refactor: remove synchronized bottleneck from LabelValidator using ThreadLocal

src/main/java/gov/nasa/pds/tools/label/CachedLSResourceResolver.java

@@ -20,8 +20,8 @@
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLConnection;
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import org.apache.commons.io.IOUtils;
 import org.w3c.dom.ls.LSInput;
 import org.w3c.dom.ls.LSResourceResolver;
@@ -135,10 +135,15 @@ public void setSystemId(String systemId) {
     }
   }
 
-  /** Hashmap to hold the entities. */
-  private Map<String, byte[]> cachedEntities = new HashMap<>();
+  /** Thread-safe cache of resolved entities keyed by systemId. */
+  private Map<String, byte[]> cachedEntities = new ConcurrentHashMap<>();
 
-  private ProblemHandler handler;
+  /**
+   * Per-thread problem handler so that concurrent callers do not overwrite
+   * each other's handler when this resolver is shared (e.g. as an
+   * {@link XMLCatalogResolver} on the singleton {@code LabelValidator}).
+   */
+  private final ThreadLocal<ProblemHandler> handlerRef = new ThreadLocal<>();
 
   /**
    * Constructor.
@@ -150,11 +155,11 @@ public CachedLSResourceResolver() {
   /**
    * Constructor.
    *
-   * @param container A container to hold messages.
+   * @param handler A handler to receive problems during resource resolution.
    */
   public CachedLSResourceResolver(ProblemHandler handler) {
-    cachedEntities = new HashMap<>();
-    this.handler = handler;
+    cachedEntities = new ConcurrentHashMap<>();
+    this.handlerRef.set(handler);
   }
 
   @Override
@@ -163,8 +168,8 @@ public LSInput resolveResource(String type, String namespaceURI, String publicId
     if (systemId == null) {
       return null;
     }
-    byte[] entity = cachedEntities.get(systemId);
     LSInputImpl input = new LSInputImpl();
+    byte[] entity = cachedEntities.get(systemId);
     if (entity == null) {
       InputStream in = null;
       URLConnection conn = null;
@@ -194,16 +199,17 @@ public LSInput resolveResource(String type, String namespaceURI, String publicId
         conn = url.openConnection();
         in = Utility.openConnection(conn);
         entity = IOUtils.toByteArray(in);
-        cachedEntities.put(systemId, entity);
+        cachedEntities.putIfAbsent(systemId, entity);
       } catch (Exception e) {
-        if (handler != null) {
+        ProblemHandler h = handlerRef.get();
+        if (h != null) {
           URL u = null;
           try {
             u = new URL(systemId);
           } catch (MalformedURLException mu) {
             u = url;
           }
-          handler.addProblem(new ValidationProblem(new ProblemDefinition(ExceptionType.FATAL,
+          h.addProblem(new ValidationProblem(new ProblemDefinition(ExceptionType.FATAL,
               ProblemType.LABEL_UNRESOLVABLE_RESOURCE, e.getMessage()), u));
         } else {
           e.printStackTrace();
@@ -233,10 +239,10 @@ public void addCachedEntities(Map<String, byte[]> entities) {
   }
 
   public void setProblemHandler(ProblemHandler handler) {
-    this.handler = handler;
+    this.handlerRef.set(handler);
   }
 
   public ProblemHandler getProblemHandler() {
-    return this.handler;
+    return this.handlerRef.get();
   }
 }