前面介绍了 arthas 启动相关的代码并聊了聊怎么到一个 shellserver 的建立。
本篇我们来探讨一下几个使用频次非常高的命令是如何实现的。

dashboard

想看这个命令的主要原因是编程这些年来从来没有开发过 terminal 的这种比较花哨的界面，和 top 命令一样。

  public void process(final CommandProcess process) {Session session = process.session();timer = new Timer("Timer-for-arthas-dashboard-" + session.getSessionId(), true);// ctrl-C supportprocess.interruptHandler(new DashboardInterruptHandler(process, timer));/** 通过handle回调，在suspend和end时停止timer，resume时重启timer*/Handler<Void> stopHandler = new Handler<Void>() {@Overridepublic void handle(Void event) {stop();}};Handler<Void> restartHandler = new Handler<Void>() {@Overridepublic void handle(Void event) {restart(process);}};process.suspendHandler(stopHandler);process.resumeHandler(restartHandler);process.endHandler(stopHandler);// q exit supportprocess.stdinHandler(new QExitHandler(process));// start the timertimer.scheduleAtFixedRate(new DashboardTimerTask(process), 0, getInterval());}

1. 获取会话

   Session session = process.session();
   

   • 调用process对象的session方法获取当前会话对象，并赋值给session变量。process

2. 创建定时器

   timer = new Timer("Timer-for-arthas-dashboard-" + session.getSessionId(), true);
   

   • 创建一个定时器对象，命名为"Timer-for-arthas-dashboard-" + session.getSessionId()，其中session.getSessionId()获取会话ID。 定时器的主要作用是进行界面刷新。
   • 第二个参数true表示这个定时器是守护线程，当所有非守护线程结束时，程序会自动退出。

3. 处理中断

   process.interruptHandler(new DashboardInterruptHandler(process, timer));
   

   • 设置一个中断处理器DashboardInterruptHandler，用于处理中断信号（如Ctrl+C）。

4. 处理挂起、恢复和结束事件

   Handler<Void> stopHandler = new Handler<Void>() {@Overridepublic void handle(Void event) {stop();}
   };Handler<Void> restartHandler = new Handler<Void>() {@Overridepublic void handle(Void event) {restart(process);}
   };
   process.suspendHandler(stopHandler);
   process.resumeHandler(restartHandler);
   process.endHandler(stopHandler);
   

   • 定义了两个处理器stopHandler和restartHandler，分别用于处理挂起、恢复和结束事件。
   • stopHandler调用stop方法停止定时器。
   • restartHandler调用restart(process)方法重新启动定时器。
   • 将这些处理器分别注册到process对象的suspendHandler、resumeHandler和endHandler中。

5. 处理退出命令

   process.stdinHandler(new QExitHandler(process));
   

   • 设置一个标准输入处理器QExitHandler，用于处理退出命令（如输入q）。

6. 启动定时器

   timer.scheduleAtFixedRate(new DashboardTimerTask(process), 0, getInterval());
   

   • 使用timer对象定期执行DashboardTimerTask任务。
   • 周期性执行 DashboardTimerTask ，统计信息从 jvm 的 MXBean 中获取并组装 dashboardModel 再后面跟踪，就能发现界面绘制是属于 ProcessImpl 对命令结果的处理，发现没有，arthas 是一个很完整的解耦设计，职责分明。如何绘制界面在 view 文件夹，比如该命令
     

再往后看，界面就是做的以行为单元的字符串，输出到标准输出中，stdout。

watch

下面的 watch 命令在分析性能问题是出场率最高的命令，极大的释放了开发的背锅压力。
WatchCommand 继承了 EnhancerCommand ，下面的代码就是增强的核心代码。一般来说我们都知道使用 javaagent 可以在加载到jvm之前做增强，也就是 premain 方法，但 jvm也提供了 attach 的增强，即 agentmain，这也是在上一篇中提到过的

