This post is part of our Java Programming: A Comprehensive Guide for Beginners series.
Multithreading and concurrency are powerful tools in Java for improving performance and responsiveness. However, they come with challenges that require careful design and implementation. Understanding thread lifecycles, synchronization, thread pools, and best practices is essential for writing robust and efficient multithreaded applications. As you gain experience in multithreading, you'll be better equipped to design scalable and responsive software systems.
7.1 Introduction to Multithreading
Multithreading is a powerful concept in Java that allows multiple threads to execute concurrently within the same program. Each thread represents an independent flow of control, enabling developers to perform tasks concurrently, leading to improved performance and responsiveness. This chapter explores the fundamentals of multithreading, synchronization, and best practices for concurrent programming in Java.7.2 Creating Threads in Java
In Java, there are two primary ways to create threads: by extending the Thread class or by implementing the Runnable interface. Extending the Thread class is straightforward, but implementing Runnable is often preferred for better design and flexibility.Example: Creating Threads with Runnable
// Example: Creating Threads with Runnable
public class MyRunnable implements Runnable {
@Override
public void run() {
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getId() + " Value " + i);
}
}
}
public class ThreadExample {
public static void main(String[] args) {
Thread t1 = new Thread(new MyRunnable());
Thread t2 = new Thread(new MyRunnable());
t1.start();
t2.start();
}
}
In this example, two threads (t1 and t2) are created by passing instances of MyRunnable to their constructors. The run method of MyRunnable defines the task each thread will perform.
7.3 Thread States and Lifecycle
A thread in Java goes through various states during its lifecycle:New: A thread that has been created but not yet started.- Runnable: A thread that is ready to run is moved to the runnable state.
- Blocked: A thread that is blocked waiting for a monitor lock is in this state.
- Waiting: A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
- Timed Waiting: A thread that is waiting for another thread to perform a particular action for up to a specified waiting time is in this state.
- Terminated: A thread that has exited is in this state.
7.4 Synchronization in Java
In multithreading, synchronization is essential to ensure that multiple threads can safely access shared resources without interference. Java provides the synchronized keyword and the ReentrantLock class for synchronization.Example: Synchronization with synchronized method
// Example: Synchronization with synchronized method
class Counter {
private int count = 0;
// Synchronized method
public synchronized void increment() {
count++;
}
public int getCount() {
return count;
}
}
public class SynchronizationExample {
public static void main(String[] args) {
Counter counter = new Counter();
// Two threads incrementing the counter concurrently
Thread t1 = new Thread(() -> {
for (int i = 0; i < 10000; i++) {
counter.increment();
}
});
Thread t2 = new Thread(() -> {
for (int i = 0; i < 10000; i++) {
counter.increment();
}
});
t1.start();
t2.start();
try {
t1.join();
t2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Count: " + counter.getCount());
}
}
In this example, two threads increment a counter concurrently. The increment method is synchronized, ensuring that only one thread can execute it at a time, preventing race conditions.
7.5 Deadlocks and Avoiding Them
A deadlock occurs when two or more threads are blocked forever, each waiting for the other to release a lock. Avoiding deadlocks involves careful design of synchronization, avoiding circular waiting, and using mechanisms like tryLock with timeouts.Example: Avoiding Deadlocks
// Example: Avoiding Deadlocks
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class DeadlockAvoidanceExample {
private static final Lock lock1 = new ReentrantLock();
private static final Lock lock2 = new ReentrantLock();
public static void main(String[] args) {
Thread t1 = new Thread(() -> {
acquireLocks(lock1, lock2);
});
Thread t2 = new Thread(() -> {
acquireLocks(lock2, lock1);
});
t1.start();
t2.start();
}
private static void acquireLocks(Lock firstLock, Lock secondLock) {
boolean gotFirstLock = false;
boolean gotSecondLock = false;
try {
while (true) {
try {
gotFirstLock = firstLock.tryLock();
gotSecondLock = secondLock.tryLock();
} finally {
if (gotFirstLock && gotSecondLock) {
return;
}
if (gotFirstLock) {
firstLock.unlock();
}
if (gotSecondLock) {
secondLock.unlock();
}
}
// Simulate waiting to avoid CPU intensive loop
Thread.sleep(1);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
In this example, two threads try to acquire locks in a specific order to avoid circular waiting, reducing the chances of a deadlock.
7.6 Thread Pools
Creating and managing threads can be resource-intensive. Thread pools provide a solution by reusing existing threads for multiple tasks, reducing the overhead of thread creation and management.Example: Using Thread Pools
In this example, a fixed-size thread pool is created using Executors.newFixedThreadPool(3). Tasks are submitted to the pool, and the pool manages the execution using available threads.
// Using Thread Pools
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ThreadPoolExample {
public static void main(String[] args) {
ExecutorService executorService = Executors.newFixedThreadPool(3);
for (int i = 1; i <= 5; i++) {
final int taskId = i;
executorService.submit(() -> {
System.out.println(
"Task " +
taskId +
" executed by thread: " +
Thread.currentThread().getName()
);
});
}
executorService.shutdown();
}
}
7.7 Java Memory Model and Volatile Keyword
Understanding the Java Memory Model is crucial for multithreading. The volatile keyword ensures visibility of changes made by one thread to other threads, preventing certain types of memory consistency errors.Example: Using volatile
In this example, the volatile keyword is used to ensure the visibility of the flag variable across threads. Thread 2 checks the flag continuously and exits when the flag becomes true.
// Example: Using volatile
public class VolatileExample {
private volatile boolean flag = false;
public static void main(String[] args) {
VolatileExample example = new VolatileExample();
// Thread 1: Setting the flag to true
new Thread(() -> {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
example.setFlag(true);
System.out.println("Flag set to true by Thread 1");
})
.start();
// Thread 2: Checking the flag
new Thread(() -> {
while (!example.isFlag()) {
// Waiting for the flag to become true
}
System.out.println("Flag is now true. Thread 2 exiting.");
})
.start();
}
public boolean isFlag() {
return flag;
}
public void setFlag(boolean flag) {
this.flag = flag;
}
}
7.8 Best Practices for Multithreading
- Use Thread Pools: Utilize thread pools to manage threads efficiently.
- Synchronize Access to Shared Resources: Ensure proper synchronization to prevent race conditions and data corruption.
- Minimize Lock Contention: Use fine-grained locks and lock-free algorithms to reduce contention.
- Avoid Deadlocks: Design synchronization to avoid circular waiting and use timeouts for locks.
- Use volatile Carefully: Apply the volatile keyword judiciously for variables that need visibility guarantees.
- Be Cautious with Thread.stop(): Avoid using the deprecated Thread.stop() method as it can leave the program in an inconsistent state.
- Consider Using Higher-Level Concurrency Utilities: Java provides higher-level abstractions like ExecutorService and CompletableFuture that simplify concurrent programming.