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<article id="post-maven-version-separation" class="article article-type-post" itemscope itemprop="blogPost">
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<time datetime="2018-09-27T15:20:37.000Z" itemprop="datePublished">2018-09-27</time>
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<a class="article-title" href="/2018/09/27/maven-version-separation/">使用maven version:set分离服务的发布版本</a>
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<p>开发过程中需要分离测试环境依赖和线上环境依赖。</p>
<p>举个简单的例子，有个依赖包hexo-api，在正式环境中是version=hexo-api-1.0，测试环境中是version=hexo-api-1.0-SNAPSHOT。</p>
<p>但实际项目中pom.xml是这样配的，如:<br><figure class="highlight xml"><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></pre></td><td class="code"><pre><span class="line">...</span><br><span class="line"><span class="tag"><<span class="name">groupId</span>></span>com.qinguan<span class="tag"></<span class="name">groupId</span>></span></span><br><span class="line"><span class="tag"><<span class="name">artifactId</span>></span>hexo<span class="tag"></<span class="name">artifactId</span>></span></span><br><span class="line"><span class="tag"><<span class="name">name</span>></span>hexo-api<span class="tag"></<span class="name">name</span>></span></span><br><span class="line"><span class="tag"><<span class="name">packaging</span>></span>jar<span class="tag"></<span class="name">packaging</span>></span></span><br><span class="line"><span class="tag"><<span class="name">version</span>></span>1.0<span class="tag"></<span class="name">version</span>></span></span><br><span class="line">...</span><br></pre></td></tr></table></figure></p>
<p>我不想因为发布一个测试版本1.0-SNAPSHOT的jar而去提交一个如下的变更。<br><figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag"><<span class="name">version</span>></span>1.0-SNAPSHOT<span class="tag"></<span class="name">version</span>></span></span><br></pre></td></tr></table></figure></p>
<p>那怎么办呢？可以试试<a href="https://www.mojohaus.org/versions-maven-plugin/index.html" target="_blank" rel="noopener">maven-version-plugin</a>。</p>
<p>具体用法:<br><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></pre></td><td class="code"><pre><span class="line"><span class="comment">//假定之前打包命令</span></span><br><span class="line">clean compile -Papi deploy </span><br><span class="line"></span><br><span class="line"><span class="comment">//测试包打包命令</span></span><br><span class="line">clean versions:set -DnewVersion=<span class="number">1.0</span>-SNAPSHOT compile -Papi deploy</span><br></pre></td></tr></table></figure></p>
<p>通过<a href="https://www.mojohaus.org/versions-maven-plugin/set-mojo.html" target="_blank" rel="noopener">versions:set</a>更改pom.xml中的version值，newVersion参数为目标版本号。</p>
<p>此时打开pom.xml，可以看到文件中的version值已由1.0更新为1.0-SNAPSHOT了，jar包也变成了hexo-api-1.0-SNAPSHOT.jar。</p>
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<a class="article-title" href="/2018/09/27/enum-compatibility-issue/">Thrift枚举对象兼容性问题</a>
