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<p><strong>Servlet的service方法</strong>，是整个HTTP请求处理流程的一个入口。在某个Controller方法上打上断点，可以观察到如下调用栈：</p>
<p><img src="http://upload-images.jianshu.io/upload_images/1754553-0c28bd83fe96ecb8.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>由于DispatcherServlet继承自HttpServlet，也就间接实现了Servlet接口。DispatcherServlet的类图如下：</p>
<p><img src="http://upload-images.jianshu.io/upload_images/1754553-07377b614e6d7193.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="image.png"></p>
<p>所以请求到来时，首先会进入DispatcherServlet的父类HttpServlet的service方法：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">service</span><span class="params">(ServletRequest req, ServletResponse res)</span></span></div><div class="line"><span class="function"> <span class="keyword">throws</span> ServletException, IOException </span>{</div><div class="line"></div><div class="line"> HttpServletRequest request;</div><div class="line"> HttpServletResponse response;</div><div class="line"></div><div class="line"> <span class="keyword">try</span> {</div><div class="line"> request = (HttpServletRequest) req;</div><div class="line"> response = (HttpServletResponse) res;</div><div class="line"> } <span class="keyword">catch</span> (ClassCastException e) {</div><div class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> ServletException(<span class="string">"non-HTTP request or response"</span>);</div><div class="line"> }</div><div class="line"> <span class="comment">// 只是做下类型强转，然后调用另外一个方法</span></div><div class="line"> service(request, response);</div><div class="line">}</div></pre></td></tr></table></figure>
<p>在上面方法的末尾，会调用另外一个与HTTP协议相关的service方法，这个方法的定义为<code>protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException</code>。FrameworkServlet实现了这个方法，所以下一步来到FrameworkServlet的协议相关的service方法：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">service</span><span class="params">(HttpServletRequest request, HttpServletResponse response)</span></span></div><div class="line"><span class="function"> <span class="keyword">throws</span> ServletException, IOException </span>{</div><div class="line"></div><div class="line"> HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());</div><div class="line"> <span class="keyword">if</span> (httpMethod == HttpMethod.PATCH || httpMethod == <span class="keyword">null</span>) {</div><div class="line"> processRequest(request, response);</div><div class="line"> }</div><div class="line"> <span class="keyword">else</span> {</div><div class="line"> <span class="keyword">super</span>.service(request, response);</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>在这段代码最后，又会委托给父类HttpServlet的协议相关的service方法，借用父类对请求方法（GET，POST等等）的判断逻辑，去调用相应的doGet或doPost方法：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div><div class="line">51</div><div class="line">52</div><div class="line">53</div><div class="line">54</div><div class="line">55</div><div class="line">56</div><div class="line">57</div><div class="line">58</div><div class="line">59</div><div class="line">60</div><div class="line">61</div><div class="line">62</div><div class="line">63</div><div class="line">64</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">void</span> <span class="title">service</span><span class="params">(HttpServletRequest req, HttpServletResponse resp)</span></span></div><div class="line"><span class="function"> <span class="keyword">throws</span> ServletException, IOException </span>{</div><div class="line"></div><div class="line"> String method = req.getMethod();</div><div class="line"></div><div class="line"> <span class="keyword">if</span> (method.equals(METHOD_GET)) {</div><div class="line"> <span class="keyword">long</span> lastModified = getLastModified(req);</div><div class="line"> <span class="keyword">if</span> (lastModified == -<span class="number">1</span>) {</div><div class="line"> <span class="comment">// servlet doesn't support if-modified-since, no reason</span></div><div class="line"> <span class="comment">// to go through further expensive logic</span></div><div class="line"> doGet(req, resp);</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> <span