Re: Fetching record counts.
Re: Fetching record counts.
- Subject: Re: Fetching record counts.
- From: Chuck Hill <email@hidden>
- Date: Tue, 01 Feb 2005 09:00:31 -0800
I don't see any locking in there. Unless you know that this is being
called from code that has locked the EC, you should add that. You can
probably avoid the Concurrency issue by wrapping these methods in a
try...finally block and locking the EOObjectStoreCoordinator:
objectStoreCoordinator.lock();
try {
// method body here
}
finally {
objectStoreCoordinator.unlock();
}
Chuck
At 03:34 PM 01/02/2005 +1100, Marek Wawrzyczny wrote:
>Thanks Arturo,
>
>To be absolutely honest I have never used synchronized keyword in this
>way before.
>
>I will still create the "count" attribute dynamically, otherwise I
>suspect there might be a risk of a deadlock if the attribute was to be
>added to two different entities simultanously. So, the given entity
>would be locked for the duration of addition/fetching count/removal of
>the "count" attribute.
>
>
>
>import com.webobjects.foundation.*;
>import com.webobjects.eocontrol.*;
>import com.webobjects.eoaccess.*;
>
>
>/**
> * This class impements methods that allow us to count the number of
> * rows for a user defined fetch specification.
> *
> * <p>The count is performed directly on the database source without
>having to load
> * them into memory.</p>
> *
> * <p>Original code by Eric Noyau. This code is based on Eric Noyau's
>Bugtracker application.</p>
> *
> * @author Eric Noyau
> * @author Marek Wawrzyczny
> * @version 1.1
> */
>
>public class EOFetchCounting extends Object {
>
> /**
> * Returns the number of rows metching the fetch specifivation.
> * @param editingContext - editing context to perform the fetch in
> * @param fetchSpecification - the fetch specification
> */
> public static Number objectCountWithFetchSpecification(
> EOEditingContext editingContext,
> EOFetchSpecification fetchSpecification) {
>
> boolean hasCountAttribute = false;
> EOFetchSpecification rawFetchspecification;
> EOEntity entity;
> EOQualifier schemaBasedQualifier;
> EOAttribute attribute;
> NSArray results = null;
>
> if (fetchSpecification != null) {
>
> try {
> rawFetchspecification =
>(EOFetchSpecification)fetchSpecification.clone();
> } catch (Exception e) {
> System.out.println("\nException in
>objectCountWithFetchSpecification :\n"+e);
> return null;
> }
>
> entity =
>EOModelGroup.defaultGroup().entityNamed(rawFetchspecification.entityName
>());
>
> // Synchronize access to the entity object, the "count" attribute
>should 'live' only for the
> // duration of the fetch.
> synchronized (entity) {
> schemaBasedQualifier =
>entity.schemaBasedQualifier(rawFetchspecification.qualifier());
> attribute = objectCountAttribute();
>
> try {
> entity.addAttribute(attribute);
> } catch (Exception e) {
> NSLog.out.appendln("FetchCounting: Exception while adding attrib "
>+ e);
> }
>
> try {
> rawFetchspecification.setQualifier(schemaBasedQualifier);
> rawFetchspecification.setRawRowKeyPaths(new
>NSArray(attribute.name()));
> results =
>editingContext.objectsWithFetchSpecification(rawFetchspecification);
> } catch (Exception e) {
> NSLog.out.appendln("FetchCounting: Exception while performing
>fetch " + e);
> } finally {
> entity.removeAttribute(attribute);
> }
> }
>
> if ((results != null) && (results.count() == 1)) {
> NSDictionary row = (NSDictionary) results.lastObject();
> return (Number)row.objectForKey(attribute.name());
> }
> }
>
> return null;
> }
>
> /**
> * Returns the number of rows metching the fetch specifivation.