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<p>最近在一个基于Thrift框架的RPC服务中碰到一个枚举值的兼容性问题。<br>简单描述如下:<br>有一个位置定义，起初只有两个枚举值，LEFT和RIGHT。然后发布schema定义版本1.0，客户端服务引用version = 1.0。<br><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></pre></td><td class="code"><pre><span class="line"><span class="comment">// commom.thrift</span></span><br><span class="line"><span class="keyword">enum</span> Position {</span><br><span class="line"> LEFT =<span class="number">0</span>,</span><br><span class="line"> RIGHT =<span class="number">1</span>,</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>经过一段时间，Position需要扩展，增加UP和DOWN，发布schema定义版本2.0。<br><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></pre></td><td class="code"><pre><span class="line"><span class="comment">// commom.thrift</span></span><br><span class="line"><span class="keyword">enum</span> Position {</span><br><span class="line"> LEFT =<span class="number">0</span>,</span><br><span class="line"> RIGHT =<span class="number">1</span>,</span><br><span class="line"> UP =<span class="number">2</span>,</span><br><span class="line"> DOWN =<span class="number">3</span>,</span><br><span class="line">}</span><br></pre></td></tr></table></figure></p>
<p>此时，客户端还在使用schema=1.0版本。若服务端返回了Position.UP或者Position.DOWN数据到客户端，客户端是不识别这两个枚举值的。问题就这样产生了，客户端有可能因为没有对未知数据进行兼容处理而直接报错异常了。</p>
<p>实际上，有几种设计是违反<strong>向后兼容</strong>原则的，上述情况就是典型的一种，即<strong>向枚举类型中新增枚举值</strong>。这种场景下，原则上要求所有客户端均先于服务端进行升级到最新版本。鉴于分布式服务应用场景，要求所有客户端都统一升级，是几乎不可能实现的。</p>
<p>针对<strong>向已有枚举类型中新增枚举值</strong>的情况，Thrift和Protobuf各有一些处理:</p>
<ol>
<li>Thrift：针对不认识的枚举值，默认返回null(通过findByValue实现)</li>
<li>Protobuf：针对不认识的枚举值，默认返回枚举类型的第一个值(或者返回default value，如有)<ul>
<li>注: 所以Protobuf定义的schema enum一般都会设第一个值为UNKNOWN以兼容低版本客户端</li>
</ul>
</li>
</ol>
<p>在阿里巴巴的Java编码规范中也有相关规则：<strong>强制“接口返回值不允许使用枚举类型或者包含枚举类型的POJO对象”</strong>。</p>
<p>Thrift/Protobuf协议本身对新增枚举值的处理没有问题(至少在反序列化方法中没有简单粗暴地直接抛出异常)，但对业务层的处理及使用提出了一定的要求，即业务层有可能读到null(Thrift)或者UNKONWN类型枚举(Protobuf)，需要做一定的防御性判断。</p>
<p>进一步阅读资料:</p>
<ol>
<li><a href="https://github.com/bkayser/thrift-versioning-doc" target="_blank" rel="noopener">thrift-versioning-doc</a></li>
<li><a href="https://developers.google.com/protocol-buffers/docs/proto#updating" target="_blank" rel="noopener">Updating A Message Type</a></li>
</ol>
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<a class="article-title" href="/2018/08/22/a-high-coupling-case/">一个内部API高度耦合的Java实现案例</a>
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<p>某项目使用了“一套API，多种底层存储”的设计方式，可以灵活切换多种数据源，但同时能够保持暴露的API接口定义一致。这种设计方式，优点是显而易见的。</p>
<p>随着业务的增长，数据量越来越大。单种类存储已经捉襟见肘，需要依赖多种存储共同保障服务的状况下，早期高度耦合的内部API实现使得存储的改造工作量增大了不少。</p>
<p>以某项目为例，原本业务数据量没那么大，选择了Redis作为线上业务的主要存储，以Couchbase为辅，同时利用HBase实现了一套在线冷备服务，其中RedisAPI和HBaseAPI互斥。但随着数据量的突飞猛进，Redis的容量已不堪重负，计划全部迁移到容量更大的分布性缓存Couchbase。同时，在Couchbase存储未命中时，回源到HBase服务进行兜底读取。</p>
<p>由于上层业务逻辑基于同一套存储API实现的，为同时使用多种存储，要求对这套存储API进行改造。</p>
<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><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//common jar</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">BaseTestRepo</span> </span>{</span><br><span class="line"></span><br><span class="line"> <span class="keyword">default</span> <span class="keyword">public</span> <T> <span class="function">T <span class="title">get</span><span class="params">(String key)</span> </span>{</span><br><span class="line"> <span class="comment">//TODO</span></span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> NotImplementedException(<span class="string">"not implement!"