# Definition of dynamic models using XML descriptors
Definition of dynamic models using XML descriptors
# Introduction
It is possible to define the underlying data model without having to write java code. To do this, you must use an XML descriptor which describes the DataNodes and their relationships. An skeleton XML descriptor has the following structure:
```XML
...
...
...
```
Within the XML tag whose root is always zk-ib, you can specify one or more DataNodes. Each DataNode has a unique name. Within each DataNode, you can define multiple finders. Each finder specifies a name and a type. The name will be used to build xpaths, whlie the type identifies the type of DataNode this xpaths refers to.
Within each finder you can define multiple search handlers. They will be responsible for retrieving data from persistent storage, just like the find method on the finder interface. Additionally, you can define one or new instance handlers. They will be responsible for creating new business objects on user request.
Finally, each DataNode can have many persistence handlers. They will act just lik the doInsert, doUpdate and doDelete methods on DataNode class. Each type of handler can be executed conditionally, depending on expressions to be evaluated at run time. These expressions can use the following predefined variables:
Additionally, EL expressions may refer to all variables defined within the DataSource. Those variables are accessed via JXPathContext.getVariables() method. To use of this type of data models, simply create a datamodel component on the ZUL page and assign the src attribute the path to the XML descriptor. The path can be a web component or a class path resource.
# Ejb-handler Handler
It is responsible for persisting the object via a stateless session bean. The following attributes are supported:
Inside the handler, you should specify the suitable insert-method, delete-method and update-method tags. There is a mandatory attribute named method. This attribute must contain the name of the method to invoke. Additionally, the parameters to use can be specified. The following example shows an ejb-handler that uses an EJB whose only parameter is the data object:
```western
...
```
This second example shows how to call a method with slightly complex parameters:
```XML
```
# Script-handler Handler
It ca be used to persist the business objects using BSH scripts. Supports the following attributes:
Inside the handler, you can use the insert-script, delete-script and update-script tags. Each contains the BSH script that the engine will execute to perform inserts, deletes or updates. Within the BSH script you can refer to the same EL expressions predefined variables:
The following example shows how to save objects in file Country:
```XML
import java.io.*;
f = new FileOutputStream ("country."+instance.abbreviation);
oos = new ObjectOutputStream (f);
oos.writeObject (instance);
oos.close ();
f.close ();
import java.io.*;
f = new FileOutputStream ("country."+instance.abbreviation);
oos = new ObjectOutputStream (f);
oos.writeObject (instance);
oos.close ();
f.close ();
import java.io.*;
f = new File ("country."+instance.abbreviation);
f.delete ();
...
```
# Collection-handler Handler
This handler is applicable when the persistence of this object is managed by the parent dataNode. The allowed attributes are:
The following example shows how you could manage objects City as a collection of objects within the object Country, which in turn is saved on a disk arhivo:
```
```
# Custom-handler Handler
The custom-handler provides coverage for situations where you need a more sophisticated handler and it is not worth to use a bsh script., In this case the persistence must be done be a java class that implements the PersistenceHandler interface, and the custom-handler specifying the name of the class used. The attributes are:
# Data validation
The validation tag is responsible for performing basic checks regarding mandatory attributes and valid attribute values before being submitted to the persistence handler. The validation tag may contain one or more attribute-validation and script-validation tags. The verification will be performed before running the insert or update handler.
attribute-validator contains the following attributes:
attribute-script contains a script that will be executed to validate the business object.
Example:
```western
if (instance.abbreviaton.equals(“CT”))
{
throw new RuntimeException (“Catalonia is not a country yet”);
}
...
```
# EJB find handler: ejb-finder
Handles the method to retrieve business objects via a stateless session bean. Supports the following attributes:
Additionally, it can specify one or more parameters in a way similar to the ejb-handler methods
ejb-finder example
```western
...
...
```
# script-finder handler
It is responsible for retrieve business objects from the persistence layer using BSH scripts. The following attributes are supported
The contained bsh script must return a collection of business objects. If the returned object is not a collection object, the engine will treat the returned object as a singleton.
In the following example, the script finder retrives all countries saved to disk in the previous example:
```western
...
import java.io.*;
files[] = new File(".").listFiles ( new FilenameFilter () {
public boolean accept (File dir, String name) {
return name.startsWith("coutnry.");
}
);
v = new java.util.Vector();
for (int i = 0; i < files.length; i++)
{
f = new FileInputStream (files[i]);
ois = new ObjectInputStream (f);
v.add (ois.readObject ();
ois.close ();
f.close();
}
return v;
...
```
# collection-finder handler
Similar to collection-handler, this handler retrieves the list objects contained on a parent collection.
The following example shows how you could retrieve City objects as a collection of objects within the County object:
```western
...
...
```
# custom-finder handler
The custom-finder provides coverage for situations where you need a more sophisticated handler and is not worth implement it using a script. In this case a class that implements the FinderHandler interface must be developed, and the custom-finder specifying the name of the class must be used
# new-instance-script handler
It is responsible for instantiating new objectswithin a finder on user request .
With the following example the model will alow to create a new city within a country:
```western
...
c = new City();
c.countryAbbreviation = instance.abbreviation;
return c;
...
```
# new-instance-bean handler
This handler allows the craetion of a new business object and assign default attribute values. The value of the bean attributes is specified using multiple instances of the bean-attribute tag
With the following example would create a new city within a country:
```western
...
...
```
# custom-attribute handler
Generates virtual attributes derived from other attributes or external elements of the application. It can be applied to any DataNode to add attributes that were not originally present at the underlying business object. Those attributes will be presented at the JXPath interface just as if they were business objects attributes
It's important to properly set the depends attribute as long as the attribute will be reevaluated whenever a dependent attribute has been changed.
The custom attribute can have an empty EL expressions and use a BeanShell script instead. Here is an example of both aproaches:
```western
@network
@mask
return instance.network + “/” + instance.mask;
...
```
# Using dynamic models
To use dynamic models XmlDataSource tag must be replaced by datamodel. The datamodel tag has the following attributes:
className usage is not compatible with src and rootNode attributes.
rootNode attribute is mandatory when using XML dynamic data models.