🔒 Custom Reader-Writer Lock in Java

📘 Problem Statement

A Reader-Writer Lock allows:

• Multiple threads to read a resource simultaneously
• But only one thread to write, and writers must have exclusive access

This solution implements: ✅ Thread-safe access
✅ Prevention of writer starvation
✅ Multiple readers, one writer at a time

❗ Constraints

1. Multiple Readers: Multiple threads can read simultaneously when no writer is active
2. Exclusive Writer: Only one writer can access the resource at a time
3. Writer Priority: Writers should not starve due to continuous reader access
4. Thread Safety: Must handle concurrent access from multiple reader and writer threads
5. No Deadlocks: Solution must not deadlock under any circumstances
6. Fair Access: Both readers and writers should get reasonable access to the resource

📥 Input / Output

Input:

• Multiple reader threads requesting read access
• Multiple writer threads requesting write access
• Shared resource that needs protection

Output:

• Controlled access to the shared resource
• Multiple concurrent readers when safe
• Exclusive writer access when needed
• Proper synchronization without deadlocks

💡 Intuition About Solving This Problem

The key insight is that we need to track three pieces of state: current readers, current writers, and pending write requests. Readers can proceed when there are no active writers or pending write requests. Writers can proceed when there are no active readers or writers. The write request counter prevents writer starvation by giving priority to writers over new readers.

🚀 Steps to Take to Solve

1. Design the state variables: Track readers, writers, and write requests
2. Implement read lock acquisition: Allow multiple readers when no writers active
3. Implement write lock acquisition: Ensure exclusive access for writers
4. Handle write requests: Track pending writes to prevent reader starvation
5. Implement proper release: Decrement counters and notify waiting threads
6. Add safety checks: Ensure proper state transitions

⚠️ Edge Cases to Consider

1. Writer starvation: Handle when readers continuously acquire locks
2. Reader starvation: Handle when writers continuously acquire locks
3. Concurrent releases: Handle multiple threads releasing locks simultaneously
4. Thread interruption: Handle InterruptedException properly
5. Spurious wakeups: Use while loops to check conditions after waking up
6. State consistency: Ensure counters never go negative

💻 Code (with comments)

/**
 * A reader-writer lock that allows multiple readers to access a resource
 * simultaneously, but only one writer at a time.
 */
public class ReaderWriterLock {
    private int readers = 0;        // Current number of active readers
    private int writers = 0;        // Current number of active writers
    private int writeRequests = 0;  // Number of threads waiting to write

    public synchronized void acquireReadLock() throws InterruptedException {
        // Wait until there are no active writers or pending write requests
        while (writers > 0 || writeRequests > 0) {
            wait();
        }
        readers++; // Increment reader count
    }

    public synchronized void releaseReadLock() {
        readers--; // Decrement reader count
        if (readers == 0) {
            notifyAll(); // Notify waiting writers when no readers remain
        }
    }

    public synchronized void acquireWriteLock() throws InterruptedException {
        writeRequests++; // Increment write request count to prevent new readers
        // Wait until there are no active readers or writers
        while (readers > 0 || writers > 0) {
            wait();
        }
        writeRequests--; // Decrement write request count
        writers++; // Increment writer count
    }

    public synchronized void releaseWriteLock() {
        writers--; // Decrement writer count
        notifyAll(); // Notify all waiting threads
    }

    public synchronized int getReaderCount() {
        return readers; // Return current number of active readers
    }

    public synchronized boolean isWriteLocked() {
        return writers > 0; // Check if any writer is currently active
    }

    public synchronized int getWriteRequests() {
        return writeRequests; // Return number of pending write requests
    }
}

🔍 Dry Run Example

Scenario: 2 readers and 1 writer competing for access

Initial State:

• readers = 0, writers = 0, writeRequests = 0

Step 1: Reader 1 wants to read

• Check: writers > 0 || writeRequests > 0 → false (0 > 0 || 0 > 0)
• Increment readers to 1
• Reader 1 can proceed

Step 2: Reader 2 wants to read

• Check: writers > 0 || writeRequests > 0 → false (0 > 0 || 0 > 0)
• Increment readers to 2
• Reader 2 can proceed

Step 3: Writer wants to write

• Increment writeRequests to 1
• Check: readers > 0 || writers > 0 → true (2 > 0 || 0 > 0)
• Writer waits

Step 4: Reader 1 finishes reading

• Decrement readers to 1
• readers != 0, so no notification
• Writer still waits

Step 5: Reader 2 finishes reading

• Decrement readers to 0
• readers == 0, so notifyAll()
• Writer wakes up and checks: readers > 0 || writers > 0 → false (0 > 0 || 0 > 0)
• Decrement writeRequests to 0
• Increment writers to 1
• Writer can proceed

Result: Multiple readers can read simultaneously, writer gets exclusive access when no readers remain.

⏱️ Time Complexity / Space Complexity

Time Complexity:

• Read lock acquisition: O(1) - constant time to check conditions and update state
• Write lock acquisition: O(1) - constant time to check conditions and update state
• Lock release: O(1) - constant time to update state and notify threads
• Waiting time: O(1) - constant time to block/unblock threads

Space Complexity:

• State variables: O(1) - constant space for counters
• Thread management: O(1) - constant space regardless of number of threads
• Synchronization overhead: O(1) - constant space for intrinsic locks

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🚀 Usage Example

ReaderWriterLock lock = new ReaderWriterLock();

// Reader thread
new Thread(() -> {
    try {
        lock.acquireReadLock();
        System.out.println("🔍 Reading data...");
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    } finally {
        lock.releaseReadLock();
    }
}).start();

// Writer thread
new Thread(() -> {
    try {
        lock.acquireWriteLock();
        System.out.println("✍️ Writing data...");
        Thread.sleep(1000);
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
    } finally {
        lock.releaseWriteLock();
    }
}).start();

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🌟 Key Features

Feature	Description
👥 Multiple Concurrent Readers	Readers can access simultaneously if no writer is active
🧍‍♂️ Exclusive Writer Access	Only one writer allowed, no readers during writing
⏳ Writer Priority	Prevents writer starvation by delaying new readers
🔐 Thread Safe	All methods are synchronized
🛡️ Deadlock Prevention	Proper wait/notify management avoids deadlocks

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✅ Summary

This ReaderWriterLock class:

• Balances concurrency and data integrity
• Enables fine-grained access control
• Prioritizes fairness and avoids starvation