锁的升降级

为什么需要升降级？
支持锁的降级，不支持升级：代码演示； 当写入过程降级为读锁，但是又不去释放锁，之后其他线程就可以读取了。
为什么不能升级？
会产生死锁，多个线程获取了读锁，然后都去竞争写锁，在多个线程互相等待对方释放读锁时，才能获取写锁，这样就陷入了死锁

public class Upgrading {
    private static ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    private static ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
    private static  ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();
    private static void readUpgrading(){
        readLock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"得到了读锁，正在读取");
            Thread.sleep(1000);
            System.out.println("");
            System.out.println("升级会带来阻塞");
            writeLock.lock();
            System.out.println("获取到了写锁，升级锁成功");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            readLock.unlock();
            System.out.println(Thread.currentThread().getName()+"释放读锁");
        }
    }
    private static void writeDownGrading(){
        writeLock.lock();
        try {
            System.out.println(Thread.currentThread().getName()+"得到了写锁，正在写入");
            Thread.sleep(1000);
            readLock.lock();
            System.out.println(Thread.currentThread().getName()+"在不释放写锁的情况下，直接获取读锁，成功降级");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println(Thread.currentThread().getName()+"释放写锁");
            writeLock.unlock();
        }
    }
    public static void main(String[] args) throws InterruptedException {
        // 演示降级
        Thread thread = new Thread(() -> {
            writeDownGrading();
        }, "Thread-1");
        thread.start();
        thread.join();
        // 演示升级
        Thread thread1 = new Thread(() -> readUpgrading(), "thread-2");
        thread1.start();
    }
}