class="keyword">long</span> ifModifiedSince;</div><div class="line"> <span class="keyword">try</span> {</div><div class="line"> ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);</div><div class="line"> } <span class="keyword">catch</span> (IllegalArgumentException iae) {</div><div class="line"> <span class="comment">// Invalid date header - proceed as if none was set</span></div><div class="line"> ifModifiedSince = -<span class="number">1</span>;</div><div class="line"> }</div><div class="line"> <span class="keyword">if</span> (ifModifiedSince < (lastModified / <span class="number">1000</span> * <span class="number">1000</span>)) {</div><div class="line"> <span class="comment">// If the servlet mod time is later, call doGet()</span></div><div class="line"> <span class="comment">// Round down to the nearest second for a proper compare</span></div><div class="line"> <span class="comment">// A ifModifiedSince of -1 will always be less</span></div><div class="line"> maybeSetLastModified(resp, lastModified);</div><div class="line"> doGet(req, resp);</div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);</div><div class="line"> }</div><div class="line"> }</div><div class="line"></div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_HEAD)) {</div><div class="line"> <span class="keyword">long</span> lastModified = getLastModified(req);</div><div class="line"> maybeSetLastModified(resp, lastModified);</div><div class="line"> doHead(req, resp);</div><div class="line"></div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_POST)) {</div><div class="line"> doPost(req, resp);</div><div class="line"></div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_PUT)) {</div><div class="line"> doPut(req, resp);</div><div class="line"></div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_DELETE)) {</div><div class="line"> doDelete(req, resp);</div><div class="line"></div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_OPTIONS)) {</div><div class="line"> doOptions(req,resp);</div><div class="line"></div><div class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> (method.equals(METHOD_TRACE)) {</div><div class="line"> doTrace(req,resp);</div><div class="line"></div><div class="line"> } <span class="keyword">else</span> {</div><div class="line"> <span class="comment">//</span></div><div class="line"> <span class="comment">// Note that this means NO servlet supports whatever</span></div><div class="line"> <span class="comment">// method was requested, anywhere on this server.</span></div><div class="line"> <span class="comment">//</span></div><div class="line"></div><div class="line"> String errMsg = lStrings.getString(<span class="string">"http.method_not_implemented"</span>);</div><div class="line"> Object[] errArgs = <span class="keyword">new</span> Object[<span class="number">1</span>];</div><div class="line"> errArgs[<span class="number">0</span>] = method;</div><div class="line"> errMsg = MessageFormat.format(errMsg, errArgs);</div><div class="line"></div><div class="line"> resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>由于子类FrameworkServlet实现了相应的doGet，doPost等方法，所以接着来到了FrameworkServlet的doGet方法：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">doGet</span><span class="params">(HttpServletRequest request, HttpServletResponse response)</span></span></div><div class="line"><span class="function"> <span class="keyword">throws</span> ServletException, IOException </span>{</div><div class="line"></div><div class="line"> processRequest(request, response);</div><div class="line">}</div><div class="line"></div><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">doPost</span><span class="params">(HttpServletRequest request, HttpServletResponse response)</span></span></div><div class="line"><span class="function"> <span class="keyword">throws</span> ServletException, IOException </span>{</div><div class="line"></div><div class="line"> processRequest(request, response);</div><div class="line">}</div></pre></td></tr></table></figure>
