解锁Spring Boot新技巧，搞懂这些神奇注解，让你的项目更有料！

作者：微信小助手

发布时间：2024-01-24T22:47:17

成长不是瞬间的华丽转身

而是每天坚持努力的积累

STRAT

乘风破浪 | 直挂云帆

本篇来聊聊Springboot中的条件注解，这些条件注解使得开发者能够根据运行时环境的不同，动态的选择性地加载或配置一些 Bean 或组件。通过合理使用条件注解，可以实现更灵活和可配置的应用程序。

条件注解介绍以及解析

ConditionalOnBean ：是否存在某个某类或某个名字的Bean。
ConditionalOnMissingBean ：是否缺失某个某类或某个名字的Bean。
ConditionalOnSingleCandidate ：是否符合指定类型的Bean只有一个。
ConditionalOnClass ：是否存在某个类。
ConditionalOnMissingClass ：是否缺失某个类。
ConditionalOnExpression ：指定的表达式返回的是true还是false。
ConditionalOnJava ：判断Java版本。
ConditionalOnJndi ：JNDI指定的资源是否存在。
ConditionalOnWebApplication ：当前应用是一个Web应用。
ConditionalOnNotWebApplication ：当前应用不是一个Web应用。
ConditionalOnProperty ：Environment中是否存在某个属性。
ConditionalOnResource ：指定的资源是否存在。
ConditionalOnWarDeployment ：项目是否以War包部署的方式运行。
ConditionalOnCloudPlatform ：是否在某个云平台上。

当Spring在解析某一个条件注解的时候，首先会来匹配 SpringBootCondition中的matches方法，该方法里面会根据不同的逻辑去处理。

        </ul>
        <pre class="code-snippet__js" data-lang="java"><code><span class="code-snippet_outer"><span class="code-snippet__meta">@Override</span></span></code><code><span class="code-snippet_outer"><span class="code-snippet__function"><span class="code-snippet__keyword">public</span> <span class="code-snippet__keyword">final</span> <span class="code-snippet__keyword">boolean</span> <span class="code-snippet__title">matches</span><span class="code-snippet__params">(ConditionContext context, AnnotatedTypeMetadata metadata)</span> </span>{</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__comment">// 针对每个条件注解进行条件判断</span></span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__comment">// 条件注解写在了哪个类上，或哪个方法上</span></span></code><code><span class="code-snippet_outer">    String classOrMethodName = getClassOrMethodName(metadata);</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">try</span> {</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 条件的判断结果</span></span></code><code><span class="code-snippet_outer">        ConditionOutcome outcome = getMatchOutcome(context, metadata);</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 如果log的日志级别为trace，那就直接记录当前条件的判断结果</span></span></code><code><span class="code-snippet_outer">        logOutcome(classOrMethodName, outcome);</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 将判断结果记录到ConditionEvaluationReport中</span></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">//ConditionEvaluationReportLoggingListener会在收到ContextRefreshedEvent事件后把判断结果用日志的方式打印出来</span></span></code><code><span class="code-snippet_outer">        recordEvaluation(context, classOrMethodName, outcome);</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">return</span> outcome.isMatch();</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">catch</span> (NoClassDefFoundError ex) {</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">throw</span> <span class="code-snippet__keyword">new</span> IllegalStateException(<span class="code-snippet__string">"Could not evaluate condition on "</span> + classOrMethodName + <span class="code-snippet__string">" due to "</span></span></code><code><span class="code-snippet_outer">                + ex.getMessage() + <span class="code-snippet__string">" not found. Make sure your own configuration does not rely on "</span></span></code><code><span class="code-snippet_outer">                + <span class="code-snippet__string">"that class. This can also happen if you are "</span></span></code><code><span class="code-snippet_outer">                + <span class="code-snippet__string">"@ComponentScanning a springframework package (e.g. if you "</span></span></code><code><span class="code-snippet_outer">                + <span class="code-snippet__string">"put a @ComponentScan in the default package by mistake)"</span>, ex);</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">catch</span> (RuntimeException ex) {</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">throw</span> <span class="code-snippet__keyword">new</span> IllegalStateException(<span class="code-snippet__string">"Error processing condition on "</span> + getName(metadata), ex);</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer">}</span></code></pre>
       </section>
      </section>
     </section>
    </section>
   </section>
  </section>
 </section>
</section>

