This is a helper class and sample how to use synchronization for simultaneous read and blocked  writing in java.
There are Sources and JavaDoc .

This source had being written as an answer on professional  interview.
Code never being tested and if You have any comments, or used it, please e-mail me about.

In proposed case we are assuming long read operation, so double lock-unlock expenses can be ignored. If reading process is enough short, simple synchronization by this object can be much sufficient.
Sample how to use RwSync class: 
try
{
	RwSync sem = new RwSync();
	/////////////////////////////////
	// read access sample
	try
	{	sem.ReadLock();
		// do reading
	}finally
	{	sem.ReadUnlock();
	}

       	/////////////////////////////////
	// write access sample
	synchronized( sem )
	{	sem.WriteAcquire();
		// write operation
	}
}catch( Exception e )
	{   e.printStackTrace();	}


FYI. Entities, responsible for synchronization in java:
synchronized keyword in metod declaration. Used to prevent simultanious access for object data members and methods.
Same as synchronized( this ) inside of method.
 
class Foo
{  
  public syncronized void aMethod()	
  {  
    // ...
  }
}
synchronized(obj) statement in block Lock access to object in scope of block
 
class Foo 
{ Object sync=new Object();
  void aMethod() 
  {
    synchronized( sync )
    { 
      // ...
    }
}
object.wait([milis[,nanos]])
 methods in java.lang.Object
( parent for any object )

 Stops current thread execution until notify() or notifyAll() will be called by another thread.
 
class Foo 
{ Object sync=new Object();
  void aMethod()
  {  // exception handler not shown
     synchronized( sync )
     {  sync.wait();
     	 // ...
     }
  }
}
object.notify()
object.notifyAll()

 methods in java.lang.Object Release threads, waiting for notification. 
class Foo 
{ Object sync=new Object();
  void aMethod() 
  {
    synchronized( sync )
    { // some locked processing..
      sync.notifyAll();
    }
}
static initializators
 static variable initialization or static block
 Lock access to class-scope variables 
class Foo 
{ 
  static Object gblSync=new Object();
  // static initializator locks class
  static 
  {  // this block locks class ...
  }
}
Detailed definitions can be found in The Java Language Specification.


© 2003 Oleksandr Firsov