java - How to correctly use entitymanager for creating entity objects? -

good day,

i on first jpa project , have difficulties using , understanding entity manager. have created several classes , assigned them entities via annotations. now, trying create class create entity object via entitymanager. instance, have following class:

@entity @table(name = "product") public class product implements defaultproduct {  @id @column(name = "product_name", nullable = false) private string productname;  @column(name = "product_price", nullable = false) private double productprice;  @column(name = "product_quantity", nullable = false) private int productquantity;  @manytoone @joincolumn(name = "product_account", nullable = false) private account parentaccount;  public product(string productname, double productprice,         int productquantity, account parentaccount)         throws illegalargumentexception {      if (productname == null || productprice == 0 || productquantity == 0) {         throw new illegalargumentexception(                 "product name/price or quantity have not been specified.");     }     this.productname = productname;     this.productprice = productprice;     this.productquantity = productquantity;     this.parentaccount = parentaccount; }  public string getproductname() {     return productname; }  public double getprice() {     return productprice * productquantity; }  public int getquantity() {     return productquantity; }  public account getaccount() {     return parentaccount; }  } 

now, trying create class:

public class createproduct {  private static final string persistence_unit_name = "product";  entitymanagerfactory factory = persistence         .createentitymanagerfactory(persistence_unit_name);  public void createproduct(product product) {     entitymanager manager = factory.createentitymanager();     manager.gettransaction().begin();  //code written here      manager.gettransaction().commit();  } } 

can please give me example of code have write down between begin() , commit() lines inside createproduct method; additionally, appreciate if explain how entitymanager works. have read several docs still need clarification.

thanks in advance

 account account - new account();  product product = new product("name", 10, 11, account);  manager.persist(product); 

depending when put annotations @column @onetoone , others on fields or getters entitymanager use way gets field values class. uses reflection read annotations , using knows table should looks like. knowing table structure creates query in background ind sends db. analysis of classes done when creating entity manager factory , operation in general time consuming (depends on db structure). understend deaply how works read more reflection. if reflection how simple is.