各条件注解的原理

@ConditionOnClass

判断某个类是否存在，执行OnClassCondition.getMatchOutcome方法。

  </ul>
  <pre class="code-snippet__js" data-lang="kotlin"><code><span class="code-snippet_outer"><span class="code-snippet__keyword">public</span> ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {</span></code><code><span class="code-snippet_outer">    ClassLoader classLoader = context.getClassLoader();</span></code><code><span class="code-snippet_outer">    ConditionMessage matchMessage = ConditionMessage.empty();</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__comment">// 拿到ConditionalOnClass注解中的value值，也就是要判断是否存在的类名</span></span></code><code><span class="code-snippet_outer">    List&lt;String&gt; onClasses = getCandidates(metadata, ConditionalOnClass.<span class="code-snippet__keyword">class</span>);</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">if</span> (onClasses != <span class="code-snippet__literal">null</span>) {</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 判断onClasses中不存在的类</span></span></code><code><span class="code-snippet_outer">        List&lt;String&gt; missing = filter(onClasses, ClassNameFilter.MISSING, classLoader);</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 如果有缺失的类，那就表示不匹配</span></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">if</span> (!missing.isEmpty()) {</span></code><code><span class="code-snippet_outer">            <span class="code-snippet__keyword">return</span> ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnClass.<span class="code-snippet__keyword">class</span>)</span></code><code><span class="code-snippet_outer">                    .didNotFind(<span class="code-snippet__string">"required class"</span>, <span class="code-snippet__string">"required classes"</span>).items(Style.QUOTE, missing));</span></code><code><span class="code-snippet_outer">        }</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 否则就表示匹配</span></span></code><code><span class="code-snippet_outer">        matchMessage = matchMessage.andCondition(ConditionalOnClass.<span class="code-snippet__keyword">class</span>)</span></code><code><span class="code-snippet_outer">                .found(<span class="code-snippet__string">"required class"</span>, <span class="code-snippet__string">"required classes"</span>)</span></code><code><span class="code-snippet_outer">                .items(Style.QUOTE, filter(onClasses, ClassNameFilter.PRESENT, classLoader));</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__comment">// 和上面类似，只不过是判断onMissingClasses是不是全部缺失，如果是则表示匹配</span></span></code><code><span class="code-snippet_outer">    List&lt;String&gt; onMissingClasses = getCandidates(metadata, ConditionalOnMissingClass.<span class="code-snippet__keyword">class</span>);</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">if</span> (onMissingClasses != <span class="code-snippet__literal">null</span>) {</span></code><code><span class="code-snippet_outer">        List&lt;String&gt; present = filter(onMissingClasses, ClassNameFilter.PRESENT, classLoader);</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">if</span> (!present.isEmpty()) {</span></code><code><span class="code-snippet_outer">            <span class="code-snippet__keyword">return</span> ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnMissingClass.