Add all 23 GoF design pattern implementations

01-creational/singleton/AppConfigDCL.java

@@ -0,0 +1,31 @@
+package singleton;
+
+public class AppConfigDCL {
+
+    // volatile is MANDATORY for DCL to work correctly.
+    // Without it, the JVM can reorder the writes inside the constructor
+    // and return a partially-constructed object to a second thread.
+    private static volatile AppConfigDCL instance;
+
+    private AppConfigDCL() { /* load config */ }
+
+    public static AppConfigDCL getInstance() {
+
+        // First check: no lock, instant return for the 99.99% case
+        // (instance already created, just return it)
+        if (instance == null) {
+
+            // Lock only when first check says we might need to create
+            synchronized (AppConfigDCL.class) {
+
+                // Second check: re-verify under the lock.
+                // Another thread might have created the instance between
+                // our first check and acquiring the lock.
+                if (instance == null) {
+                    instance = new AppConfigDCL();
+                }
+            }
+        }
+        return instance;
+    }
+}