Java.lang.InheritableThreadLocal Class with Examples

The java.lang.InheritableThreadLocal class extends ThreadLocal to provide inheritance of values from parent thread to child thread: when a child thread is created, the child receives initial values for all inheritable thread-local variables for which the parent has values.

Parent thread, ThreadLocal variable by default is not available to child thread.
Constructor :

InheritableThreadLocal gfg_tl = new InheritableThreadLocal();

It is the child class of ThreadLocal and hence all methods present in ThreadLocal by default available to InheritableThreadLocal.
It contains only one method :
Syntax :

public Object childValue(Object parentValue) 

This method is called(overridden) within the parent thread before the child thread is started.

• If we want to make parent thread, thread local variable value available to the child thread, then we should go for InheritableThreadLocal class.
• By default, child thread value is exactly the same as parent thread value. But we can provide our own customized value for child thread by overriding childValue method.

Example:

// Java program to illustrate parent thread, ThreadLocal variable
// by default not available to child thread

class ParentThread extends Thread {
    public static ThreadLocal gfg_tl = new ThreadLocal();
    public void run()
    {

        // setting the new value
        gfg_tl.set("parent data");

        // returns the ThreadLocal value associated with current thread
        System.out.println("Parent  Thread Value :" + gfg_tl.get());

        ChildThread gfg_ct = new ChildThread();
        gfg_ct.start();
    }
}

class ChildThread extends Thread {
    public void run()
    {

        // returns  the ThreadLocal value associated with current thread
        System.out.println("Child Thread Value :" + ParentThread.gfg_tl.get());
        /* null (parent thread variable
        thread local value is not available to child thread ) */
    }
}
class ThreadLocalDemo {
    public static void main(String[] args)
    {
        ParentThread gfg_pt = new ParentThread();
        gfg_pt.start();
    }
}

Output:
Parent Thread Value:parent data
Child Thread Value:null (by default initialValue is null)

// Java program to illustrate inheritance of customized value
// from parent thread to child thread

class ParentThread extends Thread {
    // anonymous inner class  for overriding childValue method.
    public static InheritableThreadLocal gfg_tl = new InheritableThreadLocal() {
        public Object childValue(Object parentValue)
        {
            return "child data";
        }
    };
    public void run()
    {
        // setting the new value
        gfg_tl.set("parent data");
        // parent data
        System.out.println("Parent Thread Value :" + gfg_tl.get());

        ChildThread gfg_ct = new ChildThread();
        gfg_ct.start();
    }
}
class ChildThread extends Thread {

    public void run()
    {
        // child data
        System.out.println("Child Thread Value :" + ParentThread.gfg_tl.get());
    }
}
class ThreadLocalDemo {

    public static void main(String[] args)
    {
        ParentThread gfg_pt = new ParentThread();
        gfg_pt.start();
    }
}

Output:
Parent Thread Value:parent data 
Child Thread Value:child data

1st Scenario : In the above program if we replace InheritableThreadLocal with ThreadLocal and we are not overriding childValue method then the output is :

Output:
Parent Thread Value: parent data 
Child Thread Value:null   (by default initialValue is null)

2nd Scenario : In the above program if we are maintaining InheritableThreadLocal and we are not overriding childValue method, then the output is :

Output :
Parent Thread Value:parent data 
Child Thread Value:parent data

3rd Scenario : In the above program if we are maintaining InheritableThreadLocal and we are also overriding childValue method, then the output is :

Output:
Parent Thread Value:parent data 
Child Thread Value:child data