1. 概述
对于开发人员来说，调试可能非常耗时，使调试过程更高效的方法之一是使用JMX(Java Management Extension)，这是一种用于Java应用程序的监视和管理技术。
在本教程中，我们将介绍如何使用JMXTerm对Java应用程序执行外部调试。
2. JMXTerm
JMX有几个可用于Java应用程序的工具，例如JConsole、VisualVM和JMXTerm。JConsole是一个用于监控性能的图形工具。VisualVM提供高级调试和分析功能，需要一个插件才能与MBean配合使用。虽然这些都是有用的工具，但JMXTerm是一个轻量级、灵活的命令行选项，可用于自动化。
2.1 设置
要使用JMXTerm，我们首先需要下载并安装它。最新版本的JMXTerm可在官方网站上找到，它被打包在一个简单的jar文件中：
$ java -jar jmxterm.jar
请注意，可以使用更高版本的Java，这可能是由于模块化JDK中对反射访问的限制而发生的。要解决这个问题，我们可以使用–add-exports：
$ java --add-exports jdk.jconsole/sun.tools.jconsole=ALL-UNNAMED -jar jmxterm.jar
2.2 连接
启动JMXTerm后，我们可以使用主机和端口连接到Java应用程序。请注意，此选项可能需要额外的步骤来配置或查找所需的端口：
$ open [host]:[port]
此外，可以在启动时传递地址：
$ java -jar jmxterm.jar -l [host]:[port]
或者，我们可以使用PID来访问和打开连接。JMXTerm允许我们使用jvms命令直接查找它：
$ jvms
83049 (m) - cn.tuyucheng.taketoday.jmxterm.GameRunner
$ open 83049
#Connection to 83049 is opened
2.3 MBean
MBean是我们可以通过JMX管理和监控的Java对象，它们公开了一个可以通过JMX代理访问和控制的接口，使诊断和故障排除变得更加容易。
要创建MBean，我们应该遵循约定，首先创建一个名称以MBean结尾的接口。在我们的例子中，它将是GuessGameMBean。这意味着该类的名称应该是GuessGame，而不是其他名称。或者，在某些情况下，可以使用MXBeans。该接口包含我们要公开给JMX的操作：
public interface GuessGameMBean {
void finishGame();
void pauseGame();
void unpauseGame();
}
GuessGame本身是一个简单的猜数字游戏：
public class GuessGame extends GuessGameMBean {
// ...
public void start() {
int randomNumber = randomNumbergenerator.getLastNumber();
while (!isFinished) {
waitASecond();
while (!isPaused && !isFinished) {
log.info("Current random number is " + randomNumber);
waitASecond();
for (Player player : players) {
int guess = player.guessNumber();
if (guess == randomNumber) {
log.info("Players " + player.getName() + " " + guess + " is correct");
player.incrementScore();
notifyAboutWinner(player);
randomNumber = randomNumbergenerator.generateRandomNumber();
break;
}
log.info("Player " + player.getName() + " guessed incorrectly with " + guess);
}
log.info("\n");
}
if (isPaused) {
log.info("Game is paused");
}
if (isFinished) {
log.info("Game is finished");
}
}
}
//...
}
我们还将通过JMX跟踪玩家：
public interface PlayerMBean {
int getGuess();
int getScore();
String getName();
}
3. 使用JMXTerm进行调试
使用JMXTerm连接到Java应用程序后，我们可以查询可用域：
$ domains
#following domains are available
JMImplementation
cn.tuyucheng.taketoday.jmxterm
com.sun.management
java.lang
java.nio
java.util.logging
jdk.management.jfr
3.1 Logger级别
让我们尝试更改正在运行的应用程序的日志记录级别，我们将使用java.util.logging.Logger进行演示，但请注意JUL有很大的缺点。JUL提供开箱即用的MBean：
$ domain java.util.logging
#domain is set to java.util.logging
现在我们可以检查域中可用的MBean：
$ beans
#domain = java.util.logging:
java.util.logging:type=Logging
下一步，我们需要检查记录器bean提供的信息：
$ bean java.util.logging:type=Logging
#bean is set to java.util.logging:type=Logging
$ info
#mbean = java.util.logging:type=Logging
#class name = sun.management.ManagementFactoryHelper$PlatformLoggingImpl
# attributes
%0 - LoggerNames ([Ljava.lang.String;, r)
%1 - ObjectName (javax.management.ObjectName, r)
# operations
%0 - java.lang.String getLoggerLevel(java.lang.String p0)
%1 - java.lang.String getParentLoggerName(java.lang.String p0)
%2 - void setLoggerLevel(java.lang.String p0,java.lang.String p1)
#there's no notifications
要访问GuessGame对象中的记录器，我们需要找到记录器的名称：
$ get LoggerNames
#mbean = java.util.logging:type=Logging:
LoggerNames = [ ..., cn.tuyucheng.taketoday.jmxterm.GuessGame, ...];
最后，检查日志记录级别：
$ run getLoggerLevel cn.tuyucheng.taketoday.jmxterm.GuessGame
#calling operation getLoggerLevel of mbean java.util.logging:type=Logging with params [cn.tuyucheng.taketoday.jmxterm.GuessGame]
#operation returns:
WARNING
要更改它，我们只需调用带有参数的setter方法：
$ run setLoggerLevel cn.tuyucheng.taketoday.jmxterm.GuessGame INFO
之后，我们可以观察应用程序的日志：
...
