12 Persistent Objects: Serialization and RelationalDatabases
Persistent Objects Basics
The concept ofsaving the state of an object so that it can be used later is called persistence.
Figure 12.2 Object life cycle with persistence.
Saving the Object to a Flat File
Java has abuilt-in mechanism for object persistence. Like other C-based languages, Javalargely uses the concept of a stream to deal with I/O.To write to a Stream, objects must implement either the Serializable or Externalizable interface.
Serializing a File
class Person implements Serializable{
…
publicSavePerson(){
…
}
}
Implementation and Interface Revisited
Person person = (Person )ois.readObject();
What About the Methods?
the methods themselves are notnecessarily kept in the data store.
Using XML in the Serialization Process
XML is thestandard for defining data, so we can create an XML model of our serializationexample that can, at least theoretically, be used across various platforms andlanguages.
Let’s first lookat the C# code.
[XmlAttribute(“name”)]
public String Name
{
get
{
return this.strName;
}
set
{
if (value == null) return;
this.strName = value;
}
}
public void Serialize()
{
Person[]myPeople = new Person[3];
myPeople[0] =new Person(“John Q. Public”, 32, 95);
myPeople[1] =new Person(“Jacob M. Smith”, 35, 67);
myPeople[2] =new Person(“Joe L. Jones”, 65, 77);
XmlSerializermySerializer = new XmlSerializer(typeof(Person[]));
TextWritermyWriter = new StreamWriter(“person.xml”);
mySerializer.Serialize(myWriter,myPeople);
myWriter.Close();
}
the fileproduced is in XML.
<?xml version=”1.0” encoding=”utf-8”?>
<ArrayOfPerson xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance”
xmlns:xsd=”http://www.w3.org/2001/XMLSchema”>
<Person name=”John Q.Public”>
<age>32</age>
</Person>
<Person name=”Jacob M.Smith”>
<age>35</age>
</Person>
<Person name=”Joe L.Jones”>
<age>65</age>
</Person>
</ArrayOfPerson>
To restore theobject,we use the following code:
public void DeSerialize()
{
Person[]myRestoredPeople;
XmlSerializermySerializer = new XmlSerializer(typeof(Person[]));
TextReadermyReader = new StreamReader(“person.xml”);
myRestoredPeople= (Person[])mySerializer.Deserialize(myReader);
Console.WriteLine(“My People restored:”);
foreach (PersonlistPerson in myRestoredPeople)
{
Console WriteLine(listPerson.Name + “ is “ +
listPerson.Age + “ years old.”);
}
Console.WriteLine(“Press any key to continue...”);
Console.ReadKey();
}
As we havenoted, one of the major advantages of this approach is that the XML file isaccessible by any and all languages and platforms that implement the XMLinterface, including Java.
Writing to a Relational Database
Accessing aRelational Database
JDBC Java Database Connectivity
Open Database Connectivity (ODBC)
Figure 12.7 Database client server model using ODBC/JDBC.
Loading the Driver
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
Making theConnection
Connection con = DriverManager.getConnection(“jdbc:odbc:myDriver”, “id”, “pwd”);
The SQL Statements
String sqlQuery= “SELECTPRODUCT FFROM SUPPLIERTABLE WHERE PRODUCT = ‘Bolts’”;
ResultSet rs = statement.executeQuery(sqlQuery);
The complete code for this exampleis as follows:
public void findVendor(String vendorId)throws SQLException{
StringreturnString = null;
StringdbUserid = “userid”; // Your Database user id
StringdbPassword = “password” ; // Your Database password
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
Connectionconnection =
DriverManager.getConnection(“jdbc:odbc:myDriver”,dbUserid ,dbPassword);
Statementstatement = connection .createStatement();
StringsqlQuery=
“selectPRODUCT from SUPPLIERTABLE where PRODUCT = ‘Bolts’”;
ResultSetrs = statement.executeQuery(sqlQuery);
if(rs.next())
{
System.out.println(“rs.getString(“SUPPLIERID”));
}
statement.close();
connection.close();
}
Conclusion
In this chapter,we covered theconcept of object persistence.