Add comment for InterceptorInstanceLoader

apm-sniffer/apm-agent-core/src/main/java/org/skywalking/apm/agent/core/plugin/loader/InterceptorInstanceLoader.java

@@ -28,6 +28,7 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.locks.ReentrantLock;
+
 import org.skywalking.apm.agent.core.boot.AgentPackageNotFoundException;
 import org.skywalking.apm.agent.core.plugin.interceptor.enhance.InstanceConstructorInterceptor;
 import org.skywalking.apm.agent.core.plugin.interceptor.enhance.InstanceMethodsAroundInterceptor;
@@ -58,8 +59,23 @@ public class InterceptorInstanceLoader {
 
     private static Map<ClassLoader, ClassLoader> EXTEND_PLUGIN_CLASSLOADERS = new HashMap<ClassLoader, ClassLoader>();
 
+    /**
+     * Load an instance of interceptor, and keep it singleton.
+     * Create {@link AgentClassLoader} for each targetClassLoader, as an extend classloader.
+     * It can load interceptor classes from plugins, activations folders.
+     *
+     * @param className
+     * @param targetClassLoader
+     * @param <T>
+     * @return
+     * @throws InvocationTargetException
+     * @throws IllegalAccessException
+     * @throws InstantiationException
+     * @throws ClassNotFoundException
+     * @throws AgentPackageNotFoundException
+     */
     public static <T> T load(String className, ClassLoader targetClassLoader)
-        throws InvocationTargetException, IllegalAccessException, InstantiationException, ClassNotFoundException, AgentPackageNotFoundException {
+            throws InvocationTargetException, IllegalAccessException, InstantiationException, ClassNotFoundException, AgentPackageNotFoundException {
         if (targetClassLoader == null) {
             targetClassLoader = InterceptorInstanceLoader.class.getClassLoader();
         }
@@ -82,7 +98,7 @@ public class InterceptorInstanceLoader {
             }
         }
 
-        return (T)inst;
+        return (T) inst;
     }
 
     /**
@@ -98,7 +114,7 @@ public class InterceptorInstanceLoader {
      * @throws ClassNotFoundException
      */
     public static <T> T load0(String className, ClassLoader targetClassLoader)
-        throws InvocationTargetException, IllegalAccessException, InstantiationException, ClassNotFoundException {
+            throws InvocationTargetException, IllegalAccessException, InstantiationException, ClassNotFoundException {
         if (targetClassLoader == null) {
             targetClassLoader = InterceptorInstanceLoader.class.getClassLoader();
         }
@@ -130,14 +146,14 @@ public class InterceptorInstanceLoader {
             }
         }
 
-        return (T)inst;
+        return (T) inst;
     }
 
     /**
      * load class from class binary files.
      * Most likely all the interceptor implementations should be loaded by this.
      *
-     * @param className interceptor class name.
+     * @param className         interceptor class name.
      * @param targetClassLoader the classloader, which should load the interceptor.
      * @param <T>
      * @return interceptor instance.
@@ -146,7 +162,7 @@ public class InterceptorInstanceLoader {
      * @throws InstantiationException
      */
     private static <T> T loadBinary(String className,
-        ClassLoader targetClassLoader) throws InvocationTargetException, IllegalAccessException, InstantiationException {
+                                    ClassLoader targetClassLoader) throws InvocationTargetException, IllegalAccessException, InstantiationException {
         String path = "/" + className.replace('.', '/').concat(".class");
         byte[] data = null;
         BufferedInputStream is = null;
@@ -186,21 +202,21 @@ public class InterceptorInstanceLoader {
         }
         defineClassMethod.setAccessible(true);
         logger.debug("load binary code of {} to classloader {}", className, targetClassLoader);
-        Class<?> type = (Class<?>)defineClassMethod.invoke(targetClassLoader, className, data, 0, data.length, null);
-        return (T)type.newInstance();
+        Class<?> type = (Class<?>) defineClassMethod.invoke(targetClassLoader, className, data, 0, data.length, null);
+        return (T) type.newInstance();
     }
 
     /**
      * Find loaded class in the current classloader.
      * Just in case some classes have already been loaded for some reason.
      *
-     * @param className interceptor class name.
+     * @param className         interceptor class name.
      * @param targetClassLoader the classloader, which should load the interceptor.
      * @param <T>
      * @return interceptor instance.
      */
     private static <T> T findLoadedClass(String className,
-        ClassLoader targetClassLoader) throws InvocationTargetException, IllegalAccessException, InstantiationException {
+                                         ClassLoader targetClassLoader) throws InvocationTargetException, IllegalAccessException, InstantiationException {
         Method defineClassMethod = null;
         Class<?> targetClassLoaderType = targetClassLoader.getClass();
         while (defineClassMethod == null && targetClassLoaderType != null) {
@@ -211,10 +227,10 @@ public class InterceptorInstanceLoader {
             }
         }
         defineClassMethod.setAccessible(true);
-        Class<?> type = (Class<?>)defineClassMethod.invoke(targetClassLoader, className);
+        Class<?> type = (Class<?>) defineClassMethod.invoke(targetClassLoader, className);
         if (type == null) {
             return null;
         }
-        return (T)type.newInstance();
+        return (T) type.newInstance();
     }
 }