StampedLock is a high-performance read-write lock mechanism introduced by Java 8, supporting three modes: 1. Write lock is an exclusive lock; 2. Optimistic read is a lightweight attempt mechanism; 3. Read lock is a shared lock. Compared with ReentrantReadWriteLock, it improves concurrency performance through the stamp version stamp mechanism, which is especially suitable for scenarios where more reads and less writes, and avoids write hunger. When using it, you should pay attention to details such as non-reentry, non-sharing of stamps, and exception handling. It is recommended to use it under high concurrency requirements.

StampedLock is a new lock mechanism introduced by Java 8, located in the java.util.concurrent.locks package. It is similar to the traditional ReentrantReadWriteLock , and also supports read and write locks, but is more efficient and flexible. Its design goal is to solve the performance bottleneck problem of read and write locks in high concurrency scenarios, especially in the case of "more reads and less writes".

What is StampedLock?

Simply put, StampedLock is a read and write lock implementation based on stamps . Unlike ordinary locks, it does not control resource access through lock() and unlock() , but instead indicates the current lock status by obtaining a "version stamp". You can understand that each lock is added, a stamp will be returned, and this stamp is required for subsequent unlocking or verification.

It mainly provides three modes:

• Write lock : Exclusive lock, other threads cannot read or write after acquisition.
• Optimistic reading : It is not a real lock, but a lightweight attempt mechanism, suitable for scenarios with frequent reading operations.
• Read lock : a shared lock that allows multiple threads to read data at the same time.

What are the advantages of StampedLock?

Compared with ReentrantReadWriteLock, the biggest highlight of StampedLock is that it supports optimistic reading , which can significantly improve performance in scenarios where read operations are far more than write operations.

For example, if you have a cache system, you read data most of the time and update it occasionally. Using ordinary read and write locks, even if they are just reading, they need to add and release the lock, which is a lot of overhead. StampedLock's optimistic reading almost does not require blocking threads. You only need to verify whether the stamp is valid after reading it.

In addition, StampedLock also avoids the "write hunger" problem (i.e. the write thread cannot acquire the lock for a long time), because the write lock has priority.

How to use StampedLock?

Here are a brief description of several common usages:

Get the write lock

 long stamp = lock.writeLock();
try {
    // Modify shared data} finally {
    lock.unlockWrite(stamp);
}

Use optimistic reading

 long stamp = lock.tryOptimisticRead();
// Read data if (!lock.validate(stamp)) {
    // If the stamp fails, it means that a write operation has occurred and you need to upgrade to read lock stamp = lock.readLock();
    try {
        // Read the data again} finally {
        lock.unlockRead(stamp);
    }
}

Get the read lock

 long stamp = lock.readLock();
try {
    // Read data} finally {
    lock.unlockRead(stamp);
}

It can be seen that each lock operation needs to be completed with stamp, which is one of the biggest differences between it and the traditional lock mechanism.

Notes on using StampedLock

Although StampedLock has good performance, some details need to be paid attention to in actual use:

• Don't switch arbitrarily between pessimistic locks and optimistic reads : frequent switching from optimistic reads to reads or writes may affect performance.
• stamp is not thread-safe : each thread should use its own stamp and cannot be shared.
• Reentry cannot be reentry : StampedLock does not support reentry, and the same thread acquires write locks multiple times will cause deadlocks.
• Be careful when handling exceptions : If an exception is thrown during locking, you must make sure that the unlock method is finally called, otherwise it will be deadlocked.

In addition, since the API is a bit complicated, it is recommended to use it only in scenarios where high-performance concurrent read and write is indeed required. For most business logic, ReentrantReadWriteLock or synchronized may be sufficient.

Basically that's it. StampedLock is a very practical but relatively easy to misuse component in Java concurrency package. Only by understanding its applicable scenarios and usage can it truly exert its value.

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