Hibernate关联关系关联指的是类之间的引用关系。如果类A与类B关联那么被引用的类B将被定义为类A的属性。例如:订单和订单项一个订单对应多个订单项Order.javapackage entity; import java.io.Serializable; import java.util.HashSet; import java.util.Set; public class Order implements Serializable { private static final long serialVersionUID -1737859893948370664L; private Integer orderId; private String orderNo; private SetOrderItem orderItem new HashSetOrderItem(); public Order() { super(); } public Integer getOrderId() { return orderId; } public void setOrderId(Integer orderId) { this.orderId orderId; } public String getOrderNo() { return orderNo; } public void setOrderNo(String orderNo) { this.orderNo orderNo; } public SetOrderItem getOrderItem() { return orderItem; } public void setOrderItem(SetOrderItem orderItem) { this.orderItem orderItem; } Override public String toString() { return Order [orderId orderId , orderNo orderNo ]; } }订单配置文件Order.hbm.xml!DOCTYPE hibernate-mapping PUBLIC -//Hibernate/Hibernate Mapping DTD 3.0//EN http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd hibernate-mapping class nameentity.Order tablet_order id nameorderId columnorder_id typeinteger generator classnative/generator /id property nameorderNo columnorder_no typestring/property !-- lazy是否懒加载默认是懒加载的true 懒汉模式 name类的关联属性名 cascade级联关系 级联新增与修改 inverse关联关系交给对方控制 默认是true当前类不维护关联关系 子标签key:column:主表的主键从表的外键 -- set nameorderItem inversetrue cascadesave-update key columnoid/key one-to-many classentity.OrderItem/ /set /class /hibernate-mapping订单项OrderItem.javapackage entity; import java.io.Serializable; public class OrderItem implements Serializable{ private static final long serialVersionUID 7224901622594643894L; private Integer orderItemId; private Integer productId; private Integer quantity; private Integer oid; //如果类OrderItem与Order关联那么被引用的类Order将被定义为类OrderItem的属性 private Order order; public OrderItem() { super(); } public Integer getOrderItemId() { return orderItemId; } public void setOrderItemId(Integer orderItemId) { this.orderItemId orderItemId; } public Integer getProductId() { return productId; } public void setProductId(Integer productId) { this.productId productId; } public Integer getQuantity() { return quantity; } public void setQuantity(Integer quantity) { this.quantity quantity; } public Integer getOid() { return oid; } public void setOid(Integer oid) { this.oid oid; } public Order getOrder() { return order; } public void setOrder(Order order) { this.order order; } Override public String toString() { return OrderItem [orderItemId orderItemId , productId productId , quantity quantity ]; } }订单项配置文件OrderItem.hbm.xml!DOCTYPE hibernate-mapping PUBLIC -//Hibernate/Hibernate Mapping DTD 3.0//EN http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd hibernate-mapping class nameentity.OrderItem tablet_order_item id nameorderItemId columnorder_item_id typeinteger generator classnative/generator /id property nameproductId columnproduct_id typeinteger/property property namequantity columnquantity typeinteger/property property insertfalse updatefalse nameoid columnoid typeinteger/property many-to-one nameorder classentity.Order columnoid/many-to-one /class /hibernate-mapping编写Junit测试类OrderDAOTestpackage dao; import org.junit.Before; import org.junit.Test; import entity.Order; import entity.OrderItem; public class OrderDAOTest { private OrderDAO orderDAO; private Order order; Before public void setUp() throws Exception { orderDAO new OrderDAO(); order new Order(); } Test public void testAdd() { order.setOrderNo(p3); OrderItem orderItem null; for (int i 1; i 10; i) { orderItem new OrderItem(); orderItem.setProductId(i); orderItem.setQuantity(i1); orderItem.setOrder(order); order.getOrderItem().add(orderItem); } orderDAO.add(order); } Test public void testLoad() { order.setOrderId(3); Order load orderDAO.load(order); System.out.println(load.getOrderId()); System.out.println(load.getOrderNo()); System.out.println(load.getOrderItem().size()); } }懒汉模式加载是不创建实例用户获取时才真正判断是否为空如果为空才创建实例public class Singleton2 { //构造方法私有化不允许外面直接创建对象 private Singleton2(){} //声明类的唯一实例使用private static 修饰 private static Singleton2 instance; //提供一个获取实例的方法使用 public staitc修饰 public static Singleton2 getInstance() { if(instancenull) { instancenew Singleton2(); } return instance; } } public class Test { public static void main(String[] args) { Singleton2 s3Singleton2.getInstance(); Singleton2 s4Singleton2.getInstance(); if(s3s4) System.out.println(s1和s2是同一个实例); else System.out.println(s1和s2不是同一个实例); } }懒加载(lazy)就是延时加载延迟加载。lazy有三个属性true、false、extra集合属性时候使用true:默认取值它的意思是只有在调用这个集合获取里面的元素对象时才发出查询语句加载其集合元素的数据。false:取消懒加载特性即在加载对象的同时就发出第二条查询语句加载其关联集合的数据。extra:一种比较聪明的懒加载策略即调用集合的size/contains等方法的时候hibernate并不会去加载整个集合的数据而是发出一条聪明的SQL语句以便获得需要的值只有在真正需要用到这些集合元素对象数据的时候才去发出查询语句加载所有对象的数据。extra能够更高性能的提升Hibernate的查询效率。