Java synchronized lock object

Kathy 184 Published: 11/25/2024

Java synchronized lock object

I'd be happy to explain the concept of a synchronized lock object in Java.

In Java, a synchronized method or block is used to ensure that only one thread can execute it at any given time. This is achieved by acquiring and releasing a mutex (mutual exclusion) lock on an object. The lock object is typically an instance variable, but it can also be a class-level variable.

When a thread attempts to enter a synchronized method or block, the following happens:

Acquire the lock: If the lock is not already held by another thread, the current thread acquires the lock. This is done using an atomic operation. Wait if locked: If another thread already holds the lock, the current thread waits until the lock is released. Execute the code: Once the lock is acquired, the thread can execute the synchronized method or block. Release the lock: When the thread finishes executing the synchronized method or block, it releases the lock, allowing other threads to acquire it.

The synchronized lock object serves as a guard, ensuring that only one thread can enter the protected code at a time. This is useful for implementing critical sections of code that must be executed in a serialized manner.

Here are some important aspects to consider when using synchronized locks:

Synchronization granularity: The granularity of synchronization refers to the amount of code or data protected by the lock. Fine-grained locking protects small sections of code, while coarse-grained locking protects larger areas. Lock contention: When multiple threads contend for the same lock, it can lead to increased latency and decreased throughput. Lock contention is mitigated through techniques like lock stripping, which removes unnecessary locks. Deadlocks and livelocks: When two or more threads are blocked indefinitely, waiting for each other to release a resource, it's called a deadlock. Livelocks occur when threads continually contend for the same lock, resulting in no progress. Starvation: Starvation occurs when a thread is unable to acquire a lock due to excessive contention or prolonged holding of the lock by another thread.

To illustrate the concept of synchronized locks, consider an example where multiple threads update shared data:

public class SynchronizedExample {
private int sharedData = 0;
private final Object LOCK = new Object();
public void updateSharedData(int value) {
synchronized (LOCK) { // Acquire the lock
sharedData += value; // Update shared data
}
}
public static void main(String[] args) {
SynchronizedExample example = new SynchronizedExample();
Thread t1 = new Thread(() -> example.updateSharedData(5));
Thread t2 = new Thread(() -> example.updateSharedData(10));
t1.start();
t2.start();
// Only one thread can execute updateSharedData at a time
}
}

In this example, the updateSharedData method is marked as synchronized, ensuring that only one thread can update shared data at any given moment. The LOCK object serves as the synchronization point, allowing threads to access and modify shared data in a thread-safe manner.

In conclusion, synchronized locks are a fundamental concept in Java concurrency. They enable developers to ensure thread-safety by serializing access to critical sections of code. Understanding the intricacies of lock contention, deadlocks, livelocks, and starvation is crucial when designing concurrent systems that rely on synchronization mechanisms like synchronized locks.

How do you get a lock on an object in Java?

I'm happy to help! However, I'll respond in English as per your request.

In Java, locking is used to control access to shared resources or synchronize the execution of code. A lock can be obtained using synchronization primitives such as synchronized methods or blocks, ReentrantLock, or java.util.concurrent.locks.Lock.

Here's how you can get a lock on an object in Java:

Using synchronized keyword: You can use the synchronized keyword to make a method or block of code thread-safe. This keyword ensures that only one thread can execute this section of code at a time. The lock is associated with the object on which the method is called.

Example:

public class BankAccount {
private int balance;
public synchronized void withdraw(int amount) {
if (balance >= amount) {
balance -= amount;
} else {
throw new InsufficientFundsException();
}
}
}

Using ReentrantLock class: The ReentrantLock class is a more fine-grained way to control access to shared resources. You can use the tryLock() or lock() method to acquire the lock.

Example:

public class BankAccount {
private final ReentrantLock lock = new ReentrantLock();
private int balance;
public void withdraw(int amount) {
lock.lock();
try {
if (balance >= amount) {
balance -= amount;
} else {
throw new InsufficientFundsException();
}
} finally {
lock.unlock();
}
}
}

Using java.util.concurrent.locks.Lock interface: The Lock interface provides a more flexible way to control access to shared resources. You can use the lock() or tryLock() method to acquire the lock.

Example:

public class BankAccount {
private final Lock lock = new ReentrantLock();
private int balance;
public void withdraw(int amount) {
lock.lock();
try {
if (balance >= amount) {
balance -= amount;
} else {
throw new InsufficientFundsException();
}
} finally {
lock.unlock();
}
}
}

In all the above cases, when a thread acquires the lock on an object, it ensures that no other thread can execute the synchronized code until the first thread releases the lock. This helps to prevent data inconsistency and ensures that shared resources are used in a consistent manner.

Remember, Java's built-in synchronization primitives provide a convenient way to control access to shared resources, but they might not be suitable for all scenarios. In certain situations, you may need more fine-grained control over concurrency, which is where the java.util.concurrent.locks package comes into play.