10.ThreadLocal

Posted 2022-07-20 fly-book

/**
 * 如果你创建了 一个 ThreadLocal 变量，那么访问这个变量的每个线程都会有这个变量的一个本地副本。
 * 当多个线程操作这个变量时，实际操作的是自己本地内存里面的变量，从而避免了线程安全问题
 */
public class ThreadLocalDemo 
    static ThreadLocal<String> local = new ThreadLocal<>();
    static void print(String s)
        System.out.println(s+":"+local.get());
        local.remove();
    
    public static void main(String[] args)
        Thread threadA = new Thread(new Runnable() 
            @Override
            public void run() 
                local.set("threadA");
                print("threadA");
                System.out.println("threadA remove after:"+ local.get());
            
        );
        Thread threadB = new Thread(new Runnable() 
            @Override
            public void run() 
                local.set("threadB");
                print("threadB");
                System.out.println("threadB remove after:"+ local.get());
            
        );
        threadA.start();
        threadB.start();
        //threadB:threadB
        //threadA:threadA
        //threadB remove after:threadB
        //threadA remove after:threadA

        //执行local.remove();
        //threadA:threadA
        //threadB:threadB
        //threadA remove after:null
        //threadB remove after:null
    

public class ThreadLocalDemo2 
    static ThreadLocal<String> local = new ThreadLocal<>();
    public static void main(String[] args)
        local.set("hello");
        Thread threadA = new Thread(new Runnable() 
            @Override
            public void run() 
                System.out.println("thread:"+local.get());
            
        );
        threadA.start();
        System.out.println("main:"+local.get());
        //thread:null
        //main:hello
        //同一个 ThreadLocal 变量在父线程中被设置值后， 在子线程中是获取不到的。
        //因为在子线程 threadA 里面调用 get 方法时当前线程 为 threadA 线程，而这里调用 set 方法设置线程变量的是 main 线程，
        // 两者是不同的线程，自然子线程访问时返回 null
    

/**
 * InheritableThreadLocal
 * 子线程获取父线程的threadlocal变量
 */
public class ThreadLocalDemo3 
    static ThreadLocal<String> local = new InheritableThreadLocal<>();
    public static void main(String[] args)
        local.set("hello");
        Thread threadA = new Thread(new Runnable() 
            @Override
            public void run() 
                System.out.println("thread:"+local.get());
            
        );
        threadA.start();
        System.out.println("main:"+local.get());
        //main:hello
        //thread:hello
        //当父线程创建子线程时，构造函数会把父线程中 inheritableThreadLocals 变量里面的本地变量复制一份保存到子线程的 inheritableThreadLocals 变量里面