在上一篇中讲了Flume NG配置模块主要的接口的类，PropertiesConfigurationProvider提供了基于properties配置文件的静态配置的能力，这篇细说一下PollingPropertiesFileConfigurationProvider提供的执行时动态改动配置并生效的能力。
要实现动态改动配置文件并生效，主要有两个待实现的功能
1. 观察配置文件是否改动
2. 假设改动，将改动的内容通知给观察者
对于第一点，监控配置文件是否改动，Flume NG定义了一个FileWatcherRunnable对象来监控配置文件，启动了一个单独的线程採用定时轮询的方式来监控，轮询频率是30毫秒一次。比較file.lastModified属性与lastChange时间戳，当file.lastModified > lastChange时表示文件被改动
public class FileWatcherRunnable implements Runnable {
private final File file;
private final CounterGroup counterGroup;
private long lastChange;
public FileWatcherRunnable(File file, CounterGroup counterGroup) {
super();
this.file = file;
this.counterGroup = counterGroup;
this.lastChange = 0L;
}
@Override
public void run() {
LOGGER.debug("Checking file:{} for changes", file);
counterGroup.incrementAndGet("file.checks");
long lastModified = file.lastModified();
if (lastModified > lastChange) {
LOGGER.info("Reloading configuration file:{}", file);
counterGroup.incrementAndGet("file.loads");
lastChange = lastModified;
try {
eventBus.post(getConfiguration());
} catch (Exception e) {
LOGGER.error("Failed to load configuration data. Exception follows.",
e);
} catch (NoClassDefFoundError e) {
LOGGER.error("Failed to start agent because dependencies were not " +
"found in classpath. Error follows.", e);
} catch (Throwable t) {
// caught because the caller does not handle or log Throwables
LOGGER.error("Unhandled error", t);
}
}
}
}
// PollingPropertiesFileConfigurationProvider.start()启动一个单独的线程来监控properties配置文件
public void start() {
LOGGER.info("Configuration provider starting");
Preconditions.checkState(file != null,
"The parameter file must not be null");
executorService = Executors.newSingleThreadScheduledExecutor(
new ThreadFactoryBuilder().setNameFormat("conf-file-poller-%d")
.build());
FileWatcherRunnable fileWatcherRunnable =
new FileWatcherRunnable(file, counterGroup);
executorService.scheduleWithFixedDelay(fileWatcherRunnable, 0, interval,
TimeUnit.SECONDS);
lifecycleState = LifecycleState.START;
LOGGER.debug("Configuration provider started");
}
对于第二点，利用Guava EventBus提供的公布订阅模式机制，将配置改动封装成事件传递给Application。来又一次载入配置
// FileWatcherRunnable.run方法 公布配置改动的事件
eventBus.post(getConfiguration()); // Application.main方法来注冊事件订阅
Application application;
if(reload) {
EventBus eventBus = new EventBus(agentName + "-event-bus");
PollingPropertiesFileConfigurationProvider configurationProvider =
new PollingPropertiesFileConfigurationProvider(agentName,
configurationFile, eventBus, 30);
components.add(configurationProvider);
application = new Application(components);
eventBus.register(application);
} // Application类採用@Subscribe标注来定义订阅方法，即配置改动后会运行handleConfigurationEvent方法，这种方法是线程安全的 @Subscribe
public synchronized void handleConfigurationEvent(MaterializedConfiguration conf) {
stopAllComponents();
startAllComponents(conf);
}