<span class="code-snippet__keyword">class</span>)</span></code><code><span class="code-snippet_outer">                    .found(<span class="code-snippet__string">"unwanted class"</span>, <span class="code-snippet__string">"unwanted classes"</span>).items(Style.QUOTE, present));</span></code><code><span class="code-snippet_outer">        }</span></code><code><span class="code-snippet_outer">        matchMessage = matchMessage.andCondition(ConditionalOnMissingClass.<span class="code-snippet__keyword">class</span>)</span></code><code><span class="code-snippet_outer">                .didNotFind(<span class="code-snippet__string">"unwanted class"</span>, <span class="code-snippet__string">"unwanted classes"</span>)</span></code><code><span class="code-snippet_outer">                .items(Style.QUOTE, filter(onMissingClasses, ClassNameFilter.MISSING, classLoader));</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">return</span> ConditionOutcome.match(matchMessage);</span></code><code><span class="code-snippet_outer">}</span></code></pre>
 </section>
 <section data-tools="135编辑器" data-id="7">
  <section style="padding-left: 10px;border-left: 5px solid rgb(170, 170, 170);margin: 10px auto;">
   <section style="font-size: 13px;letter-spacing: 1px;line-height: 1.75em;color: rgba(102, 102, 102, 0.8);">
    <section data-role="list">
     <ul class="list-paddingleft-1" style="padding-left: 30px;list-style-position: outside;margin-left: 8px;margin-right: 8px;">
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="font-size: 15px;caret-color: red;color: rgb(96, 96, 96);letter-spacing: 1px;">获取ConditionalOnClass注解中的value值。</span>
        <ne-clipboard source="https%3A%2F%2Fwww.yuque.com%2Fdream_lsj%2Fyddpep%2Fpunmt1zhmirb2y8x%23Bvjvx"></ne-clipboard>
       </section></li>
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">判断是否存在不配的类，判断逻辑为</span>
        <span style="font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);"> Class.<em>forName</em>(className);</span>
        <span style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;"> 存在就会直接返回。</span>
       </section></li>
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">不存在就会获取ConditionalOnMissingClass注解中的value值，然后处理逻辑与前面一直。</span>
       </section></li>
     </ul>
    </section>
    <section typography="classic" style="height: 0px;overflow: hidden;">
     <br>
    </section>
   </section>
  </section>
 </section>
 <section typography="classic">
  <section style="min-height: 24px;margin: 24px 8px;line-height: 1.75em;">
   <span style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">Spring中会去每个注解的解析，但是springBoot中会</span>
   <span style="font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);">提前将@ConditionalOnClass与@ConditionalOnClass</span>
   <span style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">一起解析，如果发现不匹配能够快速的处理，但是如果都成立，那么这个回去重复执行。</span>
  </section>
 </section>
</section>

