Data access object DAO in Java

Question

I was going through a document and I came across a term called DAO  I found out that it is a Data Access Object  Can someone please explain me what this actually is    I know that it is some kind of an interface for accessing data from different types of sources  in the middle of this little research of mine I bumped into a concept called data source or data source object  and things got messed up in my mind    I really want to know what a DAO is programmatically in terms of where it is used  How it is used  Any links to pages that explain this concept from the very basic stuff is also appreciated

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What is DATA ACCESS OBJECT  DAO  -    It is a object interface  which is used to access data from database of data storage       WHY WE USE DAO     it abstracts the retrieval of data from a data resource such as a database  The concept is to  separate a data resource s client interface from its data access mechanism    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    Rewrite your application to use SQL Server instead of Oracle  or add conditional code to handle the differences   or Create a layer inbetween your application logic and the data access    Its in all referred as DAO Pattern  It consist of following    Data Access Object Interface - This interface defines the standard operations to be performed on a model object s   Data Access Object concrete class -This class implements above interface  This class is responsible to get data from a datasource which can be database   xml or any other storage mechanism  Model Object or Value Object - This object is simple POJO containing get set methods to store data retrieved using DAO class       Please check this example  This will clear things more clearly     Example I assume this things must have cleared your understanding of DAO up to certain extend

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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   Data Access Object Interface - This interface defines the standard operations to be performed on a model object s    Data Access Object concrete class -This class implements above interface  This class is responsible to get data from a datasource which can be database   xml or any other storage mechanism   Model Object or Value Object - This object is simple POJO containing get set methods to store data retrieved using DAO class   Sample code here

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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

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The Data Access Object is basically an object or an interface that provides access to an underlying database or any other persistence storage    That definition from  http   en wikipedia org wiki Data access object  Check also the sequence diagram here   http   www oracle com technetwork java dataaccessobject-138824 html  Maybe a simple example can help you understand the concept   Let s say we have an entity to represent an employee   public class Employee        private int id      private String name        public int getId             return id            public void setId int id            this id   id            public String getName             return name            public void setName String name            this name   name             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   interface EmployeeDAO        List lt Employee gt  findAll        List lt Employee gt  findById        List lt Employee gt  findByName        boolean insertEmployee Employee employee       boolean updateEmployee Employee employee       boolean deleteEmployee Employee employee        Next we have to provide a concrete implementation for that interface to deal with SQL server  and another to deal with flat files  etc

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Don t get confused with too many explanations  DAO  From the name itself it means Accessing Data using Object  DAO is separated from other Business Logic

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I am going to be general and not specific to Java as DAO and ORM are used in all languages    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   type Person   name age     Now with help of DAO instead of writing some specific queries  to fetch all persons  for what ever type of db you are using  which can be error-prone  instead you do   list persons   DAO getPersons        person   DAO getPersonWithName  John    age   person age    You do not write the DAO abstraction yourself  instead it is usually part of some opensource project  depending on what language and framework you are using   Now to the main question here     where it is used     Well usually if you are writing complex business and domain specific code your life will be very difficult without DAO  Of course you do not need to use ORM and DAO provided  instead you can write your own abstraction and native queries  I have done that in the past and almost always regretted it later

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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   DAO Example  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   Hope it helps  Bye

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I just want to explain it in my own way with a small story that I experienced in one of my projects  First I want to explain Why DAO is important  rather than go to What is DAO  for better understanding  Why DAO is important  In my one project of my project  I used Client class which contains all the basic information of our system users  Where I need client then every time I need to do an ugly query where it is needed  Then I felt that decreases the readability and made a lot of redundant boilerplate code  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 - QueryUtils findAllActivatedClients     In this way  I made some optimizations of my code  But there was another problem     I felt that the QueryUtils class was growing very highly  100  methods were included in that class which was also very cumbersome to read and use  Because this class contains other queries of another domain models   For example- products  categories locations  etc    Then the superhero Mr  CTO introduced a new solution named DAO which solved the problem finally  I felt DAO is very domain-specific  For example  he created a DAO called ClientDAO class where all Client class related queries are found which seems very easy for me to use and maintain  The giant QueryUtils class was broken down into many other domain-specific DAO for example - ProductsDAO class  CategoriesDAO class  etc which made the code more Readable  more Maintainable  more Decoupled  What is DAO  It is an object or interface  which made an easy way to access data from the database without writing complex and ugly queries every time in a reusable way

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Dao clases are used to reuse the jdbc logic  amp  Dao Data Access Object  is a design pattern  dao is a simple java class which contains JDBC logic    Data Access Layer has proven good in separate business logic layer and persistent layer  The DAO design pattern completely hides the data access implementation from its clients  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

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I think the best example  along with explanations  you can find on the oracle website   here  Another good tuturial could be found here

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Pojo also consider as Model class in Java where we can create getter and setter for particular variable defined in private   Remember all variables are here declared with private modifier

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DAO is an act like  as  Persistence Manager   in 3 tier architecture as well as DAO also design pattern as you can consult  Gang of Four  book  Your application service layer just need to call the method of DAO class without knowing hidden  amp  internal details of DAO s method

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Spring JPA DAO  For example we have some entity Group   For this entity we create the repository GroupRepository   public interface GroupRepository extends JpaRepository lt Group  Long gt           Then we need to create a service layer with which we will use this repository   public interface Service lt T  ID gt         T save T entity        void deleteById ID id        List lt T gt  findAll         T getOne ID id        T editEntity T entity        Optional lt T gt  findById ID id      public abstract class AbstractService lt T  ID  R extends JpaRepository lt T  ID gt  gt  implements Service lt T  ID gt         private final R repository       protected AbstractService R repository            this repository   repository              Override     public T save T entity            return repository save entity               Override     public void deleteById ID id            repository deleteById id               Override     public List lt T gt  findAll             return repository findAll                Override     public T getOne ID id            return repository getOne id               Override     public Optional lt T gt  findById ID id            return repository findById id               Override     public T editEntity T entity            return repository saveAndFlush entity             org springframework stereotype Service public class GroupServiceImpl extends AbstractService lt Group  Long  GroupRepository gt         private final GroupRepository groupRepository        Autowired     protected GroupServiceImpl GroupRepository repository            super repository           this groupRepository   repository            And in the controller we use this service    RestController  RequestMapping   api   class GroupController        private final Logger log   LoggerFactory getLogger GroupController class        private final GroupServiceImpl groupService        Autowired     public GroupController GroupServiceImpl groupService            this groupService   groupService              GetMapping   groups       Collection lt Group gt  groups             return groupService findAll                GetMapping   group  id        ResponseEntity lt   gt  getGroup  PathVariable Long id            Optional lt Group gt  group   groupService findById id           return group map response - gt  ResponseEntity ok   body response                    orElse new ResponseEntity lt  gt  HttpStatus NOT FOUND                PostMapping   group       ResponseEntity lt Group gt  createGroup  Valid  RequestBody Group group  throws URISyntaxException           log info  Request to create group       group           Group result   groupService save group           return ResponseEntity created new URI   api group     result getId                      body result               PutMapping   group       ResponseEntity lt Group gt  updateGroup  Valid  RequestBody Group group            log info  Request to update group       group           Group result   groupService save group           return ResponseEntity ok   body result               DeleteMapping   group  id        public ResponseEntity lt   gt  deleteGroup  PathVariable Long id            log info  Request to delete group       id           groupService deleteById id           return ResponseEntity ok   build

[java] Data access object (DAO) in Java

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