If you’re working with Java or other object-oriented programming languages that support concurrent programming, understanding synchronization is crucial. A common question is: "If I synchronized two methods on the same class, can they run simultaneously?" Let's delve into this topic and explore the technical ramifications.

Understanding Synchronization

What is Synchronization?

Synchronization is a building block in concurrent programming used to control the access of multiple threads to a shared resource. In Java, synchronization can be achieved using the synchronized keyword, ensuring that only one thread can access a synchronized method or block at any given time.

How Does synchronized Work?

In Java, when a method is declared as synchronized, it is essentially locked on the object instance level (for non-static methods) or the class level (for static methods). This means:

• Instance Methods: For instance methods, a synchronized declaration locks the method to a particular object instance. Therefore, different instances of the class can have their synchronized methods accessed concurrently by different threads.
• Static Methods: In contrast, static synchronized methods synchronize on the class object itself, ensuring only one thread can execute such a method across all instances of the class.

Example Scenario

Let's dive into a scenario to demonstrate how synchronization affects method execution.

1public class SyncExample {
2
3    public synchronized void methodA() {
4        // some code
5        System.out.println("Method A executing");
6        try {
7            Thread.sleep(1000);
8        } catch (InterruptedException e) {
9            e.printStackTrace();
10        }
11    }
12
13    public synchronized void methodB() {
14        // some code
15        System.out.println("Method B executing");
16        try {
17            Thread.sleep(1000);
18        } catch (InterruptedException e) {
19            e.printStackTrace();
20        }
21    }
22
23    public static void main(String[] args) {
24        final SyncExample example = new SyncExample();
25
26        Thread thread1 = new Thread(() -> example.methodA());
27        Thread thread2 = new Thread(() -> example.methodB());
28
29        thread1.start();
30        thread2.start();
31    }
32}

In the above example, both methodA and methodB are synchronized on the same instance of SyncExample. As a result, even though there are two separate threads attempting to execute both methods simultaneously, one thread must wait for the other to finish. This is because both methods are synchronized on the same monitor — the instance of the class.

Can Synchronized Methods Run Simultaneously?

Synchronized on Same Object

When two methods are synchronized on the same object instance, they cannot run simultaneously. One thread must wait for the other to finish executing the synchronized method before acquiring the lock on the object.

Synchronized on Different Objects

However, if two synchronized methods are invoked on different object instances, they can run simultaneously. This is because each instance has its own lock.

Synchronized Static Methods

Static synchronized methods are locked on the class object. Hence, only one thread can execute any of the static synchronized methods at a time, even if they’re called on different instances.

Fine-grained Synchronization

For greater concurrency, you can use a block within a method to provide fine-grained synchronization using different locks when it makes sense.

1public void nonBlockingMethod() {
2    synchronized (this) {
3        // critical section
4        System.out.println("Synchronized block executing");
5    }
6}

Summary Table

Additional Details and Best Practices

• Avoid Over-synchronizing: Using excessive synchronization can lead to decreased performance and potential deadlock scenarios.
• Volatile Variables: Consider using volatile for variables used by multiple threads but avoid depending solely on it for synchronization.
• Concurrency Utilities: Java’s java.util.concurrent package provides utility classes such as ReentrantLock, Semaphore, and CountDownLatch that can sometimes offer more nuanced control over concurrent flows than synchronized blocks.

Understanding the nuances of synchronization in Java or any programming language with similar paradigms is fundamental for building efficient and safe multi-threaded applications. By carefully managing how threads access shared resources, developers can ensure that their applications perform optimally under concurrent loads.