</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">default</span> <span class="keyword">public</span> <span class="keyword">void</span> <span class="title">set</span><span class="params">(String key, <span class="keyword">byte</span>[] value)</span> </span>{</span><br><span class="line"> <span class="comment">//TODO</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//couchbase.repo jar</span></span><br><span class="line"><span class="meta">@Repository</span>(<span class="string">"cbDataRepo"</span>)</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">CocuhbaseRepo</span> <span class="keyword">implements</span> <span class="title">BaseTestRepo</span> </span>{</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//hbase.repo jar</span></span><br><span class="line"><span class="meta">@Repository</span>(<span class="string">"dataRepo"</span>)</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">HBaseRepo</span> <span class="keyword">implements</span> <span class="title">BaseTestRepo</span> </span>{</span><br><span class="line"> <span class="comment">//TODO</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//redis.repo jar</span></span><br><span class="line"><span class="meta">@Component</span>(<span class="string">"dataRepo"</span>)</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">RedisRepo</span> <span class="keyword">implements</span> <span class="title">BaseTestRepo</span> </span>{</span><br><span class="line"> <span class="comment">//TODO</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//serviceA</span></span><br><span class="line"><span class="meta">@Service</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TestAService</span> </span>{</span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> BaseTestRepo dataRepo;</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> BaseTestRepo cbDataRepo;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <T> <span class="function">T <span class="title">getEntity</span><span class="params">(String key)</span> </span>{</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> NotImplementedException(<span class="string">"not implement!"</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setEntity</span><span class="params">(String key, Object value)</span> </span>{</span><br><span class="line"> <span class="comment">//TODO</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//serviceB</span></span><br><span class="line"><span class="meta">@Service</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">TestBService</span> </span>{</span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> BaseTestRepo dataRepo;</span><br><span class="line"></span><br><span class="line"> <span class="meta">@Autowired</span></span><br><span class="line"> BaseTestRepo cbDataRepo;</span><br><span class="line"></span><br><span class="line"> <span class="comment">//通用API</span></span><br><span class="line"> <span class="keyword">public</span> <T> <span class="function">T <span class="title">getEntity</span><span class="params">(String key)</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> dataRepo.get(key);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">//通用API</span></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">setEntity</span><span class="params">(String key, Object value)</span> </span>{</span><br><span class="line"> dataRepo.set(key, toByte(value))</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="comment">//TODO</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">上层业务模块A需要依赖redis的高效部署业务，则可以引入service.jar、redis.repo jar、common.jar、couchbase.repo jar来实现；</span><br><span class="line">上层业务模块B需要依赖hbase来实现离线任务，则可引入service.jar、hbase.repo jar、common.jar、couchbase.repo jar来实现。</span><br></pre></td></tr></table></figure>
