[英]java multi-thread acquire lock not working
我正在尝试编写一小段代码以锁定和解锁代码块。 Acquisition_lock和release_lock函数如下:
public static void acquire_lock(long timestamp) {
synchronized(operations) {
// put the timestamp into queue
operations.add(timestamp);
// check if the head of queue is current timestamp, if not,
// this means there are some other operations ahead of current one
// so current operation has to wait
while (operations.peek() != timestamp) {
try {
operations.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void release_lock() {
synchronized(operations) {
// poll the finished operation out of queue
// and wake up all waiting operations
operations.poll();
operations.notifyAll();
}
}
但是,当我将此代码放到测试环境中时,它并不总是能正常工作,整个测试代码如下:
public class AcquireLockNotWork {
static int balance = 0;
static PriorityQueue<Long> operations = new PriorityQueue<Long>();
// withdraw money from balance
public static void withdraw(final int amt) {
// get system time
Long timestamp = System.nanoTime();
Thread t = new Thread(new Runnable() {
public void run() {
// try to use acquire_lock to lock this piece of code
acquire_lock(timestamp);
try {
Thread.sleep(500);
int holdings = balance;
balance = holdings - amt;
System.out.println("Withdrew " + amt + " from funds. Now at " + balance);
} catch (Exception e) {
e.printStackTrace();
} finally {
release_lock();
}
}
});
t.start();
}
//put money into banlance
public static void deposit(int amt) {
Thread t1 = new Thread(new Runnable() {
public void run() {
Long timestamp = System.nanoTime();
acquire_lock(timestamp);
int holdings = balance;
balance = holdings + amt;
System.out.println("deposit " + amt + ", balance: " + balance);
release_lock();
}
});
t1.start();
}
public static void acquire_lock(long timestamp) {
synchronized(operations) {
// put the timestamp into queue
operations.add(timestamp);
// check if the head of queue is current timestamp, if not,
// this means there are some other operations ahead of current one
// so current operation has to wait
while (operations.peek() != timestamp) {
try {
operations.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void release_lock() {
synchronized(operations) {
// poll the finished operation out of queue
// and wake up all waiting operations
operations.poll();
operations.notifyAll();
}
}
public static void test1() {
balance = 0;
deposit(2000);
withdraw(500);
withdraw(1000);
}
public static void main(String[] args) {
test1();
}
}
在少数情况下,结果将是这样的:
deposit 2000, balance: 2000
Withdrew 500 from funds. Now at 500
Withdrew 1000 from funds. Now at 500
这意味着acquire_lock和release_lock函数不能正常工作。 似乎最后两个线程(撤回500和撤回1000)已经同时进入了Acquisition_lock()和release_lock()之间的块,这不是我想要的。 那么acquire_lock和release_lock函数怎么了?
这里非常棘手。 发生异常是因为后面的线程首先进入了acquire_lock。 并且当较早的线程进入acquired_lock时,它将不会被阻塞,因为代码会根据其时间戳来阻塞线程。 因此,这两个线程转到相同的受保护代码区域。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.