Difficulty: Medium
Correct Answer: To prevent race conditions and inconsistent state by ensuring that only one thread at a time executes critical sections that access shared mutable data
Explanation:
Introduction / Context:
When multiple threads run in the same Java application, they often share data structures such as lists, counters, or maps. If threads read and write this shared state without coordination, subtle and hard-to-debug errors can occur, including race conditions, lost updates, and inconsistent views of data. Synchronization is a mechanism provided by Java to control access to shared resources. This question explores why synchronization is needed in a multithreaded environment.
Given Data / Assumptions:
Concept / Approach:
Synchronization in Java serves two main purposes: mutual exclusion and memory visibility. Mutual exclusion ensures that only one thread at a time executes a critical section of code that manipulates shared state. This prevents race conditions where multiple threads update the same variable concurrently, leading to unpredictable results. Memory visibility ensures that changes made by one thread become visible to others in a predictable way. Together, these properties keep shared data consistent and help maintain application correctness even when many threads are active.
Step-by-Step Solution:
Step 1: Consider a shared counter variable incremented by multiple threads without synchronization; each increment involves a read, modify, and write sequence.Step 2: Recognise that without mutual exclusion, two threads can interleave operations so that one increment overwrites the other, causing lost updates and incorrect results.Step 3: Understand that synchronized blocks or methods wrap critical sections, so only one thread at a time can execute them based on a shared lock.Step 4: Note that entering and exiting a synchronized block also establishes happens-before relationships, flushing and reading caches so that threads see up-to-date values.Step 5: Conclude that synchronization is needed to prevent race conditions and inconsistent state in shared data, which is exactly what option A describes.
Verification / Alternative check:
Empirical tests with unsynchronized counters or collections show incorrect behaviour under load, such as incorrect totals or corrupted structures. Adding synchronized around critical updates fixes these issues but may reduce throughput, demonstrating the trade-off between safety and performance. Java concurrency documentation explains that synchronization and volatile are key tools for ensuring correct visibility and atomicity of shared data, confirming the reasoning behind option A.
Why Other Options Are Wrong:
Option B wrongly claims that synchronization makes threads run faster by skipping context switches; in practice, synchronization adds overhead and may reduce performance, although it improves correctness. Option C suggests that synchronization allows more concurrent modification, but the opposite is true: it restricts concurrent execution in critical sections to avoid conflicts. Option D states that synchronization prevents exceptions entirely, which is not accurate; exceptions can still occur for many reasons unrelated to concurrency. Only option A correctly captures the purpose of synchronization.
Common Pitfalls:
A common pitfall is overusing coarse-grained synchronization, locking entire objects or methods unnecessarily and causing contention and bottlenecks. Another mistake is assuming that reading and writing primitive types like int are always safe without synchronization; even if operations appear atomic, visibility issues can still arise. Using modern libraries such as java.util.concurrent, choosing appropriate lock granularity, and understanding when synchronization is required are essential skills for building robust multithreaded Java applications.
Final Answer:
Correct answer: To prevent race conditions and inconsistent state by ensuring that only one thread at a time executes critical sections that access shared mutable data
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