ThreadLocal和本地变量区别

总结：

1. Static声明的ThreadLocal修饰的变量，为线程私有，即某个线程修改之后对其他线程没有影响
2. Static声明的基本类型变量，为全局共享，即任意一个线程修改之后，对其他线程是可见的，记得加volatile

如果在ThreadLocal变量前加volatile，不会生效

public class ThreadLocalTest {
        //声明一个Threadlocal变量，包装的是Boolean类型，初始化false
    static ThreadLocal<Boolean> local = new ThreadLocal<Boolean>(){ 
        @Override
        protected Boolean initialValue() {
            return false;
        }
    };

    private static ThreadLocalTest instance;
    //双重
    public static ThreadLocalTest getInstance(){
        if(instance==null){
            synchronized (ThreadLocalTest.class) {
                if(instance==null){
                    instance = new ThreadLocalTest();
                }
            }
        }
        return instance;
    }
    
    public void set(boolean value){
        local.set(value);
    }
    
    public boolean get(){
        return local.get();
    }

    public static void main(String[] args) {
        System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest.getInstance().get());//false
        ThreadLocalTest.getInstance().set(true);
        System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest.getInstance().get());//true
        
        new Thread(new Runnable() {
            public void run() {
                //ThreadLocalTest.getInstance().set(true);
                System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest.getInstance().get()); //true
            }
        }).start();
        
        new Thread(new Runnable() {
            public void run() {
                System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest.getInstance().get());//false
            }
        }).start();
    }
}

public class ThreadLocalTest2 {
    
    static Boolean flag  = false; //将threadlocal变量换成普通类型变量
    
    private static ThreadLocalTest2 instance;
    
    public static ThreadLocalTest2 getInstance(){
        if(instance==null){
            synchronized (ThreadLocalTest2.class) {
                if(instance==null){
                    instance = new ThreadLocalTest2();
                }
            }
        }
        return instance;
    }
    
    public void set(boolean value){
        flag = value;
    }
    
    public boolean get(){
        return flag;
    }

    public static void main(String[] args) {
                //第一次为false，因为默认是false
        System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest2.getInstance().get());//false
        
        //设置一次之后，全局共享
        ThreadLocalTest2.getInstance().set(true);
        System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest2.getInstance().get());//true
        
                //该变量对其他线程都是共享的
        new Thread(new Runnable() {
            public void run() {
                //ThreadLocalTest2.getInstance().set(true); //如果将赋值语句放在此处，将上面的set注释掉，运行几次看看结果是否还一样
                System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest2.getInstance().get()); //true
            }
        }).start();
        
        new Thread(new Runnable() {
            public void run() {
                System.out.println(Thread.currentThread().getName()+"："+ThreadLocalTest2.getInstance().get());//true
            }
        }).start();
    }
}