Question

1. When make more fair to prevent starvation in java using lock instead of declare synchronized function
   it can not instantiate the type Lock

2. do it need to implement FairLock like http://tutorials.jenkov.com/java-concurrency/starvation-and-fairness.html
   do java’s lock equal FairLock above? It said FairLock will slow performance, what will it be in real practice

   Lock lock = new Lock();
   

Answer 1

You’re probably trying to instantiate a java.util.concurrent.locks.Lock, which is an interface. That won’t work of course. You’ll want to instantiate an implementation, such as ReentrantLock, which is pretty much the standard way of doing “flexible” locks in Java, in cases where simple monitor locks aren’t sufficient. Its documentation has something to say about fairness:

The constructor for this class accepts an optional fairness parameter. When set true, under contention, locks favor granting access to the longest-waiting thread. Otherwise this lock does not guarantee any particular access order. Programs using fair locks accessed by many threads may display lower overall throughput (i.e., are slower; often much slower) than those using the default setting, but have smaller variances in times to obtain locks and guarantee lack of starvation. Note however, that fairness of locks does not guarantee fairness of thread scheduling. Thus, one of many threads using a fair lock may obtain it multiple times in succession while other active threads are not progressing and not currently holding the lock. Also note that the untimed tryLock method does not honor the fairness setting. It will succeed if the lock is available even if other threads are waiting.