regarding persistent classes & objects

Former Member — Thu, 13 Mar 2008 03:58:42 GMT

Hi all,

I am new to abap objects, please give me some simple links of persistent classes & object.

Let me know the difference between persistent classes & other simple classes..

I ensure you for points!

cheers.

Re: regarding persistent classes & objects

Former Member — Thu, 13 Mar 2008 05:17:15 GMT

Hi,

Transient and persistent data

In principle, ABAP programs work with local program data, which resides in the program’s internal session. This data lives only as long as its context – that is, as long as its associated procedure (for local procedure data); its object (for attributes of classes); or its program (for global program data). This data is known as transient. Data that can be preserved beyond the runtime of the program is known as persistent. In SAP Systems, persistent data usually occurs as the content of database tables, but also as the content of files on application and presentation servers.

To work with persistent data, the system has to load it into transient data objects belonging to the ABAP program while that program is being executed. Then, after processing has been completed, it stores the data in a persistent form again. During this time, the content of the data exists twice: once in the ABAP program (transiently), and once in the appropriate storage medium (persistently). A typical process would be reading data from a database table using the SELECT statement into a transient work area; modifying the work area; and then updating the database table (using UPDATE). In such cases, the contents of transient and persistent data are different in the interim during this process.

Data in object-oriented programming

In an ideal object-oriented application, data occurs only as the attributes of objects (if we ignore the local data in methods for the time being). Objects are an aggregation of functions (in methods) and data (in attributes). The description of an object – that is, the class – occurs persistently as a piece of source code, but its attributes exist only as long as the object. However, an object in ABAP Objects is transient in principle. It exists in the internal program session only from the time it is generated (using CREATE OBJECT) until it is deleted by the Garbage Collector. Therefore, to work with persistent data in objects, you must program access to where those objects are stored within the methods of the class.

However, in completely object-oriented business application programming, then it is pointless simply to transfer the classical separation of data and functions to the methods – that is, to work with objects, but use procedural programming within the objects themselves. Ideally you could save the encapsulation of data and functions persistently within the object instead. A program could then leave an object in a certain state and a second program could continue working on the object in that state. Classes of objects are already persistent anyway, but you need some way of saving the attributes of an object persistently and then make reference to the appropriate class. The Persistence Service allows you to do exactly that.

Persistent Classes

To use the Persistence Service for objects, the classes of these objects must be created as persistent classes in the Class Builder. The term persistent class does not imply that a class is persistent. (As a template for objects, every class is persistent). Rather, it means that the objects of that class and their state are managed by the Persistence Service. For example, the objects of these classes are instantiated in the ABAP program with a method of the Persistence Service, which ensures that the initialization is correct (not with the usual CREATE OBJECT statement). When the Class Builder creates a persistent class, it automatically generates an associated class, known as the class actor or class agent, whose methods manage the objects of persistent classes. As well as their identity, persistent classes can contain key attributes, which allow the Persistence Service to ensure that the content of each persistent object is unique.

Managed Objects

The objects of persistent classes are managed by the Persistence Service. This means, among other things, that these objects are instantiated with a method of the class actor, not with the CREATE OBJECT statement. These objects are known as managed objects. Objects managed by the Persistence Service can be either persistent or transient.

Persistent objects must be managed by the Persistence Service. The Persistence Service connects the object and the database.

Transient objects of persistent classes are also managed by the Persistence Service. For example, the Persistence Service ensures that the object is unique within a program (by checking its key attributes), but not for a connection to the database.

Plz refer to this link:

http://help.sap.com/saphelp_nw70/helpdata/en/f5/a3682ebc6911d4b2e80050dadfb92b/content.htm

Thanks,

Samantak.

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