Use synchronized To control granularity, use synchronized code blocks first; use of tool classes such as ReentrantLock and ConcurrentHashMap in the java.util.concurrent package; avoid deadlocks and use tryLock; volatile can ensure variable visibility but does not replace synchronization. Specifically: 1. When using synchronized, you should prioritize synchronizing code blocks rather than the entire method to lock resources that really need to be protected; 2. Use ReentrantLock to provide a more flexible lock mechanism, ReadWriteLock improves the performance of read more and write less scenes, and ConcurrentHashMap is more efficient than synchronizedMap; 3. To avoid deadlocks, you should unify the locking order, reduce lock nesting, and set timeout with tryLock; 4. volatile is used to ensure the visibility of variables, and is suitable for state flag variables, but it is not atomic, and needs to be used with locks. Reasonable choice of synchronization mechanism can improve concurrency performance and ensure thread safety.

Java's synchronization mechanism is a key means to ensure data consistency and thread safety in multi-threaded programming. If you don't pay attention to how you use it, it is easy to have deadlocks, race conditions or performance bottlenecks. Below are some practical suggestions and practices in actual development.

Control granularity when using synchronized keywords

synchronized is the most basic synchronization mechanism in Java. It is simple to use but easy to abuse. Many people like to directly add synchronized to the method, but this may cause the lock to be too large and affect the concurrency performance.

• Prioritize synchronous code blocks over the entire method, locking only resources that really need to be protected.
• If multiple threads operate on different object instances, try to avoid using class-level locks (such as synchronized static methods), otherwise unnecessary blockage may occur.

for example:

 public void addData(int value) {
    synchronized(dataList) {
        dataList.add(value);
    }
}

This is more flexible and more efficient than writing public synchronized void addData(...) .

Prioritize the use of tool classes in the java.util.concurrent package

JDK5 introduces the java.util.concurrent package, which contains many ready-made concurrency tool classes, such as ReentrantLock , ReadWriteLock , Semaphore and various thread-safe collections.

• ReentrantLock provides a more flexible lock mechanism than synchronized , supporting attempts to acquire locks, timeouts, etc.
• ReadWriteLock can significantly improve performance in scenarios where more reads and less writes.
• Using ConcurrentHashMap instead of Collections.synchronizedMap() , it does finer-grained segment lock optimization internally.

For example:

 private final ReentrantLock lock = new ReentrantLock();

public void doSomething() {
    lock.lock();
    try {
        // Execute critical area code} finally {
        lock.unlock();
    }
}

Although the code has a few more lines, it is more flexible and is especially suitable for complex synchronization logic.

Avoid deadlocks: Pay attention to the order and nesting of locks

Deadlocks are one of the most difficult problems in multi-threaded programs, usually because multiple threads request multiple locks in different orders.

• Unified locking order: All threads apply for locks in the same order, which can greatly reduce the probability of deadlock.
• Try to avoid acquiring another lock before it is released.
• You can use tryLock() to set the timeout time. If you can't get it, release the existing resources first and try again.

For example:

 // Wrong practice can easily cause deadlock Thread1: lockA -> lockB
Thread2: lockB -> lockA

// The correct way to do it, unify the order Thread1 & Thread2: lockA -> lockB

In addition, using tools such as jstack or VisualVM can help analyze thread state and promptly detect potential deadlock problems.

Don't ignore the role of volatile

volatile does not replace the synchronization mechanism, but it is very useful in some cases. It ensures the visibility of the variable, i.e. one thread modifies the variable and the other threads can see the changes immediately.

• Applicable to status flag variables, such as controlling whether the thread continues to run:

 private volatile boolean running = true;

while (running) {
    // Do something}

public void stop() {
    running = false;
}

If volatile is not added, the modification of running by the stop() method may not take effect immediately, because the thread may always read the value from the local cache.

However, it should be noted that volatile does not guarantee atomicity, and operations like i still need to be completed with locks.

Basically that's it. The Java synchronization mechanism does not seem complicated, but it is easy to get stuck in actual use. The key is to understand the applicable scenarios of each mechanism and select appropriate tools based on the actual situation of the project.

The above is the detailed content of Best Practices for Java Synchronization Mechanisms. For more information, please follow other related articles on the PHP Chinese website!