[BAEL-3351] - Common Concurrency Pitfalls in Java (#8104)
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<relativePath>../../parent-java</relativePath>
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</parent>
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<dependencies>
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</dependencies>
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<build>
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<finalName>core-java-concurrency-advanced-3</finalName>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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<configuration>
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<source>${maven.compiler.source}</source>
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<target>${maven.compiler.target}</target>
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</configuration>
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</plugin>
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</plugins>
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<resources>
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<resource>
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<directory>src/main/resources</directory>
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@ -28,6 +35,8 @@
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</build>
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<properties>
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<maven.compiler.source>1.8</maven.compiler.source>
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<maven.compiler.target>1.8</maven.compiler.target>
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</properties>
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</project>
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package com.baeldung.commonissues;
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import java.util.ArrayList;
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import java.util.Collections;
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import java.util.List;
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import java.util.Map;
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import java.util.concurrent.ConcurrentHashMap;
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public class CollectionsConcurrencyIssues {
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private void putIfAbsentList_NonAtomicOperation_ProneToConcurrencyIssues() {
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List<String> list = Collections.synchronizedList(new ArrayList<>());
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if (!list.contains("foo")) {
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list.add("foo");
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}
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}
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private void putIfAbsentList_AtomicOperation_ThreadSafe() {
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List<String> list = Collections.synchronizedList(new ArrayList<>());
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synchronized (list) {
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if (!list.contains("foo")) {
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list.add("foo");
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}
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}
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}
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private void putIfAbsentMap_NonAtomicOperation_ProneToConcurrencyIssues() {
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Map<String, String> map = new ConcurrentHashMap<>();
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if (!map.containsKey("foo")) {
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map.put("foo", "bar");
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}
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}
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private void putIfAbsentMap_AtomicOperation_BetterApproach() {
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Map<String, String> map = new ConcurrentHashMap<>();
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synchronized (map) {
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if (!map.containsKey("foo")) {
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map.put("foo", "bar");
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}
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}
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}
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private void putIfAbsentMap_AtomicOperation_BestApproach() {
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Map<String, String> map = new ConcurrentHashMap<>();
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map.putIfAbsent("foo", "bar");
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}
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private void computeIfAbsentMap_AtomicOperation() {
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Map<String, String> map = new ConcurrentHashMap<>();
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map.computeIfAbsent("foo", key -> key + "bar");
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}
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}
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package com.baeldung.commonissues;
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class Counter {
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private int counter = 0;
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public void increment() {
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counter++;
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}
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public int getValue() {
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return counter;
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}
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}
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package com.baeldung.commonissues;
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public class DeadlockExample {
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public static Object lock1 = new Object();
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public static Object lock2 = new Object();
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public static void main(String args[]) {
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Thread threadA = new Thread(() -> {
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synchronized (lock1) {
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System.out.println("ThreadA: Holding lock 1...");
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sleep();
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System.out.println("ThreadA: Waiting for lock 2...");
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synchronized (lock2) {
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System.out.println("ThreadA: Holding lock 1 & 2...");
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}
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}
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});
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Thread threadB = new Thread(() -> {
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synchronized (lock2) {
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System.out.println("ThreadB: Holding lock 2...");
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sleep();
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System.out.println("ThreadB: Waiting for lock 1...");
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synchronized (lock1) {
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System.out.println("ThreadB: Holding lock 1 & 2...");
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}
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}
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});
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threadA.start();
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threadB.start();
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}
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private static void sleep() {
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try {
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Thread.sleep(100);
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} catch (InterruptedException e) {
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throw new RuntimeException();
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}
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}
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}
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package com.baeldung.commonissues;
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import java.text.ParseException;
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import java.text.SimpleDateFormat;
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import java.util.Date;
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import java.util.concurrent.ExecutorService;
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import java.util.concurrent.Executors;
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public class SimpleDateFormatThreadUnsafetyExample {
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private static SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
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public static void main(String[] args) {
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String dateStr = "2019-10-29T11:12:21";
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ExecutorService executorService = Executors.newFixedThreadPool(10);
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for (int i = 0; i < 20; i++) {
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executorService.submit(() -> parseDate(dateStr));
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}
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executorService.shutdown();
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}
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private static void parseDate(String dateStr) {
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try {
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Date date = simpleDateFormat.parse(dateStr);
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System.out.println("Successfully Parsed Date " + date);
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} catch (ParseException e) {
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System.out.println("ParseError " + e.getMessage());
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} catch (Exception e) {
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e.printStackTrace();
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}
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}
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}
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@ -0,0 +1,13 @@
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package com.baeldung.commonissues;
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class SynchronizedCounter {
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private int counter = 0;
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public synchronized void increment() {
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counter++;
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}
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public synchronized int getValue() {
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return counter;
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}
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}
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@ -0,0 +1,13 @@
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package com.baeldung.commonissues;
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class SynchronizedVolatileCounter {
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private volatile int counter = 0;
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public synchronized void increment() {
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counter++;
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}
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public int getValue() {
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return counter;
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}
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}
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