read write locks and simple synchronization

[oll3i]

read write locks

public class Person
{
  int age;
  rwLockMgr lockmanager;
  public Person()
  {
    age = 17;
    lockmanager = new rwLockMgr();
  }
  public int getAge()
  {
    lockmanager.lockRead();
    return age;
  }
  public void putAge(int newage)
  {
    lockmanager.lockWrite();
    age = newage;
  }
  public void releaseLock()
  {
    lockmanager.releaseLock();
  }
 
 
 
public class PersonSyncOnly implements Runnable{
    int age;
 
      public PersonSyncOnly()
      {
        age = 17;
 
      }
      public synchronized int  getAge()
      {
 
        return age;
      }
      public synchronized  putAge(int newage)
      {
        age = newage;
      } 
 
      public void run() { 
 
              System.out.println(age); 
      } 
}
 

synchronization

public class PersonSyncOnly implements Runnable{
    int age;
 
      public PersonSyncOnly()
      {
        age = 17;
 
      }
      public synchronized int  getAge()
      {
 
        return age;
      }
      public synchronized  putAge(int newage)
      {
        age = newage;
      } 
 
      public void run() { 
 
              System.out.println(age); 
      } 
}

what would be the use of these classes to show threads working?

[JosAH]

Normally when you want to read/write from/to a shared resource you apply the
'CREW' idiom: 'Concurrent Read Exclusive Write'. That means you can have
more than one reader but just at most one writer when there are no readers.

The easiest scheduling method is to stick them all in a queue: readers can
leave the queue when there are no writers (but possibly there are other readers)
while writers can only leave the queue when there's noone else around.

kind regards,

Jos

[oll3i]

i thought that i cd share one instance of person among two threads ?
One setting and the other printing the age.
but i dont know how to do the setting part ?

[JosAH]

i thought that i cd share one instance of person among two threads ?
One setting and the other printing the age.
but i dont know how to do the setting part ?

So you don't want to be able to set something while that something is being
printed? Just synchronize them:

class Something {
   ...
   public synchronized void print() { ... }
   public synchronized void setSomething() { ... }
}

... now the two are mutually exclusive actions.

kind regards,

Jos

[oll3i]

but how to show working with threads so one prints the age and the other updates the age ?

[oll3i]

what else do i need to add to my classes to show the difference between locks and simple synchronization ?

[JosAH]

but how to show working with threads so one prints the age and the other updates the age ?

Nothing more is needed: when something is printed you don't want it updated
while printing. Synchronization can do that perfectly; don't forget synchronization
is locking, i.e. it's a language feature.

kind regards,

Jos

[oll3i]

so where to use releaseLock() ?

[JosAH]

so where to use releaseLock() ?

If we're still talking about my latest example: there is no need to release an
explicit lock, i.e. leaving the synchronized block automagically releases the
monitor (lock) on that particular object.

The entire 'synchronized' method locks the monitor on the object you synchronize.
When you leave the synchronized block/method that same monitor is unlocked
for you.

kind regards,

Jos

[oll3i]

thank you :)