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Class Class Presentation Transcript

  • JDBC – Java DataBase Connectivity java.lang Class Class<T>
  • public final class Class<T>extends Objectimplements Serializable, GenericDeclaration, Type, AnnotatedElement  Instances of the class Class represent classes and interfaces in a running Java application. An enum is a kind of class and an annotation is a kind of interface. Every array also belongs to a class that is reflected as a Class object that is shared by all arrays with the same element type and number of dimensions. The primitive Java types (boolean, byte, char, short, int, long, float, and double), and the keyword void are also represented as Class objects.  Class has no public constructor. Instead Class objects are constructed automatically by the Java Virtual Machine as classes are loaded and by calls to the defineClass method in the class loader. 2
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  • Class.forName() method  This method is used to load the class into JVM’s memory . (without creating the object also it will loads into the jvm’s memory. Class.forname(“class”); Or Reference=Class.forName(“class”) 4
  •      By using the Class.forName() we can load any class object into JVM’s memory without creating the object. Ex; Class c=Class.forName(“java.lang.ArraayList”); When the class.forName() is executed the code it will create a class boject. Inside the class object it plays the name of the class which is loaded into JVM and the name of the package of the class which is loaded into the JVM. It is a factory method( means capable of constructing its own java class object is called “factory method”. 5
  • Ex:  Class.forName("com.mysql.jdbc.Driver");  When this statement is executed, it contains the static block of given jdbc Driver class, so the static block executes automatically  In this static block, there will be a logic to create JDBC driver class object and to register that object with driver manager service through by calling DriverManager.registerDriver() 6
  •  Class.forName() loads the given jdbc Driver class bcoz of the logic available in the static block of that jdbc driver class, the jdbc driver class object will be registered with Driver Manager service 7
  • Static { mysql.jdbc.Driver jd= new mysql.jdbc.Driver(); Try { DriverManager.registerDriver(jd); } Catch(SQLException se) { se.printStackTrace(); } 8
  •  public static Class<?> forName(String className) throws ClassNotFoundException 9
  •  Returns the Class object associated with the class or interface with the given string name. Invoking this method is equivalent to: Class.forName(className, true, currentLoader) where currentLoader denotes the defining class loader of the current class.For example, the following code fragment returns the runtime Class descriptor 10
  •  Class t = Class.forName("java.lang.Thread") A call to forName("X") causes the class named X to be initialized.  Parameters:className - the fully qualified name of the desired class.Returns:the Class object for the class with the specified name.Throws:LinkageError - if the linkage failsExceptionInInitializerError - if the initialization provoked by this method failsClassNotFoundException - if the class 11
  • What is the diff b/w Class.forName() and Class.forName().newInstance()? package test; public class Demo { public Demo() { System.out.println("Hi!"); } @SuppressWarnings("unchecked") public static void main(String[] args) throws Exception { Class clazz = Class.forName("test.Demo"); Demo demo = (Demo) clazz.newInstance(); } } 12
  • What is the diff b/w Class.forName() and Class.forName().newInstance()?     calling Class.forName(String) returns the Class object associated with the class or interface with the given string name i.e. it  returns test.Demo.class which is affected to the clazz variable of  type Class. Then, calling clazz.newInstance() creates a new instance of the class represented by this Classobject. The class is instantiated as if by a new expression with an empty argument list. In other words,  this is here actually equivalent to a new Demo() and returns a new  instance of Demo. And running this Demo class thus prints the following output: Hi! 13
  • What is the diff b/w Class.forName() and Class.forName().newInstance()?   The big difference with the traditional new is  that newInstance allows to instantiate a class that you don't know  until runtime, making your code more dynamic. A typical example is the JDBC API which loads, at runtime, the  exact driver required to perform the work. EJBs containers, Servlet  containers are other good examples: they use dynamic runtime  loading to load and create components they don't know anything  before the runtime. 14
  • What is the diff b/w Class.forName() and Class.forName().newInstance()?     (...) A Driver class is loaded, and therefore automatically registered  with the DriverManager, in one of two ways: by calling the method Class.forName. This explicitly loads the driver  class. Since it does not depend on any external setup, this way of  loading a driver is the recommended one for using  theDriverManager framework. The following code loads the  class acme.db.Driver: Class.forName("acme.db.Driver");If acme.db.Driver has been written so that loading it causes an instance to be created and also calls DriverManager.registerDriver with that instance as the parameter (as it should do), then it is in the DriverManager's list  of drivers and available for creating a connection. (...) 15
  • What is the diff b/w Class.forName() and Class.forName().newInstance()?  In both of these cases, it is the responsibility of the newlyloaded Driver class to register itself by  calling DriverManager.registerDriver. As mentioned, this should be  done automatically when the class is loaded. 16