Java之jpa入门教程讲解

package com.gloomyfish.jpa.tutorial;

import java.io.Serializable;

import javax.persistence.*;

@Entity

public class Point implements Serializable {

     private static final long serialVersionUID = 1L;

     @Id @GeneratedValue

     private long id;

     private int x;

     private int y;

     public Point() {

     }

     Point( int x, int y) {

         this .x = x;

         this .y = y;

     }

     public Long getId() {

         return id;

     }

     public int getX() {

          return x;

     }

     public int getY() {

          return y;

     }

     @Override

     public String toString() {

         return String.format( "(%d, %d)" , this .x, this .y);

     }

}

package com.gloomyfish.jpa.tutorial;

import java.util.List;

import javax.persistence.EntityManager;

import javax.persistence.EntityManagerFactory;

import javax.persistence.Persistence;

import javax.persistence.Query;

import javax.persistence.TypedQuery;

public class JPAMain {

     public static void main(String[] args) {

         // Open a database connection

         // (create a new database if it doesn't exist yet):

         EntityManagerFactory emf =

             Persistence.createEntityManagerFactory( "$objectdb/db/points.odb" );

         EntityManager em = emf.createEntityManager();

         // Store 1000 Point objects in the database:

         long startTime = System.currentTimeMillis();

         em.getTransaction().begin();

         for ( int i = 0 ; i < 10 ; i++) {

             Point p = new Point(i, i);

             em.persist(p);

         }

         em.getTransaction()测试数据mit();

         long endTime = System.currentTimeMillis();

         System.out.println( "save time = " + (endTime - startTime));

         // Find the number of Point objects in the database:

         Query q1 = em.createQuery( "SELECT COUNT(p) FROM Point p" );

         System.out.println( "Total Points: " + q1.getSingleResult());

         // Find the average X value:

         Query q2 = em.createQuery( "SELECT AVG(p.x) FROM Point p" );

         System.out.println( "Average X: " + q2.getSingleResult());

         // Retrieve all the Point objects from the database:

         TypedQuery<Point> query =

             em.createQuery( "SELECT p FROM Point p" , Point. class );

         List<Point> results = query.getResultList();

         for (Point p : results) {

             System.out.println(p);

         }

         // Close the database connection:

         em.close();

         emf.close();

     }

}