读写锁ReentrantReadWriteLock锁降级的意义是什么啊

Question

• 比如如下代码，降级了并没有并发执行啊？

    public void put(Object data){
  
              rwl.writeLock().lock();
              rwl.readLock().lock();
              System.out.println(Thread.currentThread().getName() + " be ready to write data!");                    
              try {
                  Thread.sleep((long)(Math.random()*1000));
              } catch (InterruptedException e) {
                  e.printStackTrace();
              }
              this.data = data;        
              System.out.println(Thread.currentThread().getName() + " have write data: " + data);                    
              rwl.readLock().unlock();
              rwl.writeLock().unlock();
  
          }

  输出：
  Thread-0 be ready to write data!
  Thread-0 have write data: 1103
  Thread-2 be ready to write data!
  Thread-2 have write data: 4531
  Thread-1 be ready to write data!
  Thread-1 have write data: 2188
  也并没有并发执行啊

Answer 1

你这里3个线程都是要写数据 怎么可能会并发执行，Thread-0 调用put的时候占用了write锁，Thread-1和Thread-2都被挡在外面了；锁降级的意义是当前线程由write降为read，其他只需要read锁的线程可以去获取read锁，达到并发读的目的。