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Data Access Object In Vb 6.0 Pdf
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Sebrina Lobianco
Dec 21, 2023, 11:42:08 AM12/21/23
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I know that it is some kind of an interface for accessing data from different types of sources, and in the middle of this little research of mine I bumped into a concept called data source or data source object, which confused me even further.
The employee entities will be persisted into a corresponding Employee table in a database.A simple DAO interface to handle the database operation required to manipulate an employee entity will be like:
Data Access Object In Vb 6.0 Pdf
Download Zip https://7conbipinzu.blogspot.com/?mj=2wS3CF
The problem with accessing data directly is that the source of the data can change. Consider, for example, that your application is deployed in an environment that accesses an Oracle database. Then it is subsequently deployed to an environment that uses Microsoft SQL Server. If your application uses stored procedures and database-specific code (such as generating a number sequence), how do you handle that in your application? You have two options:
DAO (Data Access Object) is a very used design pattern in enterprise applications. It basically is the module that is used to access data from every source (DBMS, XML and so on). I suggest you to read some examples, like this one:
Please note that there are different ways to implements the original DAO Pattern, and there are many frameworks that can simplify your work. For example, the ORM (Object Relational Mapping) frameworks like iBatis or Hibernate, are used to map the result of SQL queries to java objects.
Data Access Object Pattern or DAO pattern is used to separate low level data accessing API or operations from high level business services. Following are the participants in Data Access Object Pattern.
To understand DAO you first need to understand ORM (Object Relational Mapping). This means that if you have a table called "person" with columns "name" and "age", then you would create object-template for that table:
The Data Access Object manages the connection with the data source to obtain and store data.It abstracts the underlying data access implementation for the Business Object to enable transparent access to the data source. A data source could be any database such as an RDBMS, XML repository or flat file system etc.
The Java Data Access Object (Java DAO) is an important component in business applications. Business applications almost always need access to data from relational or object databases and the Java platform offers many techniques for accessingthis data. The oldest and most mature technique is to use the Java Database Connectivity (JDBC)API, which provides the capability to execute SQL queries against a databaseand then fetch the results, one column at a time.
Then one of my senior developers introduced a QueryUtils.class where all queries are added using public static access modifier and then I don't need to do query everywhere. Suppose when I needed activated clients then I just call -
In software, a data access object (DAO) is a pattern that provides an abstract interface to some type of database or other persistence mechanism. By mapping application calls to the persistence layer, the DAO provides data operations without exposing database details. This isolation supports the single responsibility principle. It separates the data access the application needs, in terms of domain-specific objects and data types (the DAO's public interface), from how these needs can be satisfied with a specific DBMS (the implementation of the DAO).
Although this design pattern is applicable to most programming languages, most software with persistence needs, and most databases, it is traditionally associated with Java EE applications and with relational databases (accessed via the JDBC API because of its origin in Sun Microsystems' best practice guidelines[1] "Core J2EE Patterns".
The primary advantage of using data access objects is the rigorous separation between two parts of an application that have no need to know anything about each other, and which therefore can evolve frequently and independently. Changing business logic can rely on a constant DAO interface, while changes to persistence logic do not affect DAO clients.
In the context of the Java programming language, DAO can be implemented in various ways. This can range from a fairly simple interface that separates data access from the application logic, to frameworks and commercial products.
Potential disadvantages of using DAO include leaky abstraction,[citation needed] code duplication, and abstraction inversion. In particular, the abstraction of the DAO as a regular Java object can obscure the high cost of each database access. Developers may inadvertently make multiple database queries to retrieve information that could be returned in a single operation. If an application requires multiple DAOs, the same create, read, update, and delete code may have to be written for each DAO.[2]
A Data Access Object class can provide access to a particular data resource without coupling the resource's API to the business logic. For example, sample application classes access catalog categories, products, and items using DAO interface CatalogDAO.
Reimplementing CatalogDAO for a different data access mechanism (to use a Connector, for example), would have little or no impact on any classes that use CatalogDAO, because only the implementation would change. Each potential alternate implementation of CatalogDAO would access data for the items in the catalog in its own way, while presenting the same API to the class that uses it.
At runtime, the CatalogHelper uses the CatalogDAOFactory to create an object that implements CatalogDAO. The factory looks up the name of the class that implements the DAO interface in environment entry " param/CatalogDAOClass". The CatalogHelper accesses the catalog data source exclusively using the object created by the factory. In the example shown in the figure, the environment entry was set to the (fully-specified) name of class CloudscapeCatalogDAO . This class implements the catalog DAO interface in terms of JDBC TM data sources, accessing a Cloudscape relational database.
This approach is more flexible than using a hard-coded class. To add a new type of data source, an application developer would simply create a class that implements CatalogDAO in terms of the new data source type, specify the implementing class's name in the environment entry, and re-deploy. The factory would create an instance of the new DAO class, and the application would use the new data source type.
