写一条线程，读多条线程能够提升效率。

写写/读写 需要“互斥”;
读读 不需要互斥.

1. ReadWriteLock 维护了一对相关的锁，一个用于只读操作，另一个用于写入操作。只要没有 writer，读取锁可以由多个 reader 线程同时保持。写入锁是独占的。
2. ReadWriteLock 读取操作通常不会改变共享资源，但执行写入操作时，必须独占方式来获取锁。对于读取操作占多数的数据结构。 ReadWriteLock 能提供比独占锁更高的并发性。而对于只读的数据结构，其中包含的不变性可以完全不需要考虑加锁操作。

api相关方法：

1. Lock readLock()返回用于读取操作的锁。
2. Lock writeLock()返回用于写入操作的锁。

 demo:

package com.company;

import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/*
 * 1. ReadWriteLock : 读写锁
 * 
 * 写写/读写 需要“互斥”
 * 读读 不需要互斥
 * 
 */
public class TestReadWriteLock {

    public static void main(String[] args) {
        ReadWriteLockDemo rw = new ReadWriteLockDemo();

        new Thread(() -> rw.set(new ReadWriteLockDemo.Person("soyoungboy", 27)), "Write:").start();


        for (int i = 0; i < 10; i++) {
            new Thread(() -> rw.get()).start();
        }
    }

}

class ReadWriteLockDemo {

    private Person person;

    private ReadWriteLock lock = new ReentrantReadWriteLock();

    //读
    void get() {
        lock.readLock().lock(); //上锁

        try {
            if (person != null) {
                System.out.println(Thread.currentThread().getName() + " ==> person : " + person.toString());
            }else {
                System.out.println(Thread.currentThread().getName() + "");
            }
        } finally {
            lock.readLock().unlock(); //释放锁
        }
    }

    //写
    void set(Person person) {
        lock.writeLock().lock();

        try {
            System.out.println(Thread.currentThread().getName());
            this.person = person;
        } finally {
            lock.writeLock().unlock();
        }
    }

    static class Person {

        private String name;
        private int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getAge() {
            return age;
        }

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

        @Override
        public String toString() {
            return "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }

    }
}

结果：

Thread-0
Thread-1
Thread-3
Thread-2
Write:
Thread-4 ==> person : Person{name='soyoungboy', age=27}
Thread-6 ==> person : Person{name='soyoungboy', age=27}
Thread-8 ==> person : Person{name='soyoungboy', age=27}
Thread-5 ==> person : Person{name='soyoungboy', age=27}
Thread-7 ==> person : Person{name='soyoungboy', age=27}
Thread-9 ==> person : Person{name='soyoungboy', age=27}