@ConditionOnBean

@ConditionalOnBean和@ConditionalOnClass的底层实现应该是差不多的，一个是判断Bean存不存在，一个是判断类存不存在，事实上也确实差不多。

@ConditionalOnBean和@ConditionalOnMissingBean对应的都是OnBeanCondition类， OnBeanCondition类也是继承了SpringBootCondition，所以SpringBootCondition类中的getMatchOutcome方法才是匹配逻辑。

  </ul>
  <pre class="code-snippet__js" data-lang="kotlin"><code><span class="code-snippet_outer"><span class="code-snippet__meta">@Override</span></span></code><code><span class="code-snippet_outer"><span class="code-snippet__keyword">public</span> ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {</span></code><code><span class="code-snippet_outer">    ConditionMessage matchMessage = ConditionMessage.empty();</span></code><code><span class="code-snippet_outer">    MergedAnnotations annotations = metadata.getAnnotations();</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__comment">// 如果存在ConditionalOnBean注解</span></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">if</span> (annotations.isPresent(ConditionalOnBean.<span class="code-snippet__keyword">class</span>)) {</span></code><code><span class="code-snippet_outer">        Spec&lt;ConditionalOnBean&gt; spec = new Spec&lt;&gt;(context, metadata, annotations, ConditionalOnBean.<span class="code-snippet__keyword">class</span>);</span></code><code><span class="code-snippet_outer">        MatchResult matchResult = getMatchingBeans(context, spec);</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 如果某个Bean不存在</span></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">if</span> (!matchResult.isAllMatched()) {</span></code><code><span class="code-snippet_outer">            String reason = createOnBeanNoMatchReason(matchResult);</span></code><code><span class="code-snippet_outer">            <span class="code-snippet__keyword">return</span> ConditionOutcome.noMatch(spec.message().because(reason));</span></code><code><span class="code-snippet_outer">        }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 所有Bean都存在</span></span></code><code><span class="code-snippet_outer">        matchMessage = spec.message(matchMessage).found(<span class="code-snippet__string">"bean"</span>, <span class="code-snippet__string">"beans"</span>).items(Style.QUOTE, matchResult.getNamesOfAllMatches());</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__comment">// 如果存在ConditionalOnSingleCandidate注解</span></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">if</span> (metadata.isAnnotated(ConditionalOnSingleCandidate.<span class="code-snippet__keyword">class</span>.getName())) {</span></code><code><span class="code-snippet_outer">        Spec&lt;ConditionalOnSingleCandidate&gt; spec = new SingleCandidateSpec(context, metadata, annotations);</span></code><code><span class="code-snippet_outer">        MatchResult matchResult = getMatchingBeans(context, spec);</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// Bean不存在</span></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">if</span> (!matchResult.isAllMatched()) {</span></code><code><span class="code-snippet_outer">            <span class="code-snippet__keyword">return</span> ConditionOutcome.noMatch(spec.message().didNotFind(<span class="code-snippet__string">"any beans"</span>).atAll());</span></code><code><span class="code-snippet_outer">        }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// Bean存在</span></span></code><code><span class="code-snippet_outer">        Set&lt;String&gt; allBeans = matchResult.getNamesOfAllMatches();</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 如果只有一个</span></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">if</span> (allBeans.size() == <span class="code-snippet__number">1</span>) {</span></code><code><span class="code-snippet_outer">            matchMessage = spec.message(matchMessage).found(<span class="code-snippet__string">"a single bean"</span>).items(Style.QUOTE, allBeans);</span></code><code><span class="code-snippet_outer">        }</span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">else</span> {</span></code><code><span class="code-snippet_outer">            <span class="code-snippet__comment">// 如果有多个</span></span></code><code><span class="code-snippet_outer">            List&lt;String&gt; primaryBeans = getPrimaryBeans(context.getBeanFactory(), allBeans, spec.getStrategy() == SearchStrategy.ALL);</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">            <span class="code-snippet__comment">// 没有主Bean，那就不匹配</span></span></code><code><span class="code-snippet_outer">            <span class="code-snippet__keyword">if</span> (primaryBeans.isEmpty()) {</span></code><code><span class="code-snippet_outer">                <span class="code-snippet__keyword">return</span> ConditionOutcome.noMatch(</span></code><code><span class="code-snippet_outer">                    spec.message().didNotFind(<span class="code-snippet__string">"a primary bean from beans"</span>).items(Style.QUOTE, allBeans));</span></code><code><span class="code-snippet_outer">            }</span></code><code><span class="code-snippet_outer">            <span class="code-snippet__comment">// 有多个主Bean，那就不匹配</span></span></code><code><span class="code-snippet_outer">            <span class="code-snippet__keyword">if</span> (primaryBeans.size() &gt; <span class="code-snippet__number">1</span>) {</span></code><code><span class="code-snippet_outer">                <span class="code-snippet__keyword">return</span> ConditionOutcome</span></code><code><span class="code-snippet_outer">                    .noMatch(spec.message().found(<span class="code-snippet__string">"multiple primary beans"</span>).items(Style.QUOTE, primaryBeans));</span></code><code><span class="code-snippet_outer">            }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">            <span class="code-snippet__comment">// 只有一个主Bean</span></span></code><code><span class="code-snippet_outer">            matchMessage = spec.message(matchMessage)</span></code><code><span class="code-snippet_outer">                .found(<span class="code-snippet__string">"a single primary bean '"</span> + primaryBeans.<span class="code-snippet__keyword">get</span>(<span class="code-snippet__number">0</span>) + <span class="code-snippet__string">"' from beans"</span>)</span></code><code><span class="code-snippet_outer">                .items(Style.QUOTE, allBeans);</span></code><code><span class="code-snippet_outer">        }</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__comment">// 存在ConditionalOnMissingBean注解</span></span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">if</span> (metadata.isAnnotated(ConditionalOnMissingBean.<span class="code-snippet__keyword">class</span>.getName())) {</span></code><code><span class="code-snippet_outer">        Spec&lt;ConditionalOnMissingBean&gt; spec = new Spec&lt;&gt;(context, metadata, annotations,</span></code><code><span class="code-snippet_outer">                                                         ConditionalOnMissingBean.<span class="code-snippet__keyword">class</span>);</span></code><code><span class="code-snippet_outer">        MatchResult matchResult = getMatchingBeans(context, spec);</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">//有任意一个Bean存在，那就条件不匹配</span></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">if</span> (matchResult.isAnyMatched()) {</span></code><code><span class="code-snippet_outer">            String reason = createOnMissingBeanNoMatchReason(matchResult);</span></code><code><span class="code-snippet_outer">            <span class="code-snippet__keyword">return</span> ConditionOutcome.noMatch(spec.message().because(reason));</span></code><code><span class="code-snippet_outer">        }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__comment">// 都不存在在，则匹配</span></span></code><code><span class="code-snippet_outer">        matchMessage = spec.message(matchMessage).didNotFind(<span class="code-snippet__string">"any beans"</span>).atAll();</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer">    <span class="code-snippet__keyword">return</span> ConditionOutcome.match(matchMessage);</span></code><code><span class="code-snippet_outer">}</span></code></pre>
 </section>
 <section data-tools="135编辑器" data-id="7">
  <section style="padding-left: 10px;border-left: 5px solid rgb(170, 170, 170);margin: 10px auto;">
   <section style="font-size: 13px;letter-spacing: 1px;line-height: 1.75em;color: rgba(102, 102, 102, 0.8);">
    <section data-role="list">
     <ul class="list-paddingleft-1" style="padding-left: 30px;list-style-position: outside;margin-left: 8px;margin-right: 8px;">
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);">当前在解析的类或方法上</span>
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">，</span>
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">是否有@ConditionalOnBean注解，如果有则生成对应的Spec对象，该对象中包含了 用户指定的，要判断的是否存在的Bean的类型。</span>
       </section></li>
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">调用</span>
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);">getMatchingBeans方法进行条件判断</span>
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">&nbsp;。</span>
       </section></li>
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">只要判断出来某一个Bean不存在，则return，表示条件不匹配 。</span>
       </section></li>
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">只要所有Bean都存在，则继续执行下面代码 。</span>
       </section></li>
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);">在解析的类或方法上是否有@ConditionalOnSingleCandidate注解</span>
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">，如果有则生成对应的 SingleCandidateSpec对象，该对象中包含了用户指定的，要判断的是否存在的Bean的类型（只能指定一个类 型），并且该类型的Bean只能有一个。</span>
       </section></li>
      <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
       <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);">调用getMatchingBeans方法进行条件判断</span>
        <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(96, 96, 96);">。</span>
       </section></li>
     </ul>
    </section>
   </section>
  </section>
 </section>
</section>