> * @param editingContext - editing context to perform the fetch in
> * @param entityName - the entity to perform the count on
> * @param fetchSpecification - the fetch specification
> * @param bindings - the fetch bindings
> */
> public static Number objectCountWithFetchSpecificationAndBindings(
> EOEditingContext editingContext,
> String entityName,
> String fetchSpecName,
> NSDictionary bindings) {
>
> EOModelGroup modelGroup = EOModelGroup.defaultGroup();
> EOFetchSpecification unboundFetchSpec;
> EOFetchSpecification boundFetchSpec;
> unboundFetchSpec =
>modelGroup.fetchSpecificationNamed(fetchSpecName, entityName);
> if (unboundFetchSpec == null) {
> return null;
> }
> boundFetchSpec =
>unboundFetchSpec.fetchSpecificationWithQualifierBindings(bindings);
> return objectCountWithFetchSpecification(editingContext,
>boundFetchSpec);
> }
>
> /**
> * Returns the "count" entity attribute to perform the fetch on.
> * @return "count" entity attribute
> */
> private static EOAttribute objectCountAttribute() {
> EOAttribute objectCountAttribute = new EOAttribute();
> objectCountAttribute.setName("p_objectCountAttribute");
> objectCountAttribute.setColumnName("p_objectCountAttribute");
> objectCountAttribute.setClassName("Number");
> objectCountAttribute.setValueType("i");
> objectCountAttribute.setReadFormat("count(*)");
>
> return objectCountAttribute;
> }
>}
>
>
>On 01/02/2005, at 14:57, Arturo Pérez wrote:
>
>>
>> On Jan 31, 2005, at 9:33 PM, Marek Wawrzyczny wrote:
>>
>>> This is actually where I am stuck. I don't understand the EOModel and
>>> EOAttribute mechanism very well. One solution I tried was to add the
>>> count attribute permanently, that crashed the application. So I tried
>>> creating the attributes on the fly and removing them once finished
>>> this worked. However, it doesn't appear I can make this thread-safe
>>> since there is no locking mechanism on the EOModel (I think this is
>>> why Eric's solution used a static var) and I cannot easily test this
>>> concept under duress (when the exceptions actually start occurring).
>>>
>>> Can anyone point me in the right direction on this?
>>>
>> EOF may not provide a locking mechanism but perhaps the Java one will
>> suffice? Have you tried synchronizing on the entity, something like:
>>
>> public static Number objectCountWithFetchSpecification(
>> EOEditingContext editingContext,
>> EOFetchSpecification fetchSpecification) {
>>
>> if (fetchSpecification != null) {
>> EOFetchSpecification rawFetchspecification;
>>
>> try {
>> rawFetchspecification =
>> (EOFetchSpecification)fetchSpecification.clone();
>> } catch (Exception e) {
>> System.out.println("\nException in
>> objectCountWithFetchSpecification :\n"+e);
>> return null;
>> }
>>
>> EOEntity entity =
>> EOModelGroup.defaultGroup().entityNamed(rawFetchspecification.entityNam
>> e());
>>
>> synchronized (entity) { // ADDED SYNCHRONIZED
>> EOQualifier schemaBasedQualifier =
>> entity.schemaBasedQualifier(rawFetchspecification.qualifier());
>> EOAttribute attribute =
>> EOFetchCounting.objectCountAttribute();
>> NSArray results = null;
>> entity.addAttribute(attribute);
>>
>> // Fix to ensure that fetch counting does not trip the
>> site when
>> // the fetch fails for some reason
>> try {
>>
>> rawFetchspecification.setQualifier(schemaBasedQualifier);
>> rawFetchspecification.setRawRowKeyPaths(new
>> NSArray(attribute.name()));
>> results =
>> editingContext.objectsWithFetchSpecification(rawFetchspecification);
>> } finally {
>> // IMPORTANT!!!
>> // This needs to run every time, exception or not
>> entity.removeAttribute(attribute);
>> }
>> } // CLOSE SYNCHRONIZED
>>
>> if ((results != null) && (results.count() == 1)) {
>> NSDictionary row = (NSDictionary) results.lastObject();
>> return (Number)row.objectForKey(attribute.name());
>> }
>> }
>>
>> return null;
>> }
>>
>>
>
>Marek Wawrzyczny
>
>software engineer
>-------------------------->
>ish group pty ltd
>http://www.ish.com.au
>7 Darghan St Glebe 2037 Australia
>phone +61 2 9660 1400 fax +61 2 9660 7400
>
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--
Chuck Hill email@hidden
Global Village Consulting Inc. http://www.global-village.net
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