Double-checked locking to reduce overhead of lazy-initialization

The synchronization on the methods setPlatform and basicGetPlatform have
been removed as well, as it is not sensible to have a synchronized
setter without a synchronized getter.

Such synchronization would in any case not prevent usage of old
retrieved platforms. To prevent such cases, where the retrieved platform
is cached locally, consumers need to register as listeners for
notifications.

Author: rubenporras

com.avaloq.tools.ddk.xtext/src/com/avaloq/tools/ddk/xtext/layered/DefaultXtextTargetPlatformManager.java

@@ -35,37 +35,48 @@ public class DefaultXtextTargetPlatformManager implements IXtextTargetPlatformMa
   private IXtextTargetPlatformFactory platformFactory;
 
   /** The current platform. */
-  private IXtextTargetPlatform platform;
+  private volatile IXtextTargetPlatform platform; // NOPMD (volatile)
 
   /** Flag indicating that the target platform manager is shutting down. */
   private volatile boolean shutdownInProgress; // NOPMD (volatile)
 
+  private final Object lock = new Object();
+
   protected IXtextTargetPlatformFactory getPlatformFactory() {
     return platformFactory;
   }
 
   @Override
-  public synchronized IXtextTargetPlatform getPlatform() {
-    ensureLoaded();
-    return platform;
+  public IXtextTargetPlatform getPlatform() {
+    return ensureLoaded();
   }
 
   /**
    * Return the platform, but without making sure it's loaded. Just return the raw attribute.
    *
    * @return the current target platform, or null if none set.
    */
-  protected synchronized IXtextTargetPlatform basicGetPlatform() {
+  protected IXtextTargetPlatform basicGetPlatform() {
     return platform;
   }
 
   /**
    * Make sure the platform is loaded.
+   *
+   * @returns the loaded platform
    */
-  protected void ensureLoaded() {
-    if (platform == null) {
-      load(new NullProgressMonitor());
+  protected IXtextTargetPlatform ensureLoaded() {
+    IXtextTargetPlatform localRef = platform; // access volatile field only once when initialized
+    if (localRef == null) {
+      synchronized (this) {
+        localRef = platform;
+        if (localRef == null) {
+          load(new NullProgressMonitor());
+          localRef = platform;
+        }
+      }
     }
+    return localRef;
   }
 
   /**
@@ -156,19 +167,21 @@ public void setPlatform(final IXtextTargetPlatform newPlatform) {
    * @param mustRebuild
    *          whether a rebuild is required in any case.
    */
-  public synchronized void setPlatform(final IXtextTargetPlatform newPlatform, final Collection<IResourceDescription.Delta> deltas, final boolean mustRebuild) {
-    IXtextTargetPlatform oldPlatform = platform;
+  public void setPlatform(final IXtextTargetPlatform newPlatform, final Collection<IResourceDescription.Delta> deltas, final boolean mustRebuild) {
+    synchronized (lock) {
+      IXtextTargetPlatform oldPlatform = platform;
 
-    if (oldPlatform == null && newPlatform == null) {
-      return; // may occur during initialization...
-    }
+      if (oldPlatform == null && newPlatform == null) {
+        return; // may occur during initialization...
+      }
 
-    if (newPlatform == null) {
-      this.platform = new NullXtextTargetPlatform();
-    } else {
-      this.platform = newPlatform;
+      if (newPlatform == null) {
+        this.platform = new NullXtextTargetPlatform();
+      } else {
+        this.platform = newPlatform;
+      }
+      notifyListeners(platform, deltas, mustRebuild);
     }
-    notifyListeners(platform, deltas, mustRebuild);
   }
 
 }