In Java, thread synchronization is the process of coordinating access to shared resources or data between multiple threads in order to prevent race conditions, where two or more threads access the shared resource simultaneously and interfere with each other's operations. Synchronization in Java is typically achieved using two mechanisms: synchronized methods and synchronized blocks.

1. Synchronized Methods: In Java, a synchronized method is a method that is locked on the object's monitor, meaning that only one thread can execute the method at a time. Synchronized methods can be used to ensure that only one thread at a time can access shared data or resources. Here's an example:

public synchronized void incrementCounter() {
    counter++;
}

In this example, the incrementCounter method is synchronized, which means that only one thread can execute it at a time. This ensures that the counter variable is incremented atomically, without any race conditions.

2. Synchronized Blocks: In Java, a synchronized block is a block of code that is synchronized on a specific object's monitor. Synchronized blocks can be used to lock on different objects to control access to shared resources. Here's an example:

public void someMethod() {
    synchronized(sharedObject) {
        // synchronized block of code
    }
}

In this example, the synchronized keyword is used to lock on the sharedObject monitor, which means that only one thread at a time can execute the block of code inside the synchronized block. This ensures that shared resources accessed inside the block are accessed in a thread-safe manner.

In addition to synchronized methods and blocks, Java also provides other synchronization mechanisms such as ReentrantLock, Semaphore, and Condition, which can be used to achieve more fine-grained control over thread synchronization. It's important to note that thread synchronization in Java can lead to performance issues if not used carefully, as it can introduce contention and reduce concurrency.