Sunday, November 17, 2013

Page Refreshing

Hi,

In this post we will discuss about refreshing a web page at regular interval of time.
There are many ways to do this .
  • Using Java script
  • Setting response headers etc.

We would have seen many popular websites like Facebook, GMail etc. where a page will be refreshing automatically and updating the content at regular intervals of time.
Lets see how to refresh a JSP page with an example.

By using response header
Example :-

PageRefresh.jsp
<%@ taglib prefix="h" uri="http://java.sun.com/jsf/html"%>
<%@page import="java.util.Date" %>
<head>
<title>Page refresh</title>
</head>
<body>
<%response.setHeader("refresh","5");%>
<%Date date=new Date(); %>
<%="Current time " +date %>
</body>
</html>

Here we have used response.setHeader("refresh","5") ,which takes two parameters—refresh attribute and time interval for page refreshing in seconds. PageRefresh.jsp will be refreshing every 5 seconds and displaying the cureent date and time

Output:-

Current time Sun Jun 30 10:21:21 IST 2013

Current time Sun Jun 30 10:21:26 IST 2013

Environment details :-
  • Eclipse Juno
  • Apache Tomcat

                      Happy Learning

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Sunday, October 27, 2013

Named and Native Queries in JPA

Named  and Native Queries in JPA :-
  • Named queires are reusable and can be declared at Entity level
  • @NamedQuery annotation is used
  • multiple named queries are declared in @NamedQueries
  • For executing native queries we use @NamedNativeQueries

@NamedQuery :-
@NamedQuery(name = "allCompanyDetails", query = "SELECT c FROM Company c")

Named query is called by using query = em.createNamedQuery("allCompanyDetails");

@NamedQueries :- annotation is used for declaring multiple named queries at entity level
In the below snippet we have two named queries-"allCompanyDetails" and "onlyMailIds"

@NamedQueries({ @NamedQuery(name = "allCompanyDetails", query = "SELECT c FROM Company c"),
@NamedQuery(name="onlyMailIds",query="SELECT c.mail FROM Company c")
})

Native Queries are used to execute SQL statements directly and to call procedures and functions from JPA
@NamedNativeQueries :-
@NamedNativeQuery(name = "nativeQueryEx", query = "select * from company")

entityManager.createNativeQuery("query") is used to execute native queries

Similar to SQL in JPQL we can write join queries etc,instead of Table names we use Entity class names

Query query = em.createQuery("SELECT c from Company c,Department d where c.mail= d.mail and c.name LIKE 'T%'");

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Java Persistence Query Language-JPQL

JPQL :- Java Persistence Query Language is a query language in JPA which is similar to SQL

  • JPQL queries on Entities and their properties not on tables and queries
  • Syntax is similar to SQL
  • JPQL queries are converted to SQL using JPQL query processor
  • As the queries are based on Objects and attributes,they are independent of database
  • JPQL provides SELECT,UPDATE and DELETE operations,but doesn't support INSERT as insert operations are taken care by em.persist(Entity) method
  • EntityManager.createQuery() and EntityManager.createnamedQuery() are used to query JPQL

Entity class for database table Company

@Entity
public class Company implements Serializable {
private static final long serialVersionUID = 1L;
@Id
private BigDecimal sno;
private String mail;

private String name;

public Company() {
}

public String getMail() {
return this.mail;
}

public void setMail(String mail) {
this.mail = mail;
}

public String getName() {
return this.name;
}

public void setName(String name) {
this.name = name;
}

public BigDecimal getSno() {
return this.sno;
}

public void setSno(BigDecimal sno) {
this.sno = sno;
}

}

Using createQuery :-

Example :-In the below example we are fetching all the records from Company to List

EntityManagerFactory emf = Persistence
.createEntityManagerFactory("JPAExample_Toplink");
EntityManager em = emf.createEntityManager();
EntityTransaction et = em.getTransaction();
et.begin();

String selectQuery = "SELECT c FROM Company c";
Query rs = em.createQuery(selectQuery);

List<Company> list = rs.getResultList();
System.out.println("Printing Company Details");
for (Company company : list) {System.out.println("Name" + company.getName() + " mail id "+ company.getMail());
}
em.close();

If we want to pass values to query then we need to pass parameters.Below techniques are used for setting parameters
Named and Positional parameters :-

Named parameters :-
  • Query parameters are prefixed with colon :
  • query.setParameter() is used to set the values

Query query = em.createQuery("SELECT c FROM Company c WHERE c.name=:companyName");
query.setParameter("companyName", name);

Positional parameters :-
  • Query parameters are prefixed with '?' and followed by numeric position
  • Its similar to PreparedStatement in JDBC

Query query = em.createQuery("SELECT c FROM Company c WHERE c.name=?1");
query.setParameter(1, name);

Here 1 represent the position ? represent value to be set
Similarly we can use query.update() and query.delete() methods also








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Composite primary key -JPA

Composite Primary Key in JPA:-

Composite Primary Key :- Having more than one column as primary key in a data base table is called composite primary key

If a database table has composite primary key then how to represent it in JPA Entity bean?

