Hello, everyone , I'm Xiao Sheng ！ This part is my study ThreadLocal The notes

Thread data sharing and security - ThreadLocal

summary

ThreadLocal Is a class that will create separate copies of variables for each thread in multiple threads ; When using ThreadLocal To maintain variables , ThreadLocal A separate copy of the variable is created for each thread , Avoid data inconsistency caused by multi-threaded operation of shared variables

every last ThreadLocal object , Only one data can be associated for the current thread , If you want to associate multiple numbers for the current thread According to the , You need to use multiple ThreadLocal Object instances

Code example

public class Dog {
    }

// ThreadLocalDemo
public class ThreadLocalDemo implements Runnable{
    
    public static ThreadLocal<Object> threadLocal= new ThreadLocal<>();
    public static void main(String[] args) {
    
        //  Create a process 
        new Thread(new ThreadLocalDemo()).start();
    }

    @Override
    public void run() {
    
        Dog dog = new Dog();
        threadLocal.set(dog);
        System.out.println("ThreadLocalDemo: " + Thread.currentThread().getName());
        new Service().update();
    }
}

// Service
public class Service {
    
    public void update() {
    
        Object o = ThreadLocalDemo.threadLocal.get();
        System.out.println(o.getClass()); // threadLocal.Dog
        String name = Thread.currentThread().getName();
        System.out.println(" stay  Service  Of  update()  Threads  name= " + name);
        new Dao().update();
    }
}

// Dao
public class Dao {
    
    public void update() {
    
        Object o = ThreadLocalDemo.threadLocal.get();
        System.out.println(o.getClass()); // threadLocal.Dog
        String name = Thread.currentThread().getName();
        System.out.println(" stay  Dao  Of  update()  Thread is = " + name);
    }
}

Source code analysis

// ThreadLocal.get() Method 
public T get() {
    
    //  Get the current process t 
    Thread t = Thread.currentThread();
    //  From the container map Get the threadLocals
    ThreadLocalMap map = getMap(t);
    if (map != null) {
    
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
    
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    return setInitialValue();
}

// ThreadLocal.set( data ) Method 
public void set(T value) {
    
    //  Get the current process t 
    Thread t = Thread.currentThread();
    //  From the container map Get the threadLocals
    ThreadLocalMap map = getMap(t);
    /* ThreadLocalMap getMap(Thread t) { return t.threadLocals; } */
    if (map != null) {
    
        //  If there is , Then overwrite the  threadLocals The original data corresponding to the object （ It can be the object ）
        map.set(this, value);
    } else {
    
        //  If it doesn't exist , Create t.threadLocals, Deposit in ThreadLocalMap object 
        // key:this（ The method is called threadLocal object ）value: Data to be stored 
        createMap(t, value);
        /* void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); } */
    }
}