Java StampedLock implementation principles and best practice records

Java StampedLock: Implementation Principles and Best Practices

1. Introduction

StampedLock is a new lock mechanism introduced by Java 8. Due to its excellent performance, it is hailed as the "lock king" in the industry. This article will explore in-depth the working principle, how StampedLock works, and its best practices in practical applications.

2. StampedLock Overview

2.1 What is StampedLock?

StampedLock is a multi-mode synchronous control component that supports three modes: write lock, pessimistic reading lock and optimistic reading. Unlike the traditional ReadWriteLock, it uses the concept of "stamp" to identify the state of the lock and provides an optimistic reading mechanism, which can greatly improve system performance in specific scenarios.

2.2 Core features

• Supports three modes: write lock, pessimistic reading lock, optimistic reading
• Status control based on "stamp"
• Reentry is not supported
• Condition conditions are not supported
• Supports upgrade and downgrade of read and write locks

3. Detailed explanation of the three modes of StampedLock

3.1 Write Lock

A write lock is an exclusive lock. When one thread acquires a write lock, other threads cannot acquire any type of lock.

StampedLock lock = new StampedLock();
long stamp = (); // Get the write locktry {
    // Write to shared variables} finally {
    (stamp); // Release the write lock}

3.2 Pessimistic Read Lock

Pessimistic read locks are similar to read locks in ReadWriteLock, allowing multiple threads to acquire read locks at the same time, but are mutually exclusive to write locks.

long stamp = (); // Get the pessimistic reading locktry {
    // Read shared variables} finally {
    (stamp); // Release the read lock}

3.3 Optimistic Read

Optimistic reading is the most distinctive mode of StampedLock. It is not a real lock, but a lock-free mechanism based on version number.

long stamp = (); // Get optimistic reading stamp// Read shared variablesif (!(stamp)) { // Verify that the stamp is valid    // Upgrade to pessimistic reading lock    stamp = ();
    try {
        //Reread the shared variable    } finally {
        (stamp);
    }
}

4. Performance Advantages

4.1 Comparison with ReadWriteLock

• Read more and write less scenarios: performance improvement is about 10 times
• Read and write balance scenario: performance improvement is about 1 times
• Write more and read less scenarios: the performance is quite good

4.2 Reasons for performance advantages

• Optimistic reading mechanism avoids unnecessary locking operations
• The underlying implementation uses more CPU instruction-level optimizations
• Using lock-free algorithm, reducing thread context switching
• The spin mechanism is implemented internally to improve concurrency efficiency

5. Practical examples

5.1 Basic usage examples

public class Point {
    private double x, y;
    private final StampedLock sl = new StampedLock();
    // Write method    void move(double deltaX, double deltaY) {
        long stamp = ();
        try {
            x += deltaX;
            y += deltaY;
        } finally {
            (stamp);
        }
    }
    // Optimistic reading method    double distanceFromOrigin() {
        long stamp = ();
        double currentX = x, currentY = y;
        if (!(stamp)) {
            stamp = ();
            try {
                currentX = x;
                currentY = y;
            } finally {
                (stamp);
            }
        }
        return (currentX * currentX + currentY * currentY);
    }
}

5.2 Lock Upgrade Example

public class DataContainer {
    private final StampedLock lock = new StampedLock();
    private double data;
    public void transformData() {
        long stamp = ();
        double currentData = data;
        // Check whether updates are needed        if (needsUpdate(currentData)) {
            // Upgrade to write lock            long writeStamp = (stamp);
            if (writeStamp != 0L) {
                try {
                    data = computeNewValue(currentData);
                } finally {
                    (writeStamp);
                }
            } else {
                // Upgrade failed, fall back to normal write lock acquisition                stamp = ();
                try {
                    data = computeNewValue(data);
                } finally {
                    (stamp);
                }
            }
        }
    }
}

6. Precautions for use

6.1 Reentry is not supported

StampedLock does not support the reentry feature, and the same thread acquires locks multiple times will cause deadlocks.

6.2 Interrupt processing

When using pessimistic read locks and write locks, you need to pay attention to handling interrupts:

try {
    long stamp = ();
    try {
        // Process data    } finally {
        (stamp);
    }
} catch (InterruptedException e) {
    // Processing interrupt}

6.3 Suggestions for using optimistic reading

• Suitable for scenarios where more reads, less writes
• Few shared variables read
• The execution time of the read operation is short
• Version verification and compensation measures after failure are required

7. Summary

StampedLock can provide significant performance improvements in specific scenarios through innovative optimistic reading mechanisms and careful underlying optimization. But it is not omnipotent. When using it, you need to weigh the pros and cons according to the specific scenario, paying special attention to its non-reentrant characteristics and interrupt handling requirements. The rational use of StampedLock can greatly improve the concurrency performance of the system in appropriate scenarios.

References

• Java API Documentation
• Doug Lea's StampedLock paper
• Java Concurrency in Practice

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