From a6fc46e3f6ed643215ae21ca28d22b095c4a839c Mon Sep 17 00:00:00 2001
From: Gergo Petrik <petrikgergo@gmail.com>
Date: Mon, 4 May 2020 16:40:20 +0200
Subject: [PATCH] ABA problem in concurrency

---
 .../java/com/baeldung/abaproblem/Account.java | 34 +++++++
 .../baeldung/abaproblem/AccountUnitTest.java  | 93 +++++++++++++++++++
 2 files changed, 127 insertions(+)
 create mode 100644 core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/abaproblem/Account.java
 create mode 100644 core-java-modules/core-java-concurrency-advanced-3/src/test/java/com/baeldung/abaproblem/AccountUnitTest.java

diff --git a/core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/abaproblem/Account.java b/core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/abaproblem/Account.java
new file mode 100644
index 0000000000..0204c31fea
--- /dev/null
+++ b/core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/abaproblem/Account.java
@@ -0,0 +1,34 @@
+package com.baeldung.abaproblem;
+
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicInteger;
+
+public class Account {
+
+    private AtomicInteger balance = new AtomicInteger(0);
+
+    public int getBalance() {
+        return balance.get();
+    }
+
+    public boolean withdraw(int amount) throws InterruptedException {
+        int current = getBalance();
+        if (current < amount) {
+            throw new RuntimeException("Not sufficient balance");
+        }
+        precessBalance();
+        return balance.compareAndSet(current, current - amount);
+    }
+
+    private void precessBalance() throws InterruptedException {
+        if ("thread 1".equals(Thread.currentThread().getName())) {
+            TimeUnit.SECONDS.sleep(2);
+        }
+    }
+
+    public boolean deposit(int amount) {
+        int current = balance.get();
+        return balance.compareAndSet(current, current + amount);
+    }
+
+}
diff --git a/core-java-modules/core-java-concurrency-advanced-3/src/test/java/com/baeldung/abaproblem/AccountUnitTest.java b/core-java-modules/core-java-concurrency-advanced-3/src/test/java/com/baeldung/abaproblem/AccountUnitTest.java
new file mode 100644
index 0000000000..ab88a0f447
--- /dev/null
+++ b/core-java-modules/core-java-concurrency-advanced-3/src/test/java/com/baeldung/abaproblem/AccountUnitTest.java
@@ -0,0 +1,93 @@
+package com.baeldung.abaproblem;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class AccountUnitTest {
+
+    private Account account;
+
+    @BeforeEach
+    public void setUp() {
+        account = new Account();
+    }
+
+    @Test
+    public void zeroBalanceInitializationTest() {
+        assertEquals(0, account.getBalance());
+    }
+
+    @Test
+    public void depositTest() {
+        final int moneyToDeposit = 50;
+
+        assertTrue(account.deposit(moneyToDeposit));
+
+        assertEquals(moneyToDeposit, account.getBalance());
+    }
+
+    @Test
+    public void withdrawTest() throws InterruptedException {
+        final int defaultBalance = 50;
+        final int moneyToWithdraw = 20;
+
+        account.deposit(defaultBalance);
+
+        assertTrue(account.withdraw(moneyToWithdraw));
+
+        assertEquals(defaultBalance - moneyToWithdraw, account.getBalance());
+    }
+
+    @Test
+    public void withdrawWithoutSufficientBalanceTest() {
+        assertThrows(RuntimeException.class, () -> account.withdraw(10));
+    }
+
+    @Test
+    public void abaProblemTest() throws InterruptedException {
+        final int defaultBalance = 50;
+
+        final int amountToWithdrawByThreadA = 20;
+        final int amountToWithdrawByThreadB = 10;
+        final int amountToDepositByThreadB = 10;
+
+        account.deposit(defaultBalance);
+
+        Thread threadA = new Thread(() -> {
+            try {
+                // this will take longer due to the name of the thread
+                assertTrue(account.withdraw(amountToWithdrawByThreadA));
+            } catch (InterruptedException e) {
+                throw new RuntimeException(e);
+            }
+        }, "tread 1");
+
+        Thread threadB = new Thread(() -> {
+
+            assertTrue(account.deposit(amountToDepositByThreadB));
+            assertEquals(defaultBalance + amountToDepositByThreadB, account.getBalance());
+            try {
+                // this will be fast due to the name of the thread
+                assertTrue(account.withdraw(amountToWithdrawByThreadB));
+            } catch (InterruptedException e) {
+                throw new RuntimeException(e);
+            }
+
+            // thread 1 didn't finish yet, so the original value will be in place for it
+            assertEquals(defaultBalance, account.getBalance());
+
+        }, "thread 2");
+
+        threadA.start();
+        threadB.start();
+        threadA.join();
+        threadB.join();
+
+        // compareAndSet operation succeeds for thread 1
+        assertEquals(defaultBalance - amountToWithdrawByThreadA, account.getBalance());
+    }
+}