Add LoggingPluginStateListener as listener in DefaultPluginManager only for development mode

pf4j/src/main/java/org/pf4j/AbstractPluginManager.java

@@ -354,7 +354,7 @@ public abstract class AbstractPluginManager implements PluginManager {
             log.warn("Cannot start an unresolved plugin '{}'", getPluginLabel(pluginDescriptor));
             return pluginState;
         }
-        
+
         if (PluginState.DISABLED == pluginState) {
             // automatically enable plugin on manual plugin start
             if (!enablePlugin(pluginId)) {
@@ -795,7 +795,7 @@ public abstract class AbstractPluginManager implements PluginManager {
 
     protected synchronized void firePluginStateEvent(PluginStateEvent event) {
         for (PluginStateListener listener : pluginStateListeners) {
-            log.debug("Fire '{}' to '{}'", event, listener);
+            log.trace("Fire '{}' to '{}'", event, listener);
             listener.pluginStateChanged(event);
         }
     }

pf4j/src/main/java/org/pf4j/DefaultPluginManager.java

@@ -115,6 +115,10 @@ public class DefaultPluginManager extends AbstractPluginManager {
 
         super.initialize();
 
+        if (isDevelopment()) {
+            addPluginStateListener(new LoggingPluginStateListener());
+        }
+
         log.info("PF4J version {} in '{}' mode", getVersion(), getRuntimeMode());
 	}
 

pf4j/src/main/java/org/pf4j/LoggingPluginStateListener.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2018 Decebal Suiu
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pf4j;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * It's an implementation of {@link PluginStateListener} that writes all events to logger (DEBUG level).
+ * This listener is added automatically by {@link DefaultPluginManager} for {@code dev} mode.
+ *
+ * @author Decebal Suiu
+ */
+public class LoggingPluginStateListener implements PluginStateListener {
+
+    private static final Logger log = LoggerFactory.getLogger(LoggingPluginStateListener.class);
+
+    @Override
+    public void pluginStateChanged(PluginStateEvent event) {
+        log.debug("The state of plugin '{}' has changed from '{}' to '{}'", event.getPlugin().getPluginId(),
+            event.getOldState(), event.getPluginState());
+    }
+
+}