<p>观察上面代码可以发现，FrameworkServlet的doGet和doPost等方法都是final的，也就是不允许子类再去重写。在这些方法的内部，都统一去调用了processRequest这个方法。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">processRequest</span><span class="params">(HttpServletRequest request, HttpServletResponse response)</span></span></div><div class="line"><span class="function"> <span class="keyword">throws</span> ServletException, IOException </span>{</div><div class="line"></div><div class="line"> <span class="keyword">long</span> startTime = System.currentTimeMillis();</div><div class="line"> Throwable failureCause = <span class="keyword">null</span>;</div><div class="line"></div><div class="line"> LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();</div><div class="line"> LocaleContext localeContext = buildLocaleContext(request);</div><div class="line"></div><div class="line"> RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();</div><div class="line"> ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);</div><div class="line"></div><div class="line"> WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);</div><div class="line"> asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), <span class="keyword">new</span> RequestBindingInterceptor());</div><div class="line"></div><div class="line"> initContextHolders(request, localeContext, requestAttributes);</div><div class="line"></div><div class="line"> <span class="keyword">try</span> {</div><div class="line"> doService(request, response);</div><div class="line"> }</div><div class="line"> <span class="keyword">catch</span> (ServletException | IOException ex) {</div><div class="line"> failureCause = ex;</div><div class="line"> <span class="keyword">throw</span> ex;</div><div class="line"> }</div><div class="line"> <span class="keyword">catch</span> (Throwable ex) {</div><div class="line"> failureCause = ex;</div><div class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> NestedServletException(<span class="string">"Request processing failed"</span>, ex);</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keyword">finally</span> {</div><div class="line"> resetContextHolders(request, previousLocaleContext, previousAttributes);</div><div class="line"> <span class="keyword">if</span> (requestAttributes != <span class="keyword">null</span>) {</div><div class="line"> requestAttributes.requestCompleted();</div><div class="line"> }</div><div class="line"> logResult(request, response, failureCause, asyncManager);</div><div class="line"> publishRequestHandledEvent(request, response, startTime, failureCause);</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>这个processRequest方法也是final的，而在这个方法内部，调用了一个doService方法：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">protected</span> <span class="keyword">abstract</span> <span class="keyword">void</span> <span class="title">doService</span><span class="params">(HttpServletRequest request, HttpServletResponse response)</span> <span class="keyword">throws</span> Exception</span>;</div></pre></td></tr></table></figure>
<p>可以看到，这个doService方法是抽象的，所以强制了子类必须重写该方法。</p>
<p>这里其实用到了一种叫做“模版方法”的设计模式：在父类中用final方法规定好具体流程，流程中的某个步骤，通过抽象方法交给子类去实现。</p>
<p>DispatcherServlet重写了doService方法：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div></pre></td><td class="code"><pre><div class="line">@Override</div><div class="line"> protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {</div><div class="line"> logRequest(request);</div><div class="line"></div><div class="line"> // Keep a snapshot of the request attributes in case of an include,</div><div class="line"> // to be able to restore the original attributes after the include.</div><div class="line"> Map<String, Object> attributesSnapshot = null;</div><div class="line"> if (WebUtils.isIncludeRequest(request)) {</div><div class="line"> attributesSnapshot = new HashMap<>();</div><div class="line"> Enumeration<?> attrNames = request.getAttributeNames();</div><div class="line"> while (attrNames.hasMoreElements()) {</div><div class="line"> String attrName = (String) attrNames.nextElement();</div><div class="line"> if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {</div><div class="line"> attributesSnapshot.put(attrName, request.getAttribute(attrName));</div><div class="line"> }</div><div class="line"> }</div><div class="line"> }</div><div class="line"></div><div class="line"> // Make framework objects available to handlers and view objects.