Question

我正在使用 StampedLock 实现缓存读取或加载方法，我想知道我是否可以更好地调整javadoc中的示例。

作者Doug Lea给出了一个'乐观读'示例和一个'升级读锁定写锁'的例子，但在我的用例中我想要同时做两件事。

public String getConverted(String input) {

    String customised = null;
    long stamp = 0L;

    while (customised == null) {
        if (!lock.validate(stamp)) {
            stamp = lock.tryOptimisticRead();
        }
        customised = cached.get(input);

        // if the lock was valid we can trust the value of customised
        if (lock.validate(stamp) && customised == null) {
            long writeStamp = 0L;
            try {
                while (customised == null) {
                    writeStamp = lock.tryConvertToWriteLock(stamp);
                    if (writeStamp != 0L) {
                        stamp = writeStamp;
                        customised = convertToCustom(input);
                        cached.put(input, customised);
                    } else {

                        // so what do we do here (line 15)?

                    }
                }
            } finally {
                lock.unlock(stamp);
            }
        } else {
            // if the lock was invalid, customised could be anything, so:
            customised = null;

            // and what do we do here (line 25)?
        }
    }

    return customised;
}

因此算法中有两点需要做点什么 - 在两种情况下都是：

获取硬锁 - 在第15行：

                    lock.unlockRead(stamp);
                    stamp = lock.writeLock();

并在第25行：

                    stamp = lock.readLock();

或者是什么？
```
                    Thread.sleep(25); 
```

这对我来说并没有削减它 - 当然StampedLock可以更好地管理这个线程上的阻塞！

readLock()

writeLock()

StampedLock#tryOptimisticRead()

StampedLock#tryConvertToWriteLock()

或者这些方法背后的逻辑是否因为失败一次而被没收？

Answer 1

您可以查看以下方法以供参考。

tryConvertToWriteLock和tryOptimisticRead中没有排队机制。

方法tryOptimisticRead（）仅在锁定时返回非零标记目前未处于写入模式。方法验证（long）返回如果在获取a之后未在写入模式下获取锁定，则为true 给予邮票。这种模式可以被认为是一个非常弱的版本读锁定，可以随时由作者打破。指某东西的用途短只读代码段的乐观模式通常会减少争用并提高吞吐量。但是，它的使用本质上是有用的脆弱。乐观的阅读部分应该只读取字段并保持它们在局部变量中供以后验证后使用。字段读取而在乐观模式下可能会非常不一致，因此适用只有当您熟悉数据表示才能检查时一致性和/或反复调用方法validate（）。例如，首次阅读对象时通常需要执行此类步骤数组引用，然后访问其中一个字段，元素或方法

另外，我喜欢你检查乐观锁定有效性的控制流程，如果你发现它是无效的，那么就获得锁定，因为它避免了＆＃34; else＆＃34;代码中使用的块。

tryConvertToWriteLock 的类似代码流。

private final StampedLock sl = new StampedLock(); 

    /**


    * This method is to show the feature of tryOptimisticRead() method  
    */ 
      public double getTotalRevenueOptimisticRead() {  
        long stamp = sl.tryOptimisticRead();  
        double balance = this.totalRevenue;
        boolean lockAcquired = false;   
        //calling validate(stamp) method to ensure that stamp is valid, if not then acquiring the read lock  
        if (!sl.validate(stamp)){
          lockAcquired = true;   
          LOG.info("stamp for tryOptimisticRead() is not valid now, so acquiring the read lock");   
          stamp = sl.readLock();  
        }     
        try {    
          balance = this.totalRevenue;   
        } finally {
          if(lockAcquired){
             sl.unlockRead(stamp); 
          }    
        }  
        return balance; 
      } 



/**  
* This method is to show the feature of tryConvertToWriteLock() method  
*/ 
  public double getStateTaxReturnUisngConvertToWriteLock(TaxPayer taxPayer) { 
    double incomeTaxRetunAmount = taxPayer.getTaxAmount() * 5/100;  
    long stamp = sl.readLock();
    boolean lockAcquired = false;     
    //Trying to upgrade the lock from read to write  
    stamp = sl.tryConvertToWriteLock(stamp);    
    //Checking if tryConvertToWriteLock got success otherwise call writeLock method  
    if(stamp == 0L){   
      LOG.info("stamp is zero for tryConvertToWriteLock(), so acquiring the write lock");   
      stamp = sl.writeLock();
      lockAcquired = true;  
    }    
    try {   
      this.totalRevenue -= incomeTaxRetunAmount;  
    } finally {
      if(lockAcquired){
         sl.unlockWrite(stamp);
      }   
    }  
    return incomeTaxRetunAmount; 
  }

StampedLock＃tryOptimisticRead（）实现

1 个答案: