25 Feb 2019  Sergio Martin Rubio  3 mins read.

ThreadLocal variables are usually private static fields in classes and maintain its state inside a thread and generate unique identifiers local to each thread. This means that every time a different thread accesses the same instance, a new ThreadLocal variable is created. Therefore, information saved in a ThreadLocal can only be accessed by a single thread for a particular instance.

## Lifecycle

ThreadLocal variables are created when a thread accesses an instance, and all the copies of ThreadLocal variables are ready to be garbage collected when the thread goes away, unless there are other references to the ThreadLocal variables.

If a class is setting a ThreadLocal value in a static block, only the thread that initializes the class will be able to retrieve the value, since static blocks are only call during class initialization.

public class MyThreadLocalManager {
static {
}
public Integer getInteger() {
}
public void setInteger(Integer integer) {
}
}


Therefore, if a thread accesses the instance of the previous class, which was originally initialized by another thread, and calls getInteger(), it will return null.

MyThreadLocalManager myThreadLocalManager = new MyThreadLocalManager();
@Override
public void run() {
}
}.start();


In order to initialize our ThreadLocal with a value, we can use the new Java 8 withInitial()

static {
}


Now the output will be “10” for every thread that uses the MyThreadLocalManager instance.

## Operations

• get()
• set(value)
• remove()
• withInitial(supplier) -> since Java 8

## Use Cases

• To avoid synchronization: ThreadLocal is thread safe, since only one thread can access the data saved in it.
• To create global variables shared by a single thread.
• In web applications in order to keep a global state (context, session…) during a HTTP request. You might need some expensive data which needs to be computed multiple times for each HTTP request, and this data changes in each request, which means that we cannot use a plain cache. So ThreadLocal can be a solution to store this data, so we only have to calculate it once for each request.

## Caveats

ThreadLocal can cause memory leaks if it is not used safely. In web applications thread workers are provided by web servers like Tomcat or Jetty, and they maintain a thread pool to serve HTTP requests. Therefore, if a request creates a ThreadLocal and it is not removed before the response is returned, a copy of the object which contains the ThreadLocal will remain with a worker Thread in the web server, and since lifespan of a worker thread is longer than the web-app itself, it will prevent the object from being garbage collected. So always remember to remove ThreadLocal variables when they are not useful anymore.