java锁的简化

Posted 2021-01-22 bihang

java使用单独的锁对象的代码展示

private Lock bankLock = new ReentrantLock();
//因为sufficientFunds是锁创建的条件所以称其为条件对象也叫条件变量。
private Condition sufficientFunds = bankLock.newCondition();

public void transfer(int from, int to, double amount) throws InterruptedException
{
    bankLock.lock();
    try
    {
        while (accounts[from] < amount)
        sufficientFunds.await();
        System.out.print(Thread.currentThread());
        accounts[from] -= amount;
        System.out.printf(" %10.2f from %d to %d", amount, from, to);
        accounts[to] += amount;
        System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());
        sufficientFunds.signalAll();
    }
    finally
    {
        bankLock.unlock();
    }
}

假如线程1已经走到bankLock.lock(),就获得了这把锁，别的线程再来就会被阻塞。

然后假如线程1满足while (accounts[from] < amount)条件就会进入这把锁的等待集，此时线程1的状态就是等待状态，并且释放这把锁。

如果此时别的线程获得这把锁并且走到了sufficientFunds.signalAll();,那么处于等待状态的线程将全部转为阻塞状态,而不是在原来的位置接着执行。也就是说将重新争夺这把锁。

使用内部锁

内部对象锁只有一个相关条件。

wait方法添加一个线程到等待集中,notifyAll,notify方法解除等待线程的阻塞状态(也就是从锁的等待集释放).

换句话说,调用 wait 或 notityAll等价于

• intrinsicCondition.await();
• intrinsicCondition.signalAll();

此时代码简化为:

public synchronized void transfer(int from, int to, double amount) throws InterruptedException
{
    while (accounts[from] < amount)
        wait();
    System.out.print(Thread.currentThread());
    accounts[from] -= amount;
    System.out.printf(" %10.2f from %d to %d", amount, from, to);
    accounts[to] += amount;
    System.out.printf(" Total Balance: %10.2f%n", getTotalBalance());
    notifyAll();
}