In Java SE 17, synchronization pertains to the mechanism that controls access to multiple threads to shared resources or critical sections to prevent data inconsistency and ensure thread safety. Java provides built-in support for thread synchronization through synchronized blocks and methods, as well as through higher-level constructs from the
`java.util.concurrent` package.
Synchronized Methods
A synchronized method ensures that only one thread can execute any synchronized method on the same object instance at a time. When a thread enters a synchronized method, it acquires the intrinsic lock (or monitor lock) of the object on which the method is invoked. If another thread attempts to invoke a synchronized method on the same object, it will be blocked until the first thread exits the synchronized method and releases the lock.
Synchronized Blocks
Synchronized blocks in Java allow more fine-grained control over the synchronization. Instead of locking the entire method, a synchronized block can specify a particular object that the thread must lock on. This approach provides more flexibility and can reduce contention by not blocking access to other synchronized methods or blocks that do not require the same lock.
synchronized (object) {
// Critical section code here
}
Locks and Condition Variables
Beyond the basic `synchronized` keyword, Java SE 17 offers a rich set of concurrency utilities in the `java.util.concurrent` package, including explicit lock objects (`Lock`), which provide more extensive lock operations than intrinsic locks. Unlike synchronized methods and blocks that are implicitly managed by the JVM, explicit locks offer advanced features like try-lock, timed lock acquisition, and interruptible lock waiting. `ReentrantLock` is a commonly used implementation of `Lock` that allows the lock to be acquired multiple times by the same thread without causing a deadlock.
Condition variables (`Condition`), used in conjunction with lock objects, allow threads to wait for specific conditions to become true within a locked region, providing a means for more complex thread communication and coordination.
- Volatile Keyword: The `volatile` keyword in Java is a lighter synchronization mechanism compared to `synchronized` blocks and methods. A `volatile` variable guarantees visibility of changes to variables across threads. When a variable is declared `volatile`, the JVM ensures that any write to that variable is immediately visible to other threads, and that reads from that variable always fetch the latest written value. However, `volatile` does not provide atomicity; it's suitable for cases where only visibility concerns exist, not compound actions (like incrementing a value).
- Atomic Variables: For scenarios requiring atomic operations on single variables without using `synchronized`, Java SE 17 offers atomic classes like `AtomicInteger`, `AtomicLong`, `AtomicReference`, etc., in the `java.util.concurrent.atomic` package. These classes provide methods for performing atomic operations, such as compare-and-set, increment, and get-and-update, ensuring thread safety without the overhead of synchronization.
- Best Practices: Effective synchronization in Java requires careful consideration to balance between ensuring data integrity and minimizing thread contention, which can lead to performance bottlenecks. Overuse of synchronization can lead to thread contention and reduced parallelism, while insufficient synchronization can cause data inconsistency and subtle bugs. Developers are advised to aim for minimal locking granularity, prefer `java.util.concurrent` utilities for better scalability, and carefully assess the needs for synchronization in the context of their specific multithreading scenarios.