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<a href="https://github.com/alibaba/arthas/blob/master/site/src/site/sphinx/en/redefine.md" class="fa fa-github"> Edit on GitHub</a>
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<div class="section" id="redefine">
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<h1>redefine<a class="headerlink" href="#redefine" title="Permalink to this headline">¶</a></h1>
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<blockquote>
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<div><p>Load the external <code class="docutils literal notranslate"><span class="pre">*.class</span></code> files to re-define the loaded classes in JVM.</p>
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</div></blockquote>
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<p>Reference: <a class="reference external" href="https://docs.oracle.com/javase/8/docs/api/java/lang/instrument/Instrumentation.html#redefineClasses-java.lang.instrument.ClassDefinition...-">Instrumentation#redefineClasses</a></p>
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<blockquote>
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<div><p>Notes: Re-defined classes cannot be restored. There are chances that redefining may fail due to some reasons, for example: there’s new field introduced in the new version of the class, pls. refer to JDK’s documentation for the limitations.</p>
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</div></blockquote>
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<blockquote>
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<div><p>The <code class="docutils literal notranslate"><span class="pre">reset</span></code> command is not valid for classes that have been processed by <code class="docutils literal notranslate"><span class="pre">redefine</span></code>. If you want to reset, you need <code class="docutils literal notranslate"><span class="pre">redefine</span></code> the original bytecode.</p>
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</div></blockquote>
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<blockquote>
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<div><p>The <code class="docutils literal notranslate"><span class="pre">redefine</span></code> command will conflict with the <code class="docutils literal notranslate"><span class="pre">jad</span></code>/<code class="docutils literal notranslate"><span class="pre">watch</span></code>/<code class="docutils literal notranslate"><span class="pre">trace</span></code>/<code class="docutils literal notranslate"><span class="pre">monitor</span></code>/<code class="docutils literal notranslate"><span class="pre">tt</span></code> commands. After executing <code class="docutils literal notranslate"><span class="pre">redefine</span></code>, if you execute the above mentioned command, the bytecode of the class will be reset.
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The reason is that in the JDK <code class="docutils literal notranslate"><span class="pre">redefine</span></code> and <code class="docutils literal notranslate"><span class="pre">retransform</span></code> are different mechanisms. When two mechanisms are both used to update the bytecode, only the last modified will take effect.</p>
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</div></blockquote>
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<div class="section" id="options">
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<h2>Options<a class="headerlink" href="#options" title="Permalink to this headline">¶</a></h2>
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<table border="1" class="docutils">
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<thead>
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<tr>
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<th align="right">Name</th>
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<th align="left">Specification</th>
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</tr>
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</thead>
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<tbody>
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<tr>
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<td align="right"><code>[c:]</code></td>
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<td align="left">hashcode of the class loader</td>
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</tr>
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<tr>
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<td align="right"><code>[p:]</code></td>
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<td align="left">absolute path of the external <code>*.class</code>, multiple paths are separated with 'space'</td>
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</tr>
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</tbody>
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</table></div>
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<div class="section" id="usage">
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<h2>Usage<a class="headerlink" href="#usage" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-bash notranslate"><div class="highlight hljs"><pre class="bash">redefine /tmp/Test.class
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redefine -c 327a647b /tmp/Test.class /tmp/Test$Inner.class</pre></div>
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</div>
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</div>
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<div class="section" id="use-with-the-jad-mc-command">
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<h2>Use with the jad/mc command<a class="headerlink" href="#use-with-the-jad-mc-command" title="Permalink to this headline">¶</a></h2>
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<div class="highlight-bash notranslate"><div class="highlight hljs"><pre class="bash">jad --source-only com.example.demo.arthas.user.UserController > /tmp/UserController.java
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mc /tmp/UserController.java -d /tmp
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redefine /tmp/com/example/demo/arthas/user/UserController.class</pre></div>
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</div>
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<ul class="simple">
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<li><p>Use <code class="docutils literal notranslate"><span class="pre">jad</span></code> command to decompile bytecode, and then you can use other editors, such as vim to modify the source code.</p></li>
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<li><p><code class="docutils literal notranslate"><span class="pre">mc</span></code> command to compile the modified code</p></li>
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<li><p>Load new bytecode with <code class="docutils literal notranslate"><span class="pre">redefine</span></code> command</p></li>
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</ul>
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</div>
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<div class="section" id="tips-for-uploading-class-files-to-the-server">
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<h2>Tips for uploading .class files to the server<a class="headerlink" href="#tips-for-uploading-class-files-to-the-server" title="Permalink to this headline">¶</a></h2>
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<p>The <code class="docutils literal notranslate"><span class="pre">mc</span></code> command may fail. You can modify the code locally, compile it, and upload it to the server. Some servers do not allow direct uploading files, you can use the <code class="docutils literal notranslate"><span class="pre">base64</span></code> command to bypass.</p>
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<ol>
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<li><p>Convert the <code class="docutils literal notranslate"><span class="pre">.class</span></code> file to base64 first, then save it as result.txt</p>
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<div class="highlight-bash notranslate"><div class="highlight hljs"><pre class="bash">Base64 < Test.class > result.txt</pre></div>
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</div>
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</li>
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<li><p>Login the server, create and edit <code class="docutils literal notranslate"><span class="pre">result.txt</span></code>, copy the local content, paste and save</p></li>
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<li><p>Restore <code class="docutils literal notranslate"><span class="pre">result.txt</span></code> on the server to <code class="docutils literal notranslate"><span class="pre">.class</span></code></p>
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<div class="highlight-default notranslate"><pre class="literal-block">Base64 -d < result.txt > Test.class</pre>
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</div>
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</li>
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<li><p>Use the md5 command to verify that the <code class="docutils literal notranslate"><span class="pre">.class</span></code> files are consistent.</p></li>
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</ol>
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</div>
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<div class="section" id="restrictions-of-the-redefine-command">
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<h2>Restrictions of the redefine command<a class="headerlink" href="#restrictions-of-the-redefine-command" title="Permalink to this headline">¶</a></h2>
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<ul>
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<li><p>New field/method is not allowed</p></li>
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<li><p>The function that is running, no exit can not take effect, such as the new <code class="docutils literal notranslate"><span class="pre">System.out.println</span></code> added below, only the <code class="docutils literal notranslate"><span class="pre">run()</span></code> function will take effect.</p>
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<div class="highlight-java notranslate"><div class="highlight hljs"><pre class="java">public class MathGame {
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public static void main(String[] args) throws InterruptedException {
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MathGame game = new MathGame();
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while (true) {
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game.run();
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TimeUnit.SECONDS.sleep(1);
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// This doesn't work because the code keeps running in while
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System.out.println("in loop");
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}
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}
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public void run() throws InterruptedException {
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// This works because the run() function ends completely every time
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System.out.println("call run()");
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try {
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int number = random.nextInt();
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List<Integer> primeFactors = primeFactors(number);
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print(number, primeFactors);
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} catch (Exception e) {
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System.out.println(String.format("illegalArgumentCount:%3d, ", illegalArgumentCount) + e.getMessage());
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}
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}</pre></div>
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</div>
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</li>
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</ul>
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<div class="highlight-default notranslate"><pre class="literal-block"></pre>
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</div>
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</div>
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</div>
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