Java ThreadLocal Explained

In Java, ThreadLocal is a thread-local variable where each thread has its own instance that can independently access its value. Each ThreadLocal instance can store a value specific to the thread that other threads cannot directly access.

The mechanism of ThreadLocal is implemented through ThreadLocalMap. Each thread maintains an instance of ThreadLocalMap, which stores the value of thread-local variables using the ThreadLocal object as the key.

When a thread accesses the get() method of ThreadLocal, it first retrieves the ThreadLocalMap instance of the current thread, and then uses the ThreadLocal object as a key to retrieve the corresponding value.

When a thread accesses the set() method of ThreadLocal, it first acquires the current thread’s ThreadLocalMap instance, then stores the ThreadLocal object as the key and the value of the thread-local variable as the value.

When a thread ends, its ThreadLocalMap instance is also recycled to prevent memory leaks.

In summary, ThreadLocal works by providing each thread with its own separate storage space, allowing each thread to independently access and modify its own thread-local variables without interference.