protected void enhance(CommandProcess process) {Session session = process.session();if (!session.tryLock()) {String msg = "someone else is enhancing classes, pls. wait.";process.appendResult(new EnhancerModel(null, false, msg));process.end(-1, msg);return;}EnhancerAffect effect = null;int lock = session.getLock();try {Instrumentation inst = session.getInstrumentation();AdviceListener listener = getAdviceListenerWithId(process);if (listener == null) {logger.error("advice listener is null");String msg = "advice listener is null, check arthas log";process.appendResult(new EnhancerModel(effect, false, msg));process.end(-1, msg);return;}boolean skipJDKTrace = false;if(listener instanceof AbstractTraceAdviceListener) {skipJDKTrace = ((AbstractTraceAdviceListener) listener).getCommand().isSkipJDKTrace();}Enhancer enhancer = new Enhancer(listener, listener instanceof InvokeTraceable, skipJDKTrace, getClassNameMatcher(), getClassNameExcludeMatcher(), getMethodNameMatcher());// 注册通知监听器process.register(listener, enhancer);effect = enhancer.enhance(inst, this.maxNumOfMatchedClass);if (effect.getThrowable() != null) {String msg = "error happens when enhancing class: "+effect.getThrowable().getMessage();process.appendResult(new EnhancerModel(effect, false, msg));process.end(1, msg + ", check arthas log: " + LogUtil.loggingFile());return;}if (effect.cCnt() == 0 || effect.mCnt() == 0) {// no class effectedif (!StringUtils.isEmpty(effect.getOverLimitMsg())) {process.appendResult(new EnhancerModel(effect, false));process.end(-1);return;}// might be method code too largeprocess.appendResult(new EnhancerModel(effect, false, "No class or method is affected"));String smCommand = Ansi.ansi().fg(Ansi.Color.GREEN).a("sm CLASS_NAME METHOD_NAME").reset().toString();String optionsCommand = Ansi.ansi().fg(Ansi.Color.GREEN).a("options unsafe true").reset().toString();String javaPackage = Ansi.ansi().fg(Ansi.Color.GREEN).a("java.*").reset().toString();String resetCommand = Ansi.ansi().fg(Ansi.Color.GREEN).a("reset CLASS_NAME").reset().toString();String logStr = Ansi.ansi().fg(Ansi.Color.GREEN).a(LogUtil.loggingFile()).reset().toString();String issueStr = Ansi.ansi().fg(Ansi.Color.GREEN).a("https://github.com/alibaba/arthas/issues/47").reset().toString();String msg = "No class or method is affected, try:\n"+ "1. Execute `" + smCommand + "` to make sure the method you are tracing actually exists (it might be in your parent class).\n"+ "2. Execute `" + optionsCommand + "`, if you want to enhance the classes under the `" + javaPackage + "` package.\n"+ "3. Execute `" + resetCommand + "` and try again, your method body might be too large.\n"+ "4. Match the constructor, use `<init>`, for example: `watch demo.MathGame <init>`\n"+ "5. Check arthas log: " + logStr + "\n"+ "6. Visit " + issueStr + " for more details.";process.end(-1, msg);return;}// 这里做个补偿,如果在enhance期间,unLock被调用了,则补偿性放弃if (session.getLock() == lock) {if (process.isForeground()) {process.echoTips(Constants.Q_OR_CTRL_C_ABORT_MSG + "\n");}}process.appendResult(new EnhancerModel(effect, true));//异步执行，在AdviceListener中结束} catch (Throwable e) {String msg = "error happens when enhancing class: "+e.getMessage();logger.error(msg, e);process.appendResult(new EnhancerModel(effect, false, msg));process.end(-1, msg);} finally {if (session.getLock() == lock) {// enhance结束后解锁process.session().unLock();}}}

这个方法的核心就在这几行代码中：

			Enhancer enhancer = new Enhancer(listener, listener instanceof InvokeTraceable, skipJDKTrace, getClassNameMatcher(), getClassNameExcludeMatcher(), getMethodNameMatcher());// 注册通知监听器process.register(listener, enhancer);effect = enhancer.enhance(inst, this.maxNumOfMatchedClass);

/*** 对象增强** @param inst              inst* @param maxNumOfMatchedClass 匹配的class最大数量* @return 增强影响范围* @throws UnmodifiableClassException 增强失败*/public synchronized EnhancerAffect enhance(final Instrumentation inst, int maxNumOfMatchedClass) throws UnmodifiableClassException {// 获取需要增强的类集合this.matchingClasses = GlobalOptions.isDisableSubClass? SearchUtils.searchClass(inst, classNameMatcher): SearchUtils.searchSubClass(inst, SearchUtils.searchClass(inst, classNameMatcher));if (matchingClasses.size() > maxNumOfMatchedClass) {affect.setOverLimitMsg("The number of matched classes is " +matchingClasses.size()+ ", greater than the limit value " + maxNumOfMatchedClass + ". Try to change the limit with option '-m <arg>'.");return affect;}// 过滤掉无法被增强的类List<Pair<Class<?>, String>> filtedList = filter(matchingClasses);if (!filtedList.isEmpty()) {for (Pair<Class<?>, String> filted : filtedList) {logger.info("ignore class: {}, reason: {}", filted.getFirst().getName(), filted.getSecond());}}logger.info("enhance matched classes: {}", matchingClasses);affect.setTransformer(this);try {ArthasBootstrap.getInstance().getTransformerManager().addTransformer(this, isTracing);// 批量增强if (GlobalOptions.isBatchReTransform) {final int size = matchingClasses.size();final Class<?>[] classArray = new Class<?>[size];arraycopy(matchingClasses.toArray(), 0, classArray, 0, size);if (classArray.length > 0) {inst.retransformClasses(classArray);logger.info("Success to batch transform classes: " + Arrays.toString(classArray));}} else {// for each 增强for (Class<?> clazz : matchingClasses) {try {inst.retransformClasses(clazz);logger.info("Success to transform class: " + clazz);} catch (Throwable t) {logger.warn("retransform {} failed.", clazz, t);if (t instanceof UnmodifiableClassException) {throw (UnmodifiableClassException) t;} else if (t instanceof RuntimeException) {throw (RuntimeException) t;} else {throw new RuntimeException(t);}}}}} catch (Throwable e) {logger.error("Enhancer error, matchingClasses: {}", matchingClasses, e);affect.setThrowable(e);}return affect;}

