(#4884) Replaced synchronized with ReentrantLock in PhOnce

eo-runtime/src/main/java/org/eolang/PhOnce.java

@@ -6,18 +6,14 @@
 package org.eolang;
 
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Supplier;
 
 /**
  * An object wrapping another one.
  *
- * @since 0.1
- * @todo #4884:30min Use ReentrantLock instead of synchronized block in the constructor.
- *  This will allow to avoid blocking the whole object while fetching the wrapped one.
- *  Moreover, using 'syznchronized' is forbidden by qulice.
- *  Don't forget to remove the suppression of PMD.AvoidSynchronizedStatement in
- *  the constructor after that.
  * @checkstyle DesignForExtensionCheck (100 lines)
+ * @since 0.1
  */
 public class PhOnce implements Phi {
 
@@ -31,20 +27,33 @@ public class PhOnce implements Phi {
      */
     private final AtomicReference<Phi> ref;
 
+    /**
+     * Reentrant lock for thread-safe initialization
+     */
+    private final ReentrantLock lock;
+
+
     /**
      * Ctor.
      *
      * @param obj The object
      */
-    @SuppressWarnings("PMD.AvoidSynchronizedStatement")
     public PhOnce(final Supplier<Phi> obj) {
         this.ref = new AtomicReference<>(null);
+        this.lock = new ReentrantLock();
         this.object = () -> {
-            synchronized (this.ref) {
-                if (this.ref.get() == null) {
-                    this.ref.set(obj.get());
+            Phi cached = this.ref.get();
+            if (cached == null) {
+                this.lock.lock();
+                try {
+                    cached = this.ref.get();
+                    if (cached == null) {
+                        cached = obj.get();
+                        this.ref.set(cached);
+                    }
+                } finally {
+                    this.lock.unlock();
                 }
-                return this.ref.get();
             }
         };
     }
@@ -62,7 +71,7 @@ public int hashCode() {
     @Override
     public Phi copy() {
         return new PhOnce(
-            () -> this.object.get().copy()
+                () -> this.object.get().copy()
         );
     }