@Id represents a primary key field

There are two ways of declaring composite primary key
  1. @IdClass
  2. @EmbeddedId with @Embeddable

Suppose we have a table where we depart_name and coll_name are as composite primary key

@IdClass :- using this approach we need to create a separate class,which holds the primary key variables.This class will be referred by entity using @IdClass(CollegeDeptPK.class)

public class CollegeDeptPK implements Serializable{

/**
*
*/
private static final long serialVersionUID = 1L;

private String collname;

private String deptname;

// getter and setters are generated here
}


@Entity
@IdClass(CollegeDeptPK.class)
public class Collegedept implements Serializable {
private static final long serialVersionUID = 1L;
@Id
private String collname;
@Id
private String deptname;

public Collegedept() {
}

public String getCollname() {
return this.collname;
}

public void setCollname(String collname) {
this.collname = collname;
}

public String getDeptname() {
return this.deptname;
}

public void setDeptname(String deptname) {
this.deptname = deptname;
}

}

In the above example Collegedept refers CollegeDeptPK .
CollegeDeptPk has two variables collname and deptname which are composite priamry key
the same variables are declared in our entity class using @Id annotation

The primary key class must contain the fields that match the priamry key attributes in the entity in both name and type

Collegedept collegedept=em.find(Collegedept.class,"Primary Key");
So we need to give a primary key here ,so then how can we proceed in this scenario?
Create an object of CollegeDeptPK ,set the variable values and pass this object reference to find method.

CollegeDeptPK collegedeptpk=new CollegeDeptPK();
collegedeptpk.setCollname("GPREC");
collegedeptpk.setDeptname("EEE");

Collegedept collegedept=em.find(Collegedept.class,collegedeptpk);
Drawbacks :- If composite primary key is implemented using @IdClass then there is duplication of declaration of primary keys in both the cases.
Using @EmbeddedId will solve the above problem

@EmbeddedId :-
  • The priamry key class declaration is annotated @Embeddable and it has all the primary key declarations
  • In entity class instead of using @Id,@EmbeddedId is used.It will be annotated on the top of reference variable
    @EmbeddedId
    priavte CollegeDeptPK collegeDeptPk;
  • The Embeddable priamry key class must be configured in persistence.xml file similar to Entity class
    <class>com.CollegeDeptPK </class>

Primary Key class :-
@Embeddable
public class CollegeDeptPK implements Serializable{

/**
*
*/
private static final long serialVersionUID = 1L;

private String collname;

private String deptname;

public String getDeptname() {
return deptname;
}
//setter and getters
}

Entity class :-
@Entity
public class Collegedept implements Serializable {
private static final long serialVersionUID = 1L;
@EmbeddedId
private CollegeDeptPK collegedeptpk;

public Collegedept() {
}
//getter and setters

}


Take away points :-
  • There are two ways of declaring composite primary keys
    @IdClass
    @EmbeddedId with @Embeddable
  • @EmbeddedId is preferred over @IdClass
  • Using @IdClass will have duplication of primary declarations
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Saturday, October 26, 2013

Persistence.xml file in JPA

Persistence.xml 
How will JPA API (EntityManager)  come to know which database to connect, connection parameters,transaction types,logging level etc.? Persistence.xml is a standard configuration file which gives complete flexibility to configure EntityManager

Persistence.xml :-
  1. is to be created under META-INF/persistence.xml
  2. A persistence.xml can contain one or more unique persistence unit names
  3. Persistence units are unique values used by EntityManagerFactory/Entitymanager
  4. Entities ,Connection parameters,logging level ,transaction types etc are declared in persistence.xml

EntityManagerFactory emf =Persistence.createEntityManagerFactory("JPAExample_Toplink");

The following persistence.xml defines one persistence unit with name JPAExample_Toplink

<persistence version="2.0"
xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">

<persistence-unit name="JPAExample_Toplink"
transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>model.Company</class>
<class>model.Department</class>
<class>model.DeptEmpl</class>

<properties>
<property name="javax.persistence.jdbc.password" value="admin" />
<property name="javax.persistence.jdbc.user" value="system" />
<property name="javax.persistence.jdbc.driver" value="oracle.jdbc.OracleDriver" />
<property name="javax.persistence.jdbc.url" value="jdbc:oracle:thin:@localhost:1521:XE" />
<property name="eclipselink.logging.level" value="INFO" />
<property name="eclipselink.ddl-generation" value="create-tables" />
</properties>
</persistence-unit>
</persistence>

Eclipse automatically creates persistence.xml file when we create a JPA project.It also provides an persistence xml file editor which reduces the manual effort of writing the file.
We can edit :-
Connection
Managed classes
mapping files
Query timeouts









                         Happy Learning

Please provide your valuable comments on this article and share it across your network.