</div><div class="line"> request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());</div><div class="line"> request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);</div><div class="line"> request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);</div><div class="line"> request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());</div><div class="line"></div><div class="line"> if (this.flashMapManager != null) {</div><div class="line"> FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);</div><div class="line"> if (inputFlashMap != null) {</div><div class="line"> request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));</div><div class="line"> }</div><div class="line"> request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());</div><div class="line"> request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);</div><div class="line"> }</div><div class="line"></div><div class="line"> try {</div><div class="line"> doDispatch(request, response);</div><div class="line"> }</div><div class="line"> finally {</div><div class="line"> if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {</div><div class="line"> // Restore the original attribute snapshot, in case of an include.</div><div class="line"> if (attributesSnapshot != null) {</div><div class="line"> restoreAttributesAfterInclude(request, attributesSnapshot);</div><div class="line"> }</div><div class="line"> }</div><div class="line"> }</div><div class="line"> }</div></pre></td></tr></table></figure>
<p>在DispatcherServlet重写的doService()方法中，调用了另外一个doDispatch方法，而这个doDispatch方法便是整个处理的核心流程了。</p>
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<h1 id="概述"><a href="#概述" class="headerlink" title="概述"></a>概述</h1><p>今天说一下Spring容器的启动过程，这个启动过程中其实包含了单例Bean的生命周期。</p>
<p>下面的main方法中的两行代码，其实就是在通过指定一个配置类，然后启动Spring容器。</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException </span>{</div><div class="line"></div><div class="line"> AnnotationConfigApplicationContext applicationContext = <span class="keyword">new</span> AnnotationConfigApplicationContext(Config.class);</div><div class="line"> applicationContext.start();</div><div class="line"> </div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>跟踪代码，最后会执行到<code>AbstractApplicationContext</code>的<code>refresh()</code>方法中：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div><div class="line">42</div><div class="line">43</div><div class="line">44</div><div class="line">45</div><div class="line">46</div><div class="line">47</div><div class="line">48</div><div class="line">49</div><div class="line">50</div><div class="line">51</div><div class="line">52</div><div class="line">53</div><div class="line">54</div><div class="line">55</div><div class="line">56</div><div class="line">57</div><div class="line">58</div><div class="line">59</div><div class="line">60</div><div class="line">61</div><div class="line">62</div><div class="line">63</div><div class="line">64</div></pre></td><td class="code"><pre><div class="line"><span class="meta">@Override</span></div><div class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">refresh</span><span class="params">()</span> <span class="keyword">throws</span> BeansException, IllegalStateException </span>{</div><div class="line"> <span class="keyword">synchronized</span> (<span class="keyword">this</span>.startupShutdownMonitor) {</div><div class="line"> <span class="comment">// Prepare this context for refreshing.</span></div><div class="line"> prepareRefresh();</div><div class="line"></div><div class="line"> <span class="comment">// Tell the subclass to refresh the internal bean factory.</span></div><div class="line"> ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();</div><div class="line"></div><div class="line"> <span class="comment">// Prepare the bean factory for use in this context.</span></div><div class="line"> prepareBeanFactory(beanFactory);</div><div class="line"></div><div class="line"> <span class="keyword">try</span> {</div><div class="line"> <span class="comment">// Allows post-processing of the bean factory in context subclasses.