[BAEL-3351] - Common Concurrency Pitfalls in Java (#8104)

core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/commonissues/CollectionsConcurrencyIssues.java

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+package com.baeldung.commonissues;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class CollectionsConcurrencyIssues {
+
+    private void putIfAbsentList_NonAtomicOperation_ProneToConcurrencyIssues() {
+        List<String> list = Collections.synchronizedList(new ArrayList<>());
+        if (!list.contains("foo")) {
+            list.add("foo");
+        }
+    }
+
+    private void putIfAbsentList_AtomicOperation_ThreadSafe() {
+        List<String> list = Collections.synchronizedList(new ArrayList<>());
+        synchronized (list) {
+            if (!list.contains("foo")) {
+                list.add("foo");
+            }
+        }
+    }
+
+    private void putIfAbsentMap_NonAtomicOperation_ProneToConcurrencyIssues() {
+        Map<String, String> map = new ConcurrentHashMap<>();
+        if (!map.containsKey("foo")) {
+            map.put("foo", "bar");
+        }
+    }
+
+    private void putIfAbsentMap_AtomicOperation_BetterApproach() {
+        Map<String, String> map = new ConcurrentHashMap<>();
+        synchronized (map) {
+            if (!map.containsKey("foo")) {
+                map.put("foo", "bar");
+            }
+        }
+    }
+
+    private void putIfAbsentMap_AtomicOperation_BestApproach() {
+        Map<String, String> map = new ConcurrentHashMap<>();
+        map.putIfAbsent("foo", "bar");
+    }
+
+    private void computeIfAbsentMap_AtomicOperation() {
+        Map<String, String> map = new ConcurrentHashMap<>();
+        map.computeIfAbsent("foo", key -> key + "bar");
+    }
+
+}

core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/commonissues/Counter.java

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+package com.baeldung.commonissues;
+
+class Counter {
+    private int counter = 0;
+
+    public void increment() {
+        counter++;
+    }
+
+    public int getValue() {
+        return counter;
+    }
+}

core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/commonissues/DeadlockExample.java

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+package com.baeldung.commonissues;
+
+public class DeadlockExample {
+
+    public static Object lock1 = new Object();
+    public static Object lock2 = new Object();
+
+    public static void main(String args[]) {
+        Thread threadA = new Thread(() -> {
+            synchronized (lock1) {
+                System.out.println("ThreadA: Holding lock 1...");
+                sleep();
+                System.out.println("ThreadA: Waiting for lock 2...");
+
+                synchronized (lock2) {
+                    System.out.println("ThreadA: Holding lock 1 & 2...");
+                }
+            }
+        });
+        Thread threadB = new Thread(() -> {
+            synchronized (lock2) {
+                System.out.println("ThreadB: Holding lock 2...");
+                sleep();
+                System.out.println("ThreadB: Waiting for lock 1...");
+
+                synchronized (lock1) {
+                    System.out.println("ThreadB: Holding lock 1 & 2...");
+                }
+            }
+        });
+        threadA.start();
+        threadB.start();
+    }
+
+    private static void sleep() {
+        try {
+            Thread.sleep(100);
+        } catch (InterruptedException e) {
+            throw new RuntimeException();
+        }
+    }
+}

core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/commonissues/SimpleDateFormatThreadUnsafetyExample.java

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+package com.baeldung.commonissues;
+
+import java.text.ParseException;
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+public class SimpleDateFormatThreadUnsafetyExample {
+
+    private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
+
+    public static void main(String[] args) {
+        String dateStr = "2019-10-29T11:12:21";
+
+        ExecutorService executorService = Executors.newFixedThreadPool(10);
+
+        for (int i = 0; i < 20; i++) {
+            executorService.submit(() -> parseDate(dateStr));
+        }
+
+        executorService.shutdown();
+    }
+
+    private static void parseDate(String dateStr) {
+        try {
+            Date date = simpleDateFormat.parse(dateStr);
+            System.out.println("Successfully Parsed Date " + date);
+        } catch (ParseException e) {
+            System.out.println("ParseError " + e.getMessage());
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
+    }
+}

core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/commonissues/SynchronizedCounter.java

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+package com.baeldung.commonissues;
+
+class SynchronizedCounter {
+    private int counter = 0;
+
+    public synchronized void increment() {
+        counter++;
+    }
+
+    public synchronized int getValue() {
+        return counter;
+    }
+}

core-java-modules/core-java-concurrency-advanced-3/src/main/java/com/baeldung/commonissues/SynchronizedVolatileCounter.java

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+package com.baeldung.commonissues;
+
+class SynchronizedVolatileCounter {
+    private volatile int counter = 0;
+
+    public synchronized void increment() {
+        counter++;
+    }
+
+    public int getValue() {
+        return counter;
+    }
+}