The sample application reduces redundant code by using a "generic DAO" that externalizes the SQL for different JDBC data sources. Figure 3 below shows how the sample application uses an XML file to specify the SQL for different JDBC data sources.
In the figure, the CatalogDAOFactory has selected an object of type GenericCatalogDAO as the DAO to access the catalog. An XML file called CatalogDAOSQL.xml specifies the SQL for each supported operation on each type of database. GenericCatalogDAO configures itself by selecting a group of SQL statements from the XML file that correspond to the database type named by environment entry " param/CatalogDAODatabase". The code sample below shows the definition of the XML for the getCategory operation of the CatalogDAO. Different SQL is specified for "cloudscape" and "oracle" database types.
Method GenericCatalogDAO.getCategory chooses the SQL corresponding to the configured database type, and uses it to fetch a category from the database via JDBC. The following code excerpt shows shows the implementation of the method.
This strategy supports multiple JDBC databases with a single DAO class. It both decreases redundant code, and makes new database types easier to add. To support a new database type, a developer simply adds the SQL statements for that database type to the XML file, updates the environment entry to use the new type, and redeploys.
The pluggable DAO and generic DAO strategies can be used separately. If you know that a DAO class will only ever use JDBC databases (for example), the generic DAO class can be hardwired into the application, instead of selected by a factory. For maximum flexibility, the sample application uses both a factory method and a generic DAO.
A data access object can represent data that is not stored in a database. The sample application uses the DAO pattern to represent XML data sources as objects. Sample application screens are defined in an XML file which is interpreted by the class ScreenDefinitionDAO. Specifying screen definitions externally makes access to the screen definitions more flexible. For example, if the application designers (or maintainers) decide to change the application to store screen descriptions in the database, instead of in an XML file, they would need only to implement a single new class ( ScreenFlowCloudscapeDAO, for example). The code that uses ScreenDefinitionDAO would remain unchanged, but the data would come from the database via the new class.
The code fragment above shows how loadScreenDefinitions loads screen definitions using DOM interfaces, while hiding that fact from clients of the class. A client of this class can expect to receive a Screens object regardless of how those screens are loaded from persistent storage. Method getScreens handles all of the DOM-specific details of loading a screen from an XML file.
We are going to create a Student object acting as a Model or Value Object.StudentDao is Data Access Object Interface.StudentDaoImpl is concrete class implementing Data Access Object Interface. DaoPatternDemo, our demo class, will use StudentDao to demonstrate the use of Data Access Object pattern.
I have recently started using test-driven development and unit testing, and it has paid off immensely in the areas where I have aplied it. One area that it has been helpful in is database access. When I abstract away the data access, the testing of methods that need the data become almost ridiculously easy.
0aad45d008
The employee entities will be persisted into a corresponding Employee table in a database.A simple DAO interface to handle the database operation required to manipulate an employee entity will be like:
Data Access Object In Vb 6.0 Pdf
Download Zip https://7conbipinzu.blogspot.com/?mj=2wS3CF
The problem with accessing data directly is that the source of the data can change. Consider, for example, that your application is deployed in an environment that accesses an Oracle database. Then it is subsequently deployed to an environment that uses Microsoft SQL Server. If your application uses stored procedures and database-specific code (such as generating a number sequence), how do you handle that in your application? You have two options:
DAO (Data Access Object) is a very used design pattern in enterprise applications. It basically is the module that is used to access data from every source (DBMS, XML and so on). I suggest you to read some examples, like this one:
Please note that there are different ways to implements the original DAO Pattern, and there are many frameworks that can simplify your work. For example, the ORM (Object Relational Mapping) frameworks like iBatis or Hibernate, are used to map the result of SQL queries to java objects.
Data Access Object Pattern or DAO pattern is used to separate low level data accessing API or operations from high level business services. Following are the participants in Data Access Object Pattern.
To understand DAO you first need to understand ORM (Object Relational Mapping). This means that if you have a table called "person" with columns "name" and "age", then you would create object-template for that table:
The Data Access Object manages the connection with the data source to obtain and store data.It abstracts the underlying data access implementation for the Business Object to enable transparent access to the data source. A data source could be any database such as an RDBMS, XML repository or flat file system etc.
The Java Data Access Object (Java DAO) is an important component in business applications. Business applications almost always need access to data from relational or object databases and the Java platform offers many techniques for accessingthis data. The oldest and most mature technique is to use the Java Database Connectivity (JDBC)API, which provides the capability to execute SQL queries against a databaseand then fetch the results, one column at a time.
Then one of my senior developers introduced a QueryUtils.class where all queries are added using public static access modifier and then I don't need to do query everywhere. Suppose when I needed activated clients then I just call -
In software, a data access object (DAO) is a pattern that provides an abstract interface to some type of database or other persistence mechanism. By mapping application calls to the persistence layer, the DAO provides data operations without exposing database details. This isolation supports the single responsibility principle. It separates the data access the application needs, in terms of domain-specific objects and data types (the DAO's public interface), from how these needs can be satisfied with a specific DBMS (the implementation of the DAO).
