ReadWriteLock讀寫鎖

• ReadWriteLock只允許一個執行緒修改，但是get()方法只讀，不修改資料，實際上允許多個執行緒同時呼叫，使用ReadWriteLock可以解決這個問題

• 適合讀多寫少的場景

• 原始碼

public interface ReadWriteLock {
    Lock readLock();
    Lock writeLock();
}

• 實現類:ReentrantReadWriteLock
• 讀鎖（共享鎖），寫鎖（獨佔鎖）

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

class MyReadWriteLock {
    public static void main(String[] args) {
        MyCache myCache = new MyCache();

        for (int i = 0; i < 6; i++) {
            final int temp = i;
            new Thread(() -> {
                // lambda無法訪問到外部的變數
                myCache.put(temp + "", "value:" + temp);
            }, String.valueOf(i)).start();
        }

        for (int i = 0; i < 6; i++) {
            final int temp = i;
            new Thread(() -> {
                myCache.get(temp + "");
            }, String.valueOf(i)).start();
        }
    }
}

class MyCache {
    private final Map<String, Object> map = new HashMap<>();
    // 讀寫鎖
    private final ReadWriteLock lock = new ReentrantReadWriteLock();

    // 只有一個可以寫
    public void put(String key, Object value) {
        lock.writeLock().lock();

        try {
            System.out.println(Thread.currentThread().getName() + "寫入" + value);
            map.put(key, value);
            System.out.println(Thread.currentThread().getName() + "寫完畢");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.writeLock().unlock();
        }
    }

    // 可以多個同時讀
    public void get(String key) {
        lock.readLock().lock();

        try {
            System.out.println(Thread.currentThread().getName() + "讀取" + map.get(key));
            System.out.println(Thread.currentThread().getName() + "讀完畢");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.readLock().unlock();
        }
    }
}