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<div class="section" id="critical-fields-in-expressions">
<span id="critical-fields-in-expressions"></span><h1>Critical Fields in Expressions<a class="headerlink" href="#critical-fields-in-expressions" title="Permalink to this headline">ΒΆ</a></h1>
<p>There is a very fundamental class <code class="docutils literal notranslate"><span class="pre">Advice</span></code> for the expressions used in filtering, tracing or monitoring and other aspects in commands.</p>
<div class="highlight-java notranslate"><div class="highlight"><pre><span></span><span class="kd">public</span> <span class="kd">class</span> <span class="nc">Advice</span> <span class="o">{</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">ClassLoader</span> <span class="n">loader</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">Class</span><span class="o"><?></span> <span class="n">clazz</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">ArthasMethod</span> <span class="n">method</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">Object</span> <span class="n">target</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">Object</span><span class="o">[]</span> <span class="n">params</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">Object</span> <span class="n">returnObj</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="n">Throwable</span> <span class="n">throwExp</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">boolean</span> <span class="n">isBefore</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">boolean</span> <span class="n">isThrow</span><span class="o">;</span>
<span class="kd">private</span> <span class="kd">final</span> <span class="kt">boolean</span> <span class="n">isReturn</span><span class="o">;</span>
<span class="o">...</span>
<span class="c1">// getter/setter </span>
<span class="o">}</span>
</pre></div>
</div>
<table border="1" class="docutils">
<thead>
<tr>
<th align="right">Name</th>
<th align="left">Specification</th>
</tr>
</thead>
<tbody>
<tr>
<td align="right">loader</td>
<td align="left">class loader of the class</td>
</tr>
<tr>
<td align="right">clazz</td>
<td align="left">the reference of the class</td>
</tr>
<tr>
<td align="right">method</td>
<td align="left">the reflective reference of the method</td>
</tr>
<tr>
<td align="right">target</td>
<td align="left">the instance of the class</td>
</tr>
<tr>
<td align="right">params</td>
<td align="left">the parameters of the method, which is an array (when there is no argument in the method, it will be an empty array)</td>
</tr>
<tr>
<td align="right">returnObj</td>
<td align="left">the return value of the method - only when <code>isReturn==true</code>, it's a valid result but if the return value is <code>void</code> then it will be a <code>null</code></td>
</tr>
<tr>
<td align="right">throwExp</td>
<td align="left">the exceptions thrown by the method invoking - only when <code>isThrow==true</code>, it's a valid thrown exception</td>
</tr>
<tr>
<td align="right">isBefore</td>
<td align="left">assistant checking flag used in <a href="watch.md"><code>before-watching</code> point</a> and at this very moment: <code>isBefore==true</code>, <code>isThrow==false</code> and <code>isReturn==false</code> since it's before we invoke the method</td>
</tr>
<tr>
<td align="right">isThrow</td>
<td align="left">assistant checking flag: whether the current method invoking ends with exceptions</td>
</tr>
<tr>
<td align="right">isReturn</td>
<td align="left">assistant checking flag: whether the method invoking exits normally without exceptions</td>
</tr>
</tbody>
</table><p>F.Y.I</p>
<ol class="simple">
<li>all the <em>fields</em> mentioned in the table above can be used directly in the <code class="docutils literal notranslate"><span class="pre">expressions</span></code>;</li>
<li>if the expressions are <a class="reference external" href="https://en.wikipedia.org/wiki/OGNL">invalid OGNL</a>, the command will be cancelled automatically with hints to correct the expressions;</li>
<li><a class="reference external" href="https://github.com/alibaba/arthas/issues/71">typical use cases</a>;</li>
<li><a class="reference external" href="https://commons.apache.org/proper/commons-ognl/language-guide.html">OGNL official usage guide</a>.</li>
</ol>
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