Add monitor module javadoc (#3121)

dubbo-monitor/dubbo-monitor-api/src/main/java/org/apache/dubbo/monitor/support/AbstractMonitorFactory.java

@@ -44,14 +44,21 @@
 public abstract class AbstractMonitorFactory implements MonitorFactory {
     private static final Logger logger = LoggerFactory.getLogger(AbstractMonitorFactory.class);
 
-    // lock for getting monitor center
+    /**
+     * The lock for getting monitor center
+     */
     private static final ReentrantLock LOCK = new ReentrantLock();
 
-    // monitor centers Map<RegistryAddress, Registry>
+    /**
+     * The monitor centers Map<RegistryAddress, Registry>
+     */
     private static final Map<String, Monitor> MONITORS = new ConcurrentHashMap<String, Monitor>();
 
     private static final Map<String, CompletableFuture<Monitor>> FUTURES = new ConcurrentHashMap<String, CompletableFuture<Monitor>>();
 
+    /**
+     * The monitor create executor
+     */
     private static final ExecutorService executor = new ThreadPoolExecutor(0, 10, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), new NamedThreadFactory("DubboMonitorCreator", true));
 
     public static Collection<Monitor> getMonitors() {

dubbo-monitor/dubbo-monitor-api/src/main/java/org/apache/dubbo/monitor/support/MonitorFilter.java

@@ -45,15 +45,28 @@ public class MonitorFilter implements Filter {
 
     private static final Logger logger = LoggerFactory.getLogger(MonitorFilter.class);
 
+    /**
+     * The Concurrent counter
+     */
     private final ConcurrentMap<String, AtomicInteger> concurrents = new ConcurrentHashMap<String, AtomicInteger>();
 
+    /**
+     * The MonitorFactory
+     */
     private MonitorFactory monitorFactory;
 
     public void setMonitorFactory(MonitorFactory monitorFactory) {
         this.monitorFactory = monitorFactory;
     }
 
-    // intercepting invocation
+    /**
+     * The invocation interceptor,it will collect the invoke data about this invocation and send it to monitor center
+     *
+     * @param invoker    service
+     * @param invocation invocation.
+     * @return {@link Result} the invoke result
+     * @throws RpcException
+     */
     @Override
     public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcException {
         if (invoker.getUrl().hasParameter(Constants.MONITOR_KEY)) {
@@ -76,7 +89,16 @@ public Result invoke(Invoker<?> invoker, Invocation invocation) throws RpcExcept
         }
     }
 
-    // collect info
+    /**
+     * The collector logic, it will be handled by the default monitor
+     *
+     * @param invoker
+     * @param invocation
+     * @param result     the invoke result
+     * @param remoteHost the remote host address
+     * @param start      the timestamp the invoke begin
+     * @param error      if there is an error on the invoke
+     */
     private void collect(Invoker<?> invoker, Invocation invocation, Result result, String remoteHost, long start, boolean error) {
         try {
             URL monitorUrl = invoker.getUrl().getUrlParameter(Constants.MONITOR_KEY);
@@ -91,6 +113,17 @@ private void collect(Invoker<?> invoker, Invocation invocation, Result result, S
         }
     }
 
+    /**
+     * Create statistics url
+     *
+     * @param invoker
+     * @param invocation
+     * @param result
+     * @param remoteHost
+     * @param start
+     * @param error
+     * @return
+     */
     private URL createStatisticsUrl(Invoker<?> invoker, Invocation invocation, Result result, String remoteHost, long start, boolean error) {
         // ---- service statistics ----
         long elapsed = System.currentTimeMillis() - start; // invocation cost
@@ -114,7 +147,6 @@ private URL createStatisticsUrl(Invoker<?> invoker, Invocation invocation, Resul
             remoteKey = MonitorService.CONSUMER;
             remoteValue = remoteHost;
         }
-
         String input = "", output = "";
         if (invocation.getAttachment(Constants.INPUT_KEY) != null) {
             input = invocation.getAttachment(Constants.INPUT_KEY);

dubbo-monitor/dubbo-monitor-default/src/main/java/org/apache/dubbo/monitor/dubbo/DubboMonitor.java

@@ -42,19 +42,28 @@ public class DubboMonitor implements Monitor {
 
     private static final Logger logger = LoggerFactory.getLogger(DubboMonitor.class);
 
+    /**
+     * The length of the array which is a container of the statistics
+     */
     private static final int LENGTH = 10;
 
     /**
-     * The timer sends the statistics data to monitor center
+     * The timer for sending statistics
      */
     private final ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(3, new NamedThreadFactory("DubboMonitorSendTimer", true));
 
+    /**
+     * The future that can cancel the <b>scheduledExecutorService</b>
+     */
     private final ScheduledFuture<?> sendFuture;
 
     private final Invoker<MonitorService> monitorInvoker;
 
     private final MonitorService monitorService;
 
+    /**
+     * The time interval for timer <b>scheduledExecutorService</b> to send data
+     */
     private final long monitorInterval;
 
     private final ConcurrentMap<Statistics, AtomicReference<long[]>> statisticsMap = new ConcurrentHashMap<Statistics, AtomicReference<long[]>>();