Although this design pattern is applicable to most programming languages, most software with persistence needs, and most databases, it is traditionally associated with Java EE applications and with relational databases (accessed via the JDBC API because of its origin in Sun Microsystems' best practice guidelines[1] "Core J2EE Patterns".
The primary advantage of using data access objects is the rigorous separation between two parts of an application that have no need to know anything about each other, and which therefore can evolve frequently and independently. Changing business logic can rely on a constant DAO interface, while changes to persistence logic do not affect DAO clients.
In the context of the Java programming language, DAO can be implemented in various ways. This can range from a fairly simple interface that separates data access from the application logic, to frameworks and commercial products.
Potential disadvantages of using DAO include leaky abstraction,[citation needed] code duplication, and abstraction inversion. In particular, the abstraction of the DAO as a regular Java object can obscure the high cost of each database access. Developers may inadvertently make multiple database queries to retrieve information that could be returned in a single operation. If an application requires multiple DAOs, the same create, read, update, and delete code may have to be written for each DAO.[2]
A Data Access Object class can provide access to a particular data resource without coupling the resource's API to the business logic. For example, sample application classes access catalog categories, products, and items using DAO interface CatalogDAO.
Reimplementing CatalogDAO for a different data access mechanism (to use a Connector, for example), would have little or no impact on any classes that use CatalogDAO, because only the implementation would change. Each potential alternate implementation of CatalogDAO would access data for the items in the catalog in its own way, while presenting the same API to the class that uses it.
At runtime, the CatalogHelper uses the CatalogDAOFactory to create an object that implements CatalogDAO. The factory looks up the name of the class that implements the DAO interface in environment entry " param/CatalogDAOClass". The CatalogHelper accesses the catalog data source exclusively using the object created by the factory. In the example shown in the figure, the environment entry was set to the (fully-specified) name of class CloudscapeCatalogDAO . This class implements the catalog DAO interface in terms of JDBC TM data sources, accessing a Cloudscape relational database.
This approach is more flexible than using a hard-coded class. To add a new type of data source, an application developer would simply create a class that implements CatalogDAO in terms of the new data source type, specify the implementing class's name in the environment entry, and re-deploy. The factory would create an instance of the new DAO class, and the application would use the new data source type.
The sample application reduces redundant code by using a "generic DAO" that externalizes the SQL for different JDBC data sources. Figure 3 below shows how the sample application uses an XML file to specify the SQL for different JDBC data sources.
In the figure, the CatalogDAOFactory has selected an object of type GenericCatalogDAO as the DAO to access the catalog. An XML file called CatalogDAOSQL.xml specifies the SQL for each supported operation on each type of database. GenericCatalogDAO configures itself by selecting a group of SQL statements from the XML file that correspond to the database type named by environment entry " param/CatalogDAODatabase". The code sample below shows the definition of the XML for the getCategory operation of the CatalogDAO. Different SQL is specified for "cloudscape" and "oracle" database types.
Method GenericCatalogDAO.getCategory chooses the SQL corresponding to the configured database type, and uses it to fetch a category from the database via JDBC. The following code excerpt shows shows the implementation of the method.
This strategy supports multiple JDBC databases with a single DAO class. It both decreases redundant code, and makes new database types easier to add. To support a new database type, a developer simply adds the SQL statements for that database type to the XML file, updates the environment entry to use the new type, and redeploys.
The pluggable DAO and generic DAO strategies can be used separately. If you know that a DAO class will only ever use JDBC databases (for example), the generic DAO class can be hardwired into the application, instead of selected by a factory. For maximum flexibility, the sample application uses both a factory method and a generic DAO.
A data access object can represent data that is not stored in a database. The sample application uses the DAO pattern to represent XML data sources as objects. Sample application screens are defined in an XML file which is interpreted by the class ScreenDefinitionDAO. Specifying screen definitions externally makes access to the screen definitions more flexible. For example, if the application designers (or maintainers) decide to change the application to store screen descriptions in the database, instead of in an XML file, they would need only to implement a single new class ( ScreenFlowCloudscapeDAO, for example). The code that uses ScreenDefinitionDAO would remain unchanged, but the data would come from the database via the new class.
The code fragment above shows how loadScreenDefinitions loads screen definitions using DOM interfaces, while hiding that fact from clients of the class. A client of this class can expect to receive a Screens object regardless of how those screens are loaded from persistent storage. Method getScreens handles all of the DOM-specific details of loading a screen from an XML file.
We are going to create a Student object acting as a Model or Value Object.StudentDao is Data Access Object Interface.StudentDaoImpl is concrete class implementing Data Access Object Interface. DaoPatternDemo, our demo class, will use StudentDao to demonstrate the use of Data Access Object pattern.
I have recently started using test-driven development and unit testing, and it has paid off immensely in the areas where I have aplied it. One area that it has been helpful in is database access. When I abstract away the data access, the testing of methods that need the data become almost ridiculously easy.
0aad45d008
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