<p>如上所示，上层业务模块只需要按需导入hbase.repo jar或者redis.repo jar，然后利用Spring自动注入机制，获取dataRepo实例，基于同一套get/set API进行存储的读写操作。对上层业务而言，它是无需感知底层业务代码使用了哪一套存储。</p>
<p>此时，若将RedisRepo全部迁移到CouchbaseRepo，则会涉及到dataRepoAPI的使用变更，redis部分变化，但必须保持HBase不变。</p>
<p>解决方案:<br>1、新增Proxy，将底层RepoAPI再加一层代理，按需路由到不同的repo实现。<br>2、基于Spring Condition条件化构建Repo Bean。</p>
<p>话说回来，这类还是场景考虑不全，实现的不够理想。</p>
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<a class="article-title" href="/2018/08/15/netty-version-conflicting-exception/">Netty版本冲突引起Cassandra访问异常</a>
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<p>Java RPC服务单元测试访问Cassandra时，发现始终报如下错误：<br><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></pre></td><td class="code"><pre><span class="line">java.lang.AbstractMethodError: </span><br><span class="line">io.netty.util.concurrent.MultithreadEventExecutorGroup.newChild(Ljava/util/</span><br><span class="line"> concurrent/Executor;[Ljava/lang/Object;)Lio/netty/util/concurrent/EventExecutor</span><br></pre></td></tr></table></figure></p>
<p>排查了一圈，最终发现是Netty引起的。<br>Java RPC服务依赖的是Netty3.X版本，Cassandra驱动测试包依赖的是Netty4.X版本，将Netty版本调整成一致解决。</p>
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<a class="article-title" href="/2018/08/13/rmi-deserialization-vulnerability/">RMI反序列化漏洞</a>
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<p>接到公司安全组提的漏洞单，说部分Java机器开了jmxremote端口，存在安全隐患。</p>
<p>查了下原来有不少Java服务默认开启了非安全模式的jmxremote端口。</p>
<p>修复(降低风险)方案有如下几种方式：</p>
<ol>
<li>使用<a href="https://github.com/ikkisoft/SerialKiller" target="_blank" rel="noopener">SerialKiller</a>,详细的在RMI反序列化漏洞文章中有介绍，略复杂</li>
<li>对于jmxremote没有强依赖的服务，可以直接禁用jmxremote端口，删除相关配置项</li>
<li>授权访问：配置认证密码或者ssl，或者配置ip白名单 -Dcom.sun.management.jmxremote.host=serverXXX</li>
</ol>
<p>关于RMI反序列化漏洞的介绍:<br><a href="http://blog.nsfocus.net/java-deserialization-vulnerability-overlooked-mass-destruction/" target="_blank" rel="noopener">http://blog.nsfocus.net/java-deserialization-vulnerability-overlooked-mass-destruction/</a><br>关于JMX配置的一些介绍:<br><a href="https://docs.oracle.com/javase/7/docs/technotes/guides/management/agent.html" target="_blank" rel="noopener">https://docs.oracle.com/javase/7/docs/technotes/guides/management/agent.html</a><br><a href="https://docs.oracle.com/javase/8/docs/technotes/guides/management/agent.html" target="_blank" rel="noopener">https://docs.oracle.com/javase/8/docs/technotes/guides/management/agent.html</a></p>
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<a class="article-title" href="/2018/06/05/https-package-capture-by-charles/">使用charles抓取iOS数据包</a>
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<p>由于分析竞手的项目涉及到iOS端的请求抓包，翻阅并验证了一下，以下是整理的大致抓包流程。</p>
<p>工具软件: charles 3.11.5版本</p>
<ol>
<li>安装mac端证书<ul>
<li>Help->SSL Proxy->Install Charles Root Certificate.</li>
</ul>
</li>
<li>开启代理设置<ul>
<li>Proxy->Proxy Setting…->Proxies </li>
