Welcome to the next step in your journey through Java concurrency! In this lesson, we will delve into the intricacies of deadlocks and lock mechanisms. Mastering these concepts is essential for building robust, error-free applications and ensuring your multi-threaded programs run smoothly.

By the end of this lesson, you will:

• Understand the concept of deadlocks and their causes.
• Learn techniques to prevent deadlocks through lock reordering.
• Gain proficiency in using ReentrantLock for improved control over locking.
• Understand the role of locks in managing shared resources.

Let’s start by exploring the concept of locks in Java.

In previous lessons, we discussed synchronized(this), where the current instance of the class serves as the monitor object or lock. This is a type of intrinsic lock, meaning that every object in Java has an implicit lock. When a thread acquires this lock, it prevents other threads from entering synchronized sections of code for that object until the lock is released.

For example:

Java
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1public void increment() {
2    synchronized (this) { 
3        // Critical section
4        count++;
5    }
6}

In the code above, the lock on this ensures that only one thread at a time can execute this critical section on the given instance of the class. This helps prevent race conditions and ensures consistent updates to shared data.

Locks are mechanisms used to synchronize access to shared resources in multi-threaded programs. When a thread acquires a lock, it prevents other threads from accessing the locked resource until it is released. The synchronized keyword is Java’s built-in mechanism for managing these intrinsic locks.

Types of Locks in Java:

• Object Locks (synchronized(this) or synchronized(someObject)): These are associated with an individual instance of an object. Only one thread can execute any synchronized method of that instance at a time.
• Class Locks (synchronized(ClassName.class)): These are used to lock at the class level, preventing multiple threads from executing static synchronized methods simultaneously.

Using locks correctly can help manage access to shared resources, but it also comes with the risk of deadlocks if not used properly.

A deadlock occurs when two or more threads are blocked forever, each waiting for a resource that another thread holds. This creates a situation where no thread can proceed, leading to a halt in the program. Essentially, it’s like two people who are each holding a key to a door that the other person needs—they're both stuck, waiting for the other to act.

Causes of Deadlocks:

Deadlocks happen because of four main conditions, which can occur simultaneously:

• Mutual Exclusion: Resources involved are non-shareable, meaning only one thread can access the resource at a time.
• Hold and Wait: A thread holds at least one resource and waits for additional resources held by other threads.
• No Preemption: Resources cannot be forcibly taken away from a thread; they can only be released voluntarily.
• Circular Wait: A closed chain of threads exists, where each thread holds a resource that the next thread needs, forming a cycle.

When these four conditions exist, the system ends up in a deadlock state, where threads cannot continue to execute.

Here's an example of a deadlock scenario:

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1public class DeadlockExample {
2    private final Object lock1 = new Object();
3    private final Object lock2 = new Object();
4
5    public void methodOne() {
6        synchronized (lock1) {
7            System.out.println("Thread 1 acquired lock1");
8            try { Thread.sleep(50); } catch (InterruptedException e) {}
9            synchronized (lock2) {
10                System.out.println("Thread 1 acquired lock2");
11            }
12        }
13    }
14
15    public void methodTwo() {
16        synchronized (lock2) {
17            System.out.println("Thread 2 acquired lock2");
18            try { Thread.sleep(50); } catch (InterruptedException e) {}
19            synchronized (lock1) {
20                System.out.println("Thread 2 acquired lock1");
21            }
22        }
23    }
24}

Here's how you can run this deadlock scenario:

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1public class Main {
2    public static void main(String[] args) {
3        DeadlockExample example = new DeadlockExample();
4        Thread t1 = new Thread(example::methodOne);
5        Thread t2 = new Thread(example::methodTwo);
6        t1.start();
7        t2.start();
8    }
9}

In this example, there are two locks (lock1 and lock2). Thread 1 acquires lock1 first and then tries to acquire lock2. At the same time, Thread 2 acquires lock2 first and then tries to acquire lock1. This creates a circular dependency:

• Thread 1: Holds lock1 and waits for lock2.
• Thread 2: Holds lock2 and waits for lock1.

Since both threads are waiting for each other to release the resources they need, neither can proceed, resulting in a deadlock.

This example helps illustrate how easy it is for deadlocks to occur when multiple threads are acquiring multiple locks in inconsistent orders.

