Java并发中的同步块/方法和调度规则

Question

I'm investigate about synchronized block and scheduling rules. I know that both methods are used to guarantee synchronous data. But I don't understand them, how they work. What are the advantage and disadvantage of synchronized and scheduling rules? I referd the instruction about scheduling rules here: http://help.eclipse.org/mars/index.jsp?topic=%2Forg.eclipse.platform.doc.isv%2Fguide%2Fruntime_jobs_rules.htm In this document, it represented about a rule as below:

We can fix this example by creating a simple scheduling rule that acts as a mutex (also known as a binary semaphore):

   class Mutex implements ISchedulingRule {
      public boolean isConflicting(ISchedulingRule rule) {
         return rule == this;
      }
      public boolean contains(ISchedulingRule rule) {
         return rule == this;
      }    }

Then, the rule is set into a object or method to control jobs. In this code, I don't see rule as well as how to check rule. And when is the scheduling rule or synchronized used?

Thank in advance

Answer 1

schedule and synchronization are used when threads need to access the same data. It would be a major problem if one thread reads data, mutates it, and before it can write the data back, another thread reads the data.

This cross-section needs to be carefully attended to to ensure only one process can access the shared resource at a time. Synchronization allows only one process at a time to utilize the resource (in a more advanced way, the synchronization can allow a pre-determined number of multiple accesses if so inclined; ie semaphores)

The schedule is used to address which order the threads can access (timing) the shared resource.

If a thread does not have access to a resource that another thread would, there is no reason to worry about synchronization as it is the only one using that resource

specific to your code snippet, it creates a mutex (which it clearly states) which allows access of the resource to only one process- the note above it calls it a binary semaphore, and for all intents-and-purposes, it works like one. However, java uses monitors in their pre-defined mutex locks; not semaphores.