增强的关于字节码增强部分在 WatchAdviceListener 中，相当于做了一个 AOP。而这些 AdviceListenerAdapter 的调用链： SpyImpl ---- SpyAPI — SpyInterceptors — Enhancer.transform

class WatchAdviceListener extends AdviceListenerAdapter {private static final Logger logger = LoggerFactory.getLogger(WatchAdviceListener.class);private final ThreadLocalWatch threadLocalWatch = new ThreadLocalWatch();private WatchCommand command;private CommandProcess process;public WatchAdviceListener(WatchCommand command, CommandProcess process, boolean verbose) {this.command = command;this.process = process;super.setVerbose(verbose);}private boolean isFinish() {return command.isFinish() || !command.isBefore() && !command.isException() && !command.isSuccess();}@Overridepublic void before(ClassLoader loader, Class<?> clazz, ArthasMethod method, Object target, Object[] args)throws Throwable {// 开始计算本次方法调用耗时threadLocalWatch.start();if (command.isBefore()) {watching(Advice.newForBefore(loader, clazz, method, target, args));}}@Overridepublic void afterReturning(ClassLoader loader, Class<?> clazz, ArthasMethod method, Object target, Object[] args,Object returnObject) throws Throwable {Advice advice = Advice.newForAfterReturning(loader, clazz, method, target, args, returnObject);if (command.isSuccess()) {watching(advice);}finishing(advice);}@Overridepublic void afterThrowing(ClassLoader loader, Class<?> clazz, ArthasMethod method, Object target, Object[] args,Throwable throwable) {Advice advice = Advice.newForAfterThrowing(loader, clazz, method, target, args, throwable);if (command.isException()) {watching(advice);}finishing(advice);}private void finishing(Advice advice) {if (isFinish()) {watching(advice);}}private void watching(Advice advice) {try {// 本次调用的耗时double cost = threadLocalWatch.costInMillis();boolean conditionResult = isConditionMet(command.getConditionExpress(), advice, cost);if (this.isVerbose()) {process.write("Condition express: " + command.getConditionExpress() + " , result: " + conditionResult + "\n");}if (conditionResult) {// TODO: concurrency issues for process.writeObject value = getExpressionResult(command.getExpress(), advice, cost);WatchModel model = new WatchModel();model.setTs(new Date());model.setCost(cost);model.setValue(new ObjectVO(value, command.getExpand()));model.setSizeLimit(command.getSizeLimit());model.setClassName(advice.getClazz().getName());model.setMethodName(advice.getMethod().getName());if (advice.isBefore()) {model.setAccessPoint(AccessPoint.ACCESS_BEFORE.getKey());} else if (advice.isAfterReturning()) {model.setAccessPoint(AccessPoint.ACCESS_AFTER_RETUNING.getKey());} else if (advice.isAfterThrowing()) {model.setAccessPoint(AccessPoint.ACCESS_AFTER_THROWING.getKey());}process.appendResult(model);process.times().incrementAndGet();if (isLimitExceeded(command.getNumberOfLimit(), process.times().get())) {abortProcess(process, command.getNumberOfLimit());}}} catch (Throwable e) {logger.warn("watch failed.", e);process.end(-1, "watch failed, condition is: " + command.getConditionExpress() + ", express is: "+ command.getExpress() + ", " + e.getMessage() + ", visit " + LogUtil.loggingFile()+ " for more details.");}}
}

Enhance 是个自定义的 ClassTransformer ，在 Instrumentation 进行 addTransformer 时，将 enhance 增强纳进管理，在 redefined 或者 retransformed 的时候调用 transform 方法。。
Instrumentation 的方法：

retransform

retransform 这个命令的牛逼之处是可以进行灵活的热加载，用过 jrebel 的同学应该比较清除，在开发时能使用热加载，compile 一下修改的功能就能生效在开发时能节省大量的时间。
在arthas 中 jad–mc–retransform 工具链实现热加载也是一个在线上快速修复的一个方法。