</span></div><div class="line"> postProcessBeanFactory(beanFactory);</div><div class="line"></div><div class="line"> <span class="comment">// Invoke factory processors registered as beans in the context.</span></div><div class="line"> invokeBeanFactoryPostProcessors(beanFactory);</div><div class="line"></div><div class="line"> <span class="comment">// Register bean processors that intercept bean creation.</span></div><div class="line"> registerBeanPostProcessors(beanFactory);</div><div class="line"></div><div class="line"> <span class="comment">// Initialize message source for this context.</span></div><div class="line"> initMessageSource();</div><div class="line"></div><div class="line"> <span class="comment">// Initialize event multicaster for this context.</span></div><div class="line"> initApplicationEventMulticaster();</div><div class="line"></div><div class="line"> <span class="comment">// Initialize other special beans in specific context subclasses.</span></div><div class="line"> onRefresh();</div><div class="line"></div><div class="line"> <span class="comment">// Check for listener beans and register them.</span></div><div class="line"> registerListeners();</div><div class="line"></div><div class="line"> <span class="comment">// Instantiate all remaining (non-lazy-init) singletons.</span></div><div class="line"> finishBeanFactoryInitialization(beanFactory);</div><div class="line"></div><div class="line"> <span class="comment">// Last step: publish corresponding event.</span></div><div class="line"> finishRefresh();</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keyword">catch</span> (BeansException ex) {</div><div class="line"> <span class="keyword">if</span> (logger.isWarnEnabled()) {</div><div class="line"> logger.warn(<span class="string">"Exception encountered during context initialization - "</span> +</div><div class="line"> <span class="string">"cancelling refresh attempt: "</span> + ex);</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="comment">// Destroy already created singletons to avoid dangling resources.</span></div><div class="line"> destroyBeans();</div><div class="line"></div><div class="line"> <span class="comment">// Reset 'active' flag.</span></div><div class="line"> cancelRefresh(ex);</div><div class="line"></div><div class="line"> <span class="comment">// Propagate exception to caller.</span></div><div class="line"> <span class="keyword">throw</span> ex;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keyword">finally</span> {</div><div class="line"> <span class="comment">// Reset common introspection caches in Spring's core, since we</span></div><div class="line"> <span class="comment">// might not ever need metadata for singleton beans anymore...</span></div><div class="line"> resetCommonCaches();</div><div class="line"> }</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
<p>整个容器启动过程如下图：(<img src="http://fanchaoo-notebook.oss-cn-beijing.aliyuncs.com/img/1754553-c418cebd6e571d1d.jpg" alt="Spring容器启动过程">)</p>
<h1 id="BeanDefinition和Bean"><a href="#BeanDefinition和Bean" class="headerlink" title="BeanDefinition和Bean"></a>BeanDefinition和Bean</h1><p>在Spring容器中，每一个Bean都会有一个对应的BeanDefinition。这个BeanDefinition，是通过各种解析之后，生成的关于Bean的定义信息。</p>
<p>粗略地说，整个启动过程可以分为两个主要步骤：1.创建<code>BeanDefinition</code>；2.创建<code>Bean</code>。</p>
<ol>
<li>注册所有的Bean定义信息（standard initialization）。ApplicationContext在启动时，首先将解析各种配置信息（比如<bean>标签，@Bean注解，@Component注解等等），为每个需要创建的Bean，都生成一个BeanDefinition对象，并注册到DefaultListableBeanFactory的beanDefinitionMap属性中（通过BeanDefinitionRegistry的registerBeanDefinition方法）。</bean></li>
<li>根据BeanDefinition创建Bean，并建立Bean和Bean之间的依赖关系，最终会将这些Bean放入DefaultListableBeanFactory的singletonObjects属性（继承自DefaultSingletonBeanRegistry）中，以供外层应用程序使用。</li>
</ol>
<h1 id="拓展点"><a href="#拓展点" class="headerlink" title="拓展点"></a>拓展点</h1><p>容器的启动会经过以下步骤：</p>
<p>创建BeanDefinition –> 创建BeanPostProcessor –> 注册ApplicationListener –> Bean实例化 –> 为Bean设置属性 –> 调用Bean的初始化方法。</p>
<p>在容器的启动过程中，Spring提供了很多拓展点（BeanDefinition创建完成后，Bean的实例化前后，Bean的初始化前后等等），可以让用户自定义一些操作，然后Spring会在启动过程中的特定位置，执行用户自定义的操作。</p>
<p>这些拓展点包括：</p>
