ThreadLocal: Each thread stores its own local and local areas, similar to containers, in which each thread has a certain storage space.
Common methods get/set/initialValue
The official recommendation is private static
Each thread stores its own data, and changes do not affect other threads
The ThreadLocal subclass InheritableThreadLocal:
Data inherited from context environment

    public class my {

//Integer initial value is null
//private static ThreadLocal<Integer> threadlocal=new ThreadLocal<>();
//To change the initial value, you need to create a subclass of ThreadLocal to override the initial value or use lambda(jdk8)
private static ThreadLocal<Integer> threadlocal=new ThreadLocal <Integer>()
        {//Parent Anonymous Internal Class Rewriting Method
            protected Integer initialValue()
            {
                return 200;
            }
        };
//private static ThreadLocal<Integer> threadlocal=new ThreadLocal.withInitial(()->200);

public static void main(String[]args) throws InterruptedException
{
    //Get value
    System.out.println(Thread.currentThread().getName()+"-->"+threadlocal.get());
    //Set value
    threadlocal.set(99);
    System.out.println(Thread.currentThread().getName()+"-->"+threadlocal.get());

    new Thread(new test()).start();
    new Thread(new test()).start();
}

public static class test implements Runnable
{
    public void run()
    {
        threadlocal.set((int)(Math.random()*100));
        System.out.println(Thread.currentThread().getName()+"-->"+threadlocal.get());
    }
}

}

Each thread's own data changes do not affect other threads

public class my {

private static ThreadLocal<Integer> threadlocal=new ThreadLocal <Integer>()
        {//Parent Anonymous Internal Class Rewriting Method
            protected Integer initialValue()
            {
                return 1;
            }
        };

public static void main(String[]args) throws InterruptedException
{
    for(int i=0;i<5;i++)
    {
        new Thread(new test()).start();
    }
}

public static class test implements Runnable
{
    public void run()
    {
        //Modifications do not affect other threads
        Integer left=threadlocal.get();
        System.out.println(Thread.currentThread().getName()+"-->"+left);
        threadlocal.set(left-1);
        System.out.println(Thread.currentThread().getName()+"Still left"+threadlocal.get());
    }
}

}

Analyse the ThreadLocal operating environment:

public class my {

private static ThreadLocal<Integer> threadlocal=new ThreadLocal <Integer>()
        {//Parent Anonymous Internal Class Rewriting Method
            protected Integer initialValue()
            {
                return 1;
            }
        };

public static void main(String[]args) throws InterruptedException
{
    new Thread(new test()).start();//Two printed contents are in different threads
    //After start(), the thread is converted from main to other threads, so the main thread is in the constructor.
}

public static class test implements Runnable
{
    public test()
    {
        threadlocal.set(100);  //It belongs to the main thread, and modifications do not affect threads within the start
        System.out.println(Thread.currentThread().getName()+"-->"+threadlocal.get());
    }
    public void run()
    {
        System.out.println(Thread.currentThread().getName()+"Still left"+threadlocal.get());
    }
}

}

Subclass Inheritable ThreadLocal:

public class my {

private static ThreadLocal<Integer> threadlocal=new InheritableThreadLocal<>();

public static void main(String[]args) throws InterruptedException
{
    System.out.println(Thread.currentThread().getName()+"-->"+threadlocal.get());
    threadlocal.set(2);

    //This thread is created by the main thread and copies a data of the main thread to this thread.
    //Give 2 of the main thread to this thread
    new Thread(()->{
        System.out.println(Thread.currentThread().getName()+"-->"+threadlocal.get());
        threadlocal.set(200);
        System.out.println(Thread.currentThread().getName()+"-->"+threadlocal.get());
    }).start();
}

}