BAEL-6428 Make a Method Execute Only Once in Java

core-java-modules/core-java-methods/src/main/java/com/baeldung/executeonce/AtomicBooleanInitializer.java

@@ -0,0 +1,19 @@
+package com.baeldung.executeonce;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+final class AtomicBooleanInitializer {
+
+    private final AtomicBoolean isInitialized = new AtomicBoolean(false);
+    int callCount = 0;
+
+    void initialize() {
+        if (isInitialized.compareAndSet(false, true)) {
+            initializationLogic();
+        }
+    }
+
+    private void initializationLogic() {
+        callCount++;
+    }
+}

core-java-modules/core-java-methods/src/main/java/com/baeldung/executeonce/StaticInitializer.java

@@ -0,0 +1,14 @@
+package com.baeldung.executeonce;
+
+final class StaticInitializer {
+
+    static int CALL_COUNT = 0;
+
+    static {
+        initializationLogic();
+    }
+
+    private static void initializationLogic() {
+        CALL_COUNT++;
+    }
+}

core-java-modules/core-java-methods/src/main/java/com/baeldung/executeonce/SynchronizedInitializer.java

@@ -0,0 +1,18 @@
+package com.baeldung.executeonce;
+
+final class SynchronizedInitializer {
+
+    private static volatile boolean isInitialized = false;
+    int callCount = 0;
+
+    synchronized void initialize() {
+        if (!isInitialized) {
+            initializationLogic();
+            isInitialized = true;
+        }
+    }
+
+    private void initializationLogic() {
+        callCount++;
+    }
+}

core-java-modules/core-java-methods/src/test/java/com/baeldung/executeonce/AtomicBooleanInitializerUnitTest.java

@@ -0,0 +1,20 @@
+package com.baeldung.executeonce;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+final class AtomicBooleanInitializerUnitTest {
+
+    @Test
+    void givenAtomicBooleanInitializer_whenRepeatedlyCallingInitialize_thenCallCountIsOne() {
+        AtomicBooleanInitializer atomicBooleanInitializer = new AtomicBooleanInitializer();
+        assertEquals(0, atomicBooleanInitializer.callCount);
+
+        atomicBooleanInitializer.initialize();
+        atomicBooleanInitializer.initialize();
+        atomicBooleanInitializer.initialize();
+
+        assertEquals(1, atomicBooleanInitializer.callCount);
+    }
+}

core-java-modules/core-java-methods/src/test/java/com/baeldung/executeonce/StaticInitializerUnitTest.java

@@ -0,0 +1,19 @@
+package com.baeldung.executeonce;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+final class StaticInitializerUnitTest {
+
+    @Test
+    void whenLoadingStaticInitializer_thenCallCountIsOne() {
+        assertEquals(1, StaticInitializer.CALL_COUNT);
+    }
+
+    @Test
+    void whenInitializingStaticInitializer_thenCallCountStaysOne() {
+        StaticInitializer staticInitializer = new StaticInitializer();
+        assertEquals(1, StaticInitializer.CALL_COUNT);
+    }
+}

core-java-modules/core-java-methods/src/test/java/com/baeldung/executeonce/SynchronizedInitializerUnitTest.java

@@ -0,0 +1,20 @@
+package com.baeldung.executeonce;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+final class SynchronizedInitializerUnitTest {
+
+    @Test
+    void givenSynchronizedInitializer_whenRepeatedlyCallingInitialize_thenCallCountIsOne() {
+        SynchronizedInitializer synchronizedInitializer = new SynchronizedInitializer();
+        assertEquals(0, synchronizedInitializer.callCount);
+
+        synchronizedInitializer.initialize();
+        synchronizedInitializer.initialize();
+        synchronizedInitializer.initialize();
+
+        assertEquals(1, synchronizedInitializer.callCount);
+    }
+}