<h2 id="BeanDefinitionRegistryPostProcessor"><a href="#BeanDefinitionRegistryPostProcessor" class="headerlink" title="BeanDefinitionRegistryPostProcessor"></a>BeanDefinitionRegistryPostProcessor</h2><p>这个接口是<code>BeanFactoryPostProcessor</code>的子接口。</p>
<p>在BeanDefinition解析并创建完成后，会执行这个接口的实现类的<code>postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry)</code>方法。可以通过这个接口，为容器增加一些自己需要的BeanDefinition。</p>
<h2 id="BeanFactoryPostProcessor"><a href="#BeanFactoryPostProcessor" class="headerlink" title="BeanFactoryPostProcessor"></a>BeanFactoryPostProcessor</h2><p>同样是在BeanDefinition解析完并创建完成后，会执行这个接口的实现类的<code>postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)</code>方法。</p>
<h2 id="InstantiationAwareBeanPostProcessor"><a href="#InstantiationAwareBeanPostProcessor" class="headerlink" title="InstantiationAwareBeanPostProcessor"></a>InstantiationAwareBeanPostProcessor</h2><p>这个接口是<code>BeanPostProcessor</code>的子接口。</p>
<p>在Bean的实例化（调用构造器）前，会执行这个接口的<code>postProcessBeforeInstantiation(Class<?> beanClass, String beanName)</code>方法；在Bean的实例化之后，会先后调用该接口的<code>boolean postProcessAfterInstantiation(Object bean, String beanName)</code>方法和<code>PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName)</code>方法。</p>
<h2 id="BeanPostProcessor"><a href="#BeanPostProcessor" class="headerlink" title="BeanPostProcessor"></a>BeanPostProcessor</h2><p>在Bean的初始化前，会执行该接口的<code>Object postProcessBeforeInitialization(Object bean, String beanName)</code>方法；在Bean的初始化之后，会执行该接口的<code>Object postProcessAfterInitialization(Object bean, String beanName)</code>方法。</p>
<h2 id="ApplicationListener"><a href="#ApplicationListener" class="headerlink" title="ApplicationListener"></a>ApplicationListener</h2><p>用户可以实现该接口的<code>void onApplicationEvent(E event)</code>方法，监听某个事件。之后在该时间发生时，容器会通过<code>ApplicationEventMulticaster</code>调用该监听器的onApplicationEvent方法。</p>
<p>需要注意的是，这些拓展接口的实现类，也都必须作为Bean，被注册到容器中才可以生效。</p>
<h1 id="几个重要的属性"><a href="#几个重要的属性" class="headerlink" title="几个重要的属性"></a>几个重要的属性</h1><p>所谓的Spring容器，往简单了说其实就是个Map，这个Map的键是Bean的名称，值是这个Bean的实例。</p>
<p>在Spring中，有几个比较重要的属性，如下：</p>
<h2 id="beanFactory"><a href="#beanFactory" class="headerlink" title="beanFactory"></a>beanFactory</h2><p>该属性在<code>GenericApplicationContext</code>中。Bean工厂，顾名思义，这个对象存储了Spring容器中所有的Bean定义信息和Bean实例。</p>
<p>完整定义为<code>private final DefaultListableBeanFactory beanFactory;</code>。</p>
<p>DefaultListableBeanFactory的继承体系如下图所示：<img src="http://upload-images.jianshu.io/upload_images/1754553-36f5e89b0262e4db.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt="DefaultListableBeanFactory继承体系"></p>
<h2 id="beanDefinitionMap"><a href="#beanDefinitionMap" class="headerlink" title="beanDefinitionMap"></a>beanDefinitionMap</h2><p>该属性在<code>DefaultListableBeanFactory</code>中，类型为Map，键为Bean名称，值为BeanDefinition对象。</p>
<p>完整定义为<code>private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);</code>。</p>
<h2 id="beanDefinitionNames"><a href="#beanDefinitionNames" class="headerlink" title="beanDefinitionNames"></a>beanDefinitionNames</h2><p>该属性在在<code>DefaultListableBeanFactory</code>中，类型为List，存储了容器中所有的Bean的名称。</p>
<p>完整定义为<code>private volatile List<String> beanDefinitionNames = new ArrayList<>(256);</code>。</p>
<h2 id="singletonObjects"><a href="#singletonObjects" class="headerlink" title="singletonObjects"></a>singletonObjects</h2><p>该属性在<code>DefaultSingletonBeanRegistry</code>中，类型为Map，存储了容器中所有的单例Bean实例，键为Bean名称，值为单例Bean实例。</p>
<p>DefaultListableBeanFactory继承了DefaultSingletonBeanRegistry，所以拥有这个属性。</p>
<p>完整定义为<code>private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);</code>。</p>
<h2 id="beanPostProcessors"><a href="#beanPostProcessors" class="headerlink" title="beanPostProcessors"></a>beanPostProcessors</h2><p>该属性在<code>AbstractBeanFactory</code>中，类型为List，存储了容器中所有的BeanPostProcessor。</p>
<p>DefaultListableBeanFactory继承了AbstractBeanFactory，所以拥有这个属性。</p>
<p>完整定义为<code>private final List<BeanPostProcessor> beanPostProcessors = new CopyOnWriteArrayList<>();</code>。</p>
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<p>整个IoC有两个重要的部分：1. 让容器创建Bean；2. 让容器为Bean设置属性。</p>
