When and how should I use a ThreadLocal variable?

Question

When should I use a ThreadLocal variable?

How is it used?

Answer 1

Caching, sometime you have to calculate the same value lots of time so by storing the last set of inputs to a method and the result you can speed the code up. By using Thread Local Storage you avoid having to think about locking.

Answer 2

There is very good example in book Java Concurrency in Practice. Where author (Joshua Bloch) explains how Thread confinement is one of the simplest ways to achieve thread safety and ThreadLocal is more formal means of maintaining thread confinement. In the end he also explain how people can abuse it by using it as global variables.

I have copied the text from the mentioned book but code 3.10 is missing as it is not much important to understand where ThreadLocal should be use.

Thread-local variables are often used to prevent sharing in designs based on mutable Singletons or global variables. For example, a single-threaded application might maintain a global database connection that is initialized at startup to avoid having to pass a Connection to every method. Since JDBC connections may not be thread-safe, a multithreaded application that uses a global connection without additional coordination is not thread-safe either. By using a ThreadLocal to store the JDBC connection, as in ConnectionHolder in Listing 3.10, each thread will have its own connection.

ThreadLocal is widely used in implementing application frameworks. For example, J2EE containers associate a transaction context with an executing thread for the duration of an EJB call. This is easily implemented using a static Thread-Local holding the transaction context: when framework code needs to determine what transaction is currently running, it fetches the transaction context from this ThreadLocal. This is convenient in that it reduces the need to pass execution context information into every method, but couples any code that uses this mechanism to the framework.

It is easy to abuse ThreadLocal by treating its thread confinement property as a license to use global variables or as a means of creating “hidden” method arguments. Like global variables, thread-local variables can detract from reusability and introduce hidden couplings among classes, and should therefore be used with care.

Answer 3

[For Reference]ThreadLocal cannot solve update problems of shared object. It is recommended to use a staticThreadLocal object which is shared by all operations in the same thread. [Mandatory]remove() method must be implemented by ThreadLocal variables, especially when using thread pools in which threads are often reused. Otherwise, it may affect subsequent business logic and cause unexpected problems such as memory leak.

Answer 4

ThreadLocal is a specially provisioned functionality by JVM to provide an isolated storage space for threads only. like the value of instance scoped variable are bound to a given instance of a class only. each object has its only values and they can not see each other value. so is the concept of ThreadLocal variables, they are local to the thread in the sense of object instances other thread except for the one which created it, can not see it. See Here

import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.IntStream;


public class ThreadId {
private static final AtomicInteger nextId = new AtomicInteger(1000);

// Thread local variable containing each thread's ID
private static final ThreadLocal<Integer> threadId = ThreadLocal.withInitial(() -> nextId.getAndIncrement());


// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
    return threadId.get();
}

public static void main(String[] args) {

    new Thread(() -> IntStream.range(1, 3).forEach(i -> {
        System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
    })).start();

    new Thread(() -> IntStream.range(1, 3).forEach(i -> {
        System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
    })).start();

    new Thread(() -> IntStream.range(1, 3).forEach(i -> {
        System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
    })).start();

}
}

Answer 5

The ThreadLocal class in Java enables you to create variables that can only be read and written by the same thread. Thus, even if two threads are executing the same code, and the code has a reference to a ThreadLocal variable, then the two threads cannot see each other's ThreadLocal variables.

Read more

Answer 6

ThreadLocal is useful, when you want to have some state that should not be shared amongst different threads, but it should be accessible from each thread during its whole lifetime.

As an example, imagine a web application, where each request is served by a different thread. Imagine that for each request you need a piece of data multiple times, which is quite expensive to compute. However, that data might have changed for each incoming request, which means that you can't use a plain cache. A simple, quick solution to this problem would be to have a ThreadLocal variable holding access to this data, so that you have to calculate it only once for each request. Of course, this problem can also be solved without the use of ThreadLocal, but I devised it for illustration purposes.

That said, have in mind that ThreadLocals are essentially a form of global state. As a result, it has many other implications and should be used only after considering all the other possible solutions.