<li>HTTP Proxy 可以默认一个端口如8888，并勾选Enable transparent HTTP proxying</li>
<li>Socks Proxy可以Enable，并且Enable HTTP proxying over SOCKS，设置端口如80，443等</li>
</ul>
</li>
<li>开启SSL代理(支持HTTPS抓包)<ul>
<li>Proxy->SSL Proxying Setting，选中Enable SSL Proxying</li>
<li>在Locations里面添加*:443，表示匹配所有的HTTPS网站</li>
</ul>
</li>
<li>给iPhone安装证书<ul>
<li>Help->SSL Proxy->Install Charles Root Certificate On a Mobile Device or remote Browers…</li>
<li>根据弹出的提示框进行操作，也就是用iPhone的Safari打开地址<a href="http://charlesproxy.com/getssl" target="_blank" rel="noopener">http://charlesproxy.com/getssl</a> 下载安装。</li>
<li>iOS11以上系统需要在设置 > 关于本机 > 证书信任设置，信任该证书。</li>
</ul>
</li>
<li>手机连接wifi(同电脑一个局域网或者直接连电脑的热点)，配置HTTP代理，选择’手动’，输入电脑的ip地址和端口号(默认8888)</li>
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<a class="article-title" href="/2018/06/05/java-unused-import-statment/">Java import 失效</a>
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<p>下午碰到一个小问题，IDEA 中的springboot项目里所有的import都无效了，显示unused import statment。</p>
<p>解决方案：File->Invalidate Caches/Restart，选择Invalidate and Restart。</p>
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<a class="article-title" href="/2018/05/10/zero-width-space/">看不见的字符-<200b></a>
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<p>今天配置一个数据库链接地址时，报URL地址有问题，但在sublime Text里明明是正常的。</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">http://shgq.1.abc:1234/pools;http://shgq.2.abc:1234/pools#xxf</span><br></pre></td></tr></table></figure>
<p>打开debug日志发现是地址解析错误，无法识别<code><200b></code>，但字符串中没有这个<code><200b></code>，是哪里冒出来的呢？</p>
<p>于是，我重新把配置贴到用vim打开的文本中，才发现了隐藏字符。<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">http://shgq.1.abc:1234/pools;http://shgq.2.abc:1234<200b>/pools#xxf</span><br></pre></td></tr></table></figure></p>
<p>搜了一把这个<code><200></code>，学名<a href="https://en.wikipedia.org/wiki/Zero-width_space" target="_blank" rel="noopener">zero-width space(ZWSP)</a>，是一种不可见的打印字符。</p>
<p>wikipedia给出一种实际的例子，可以很明显地看出区别，如下：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">LoremIpsumDolorSitAmetConsecteturAdipiscingElitSedDoEiusmodTemporIncididuntUtLaboreEtDoloreMagnaAliquaUtEnimAdMinimVeniamQuisNostrudExercitationUllamcoLaborisNisiUtAliquipExEaCommodoConsequatDuisAuteIrureDolorInReprehenderitInVoluptateVelitEsseCillumDoloreEuFugiatNullaPariaturExcepteurSintOccaecatCupidatatNonProidentSuntInCulpaQuiOfficiaDeseruntMollitAnimIdEstLaborum</span><br></pre></td></tr></table></figure>
<p>通过vim编辑时变成如下文本，中间多了很多<code><200b></code>。<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Lorem<200b>Ipsum<200b>Dolor<200b>Sit<200b>Amet<200b>Consectetur<200b>Adipiscing<200b>Elit<2b>Sed<200b>Do<200b>Eiusmod<200b>Tempor<200b>Incididunt<200b>Ut<200b>Labore<200b>Et<200b>Dole<200b>Magna<200b>Aliqua<200b>Ut<200b>Enim<200b>Ad<200b>Minim<200b>Veniam<200b>Quis<200b>Norud<200b>Exercitation<200b>Ullamco<200b>Laboris<200b>Nisi<200b>Ut<200b>Aliquip<200b>Ex<200ba<200b>Commodo<200b>Consequat<200b>Duis<200b>Aute<200b>Irure<200b>Dolor<200b>In<200b>Reprehderit<200b>In<200b>Voluptate<200b>Velit<200b>Esse<200b>Cillum<200b>Dolore<200b>Eu<200b>Fugi<200b>Nulla<200b>Pariatur<200b>Excepteur<200b>Sint<200b>Occaecat<200b>Cupidatat<200b>Non<20>Proident<200b>Sunt<200b>In<200b>Culpa<200b>Qui<200b>Officia<200b>Deserunt<200b>Mollit<200bnim<200b>Id<200b>Est<200b>Laborum</span><br></pre></td></tr></table></figure></p>