<h2 id="1-概述"><a href="#1-概述" class="headerlink" title="1. 概述"></a>1. 概述</h2><h3 id="1-1-IoC"><a href="#1-1-IoC" class="headerlink" title="1.1. IoC"></a>1.1. IoC</h3><p>IoC的全称是Inverse of Control，就是“控制反转”的意思。控制反转这个概念其实包含两个层面的意思：“控制”是接口实现类的选择控制权；而“反转”是指这种控制权从调用类转移到了第三方容器的手中。</p>
<p>IOC的思想最核心的地方在于，资源交由第三方去集中管理，使得资源可配置，易管理。</p>
<p>在Spring中，可以“使用配置信息（XML配置文件，注解）来描述需要实例化的类的信息，以及对象与对象之间的依赖关系”。而Spring容器具体做的事情，其实就是读取我们设定好的“配置信息”，然后通过“Java语言的反射机制”，帮我们去统一地“创建Bean，并建立Bean之间的依赖关系”。</p>
<p>Spring以框架的方式，提供了“工厂方法”的功能：<img src="http://upload-images.jianshu.io/upload_images/1754553-259d04d43d09537e.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>Spring容器与单例模式：<img src="http://upload-images.jianshu.io/upload_images/1754553-2022eb06ba883a03.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<h3 id="1-2-反射"><a href="#1-2-反射" class="headerlink" title="1.2. 反射"></a>1.2. 反射</h3><p>通过Java的反射机制，我们可以通过程序化的方式，访问目标类中的所有元素，比如构造器，字段，方法等等。</p>
<p>关于“反射”，有几个很重要的类：ClassLoader，Class，Constructor，Method，Field。</p>
<p>在Java中，可以通过一个“全限定类名的字符串”，来获取（其实就是通过类加载器来加载这个类）这个类的<code>Class</code>对象。有了这个对象，就可以进行各种反射操作了。</p>
<p>关于JVM的类加载机制：<img src="http://upload-images.jianshu.io/upload_images/1754553-4c3322c622e1aac0.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>JVM中三个类加载器：Bootstrap类加载器，扩展类加载器，应用类加载器。</p>
<h3 id="1-3-Resource和ResourceLoader"><a href="#1-3-Resource和ResourceLoader" class="headerlink" title="1.3. Resource和ResourceLoader"></a>1.3. Resource和ResourceLoader</h3><p>Spring提供了一个<code>Resource</code>接口，用于处理对各种“底层资源”的访问。</p>
<p><img src="http://upload-images.jianshu.io/upload_images/1754553-adc2423259d8e852.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>资源加载器ResourceLoader：<img src="http://upload-images.jianshu.io/upload_images/1754553-5fabf0114cec5431.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<h3 id="1-4-BeanFactory，ApplicationContext，WebApplicationContext"><a href="#1-4-BeanFactory，ApplicationContext，WebApplicationContext" class="headerlink" title="1.4. BeanFactory，ApplicationContext，WebApplicationContext"></a>1.4. BeanFactory，ApplicationContext，WebApplicationContext</h3><p>BeanFactory（Bean工厂）是Spring框架的基础设施，主要面向Spring本身；ApplicationContext（应用上下文）主要面向使用Spring框架的开发者。</p>
<p>BeanFactory类继承体系：<img src="http://upload-images.jianshu.io/upload_images/1754553-83196f79b76ac3f4.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>ApplicationContext类集成体系：<img src="http://upload-images.jianshu.io/upload_images/1754553-7b70c3dcc4c59580.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>ServletContext和WebApplicationContext：<img src="http://upload-images.jianshu.io/upload_images/1754553-1684b75c8099f76e.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<h3 id="1-5-Bean的生命周期"><a href="#1-5-Bean的生命周期" class="headerlink" title="1.5. Bean的生命周期"></a>1.5. Bean的生命周期</h3><p>BeanFactory和ApplicationContext关于BeanPostprocessor的处理：<img src="http://upload-images.jianshu.io/upload_images/1754553-f50da6f2a37d5459.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>创建一个Bean的完整流程：<img src="http://upload-images.jianshu.io/upload_images/1754553-5793803ba74780e1.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>关于BeanDefinition：<img src="http://upload-images.jianshu.io/upload_images/1754553-2761823629ab926f.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>InstantiationStrategy组件负责进行Bean的实例化工作，即调用构造器创建Bean。</p>
<p>BeanWrapper组件负责进行Bean的属性填充工作。</p>
<p>PropertyEditor负责进行“属性类型转换”的工作。在PropertyEditorRegistrySupport提供了很多预制的“属性编辑器”，可用于将配置信息中的“属性值字符串”，转换为Java中某种类型的值。同时，如果想自定义“属性编辑器”，可以在CustomEditorConfigurer中进行添加。</p>
<p>PropertyPlaceholderConfigurer组件负责“属性占位符的解析”工作。</p>
<p>可以用“#{BeanName.someProperty}”的方式，来引用其它Bean的属性的值。</p>
<p>为什么要设计FactoryBean这个接口：<img src="http://upload-images.jianshu.io/upload_images/1754553-85a597c3b065e47a.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>Spring会被配置类中，所有标注“@Bean”的方法进行AOP增强，将对Bean生命周期管理的逻辑织入进来。所以，@Bean所修饰的方法被调用时，并不只是简单的执行方法中的逻辑。</p>
<p>容器事件：<img src="http://upload-images.jianshu.io/upload_images/1754553-829c91f3077ebb28.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240" alt=""></p>
<p>关于“ComponentScan”的过滤：如果想实现只扫描某个注解标注的类，需要同时设置“useDefaultFilters=false”和“includeFilters”。</p>
