Improve CustomizeConfiguration by avoiding repeatedly resolve file config (#730)

Author: JoeCqupt

CHANGES.md

@@ -11,6 +11,7 @@ Release Notes.
 * Add witness class/method for resteasy-server plugin(v3/v4/v6)
 * Add async-profiler feature for performance analysis 
 * Support db.instance tag,db.collection tag and AggregateOperation span for mongodb plugin(3.x/4.x)
+* Improve CustomizeConfiguration by avoiding repeatedly resolve file config
 
 All issues and pull requests are [here](https://github.com/apache/skywalking/milestone/222?closed=1)
 

apm-sniffer/optional-plugins/customize-enhance-plugin/src/main/java/org/apache/skywalking/apm/plugin/customize/conf/CustomizeConfiguration.java

@@ -27,6 +27,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.locks.ReentrantLock;
 import javax.xml.parsers.DocumentBuilder;
 import javax.xml.parsers.DocumentBuilderFactory;
 import javax.xml.parsers.ParserConfigurationException;
@@ -67,6 +68,8 @@ public enum CustomizeConfiguration {
     private static final Map<String, ElementMatcher> CONTEXT_ENHANCE_CLASSES = new HashMap<>();
     private static final AtomicBoolean LOAD_FOR_CONFIGURATION = new AtomicBoolean(false);
 
+    private static volatile List<Map<String, Object>> RESOLVED_FILE_CONFIGURATIONS;
+    private static final ReentrantLock RESOLVED_FILE_LOCK = new ReentrantLock();
     /**
      * The loadForEnhance method is resolver configuration file, and parse it
      */
@@ -107,13 +110,26 @@ public synchronized void loadForConfiguration() {
      * @throws SAXException                 link {@link SAXException}
      */
     private List<Map<String, Object>> resolver() throws ParserConfigurationException, IOException, SAXException {
-        List<Map<String, Object>> customizeMethods = new ArrayList<Map<String, Object>>();
-        File file = new File(CustomizePluginConfig.Plugin.Customize.ENHANCE_FILE);
-        if (file.exists() && file.isFile()) {
-            NodeList classNodeList = resolverFileClassDesc(file);
-            resolverClassNodeList(classNodeList, customizeMethods);
+        if (RESOLVED_FILE_CONFIGURATIONS != null) {
+            return RESOLVED_FILE_CONFIGURATIONS;
         }
-        return customizeMethods;
+
+        RESOLVED_FILE_LOCK.lock();
+        try {
+            if (RESOLVED_FILE_CONFIGURATIONS != null) {
+                return RESOLVED_FILE_CONFIGURATIONS;
+            }
+            List<Map<String, Object>> customizeMethods = new ArrayList<Map<String, Object>>();
+            File file = new File(CustomizePluginConfig.Plugin.Customize.ENHANCE_FILE);
+            if (file.exists() && file.isFile()) {
+                NodeList classNodeList = resolverFileClassDesc(file);
+                resolverClassNodeList(classNodeList, customizeMethods);
+            }
+            RESOLVED_FILE_CONFIGURATIONS = customizeMethods;
+        } finally {
+            RESOLVED_FILE_LOCK.unlock();
+        }
+        return RESOLVED_FILE_CONFIGURATIONS;
     }
 
     /**