scala 并行集合、并行并发、线程安全问题、ThreadLocal

---

1. scala并行集合

  Scala提供并行集合，用于多核环境的并行计算，充分使用多核CPU

object T34 {
    
  def main(args: Array[String]): Unit = {
    
    val result = (0 to 20).par.map {
     x => Thread.currentThread.getName }
    println(result)
  }
}

并行：两个或多个事件在同一时刻点发生（多核CPU）
并发：两个或多个事件在同一时间段内发生（多线程竞争资源）

---

2. 线程安全问题

   多线程并发修改，对共享内存中共享对象属性进行修改所导致的数据冲突问题

public class T45 {
    
    public static void main(String[] args) {
    
        User user = new User();
        Thread thread1 = new Thread(new Runnable() {
    
            @Override
            public void run() {
    
                user.name = "zs";
                try {
    
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
    
                    e.printStackTrace();
                }
                System.out.println(user.name);
            }
        });
        Thread thread2 = new Thread(new Runnable() {
    
            @Override
            public void run() {
    
                user.name = "lisi";
                try {
    
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
    
                    e.printStackTrace();
                }
                System.out.println(user.name);
            }
        });
        thread1.start();
        thread2.start();
        System.out.println("main");
    }
}

class User {
    
     public String name;

}

---

3. ThreadLocal

   ThreadLocal中填充的变量属于当前线程，对其他线程而言是隔离的。ThreadLocal为变量在每个线程中都创建了一个副本，每个线程访问自己内部的副本变量。因此不存在线程安全问题

public class T45 {
    
    public static void main(String[] args) {
    
        User user = new User();
        //线程安全问题： 多线程并发修改，对共享内存中共享对象属性进行修改所导致的数据冲突问题
        Thread thread1 = new Thread(new Runnable() {
    
            @Override
            public void run() {
    
                user.name.set("zs");
                try {
    
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
    
                    e.printStackTrace();
                }
                System.out.println(user.name.get());
            }
        });
        Thread thread2 = new Thread(new Runnable() {
    
            @Override
            public void run() {
    
                user.name.set("lisi");
                try {
    
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
    
                    e.printStackTrace();
                }
                System.out.println(user.name.get());
            }
        });
        thread1.start();
        thread2.start();
        System.out.println("main");
    }
}

class User {
    
    public ThreadLocal<String> name=new ThreadLocal<>();
}