从这个问题如何理解 ReentrantReadWriteLock 的“非公平”模式?,我认为所有线程都有相同的机会获得锁,无论哪个先来。
所以我写了这段代码来测试它:
public static void main(String[] args) {
ReentrantReadWriteLock lock = new ReentrantReadWriteLock(true);
final ReadLock readLock = lock.readLock();
final WriteLock writeLock = lock.writeLock();
// hold the write lock 3s at first
new Thread() {
public void run() {
writeLock.lock();
System.out.println(Thread.currentThread().getName() + " got the write lock");
quietSleep(3);
writeLock.unlock();
System.out.println(Thread.currentThread().getName() + " released the write lock");
};
}.start();
// a thread want to get the read lock 1s later
new Thread() {
public void run() {
quietSleep(1);
readLock.lock();
System.out.println(Thread.currentThread().getName() + " got the read lock");
};
}.start();
// 1000 threads want to get the write lock 2s later
for (int i = 0; i < 1000; i++) {
new Thread() {
public void run() {
quietSleep(2);
writeLock.lock();
System.out.println(Thread.currentThread().getName() + " got the write lock");
};
}.start();
}
}
private static void quietSleep(int seconds) {
try {
Thread.sleep(seconds * 1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
一开始,有一个线程拿到了写锁,并保持了3s。在此期间,一个线程想要获得读锁,然后有 1000 个线程想要获得写锁。
由于 ReentrantReadWriteLock 默认使用非公平模式,我认为写线程有很大的机会获得写锁。但是我运行了很多次,每次读取线程都赢了!
输出是:
Thread-0 got the write lock
Thread-0 released the write lock
Thread-1 got the read lock
我是否理解“不公平”错误?
更新 根据 paxdiablo 的回答,我将代码修改为:
new Thread() {
public void run() {
quietSleep(1);
writeLock.lock();
System.out.println(Thread.currentThread().getName() + " got the write lock");
};
}.start();
for (int i = 0; i < 1000; i++) {
new Thread() {
public void run() {
quietSleep(2);
readLock.lock();
System.out.println(Thread.currentThread().getName() + " got the read lock");
};
}.start();
}
现在有一个线程想要写锁,1000 个读线程想要读锁。但输出是:
Thread-0 got the write lock
Thread-0 released the write lock
Thread-1 got the write lock
似乎它仍然是“先到先得”。