<p>如果使用chrome浏览器，打开<code>检查</code>功能查看wikipedia的这个例子，在<code>Elements</code> Tab页下面，找到上述字符串对应的html页面代码，可以看到上面<200b>均表示成了<code>&#8203;</code>。</200b></p>
<p>好了，再测试一下，​<code>这</code>前面隐藏了一个分隔符！不信的话，试试复制一下，用vim编辑看看。:-D</p>
<p>零宽字符有什么用呢，可以作为内容的水印，这是另外一个话题了，可以发掘利用一下。</p>
<p>以后粘贴复制文本时需要多长一个心眼了。</p>
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<a class="article-title" href="/2018/05/08/nginx-resolver-vaild-and-timeout/">nginx resolver数据缓存及解析超时时间参数</a>
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<p>在配置nginx的resolver时，有两个与时间相关的参数。</p>
<p>一、valid<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">Syntax: resolver address ... [valid=time] [ipv6=on|off];</span><br><span class="line">Default: —</span><br><span class="line">Context: http, server, location</span><br><span class="line"></span><br><span class="line">如：</span><br><span class="line">resolver 127.0.0.1 [::1]:5353 valid=30s;</span><br></pre></td></tr></table></figure></p>
<p><code>valid</code> flag means how long nginx will consider answer from resolver as valid and will not ask resolver for that period.<br>即DNS返回结果的缓存时间，在缓存有效时间内，nginx不会再次向DNS请求解析结果。</p>
<p>二、resolver_timeout<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">Syntax: resolver_timeout time;</span><br><span class="line">Default: </span><br><span class="line">resolver_timeout 30s;</span><br><span class="line">Context: http, server, location</span><br></pre></td></tr></table></figure></p>
<p><code>resolve_timeout</code> sets how long nginx will wait for anwser from resolver (DNS).<br>即nginx发起的DNS查询请求超时时间。</p>
<p>更多参考：</p>
<ul>
<li><a href="http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver" target="_blank" rel="noopener">http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver</a></li>
<li><a href="https://stackoverflow.com/questions/28606696/what-is-the-difference-in-resolver-valid-time-and-resolver-timeout-in-nginx" target="_blank" rel="noopener">https://stackoverflow.com/questions/28606696/what-is-the-difference-in-resolver-valid-time-and-resolver-timeout-in-nginx</a> </li>
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<a class="article-title" href="/2018/05/04/slf4j-stackoverflowerror/">SLF4J及日志栈溢出问题</a>
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<p>新接手一项目，在本地测试运行时，发现报如下错误:<br><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">Detected both log4j-over-slf4j.jar AND slf4j-log4j12.jar on the <span class="class"><span class="keyword">class</span> <span class="title">path</span>, <span class="title">preempting</span> <span class="title">StackOverflowError</span>.</span></span><br></pre></td></tr></table></figure></p>
<p>很明显，是项目的classpath中同时存在log4j-over-slf4j.jar和slf4j-log4j12.jar，引起死循环。但为什么项目在线上能好好运行呢？<br>原来线上运行的程序在打包时做了手脚，在最终的依赖包中，删除了log4j和slf4j-log4j12，如下：<br><figure class="highlight"><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></pre></td><td class="code"><pre><span class="line"><dependencySets></span><br><span class="line"> <dependencySet></span><br><span class="line"> <outputDirectory>lib</outputDirectory></span><br><span class="line"> <excludes></span><br><span class="line"> <exclude>log4j:log4j</exclude></span><br><span class="line"> <exclude>org.slf4j:slf4j-log4j12</exclude></span><br><span class="line"> </excludes></span><br><span class="line"> <useProjectArtifact>false</useProjectArtifact></span><br><span class="line"> </dependencySet></span><br><span class="line"></dependencySets></span><br></pre></td></tr></table></figure></p>