One effective way to prevent deadlocks is to always acquire locks in a consistent order. By ensuring that all threads acquire locks in the same order, you can eliminate the circular waiting condition that leads to deadlocks.

Here’s how we can modify the previous example to avoid deadlock:

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1public void methodTwo() {
2    synchronized (lock1) { // Changed lock order to be consistent with methodOne
3        System.out.println("Thread 2 acquired lock1");
4        try { Thread.sleep(50); } catch (InterruptedException e) {}
5        synchronized (lock2) {
6            System.out.println("Thread 2 acquired lock2");
7        }
8    }
9}

In this version:

• Both methodOne and methodTwo acquire lock1 first, then lock2.
• By using a consistent lock order, we eliminate the possibility of circular waiting.

This change prevents a deadlock from occurring because the threads are now acquiring locks in a predetermined order, avoiding the conflicting dependencies.

Java also provides a more advanced locking mechanism with the ReentrantLock class, part of the java.util.concurrent.locks package. Unlike the synchronized keyword, ReentrantLock offers greater flexibility, such as the ability to try acquiring a lock without blocking forever or to interrupt a thread waiting for a lock.

To illustrate the flexibility and control provided by ReentrantLock, here’s an example of managing locks explicitly in a multi-threaded context:

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1import java.util.concurrent.locks.Lock;
2import java.util.concurrent.locks.ReentrantLock;
3
4public class LockExample {
5    private final Lock lock1 = new ReentrantLock();
6    private final Lock lock2 = new ReentrantLock();
7
8    public void methodOne() {
9        lock1.lock();
10        try {
11            System.out.println("Thread 1 acquired lock1");
12            try { Thread.sleep(50); } catch (InterruptedException e) {}
13
14            lock2.lock();
15            try {
16                System.out.println("Thread 1 acquired lock2");
17            } finally {
18                lock2.unlock();
19            }
20        } finally {
21            lock1.unlock();
22        }
23    }
24
25    public void methodTwo() {
26        lock1.lock(); // Consistent lock order
27        try {
28            System.out.println("Thread 2 acquired lock1");
29            try { Thread.sleep(50); } catch (InterruptedException e) {}
30
31            lock2.lock();
32            try {
33                System.out.println("Thread 2 acquired lock2");
34            } finally {
35                lock2.unlock();
36            }
37        } finally {
38            lock1.unlock();
39        }
40    }
41}

In this example, we use ReentrantLock instead of synchronized to explicitly control the locking process.

• Acquiring the Lock: The lock() method acquires the lock for the current thread, preventing others from entering the critical section.
• Releasing the Lock: The unlock() method releases the lock, allowing other threads to acquire it.
• Consistent Lock Order: We still acquire lock1 before lock2, ensuring that deadlocks do not occur.

One additional benefit of ReentrantLock is its tryLock() method. This method attempts to acquire the lock without blocking the thread indefinitely. If the lock is not available, the thread can perform other tasks rather than waiting:

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1if (lock1.tryLock()) {
2    try {
3        // Critical section
4    } finally {
5        lock1.unlock();
6    }
7} else {
8    System.out.println("Could not acquire lock1");
9}

The tryLock() method is useful in situations where a thread should avoid waiting indefinitely for a lock. If the lock is available, the thread acquires it and proceeds; otherwise, it skips the critical section or takes an alternate action.

This added flexibility with ReentrantLock allows for more control over concurrency, including the option to try acquiring a lock without forcing a thread to wait indefinitely, ensuring smoother and more responsive thread execution.

Deadlocks are dangerous because they can cause a program to halt without throwing an error, making them challenging to debug and fix. Proper use of locks and advanced mechanisms like ReentrantLock can help prevent these issues.

Key points to remember:

• Avoid Deadlocks: Use consistent lock ordering and advanced locking techniques to prevent deadlocks.
• Better Control with Locks: Using ReentrantLock provides more control, including interruptibility and the ability to try acquiring locks.
• Efficient Resource Management: Proper use of locks ensures that threads do not get stuck indefinitely, allowing for better resource utilization.

Incorporating these techniques will help ensure your applications are resilient, efficient, and can manage shared resources safely in a multi-threaded environment.

Now that you've learned about deadlocks, their causes, and how to prevent them, it's time to put this knowledge into practice. Experiment with the examples provided, observe deadlock behaviors, and apply different prevention techniques. Let's move forward to solidify your understanding through hands-on exercises!