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<title>JDB(Java Debugger)</title>
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<h1>JDB(Java Debugger)</h1>
<div class="details">
<span id="author" class="author">pxzxj</span><br>
<span id="author2" class="author">pudge.zxj@gmail.com</span><br>
<span id="revnumber">version v1.0,</span>
<span id="revdate">2022/05/23</span>
</div>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<span id="back-to-index"><a href="index.html">Back to index</a></span><ul class="sectlevel1">
<li><a href="#_简介">1. 简介</a></li>
<li><a href="#_用法">2. 用法</a></li>
<li><a href="#option">3. 选项</a></li>
<li><a href="#example">4. 示例</a>
<ul class="sectlevel2">
<li><a href="#_创建项目">4.1. 创建项目</a></li>
<li><a href="#_启动项目">4.2. 启动项目</a></li>
<li><a href="#_添加断点">4.3. 添加断点</a></li>
<li><a href="#_断点调试">4.4. 断点调试</a>
<ul class="sectlevel3">
<li><a href="#_step">4.4.1. step</a></li>
<li><a href="#_next">4.4.2. next</a></li>
<li><a href="#_step_up">4.4.3. step up</a></li>
<li><a href="#_stepi">4.4.4. stepi</a></li>
<li><a href="#_查看变量和源码">4.4.5. 查看变量和源码</a></li>
</ul>
</li>
<li><a href="#_清除断点">4.5. 清除断点</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>调试器是软件开发领域最重要的工具之一，对Java开发来说，在开发环境可以使用Intellij IDEA进行debug从而快速解决问题，但在部署环境由于无法debug
经常需要耗时费力地添加打印语句并替换部署包。实际上JDK已经提供了一个命令行debug工具jdb，可以在任何环境使用它实现快速有效地问题定位。</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_简介"><a class="anchor" href="#_简介"></a>1. 简介</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The Java Debugger (<a href="https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jdb.html">JDB</a>) 是一个简单的命令行调试工具，
使用它可以像Intellij IDEA一样进行添加断点、单步执行、变量查看等操作，实际上jdb和Intellij IDEA底层都利用了
Java Platform Debugger Architecture (<a href="http://docs.oracle.com/javase/8/docs/technotes/guides/jpda/index.html">JDBA</a>)
实现调试功能，只是展现方式不同而已。</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_用法"><a class="anchor" href="#_用法"></a>2. 用法</h2>
<div class="sectionbody">
<div class="paragraph">
<p>jdk的bin目录下包含了jdb，jdb命令的基础语法是 <code><strong>jdb</strong> [options] [classname] [arguments]</code>，其中 <code>options</code> 是命令的选项，<code>classname</code> 和 <code>arguments</code> 是要调试的main类名和main方法参数。</p>
</div>
<div class="paragraph">
<p>可以注意到jdb命令的 <code>classname</code> 和 <code>arguments</code> 都使用了中括号，表示它们都不是必须的，因为jdb还可以attache到一个已经在运行的JVM上，方法与普通的 <code>Remote Debug</code> 相同，调试Spring Boot应用时使用这种方法更方便。</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-bash" data-lang="bash">#启动时添加debug参数
java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005 -jar app.jar

#attach到已经启动的JVM上
jdb -attach 5005

#与attach效果相同，attach失败时可以使用此命令
jdb -connect com.sun.jdi.SocketAttach:hostname=127.0.0.1,port=5005</code></pre>
</div>
</div>
<div class="paragraph">
<p>jdb执行成功后会进入到一个交互式shell页面中，后续的调试操作都在此shell中进行，具体调试方法参考<a href="#example">示例</a>章节</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/jdb-connect.png" alt="jdb connect">
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="option"><a class="anchor" href="#option"></a>3. 选项</h2>
<div class="sectionbody">
<div class="paragraph">
<p>使用help可以查看jdb的选项，不同版本JDK的选项有所不同，Windows下jdk8支持如下选项</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/jdb-options.png" alt="jdb options">
</div>
</div>
<div class="paragraph">
<p>进入调试shell后可以查看debug相关命令，<a href="#example">示例</a>章节会演示主要的添加断点、单步执行等命令</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/jdb-options2.png" alt="jdb options2">
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="example"><a class="anchor" href="#example"></a>4. 示例</h2>
<div class="sectionbody">
<div class="paragraph">
<p>本节演示如下比较常用的调试命令</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 50%;">
<col style="width: 50%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">命令</th>
<th class="tableblock halign-left valign-top">描述</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>stop in &lt;class id&gt;.&lt;method&gt;[(argument_type,&#8230;&#8203;)]</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">在方法中设置断点</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>stop at &lt;class id&gt;:&lt;line&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">在行中设置断点</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>clear</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">查看所有断点</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>clear &lt;class id&gt;.&lt;method&gt;[(argument_type,&#8230;&#8203;)]</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">清除方法中的断点</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>clear &lt;class id&gt;:&lt;line&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">清除行中的断点</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>step</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">执行当前行</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>step up</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">一直执行, 直到当前方法返回到其调用方</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>stepi</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">执行当前指令</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>next</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">步进一行 (步过调用)</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>cont</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">从断点处继续执行</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>locals</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">输出当前堆栈帧中的所有本地变量</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>print &lt;expr&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">输出表达式的值</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>dump &lt;expr&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">输出所有对象信息</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>eval &lt;expr&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">对表达式求值 (与 print 相同)</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>list [line number|method]</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">输出源代码</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>methods &lt;class id&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">列出类的方法</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>fields &lt;class id&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">列出类的字段</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>!!</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">重复执行最后一个命令</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>&lt;n&gt; &lt;command&gt;</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">将命令重复执行 n 次</p></td>
</tr>
</tbody>
</table>
<div class="sect2">
<h3 id="_创建项目"><a class="anchor" href="#_创建项目"></a>4.1. 创建项目</h3>
<div class="paragraph">
<p>创建一个Spring Boot项目添加 <code>starter-web</code> 依赖，并添加如下Controller</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-java" data-lang="java"><span class="fold-block">package io.github.c;

