Java provides a synchronized keyword to write synchronized code, but it is difficult to correctly write synchronized code through the synchronized keyword alone. The java.util.concurrent package provides various utility classes, such as CountDownLatch, CyclicBarrier, Exchanger, Semaphore, and Phaser, which are known as synchronizers. Synchronizers are concurrency utilities that provide thread synchronization without using the wait() and notify() methods. Let's have a look at the following classes:

CountDownLatch: This allows one thread to wait for one or more threads to complete before it can start processing.

CyclicBarrier: This is very similar to CountdownLatch, but it allows multiple threads to wait for each other before they can start processing.

Semaphore: This maintains a set of permits for restricting the number of threads that can access a shared resource. Threads require a permit from Semaphore before accessing a shared resource. It provides two main methods, acquire() and release(), for getting and releasing permits, respectively.

Exchanger: This provides a synchronization point where threads can exchange objects.

Phaser: This provides a thread synchronization mechanism similar to CyclicBarrier and CountDownLatch, but it supports more flexible usage. It allows a group of threads to wait on a barrier and then proceed after the last thread arrives, and it also supports multiple phases of execution.