@ConditionOnMissingBean

@ConditionalOnMissingBean的作用是用来判断某个Bean是否缺失，如果不存在某个Bean，那就符合该条件。

  </ul>
  <pre class="code-snippet__js" data-lang="ruby"><code><span class="code-snippet_outer">@Configuration(proxyBeanMethods = <span class="code-snippet__literal">false</span>)</span></code><code><span class="code-snippet_outer">@ConditionalOnClass({ Servlet.<span class="code-snippet__keyword">class</span>, Tomcat.<span class="code-snippet__keyword">class</span>, UpgradeProtocol.<span class="code-snippet__keyword">class</span> })</span></code><code><span class="code-snippet_outer">@ConditionalOnMissingBean(value = ServletWebServerFactory.<span class="code-snippet__keyword">class</span>, search = SearchStrategy.CURRENT)</span></code><code><span class="code-snippet_outer">static <span class="code-snippet__class"><span class="code-snippet__keyword">class</span> <span class="code-snippet__title">EmbeddedTomcat</span> {</span></span></code><code><span class="code-snippet_outer">    @Bean</span></code><code><span class="code-snippet_outer">    TomcatServletWebServerFactory tomcatServletWebServerFactory(</span></code><code><span class="code-snippet_outer">        ObjectProvider&lt;TomcatConnectorCustomizer&gt; connectorCustomizers,</span></code><code><span class="code-snippet_outer">        ObjectProvider&lt;TomcatContextCustomizer&gt; contextCustomizers,</span></code><code><span class="code-snippet_outer">        ObjectProvider&lt;TomcatProtocolHandlerCustomizer&lt;?<span class="code-snippet__meta">&gt;&gt; </span>protocolHandlerCustomizers) {</span></code><code><span class="code-snippet_outer">        TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__regexp">//</span> orderedStream()调用时会去Spring容器中找到TomcatConnectorCustomizer类型的Bean，</span></code><code><span class="code-snippet_outer">        默认是没有的，程序员可以自己定义</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        factory.getTomcatConnectorCustomizers()</span></code><code><span class="code-snippet_outer">            .addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));</span></code><code><span class="code-snippet_outer">        factory.getTomcatContextCustomizers()</span></code><code><span class="code-snippet_outer">            .addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));</span></code><code><span class="code-snippet_outer">        factory.getTomcatProtocolHandlerCustomizers()</span></code><code><span class="code-snippet_outer">            .addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">        <span class="code-snippet__keyword">return</span> factory;</span></code><code><span class="code-snippet_outer">    }</span></code><code><span class="code-snippet_outer"><br></span></code><code><span class="code-snippet_outer">}</span></code></pre>
 </section>
 <section data-tools="135编辑器" data-id="7">
  <section style="padding-left: 10px;border-left: 5px solid rgb(170, 170, 170);margin: 10px auto;">
   <section style="font-size: 13px;letter-spacing: 1px;line-height: 1.75em;color: rgba(102, 102, 102, 0.8);" data-role="list">
    <ul class="list-paddingleft-1" style="padding-left: 30px;list-style-position: outside;margin-left: 8px;margin-right: 8px;">
     <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
      <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
       <span style="caret-color: red;font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">如果用户自己没有定义</span>
       <span style="caret-color: red;font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);">ServletWebServerFactory类型</span>
       <span style="caret-color: red;font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">的Bean，那代码中所定义的Bean就会生效。</span>
      </section></li>
     <li style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">
      <section style="color: rgb(102, 102, 102);font-size: 14px;letter-spacing: 1.5px;caret-color: red;margin-top: 8px;margin-bottom: 8px;line-height: 1.75em;">
       <span style="caret-color: red;font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">如果用户自己定义了ServletWebServerFactory类型的Bean，那代码中定义的Bean就不生效 。</span>
      </section></li>
    </ul>
   </section>
  </section>
 </section>
 <section typography="classic">
  <p style="min-height: 24px;margin: 24px 8px;line-height: 1.75em;"><span style="letter-spacing: 1px;font-size: 16px;color: rgb(255, 76, 0);">SpringBoot利用该注解来决定到底用用户自己的Bean还是用SpringBoot自定配置的Bean。</span></p>
  <p style="min-height: 24px;margin: 24px 8px;line-height: 1.75em;"><span style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">如果先解析自动配置然后再解析程序员定义的，那么就会发现该注解里面的判断是错误的，因为你首先来判断是否成立，是在还没有解析程序员定义的Bean之前，所以就会认为是符合该注解，</span><span style="font-size: 15px;letter-spacing: 1px;color: rgb(255, 76, 0);">实际上只是没有加载程序员定义的，所以必须先去解析程序员定义的，然后最后执行自动配置的</span><span style="font-size: 15px;color: rgb(96, 96, 96);letter-spacing: 1px;">。</span></p>
 </section>
</section>

今天先说到这里明天我们继续，这三个注解是我们自定义扩展SpringBoot相关知识点的重要内容。

文章有帮助的话，

点赞、在看、转发吧。

一起讨论，谢谢支持哟

长按二维码关注我

每日分享干货