Contact me @ sudheer@javarecent.com orjavarecent@gmail.com
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JDBC statements-Prepared,Callable

Hi All,

In this post we will discuss about different connection statements used in JDBC.
There are primarily three types of connection statements:-
  1. Statement
  2. PreparedStatement
  3. CallableStatement

Lets discuss each one of them in detail
Statement interface :- is used to query database from Java,it provides different methods like executeQuery(), executeUpdate() etc.
  • Resultset executeQuery(String) :- is used to execute SQL SELECT queries,returns ResultSet.
  •  Int executeUpdate(String) :- is used to execute SQL(INSERT,UPDATE,DELETE) queries,returns int value indicating number of rows affected
  • boolean execute(String) :- returns true if a resultSet is retrieved successfully, generally used to execute DML statements
  • Statement query gets compiled and executed every time when it is called
  • It is used in case where there is no repetition of a query
Syntax :- statement.executeQuery("select mail from COMPANY where name="+"'"+uName+"'");

PreparedStatement interface :-
  • Prepared statement queries are precompiled and the fetch plan will be stored in cache,so for subsequent requests only execution will happen
  • They are faster than Statement queries because Statement queries will get compiled every time
  • Used in case there is repetition of a query
  • Prepared statements can be parameterized,parameterization of query values is done by using '?' - place holder in setXXX() method
  • Prepared Statement prevents SQL injection
    For detailed explanation on SQL injection with example refer SQL injection
Syntax :-There are two ways of using prepared statement
1.PreparedStatement preparedStatement = connection.prepareStatement("select * from COMPANY where sid="+"'"+emailId+"'");

2. PreparedStatement preparedStatement = connection.prepareStatement("select * from COMPANY where sid=? AND name=?");
preparedStatement.setInt(1, 101);
preparedStatement.setString(2, "Google");

setXXX() -- takes two parameters first one tells position of the value to be placed,
this starts from 1 .
Second parameter is the respective value to be passed for ?-place holder

The first type of declaration will not prevent SQL injections because we are hard coding the where clause with a variable.
Second type of declaration will prevent SQL injection because all the parameters passed will be escaped by JDBC


CallableStatement :-
  • is an interface used to execute stored procedures
  • There are three types of parameters for a stored procedure
    • IN :- used to provide input to stored procedure,this is set by using setXXX()
    • OUT :- is used to hold values from a procedure,this is retrieved using getXXX()
    • INOUT :- acts as both input and output parameters

Code snippets :-

Procedure

create or replace procedure "P_COMPANY" (pname IN VARCHAR2, pemailid OUT VARCHAR2)
is begin
select mail into pemailid from COMPANY where name=pname;
end;

Procedure P_COMPANY takes two parameters and select query put  the  mail Id into pemailid(OUT parameter)

//CallableStatement usage
1 CallableStatement cs = connection.prepareCall("{call P_COMPANY(?,?)}");
2 cs.setString(1, name);
3 cs.registerOutParameter(2, java.sql.Types.VARCHAR);
4 cs.execute();
5 String mailId = cs.getString(2);
  • Line 1 we are calling the the procedure P_COMPANY
  • setting the IN parameter using setString(1, name);
  • registering OUT parameter ,we have to mention sql data type also
  • execute() or executeQuery() or executeUpdate() can be used to execute the procedure
  • Line 5 getting the output from pemailid OUT variable
Complete code
JDBCConnection.java

public class JDBCConnection {
public void connectDB(String name) throws ClassNotFoundException,
SQLException {
Connection connection = null;
;
CallableStatement cs = null;
try {
Class.forName("oracle.jdbc.driver.OracleDriver");

// Creating connection object
connection = DriverManager.getConnection(
"jdbc:oracle:thin:@localhost:1521:xe", "system", "admin");

cs = connection.prepareCall("{call P_COMPANY(?,?)}");
cs.setString(1, name);
cs.registerOutParameter(2, java.sql.Types.VARCHAR);
cs.execute();
String mailId = cs.getString(2);

System.out.println("Required Mail id" + mailId);

} finally {
try {
connection.close();
cs.close();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

public static void main(String[] args) {
JDBCConnection jdbc = new JDBCConnection();
try {
jdbc.connectDB("Yahoo");

} catch (ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

}


Output :- Required Mail id yahoo@yahoo.com


Key Points :-
  • Statement is used for non-repeating queries
  • Prepared Statement will be pre-compiled,on subsequent requests the query will get executes
  • Action/Execution plan is stored in cache which helps in faster execution
  • Prepared Statements will prevent SQL injections
  • Prepared Statements allows parameterization of values by using place holder [?]
  • Procedure can be executed using callable interface
  • IN parameter is set using setXXX()
  • OUT parameter is retrived by first registering the parameter and using getXXX()




                          Happy Learning

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