Apr 14, 2023 12:04:30 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Current random number is 7
Apr 14, 2023 12:04:31 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Player Bob guessed incorrectly with 10
Apr 14, 2023 12:04:31 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Player Alice guessed incorrectly with 5
Apr 14, 2023 12:04:31 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Player John guessed incorrectly with 4
...
3.2 使用域Bean
让我们尝试从我们的应用程序外部停止我们的游戏，这些步骤与记录器示例中的步骤相同：
$ domain cn.tuyucheng.taketoday.jmxterm
#domain is set to cn.tuyucheng.taketoday.jmxterm
$ beans
#domain = cn.tuyucheng.taketoday.jmxterm:
cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
$ bean cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
#bean is set to cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
$ info
#mbean = cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
#class name = cn.tuyucheng.taketoday.jmxterm.GuessGame
#there is no attribute
# operations
%0 - void finishGame()
%1 - void pauseGame()
%2 - void unpauseGame()
#there's no notifications
$ run pauseGame
#calling operation pauseGame of mbean cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game with params []
我们应该在输出中看到游戏已暂停：
...
Apr 14, 2023 12:17:01 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Game is paused
Apr 14, 2023 12:17:02 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Game is paused
Apr 14, 2023 12:17:03 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Game is paused
Apr 14, 2023 12:17:04 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Game is paused
...
另外，我们可以完成游戏：
$ run finishGame
输出应该包含游戏结束的信息：
...
Apr 14, 2023 12:17:47 PM cn.tuyucheng.taketoday.jmxterm.GuessGame start
INFO: Game is finished
3.3 watch
此外，我们可以使用watch命令跟踪属性的值：
$ info
# attributes
#mbean = cn.tuyucheng.taketoday.jmxterm:id=Bobd661ee89-b972-433c-adff-93e7495c7e0a,type=player
#class name = cn.tuyucheng.taketoday.jmxterm.Player
#there's no operations
#there's no notifications
%0 - Guess (int, r)
%1 - Name (java.lang.String, r)
%2 - Score (int, r)
$ watch Score
#press any key to stop. DO NOT press Ctrl+C !!!
683683683683683683683
原始watch输出很难阅读，但我们可以为其提供一种格式：
$ watch --format Score\\ {0}\\ Score
#press any key to stop. DO NOT press Ctrl+C !!!
Score 707 Score 707 Score 707 Score 707 Score 707
但是，我们可以通过–report和–stopafter选项进一步改进它：
$ watch --report --stopafter 10 --format The\\ score\\ is\\ {0} Score
The score is 727
The score is 727
The score is 727
The score is 728
The score is 728
3.4 通知
另一个重要的调试功能是MBean通知，但是，这需要对我们的代码进行最少的更改。首先，我们必须实现javax.management.NotificationBroadcaster接口：
public interface NotificationBroadcaster {
void addNotificationListener(NotificationListener listener, NotificationFilter filter, Object handback) throws java.lang.IllegalArgumentException;
void removeNotificationListener(NotificationListener listener) throws ListenerNotFoundException;
MBeanNotificationInfo[] getNotificationInfo();
}
要发送有关获胜者的通知，我们将使用javax.management.NotificationBroadcasterSupport：
public abstract class BroadcastingGuessGame implements NotificationBroadcaster, GuessGameMBean {
private NotificationBroadcasterSupport broadcaster = new NotificationBroadcasterSupport();
private long notificationSequence = 0;
private MBeanNotificationInfo[] notificationInfo;
public BroadcastingGuessGame() {
this.notificationInfo = new MBeanNotificationInfo[]{
new MBeanNotificationInfo(new String[]{"game"}, Notification.class.getName(),"Game notification")
};
}
protected void notifyAboutWinner(Player winner) {
String message = "Winner is " + winner.getName() + " with score " + winner.getScore();
Notification notification = new Notification("game.winner", this, notificationSequence++, message);
broadcaster.sendNotification(notification);
}
public void addNotificationListener(NotificationListener listener, NotificationFilter filter, Object handback) {
broadcaster.addNotificationListener(listener, filter, handback);
}
public void removeNotificationListener(NotificationListener listener) throws ListenerNotFoundException {
broadcaster.removeNotificationListener(listener);
}
public MBeanNotificationInfo[] getNotificationInfo() {
return notificationInfo;
}
}
之后，我们可以在bean上看到通知：
$ bean cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
#bean is set to cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
$ info
#mbean = cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
#class name = cn.tuyucheng.taketoday.jmxterm.GuessGame
#there is no attribute
# operations
%0 - void finishGame()
%1 - void pauseGame()
%2 - void unpauseGame()
# notifications
%0 - javax.management.Notification(game.winner)
随后，我们可以订阅通知：
$ subscribe
#Subscribed to cn.tuyucheng.taketoday.jmxterm:id=singlegame,type=game
notification received: ...,message=Winner is John with score 10
notification received: ...,message=Winner is Alice with score 9
notification received: ...,message=Winner is Bob with score 13
notification received: ...,message=Winner is Bob with score 14
notification received: ...,message=Winner is John with score 11
通过提供–domain和–bean选项，我们可以订阅多个bean。
4. 总结
JMXTerm是一个强大而灵活的工具，用于通过JMX管理和监视Java应用程序。通过为JMX操作提供命令行接口，JMXTerm允许开发人员和管理员快速轻松地执行监控属性值、调用操作和更改配置设置等任务。
与往常一样，本教程的完整源代码可在GitHub上获得。
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