的插件注入到<code>pkg1</code>里去就行
但是这样的尝试失败了，我们先来看一下代码
<pre><code class="lang-go hljs"><span class="token comment">// pkg1/main.go</span> <span class="token keyword">package</span> pkg1 <span class="token keyword">import</span> <span class="token string">"pkg2"</span> <span class="token keyword">type</span> Plugin <span class="token keyword">interface</span> <span class="token punctuation">{</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s pkg2<span class="token punctuation">.</span>S<span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token keyword">var</span> plugin Plugin <span class="token keyword">func</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> plugin <span class="token operator">!=</span> <span class="token boolean">nil</span> <span class="token punctuation">{</span> plugin<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span>pkg2<span class="token punctuation">.</span>S<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token function">SetPlugin</span><span class="token punctuation">(</span>p Plugin<span class="token punctuation">)</span> <span class="token punctuation">{</span> plugin <span class="token operator">=</span> p <span class="token punctuation">}</span> </code></pre> <pre><code class="lang-go hljs"><span class="token comment">// pkg2/main.go</span> <span class="token keyword">package</span> pkg2 <span class="token keyword">type</span> S <span class="token keyword">struct</span> <span class="token punctuation">{</span> param1 <span class="token builtin">int</span> <span class="token punctuation">}</span> <span class="token keyword">type</span> Plugin <span class="token keyword">struct</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token punctuation">(</span>p <span class="token operator">*</span>Plugin<span class="token punctuation">)</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s S<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s S<span class="token punctuation">)</span> <span class="token punctuation">{</span> p <span class="token operator">:=</span> Plugin<span class="token punctuation">{</span><span class="token punctuation">}</span> p<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token punctuation">}</span> </code></pre> <pre><code class="lang-go hljs"><span class="token comment">// business_logic/main.go</span> <span class="token keyword">package</span> main <span class="token keyword">import</span> <span class="token punctuation">(</span> <span class="token string">"pkg1"</span> <span class="token string">"pkg2"</span> <span class="token punctuation">)</span> <span class="token keyword">func</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> pkg1<span class="token punctuation">.</span><span class="token function">SetPlugin</span><span class="token punctuation">(</span><span class="token operator">&</span>pkg2<span class="token punctuation">.</span>Plugin<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> pkg1<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token punctuation">)</span> pkg2<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span>pkg2<span class="token punctuation">.</span>S<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> </code></pre>我们发现，<code>pkg1</code>对<code>pkg2</code>的引用仍旧存在，其原因在于抽取出来的接口函数中的参数是属于<code>pkg2</code>的
<pre><code class="lang-go hljs"><span class="token keyword">type</span> Plugin <span class="token keyword">interface</span> <span class="token punctuation">{</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s pkg2<span class="token punctuation">.</span>S<span class="token punctuation">)</span> <span class="token punctuation">}</span> </code></pre> <h2>最终解决方案</h2>由于<code>pkg2</code>是新库，所以我们决定更改它的接口，最终的代码如下
<pre><code class="lang-go hljs"><span class="token comment">// pkg1/main.go</span> <span class="token keyword">package</span> pkg1 <span class="token keyword">type</span> Plugin <span class="token keyword">interface</span> <span class="token punctuation">{</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>param <span class="token builtin">int</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token keyword">var</span> plugin Plugin <span class="token keyword">func</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token keyword">if</span> plugin <span class="token operator">!=</span> <span class="token boolean">nil</span> <span class="token punctuation">{</span> plugin<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token function">SetPlugin</span><span class="token punctuation">(</span>p Plugin<span class="token punctuation">)</span> <span class="token punctuation">{</span> plugin <span class="token operator">=</span> p <span class="token punctuation">}</span> </code></pre> <pre><code class="lang-go hljs"><span class="token comment">// pkg2/main.go</span> <span class="token keyword">package</span> pkg2 <span class="token keyword">type</span> Plugin <span class="token keyword">struct</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token punctuation">(</span>p <span class="token operator">*</span>Plugin<span class="token punctuation">)</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s <span class="token builtin">int</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s <span class="token builtin">int</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> p <span class="token operator">:=</span> Plugin<span class="token punctuation">{</span><span class="token punctuation">}</span> p<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token punctuation">}</span> </code></pre> <pre><code class="lang-go hljs"><span class="token comment">// business_logic/main.go</span> <span class="token keyword">package</span> main <span class="token keyword">import</span> <span class="token punctuation">(</span> <span class="token string">"pkg1"</span> <span class="token string">"pkg2"</span> <span class="token punctuation">)</span> <span class="token keyword">func</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> pkg1<span class="token punctuation">.</span><span class="token function">SetPlugin</span><span class="token punctuation">(</span><span class="token operator">&</span>pkg2<span class="token punctuation">.</span>Plugin<span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> pkg1<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token punctuation">)</span> pkg2<span class="token punctuation">.</span><span class="token function">ExternalAPI</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> </code></pre>可以看到，这回彻底解决了<code>pkg1</code>引用<code>pkg2</code>的问题，代价就是将<code>pkg2.S</code>这个结构体参数展开了
视具体业务情况而定，我们可以通过：
<ol><li>展开结构体</li><li>将结构体换做<code>map[string]interface{}</code>（当然需要手动做字段的提取和塞入）</li><li>将结构体换做<code>string</code>，用JSON传参（手动Marshal和Unmarshal）</li><li>将参数类型放到新的第三方库<code>pkg3</code>中（这样就又要维护引用的<code>pkg3</code>版本一致）</li></ol>软件开发中没有silver-bullet，只有trade-off，这次的方案，也还算满意
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