<h2 id="2-使用方式"><a href="#2-使用方式" class="headerlink" title="2. 使用方式"></a>2. 使用方式</h2><h3 id="让容器创建Bean"><a href="#让容器创建Bean" class="headerlink" title="让容器创建Bean"></a>让容器创建Bean</h3><p>我们需要提供一些配置信息给Spring。Spring在容器创建的时候，会读取这些配置信息，并解析成相应的<code>BeanDefinition</code>，最终根据这些<code>BeanDefinition</code>创建出Bean的实例。</p>
<ul>
<li><code>@Bean</code>注解</li>
<li><code>@ComponentScan</code>注解 + <code>@Component</code>（<code>@Controller</code>，<code>@Service</code>，<code>@Repository</code>）注解</li>
<li><code>@Import</code>注解 + <code>ImportSelector</code>接口 + <code>ImportBeanDefinitionRegistrar</code>接口</li>
<li><code>FactoryBean</code>接口</li>
</ul>
<h3 id="带条件的创建"><a href="#带条件的创建" class="headerlink" title="带条件的创建"></a>带条件的创建</h3><ul>
<li><code>@Conditional</code>注解：通过创建<code>Condition</code>接口的实现类，覆盖<code>match</code>方法，来自定义如何加载该bean</li>
<li><code>@Profile</code>注解：通过虚拟机参数或环境变量中<code>spring.profiles.active</code>的值，来确定是否创建该bean</li>
</ul>
<h3 id="让容器为Bean设置属性"><a href="#让容器为Bean设置属性" class="headerlink" title="让容器为Bean设置属性"></a>让容器为Bean设置属性</h3><p>除了可以通过在构造器中设置属性值，或者在工厂方法里设置属性值，还可以通过下面方式为Bean注入属性值：</p>
<h4 id="Value"><a href="#Value" class="headerlink" title="@Value"></a>@Value</h4><p><code>@Value</code>注解可以为对象注入“基本类型（及其包装类型）的值”和“String类型的值”。</p>
<p>用法：</p>
<ul>
<li>字面量，比如<code>@Value("tom")</code></li>
<li>SpringEL表达式，比如<code>@Value("#{1 + 1}")</code></li>
<li>读取环境变量中的值，比如<code>@Value("${os.name}")</code>，读取当前操作系统的名称</li>
<li>配合<code>@PropertySource</code>注解，读取<code>.properties</code>配置文件中的值，比如<code>@Value(${name})</code></li>
</ul>
<h4 id="Autowired"><a href="#Autowired" class="headerlink" title="@Autowired"></a>@Autowired</h4><p><code>@Autowired</code>注解可以为一个对象的属性，注入实例（从容器中找出一个实例，然后注入进去）。</p>
<p><code>@Autowired</code>也可以注入String对象，和基本类型的值及其包装类对象，但很少这么用。</p>
<p><code>@Autowired</code>可以标注在“属性，构造器，方法，参数”上。</p>
<p><code>@Autowired</code>标注在属性上的时候，注入规则：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div><div class="line">18</div><div class="line">19</div><div class="line">20</div><div class="line">21</div><div class="line">22</div><div class="line">23</div><div class="line">24</div><div class="line">25</div><div class="line">26</div><div class="line">27</div><div class="line">28</div><div class="line">29</div><div class="line">30</div><div class="line">31</div><div class="line">32</div><div class="line">33</div><div class="line">34</div><div class="line">35</div><div class="line">36</div><div class="line">37</div><div class="line">38</div><div class="line">39</div><div class="line">40</div><div class="line">41</div></pre></td><td class="code"><pre><div class="line">if (有@Qualifier注解) {</div><div class="line"> 直接根据“名称”查找</div><div class="line"> if (找到) {</div><div class="line"> 直接注入，结束</div><div class="line"> } else {</div><div class="line"> if (@Autowired的required=true) {</div><div class="line"> 抛异常NoSuchBeanDefinitionException，结束</div><div class="line"> } else {</div><div class="line"> 注入null，结束</div><div class="line"> }</div><div class="line"> }</div><div class="line">} else {</div><div class="line"> 根据属性“类型”查找</div><div class="line"> if (只找到1个) {</div><div class="line"> 直接注入，结束</div><div class="line"> } else if (找到多个) {</div><div class="line"> if (如果多个bean中有一个被@Primary修饰) {</div><div class="line"> 直接注入该bean，结束</div><div class="line"> } else {</div><div class="line"> 再根据bean名称进行查找，bean名称默认为属性名</div><div class="line"> if (找到) {</div><div class="line"> 直接注入，结束</div><div class="line"> } else {</div><div class="line"> if (找到多个该类型的) {</div><div class="line"> 抛异常NoUniqueBeanDefinitionException，结束</div><div class="line"> }</div><div class="line"> if (@Autowired的required=true) {</div><div class="line"> 抛异常NoSuchBeanDefinitionException，结束</div><div class="line"> } else {</div><div class="line"> 注入null，结束</div><div class="line"> }</div><div class="line"> }</div><div class="line"> }</div><div class="line"> } else {</div><div class="line"> if (@Autowired的required=true) {</div><div class="line"> 抛异常NoSuchBeanDefinitionException，结束</div><div class="line"> } else {</div><div class="line"> 注入null，结束</div><div class="line"> }</div><div class="line"> }</div><div class="line">}</div></pre></td></tr></table></figure>
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