</span><span class="fold-block hide-when-folded">import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

</span><span class="fold-block">@RestController
public class A {

    private int counter = 0;

    @GetMapping("/s")
    public String s() {
        counter++;
        System.out.println("counter = " + counter);
        return a();
    }

    public String a() {
        String b = b();
        for(int i = 0; i &lt; 3; i++) {
            b += i;
        }
        return b;
    }

    public String b() {
        String s = "b";
        s += "d";
        return s;
    }
}
</span></code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_启动项目"><a class="anchor" href="#_启动项目"></a>4.2. 启动项目</h3>
<div class="paragraph">
<p>项目打包后使用如下命令启动</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-bash" data-lang="bash">java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005 -jar app.jar</code></pre>
</div>
</div>
<div class="paragraph">
<p>在另一个shell中使用jdb命令attach到启动的JVM上，注意命令末尾使用 <code>-sourcepath</code> 参数指定了项目的源码位置，这样在调试时可以查看对应的源码</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlight"><code class="language-bash" data-lang="bash">jdb -connect com.sun.jdi.SocketAttach:hostname=127.0.0.1,port=5005 -sourcepath D:\ideaprojects\boottest\src\main\java</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_添加断点"><a class="anchor" href="#_添加断点"></a>4.3. 添加断点</h3>
<div class="paragraph">
<p>使用 <code>stop in io.github.c.A.s</code> 添加断点，然后调用 <code>/s</code> 接口可以看到jdb会话中出现断点命中的提升并自动进入
对应线程，使用 <code>cont</code> 可以继续执行</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/breakpoint1.gif" alt="breakpoint1">
</div>
</div>
</div>
<div class="sect2">
<h3 id="_断点调试"><a class="anchor" href="#_断点调试"></a>4.4. 断点调试</h3>
<div class="paragraph">
<p>断点调试的命令有 <code>step</code>、<code>step up</code>、<code>next</code>、<code>stepi</code> 这几条，下面演示其不同效果</p>
</div>
<div class="sect3">
<h4 id="_step"><a class="anchor" href="#_step"></a>4.4.1. step</h4>
<div class="paragraph">
<p>step命令调试的效果如下所示，可以看到如果存在方法调用，step会自动进入调用的方法一步步执行，这也是它与下面的next命令的主要区别</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/breakpoint3.gif" alt="breakpoint3">
</div>
</div>
</div>
<div class="sect3">
<h4 id="_next"><a class="anchor" href="#_next"></a>4.4.2. next</h4>
<div class="paragraph">
<p>next命令调试效果如下所示，可以看到即使存在方法调用它也不会进入调用的方法</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/breakpoint4.gif" alt="breakpoint4">
</div>
</div>
</div>
<div class="sect3">
<h4 id="_step_up"><a class="anchor" href="#_step_up"></a>4.4.3. step up</h4>
<div class="paragraph">
<p>step up命令会执行完成当前方法并返回，效果如下所示</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/breakpoint5.gif" alt="breakpoint5">
</div>
</div>
</div>
<div class="sect3">
<h4 id="_stepi"><a class="anchor" href="#_stepi"></a>4.4.4. stepi</h4>
<div class="paragraph">
<p>stepi命令不是按照源码中的语句执行的，而是按照编译后的字节码指令执行的，所以会看到
连续多次都在执行 <code>counter++</code>，但后面的 <code>bci</code> 不断增加，bci是字节码序号(byte code index)的缩写</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/breakpoint6.gif" alt="breakpoint6">
</div>
</div>
</div>
<div class="sect3">
<h4 id="_查看变量和源码"><a class="anchor" href="#_查看变量和源码"></a>4.4.5. 查看变量和源码</h4>
<div class="paragraph">
<p>print命令和list命令使用效果如下</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/breakpoint7.gif" alt="breakpoint7">
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_清除断点"><a class="anchor" href="#_清除断点"></a>4.5. 清除断点</h3>
<div class="paragraph">
<p>使用 <code>clear</code> 查看已经添加的断点，使用 <code>clear &lt;class&gt;:&lt;line_number&gt;</code> 或者 <code>clear &lt;class&gt;.&lt;method_name&gt;[(argument_type,&#8230;&#8203;)]</code>
删除已经添加的断点</p>
</div>
<div class="imageblock">
<div class="content">
<img src="images/breakpoint2.gif" alt="breakpoint2">
</div>
</div>
</div>
</div>
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