1. Overview
In this tutorial, we’ll explore various methods to implement a mutex in Java, essential for managing shared resources in multithreaded environments.
2. Mutex
In the realm of multithreaded applications, the concurrent access of shared resources by two or more threads can lead to unpredictable outcomes, a phenomenon known as a race condition. These shared resources encompass vital elements such as data structures, input-output devices, files, and network connections.
A critical section of code refers to the segment of a program where shared resources are accessed. To prevent race conditions and maintain data integrity, synchronization of access to this critical section is paramount.
A mutex, or mutual exclusion, emerges as the fundamental synchronizer in this context. Acting as a lock, a mutex ensures that only one thread can execute the critical section at any given time. The process involves a thread acquiring the mutex before accessing the critical section and releasing it afterward. During this time, other threads attempting to enter the critical section are blocked, thus ensuring exclusive access and averting potential data corruption.
3. Synchronized Keyword
The synchronized keyword in Java is a built-in mechanism for synchronizing access to shared resources in multi threaded environments.
Explanation:
Counterclass: This class represents a simple counter with an integer valuecount.increment()method: This method increments thecountvariable in a synchronized manner. Thesynchronizedkeyword ensures that only one thread can execute this method at a time. When a thread enters this method, it locks the object (counterin this case), preventing other threads from accessing this method until the lock is released.SynchronizedExampleclass: This class contains themain()method where we create instances of theCounterclass and multiple threads to access the counter concurrently.Threadobjects: We create twoThreadobjects,thread1andthread2, each with a lambda expression defining their behavior. Each thread calls theincrement()method of theCounterobject five times.- Starting threads: We start both threads using the
start()method.
When you run this program, you’ll see that the output demonstrates the synchronized behavior, ensuring that the count is incremented correctly without any race conditions. The synchronized keyword ensures mutual exclusion, preventing multiple threads from accessing the critical section simultaneously, thus maintaining data consistency.
4. ReentrantLock
ReentrantLock is a class that provides a more flexible alternative to intrinsic locks (synchronized blocks) in Java.
Explanation:
Counterclass: This class represents a simple counter with an integer valuecountand aReentrantLocknamedlock.increment()method: This method increments thecountvariable using theReentrantLock. We calllock.lock()to acquire the lock before accessing the critical section. We use a try-finally block to ensure that the lock is released even if an exception occurs during the execution of the critical section. After incrementing the count, we calllock.unlock()to release the lock.ReentrantLockExampleclass: This class contains themain()method where we create an instance of theCounterclass and multiple threads to access the counter concurrently.Threadobjects: We create twoThreadobjects,thread1andthread2, each with a lambda expression defining their behavior. Each thread calls theincrement()method of theCounterobject five times.- Starting threads: We start both threads using the
start()method.
When you run this program, you’ll see that the output demonstrates the synchronized behavior achieved using ReentrantLock. The ReentrantLock provides more flexibility than synchronized keyword, such as the ability to try to acquire the lock without blocking, timed waiting, and more complex locking scenarios.
5. Semaphore
Semaphore is a synchronization primitive in Java that maintains a set of permits, which are used to control access to a shared resource.
Explanation:
Printerclass: This class represents a printer with aSemaphorenamedsemaphore.- Constructor: We initialize the semaphore with one permit, meaning only one thread can access the printer at a time.
printDocument()method: This method simulates printing a document by acquiring a permit from the semaphore before printing each copy. Theacquire()method blocks if no permit is available. After printing all copies, the permit is released using therelease()method.SemaphoreExampleclass: This class contains themain()method where we create an instance of thePrinterclass and multiple threads to access the printer concurrently.Threadobjects: We create twoThreadobjects,thread1andthread2, each with a lambda expression defining their behavior. Each thread calls theprintDocument()method of thePrinterobject to print documents concurrently.- Starting threads: We start both threads using the
start()method.
When you run this program, you’ll see that the output demonstrates how Semaphore ensures that only one thread can access the printer at a time, despite multiple threads attempting to print documents concurrently.
6. Conclusion
Each synchronization mechanism has its strengths and use cases. While synchronized keyword is suitable for simpler synchronization needs, ReentrantLock and Semaphore offer greater flexibility and control, making them preferable for more complex multi threaded applications.
By understanding the characteristics and capabilities of these synchronization mechanisms, Java developers can make informed decisions when designing and implementing multi threaded applications, ensuring thread safety and efficient resource management.