<p>让我们来重头捋一下Java日志框架及该问题的成因。</p>
<p>我们用的最多的几个日志框架有Log4j、Jakarta Commons Logging以及Logback-classic。尤其值得一提的是Logback-classic，它是Log4j作者写的又一个开源日志组件，由于实现了SLF4J’s的Logger接口，native，性能优越，可以直接作为SLF4J的实现。</p>
<p><a href="https://www.slf4j.org/index.html" target="_blank" rel="noopener">SLF4J</a>，全称Simple Logging Facade for Java，是一个抽象的日志框架，支持程序动态绑定日志框架实现，如绑定到<code>java.util.logging</code>、<code>logback</code>以及 <code>log4j</code>等，使用时需要在应用中添加依赖slf4j-api。</p>
<p><strong>SLF4J对应用日志做了封装抽象，因此，业务可以针对SLF4J进行编程，再按需绑定具体的日志框架。无论是灵活性还是可扩展性，都有很多的提升。</strong></p>
<p>为了支持动态绑定不同的日志框架，SLF4J提供了几个绑定工具，如下：</p>
<table>
<thead>
<tr>
<th>绑定</th>
<th>说明</th>
</tr>
</thead>
<tbody>
<tr>
<td><strong>slf4j-log4j12</strong></td>
<td>Log4j绑定，Log4j是一个使用非常广泛的日志组件</td>
</tr>
<tr>
<td><strong>slf4j-jcl</strong></td>
<td>Jakarta Commons Logging绑定</td>
</tr>
<tr>
<td><strong>slf4j-jdk14</strong></td>
<td>java.util.logging, JDK绑定</td>
</tr>
<tr>
<td><strong>slf4j-simple</strong></td>
<td>输出会定向到System.err，只有INFO级别以上的才会输出</td>
</tr>
<tr>
<td><strong>slf4j-nop</strong></td>
<td>丢弃日志绑定</td>
</tr>
</tbody>
</table>
<p>为了方便jcl、log4j、jul用户迁移到SLF4J，SLF4J提供了几个桥接模块。</p>
<table>
<thead>
<tr>
<th style="text-align:left">模块</th>
<th style="text-align:left">目标对象</th>
<th style="text-align:left">使用说明</th>
<th style="text-align:left">实现原理</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:left"><strong>log4j-over-slf4j</strong></td>
<td style="text-align:left">log4j用户</td>
<td style="text-align:left">将log4j.jar替换为log4j-over-slf4j.jar</td>
<td style="text-align:left">直接替换了log4j中的大量同名类，如Logger、Category、Level等</td>
</tr>
<tr>
<td style="text-align:left"><strong>jcl-over-slf4j</strong></td>
<td style="text-align:left">jcl用户</td>
<td style="text-align:left">将commons-logging.jar替换为jcl-over-slf4j.jar</td>
<td style="text-align:left">保留了jcl的接口，但底层实现采用了slf4j</td>
</tr>
<tr>
<td style="text-align:left"><strong>jul-to-slf4j</strong></td>
<td style="text-align:left">jul用户</td>
<td style="text-align:left">直接依赖</td>
<td style="text-align:left">由于jul在java.*namespace下，无法替换。该模块采用了翻译手段，将LogRecord转换为slf4j类似对象。<strong> 该方案存在一定的性能损失，不推荐使用</strong></td>
</tr>
</tbody>
</table>
<p>按上面的介绍，使用slf4j时就需要注意一下几个问题：</p>
<ol>
<li><p><strong><code>jcl-over-slf4j.jar</code>和<code>slf4j-jcl.jar</code>不能同时依赖！</strong></p>
<ul>
<li>原因：jcl会将实现委托到slf4j，而slf4j又会绑定到jcl。</li>
</ul>
</li>
<li><p><strong><code>log4j-over-slf4j.jar</code>和<code>slf4j-log4j12.jar</code>不能同时依赖！</strong></p>
<ul>
<li>原因: log4j会将实现委托到slf4j，而slf4j又会绑定到log4j实现。</li>
</ul>
</li>
<li><p><strong><code>jul-to-slf4j.jar</code>和<code>slf4j-jdk14.jar</code>不能同时依赖！</strong></p>
<ul>
<li>原因：jul会将实现委托到slf4j，而slf4j又会绑定到jdk实现。</li>
</ul>
</li>
</ol>
<p><strong>上述三个情况下，若同时依赖均会产生循环依赖问题，导致StackOverflowError!</strong></p>
<p>回到开头的问题，即上述第2个注意问题，怎么解决也已经很明显了。</p>
<p><strong>参考资料</strong></p>
<ol>
<li><a href="https://www.slf4j.org/manual.html" target="_blank" rel="noopener">SLF4J user manual</a></li>
<li><a href="https://www.slf4j.org/legacy.html" target="_blank" rel="noopener">Bridging legacy APIs</a></li>
</ol>
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