How to listen new db records through java

问题

Currently I use while(true) and Thread.sleep() for checking for new records in the db and execute java code.

Here is an example:

public class StartCommands implements Runnable{
  private Active_Job activeJob;
  Runnable execute_command;

  public StartCommands(){
     activeJobs = new Active_Job();
  }

  @Override
  public void run(){
    int jobId = 0;

    while(true){
      //access the db and get one row from the table by the status
      jobId = activeJobs.get(Status.NEW);
      if (jobId > 0){
        activeJob.updateStatus(Status.INIT);
        execute_command = activeJob.getCommand();

        new Thread(execute_command).start();

        activeJob = new Active_Job();
        jobId = 0;
      }

      Thread.sleep(10*1000);
    } 
  }
}

I've few places in the code that I use this method. But I dont like the endless loop and check every 10 seconds for new row.

So what I'm looking for is some kind of listener: once new record has been entered - execute java code. Some of the inserts executed from the application and some are not.

回答1:

The technique you are using is called polling. You are checking for new records, waiting a set amount of time, then checking again for new records. One good way to respond to new records might be to create a controller that handles inserting new records into the database and force all clients (who update database records) to use the controller to do so. Then the controller can alert you when there is a new record. To facilitate the controller's alerts, you can set up a web service where the controller can contact you.

I say that this "might" be a good way to do it because creating a controller and a web service is obviously extra work. However, it would make polling unnecessary. If you want to continue using your polling technique, you could make a service (producer) that does the polling and fills a queue with the new results. Your other program (consumer) can then retrieve items from the queue and do something with them.

回答2:

There is no builtin "update listener" in MySQL (or any SQL database I'm aware of), so you have to build your own.

Notice that in your implementation, if two new rows are added you will handle one, wait 10 seconds, then handle the next one. Your code cannot handle more than one event every 10 seconds.

What you want to do is separate the polling of the database from the dispatching of the worker threads. Have the polling loop wake up every n seconds, read ALL new records from the database, and add them to a work queue. Have a consumer thread that is waiting on the queue and launches processors as messages appear on the queue. using a thread pool implementation.

回答3:

@nir, Since there is no mysql database update listener in java so far, so what you can do is, create a database update trigger against the table, the change of which you want to listen. Within the trigger statement or code construct a function.Now from within that function call java function. Java function should be such that it modify some text, say "a". Now register the listener against the change in "a". And within the class implementing the text change listener of "a",put the code you want to execute.

回答4:

The Condition Interface would work nicely for your needs. It will give you the granular control you are looking for, and it will avoid the problem of spinning the thread constantly.

http://docs.oracle.com/javase/tutorial/essential/concurrency/newlocks.html http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/Condition.html

回答5:

Use a trigger, call a User Defined Function that uses sys_exec() to run an external app that signals an inter-process semaphore. Your listener thread can wait on that and, when signaled, process the new records.

回答6:

In oracle exists something called database change notification http://docs.oracle.com/cd/E11882_01/java.112/e16548/dbchgnf.htm and I just implement a component like yours is there something like that in mysql or what approach you arrived?

来源：https://stackoverflow.com/questions/18260593/how-to-listen-new-db-records-through-java