ThreadLocal 多线程并发，数据隔离

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ThreadLocal: 创建一个线程本地变量。

本质：在ThreadLocal类中有一个Map，用于存储每一个线程的变量的副本。

优点：既实现多线程并发，游兼顾数据的安全性。

区别：Synchronized用于线程间的数据共享，而ThreadLocal则用于线程间的数据隔离。

示例：

public class Student {
private int age = 0; //年龄

    public int getAge() {
        return this.age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

public class ThreadLocalDemo implements Runnable {
//创建线程局部变量studentLocal，在后面你会发现用来保存Student对象
private final static ThreadLocal studentLocal = new ThreadLocal();

    public static void main(String[] agrs) {
        ThreadLocalDemo td = new ThreadLocalDemo();
        Thread t1 = new Thread(td, "a");
        Thread t2 = new Thread(td, "b");
        t1.start();
        t2.start();
    }

    public void run() {
        accessStudent();
    }

    /**
     * 示例业务方法，用来测试
     */
    public void accessStudent() {
        //获取当前线程的名字
        String currentThreadName = Thread.currentThread().getName();
        System.out.println(currentThreadName + " is running!");

//产生一个随机数并打印
        Random random = new Random();
        int age = random.nextInt(100);
        System.out.println("thread " + currentThreadName + " set age to:" + age);

        //获取一个Student对象，并将随机数年龄插入到对象属性中
        Student student = getStudent();
        student.setAge(age);
        System.out.println("thread " + currentThreadName + " first read age is:" + student.getAge());

        try {
            Thread.sleep(500);
}
        catch (InterruptedException ex) {
            ex.printStackTrace();
}

System.out.println("thread " + currentThreadName + " second read age is:" + student.getAge());
}

    protected Student getStudent() {
        //获取本地线程变量并强制转换为Student类型
        Student student = (Student) studentLocal.get();
        //线程首次执行此方法的时候，studentLocal.get()肯定为null
        if (student == null) {
            //创建一个Student对象，并保存到本地线程变量studentLocal中

student = new Student();
            studentLocal.set(student);
        }
        return student;
    }
}

结果：

a is running!
thread a set age to:76
b is running!
thread b set age to:27
thread a first read age is:76
thread b first read age is:27
thread a second read age is:76
thread b second read age is:27