Hi!
In a concurrent system, if a class is immutable, the problem is simplified and
the class can be used without fear by multiple threads because (i) it’s state
does not change, and (2) it’s state is guaranteed to be visible;
Example:
/**
* The class is immutable because the fields are both immutable types
* and are "private + final”. The fields are guaranteed to be visible
* to all threads after construction. In other words, there is a
* “happens-before” constraint on the fields.
*/
public class SimpleImmutableClass {
private final String value1;
private final int value2;
public SimpleImmutableClass( String aString, int anInt ) {
value1 = aString;
value2 = anInt;
}
public String getValue1() {
return value1;
}
public int getvalue2() {
return value2;
}
}
My understanding is that DS will provide the same happens-before constraint to
the fields in the following service, so presuming that there is no method
exposed to change the field values, the service is effectively immutable and
can be used without fear in a concurrent context. So in the following, value1
and value2 are guaranteed to be visible to all threads thanks to the
happens-before constraint placed on the fields during activation:
/**
* The LogService is basically just added to show that the component is used
* in a static way, as only a static component can be effectively immutable.
*/
@Component
public class EffectivelyImmutableService {
private String value1;
private int value2;
@Reference private LogService logger;
@Activate
void activate( Map<String, Object> properties ) {
value1 = (String)properties.get( "value1" );
value2 = (int)properties.get( "value2" );
}
/**
* Hmmmm, but if an instance is never reused, then wouldn't it be completely
* unnecessary to deactivate()??
*/
void deactivate() {
value1 = null;
value2 = -1;
}
public String getValue1() {
logger.log( LogService.LOG_INFO, String.format( "Value of String is
%s", value1 ) );
return value1;
}
public int getvalue2() {
logger.log( LogService.LOG_INFO, String.format( "Value of int is %s",
value2 ) );
return value2;
}
}
Is somebody able to confirm my understanding?
Thanks!!
=David
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