hibernate的查詢方式及抓取策略
查詢方式
根據唯一標識id進行檢索：
get與load方法(https://blog.csdn.net/qq_40605913/article/details/81460410)。
物件導航檢索：
根據一個已經查詢到的物件，獲得其關聯的物件。
HQL查詢：
Hibernate Query Language，Hibernate的查詢語言，是一種面向物件的方式的查詢語言，語法類似SQL。通過session.createQuery()，用於接收一個HQL進行查詢方式。
簡單查詢：
//HQL的簡單查詢 public void test(){ Session session = HibernateUtils.getCurrentSession(); Transaction transaction = session.beginTransaction(); Query query = session.createQuery("from Customer"); List<Customer> list = query.list(); for (Customer customer : list) { System.out.println(customer); } transaction.commit(); }
//HQL的簡單查詢之別名查詢 public void test(){ Session session = HibernateUtils.getCurrentSession(); Transaction transaction = session.beginTransaction(); //c是別名，但HQL不支援*,聚合函式中的*除外 Query query = session.createQuery("select c from Customer c"); List<Customer> list = query.list(); for (Customer customer : list) { System.out.println(customer); } transaction.commit(); }
排序查詢：
//HQL的排序查詢
public void test3(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//預設
// Query query = session.createQuery(" from Customer order by cust_id");
//降序
Query query = session.createQuery(" from Customer order by cust_id desc");
//升序
// Query query = session.createQuery(" from Customer order by cust_id asc");
List<Customer> list = query.list();
for (Customer customer : list) {
System.out.println(customer);
}
transaction.commit();
}條件查詢：
public void test3(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//條件查詢
//1.按位置繫結，多個條件時類推
// Query query = session.createQuery("from Customer where cust_id=?");
// query.setParameter(0, 1l);
// List<Customer> list = query.list();
//2.按名稱繫結--根據引數位置進行繫結
//aaa,bbb這些名字隨便起，但與:之間不能有空格
Query query = session.createQuery("from Customer where cust_id = :aaa and cust_name = :bbb");
//設定引數
query.setParameter("aaa", 1l);
query.setParameter("bbb", "22");
List<Customer> list = query.list();
transaction.commit();
}投影查詢：
public void test3(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//單個屬性
// List<Object> list = session.createQuery("select c.cust_name from Customer c").list();
//多個屬性
// List<Object []> list = session.createQuery("select c.cust_name,c.cust_id from Customer c").list();
// for (Object [] object : list) {
// System.out.println(Arrays.toString(object));
// }
//將查詢結果封裝到物件中
List<Customer> list = session.createQuery("select new Customer(cust_name,cust_id) from Customer").list();
for (Customer customer : list) {
System.out.println(customer);
}
transaction.commit();
}
分頁查詢：
//HQL的分頁查詢
public void test3(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
Query query = session.createQuery("from Customer");
query.setFirstResult(0);
query.setMaxResults(2);
List<Customer> list = query.list();
for (Object object : list) {
System.out.println(object);
}
transaction.commit();
}分組統計查詢：
//HQL的分組統計查詢
public void test3(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//聚合函式：count(),sum(),max(),min(),avg()
// Object object = session.createQuery("select count(*) from Customer").uniqueResult();
// System.out.println(object);
Query query = session.createQuery("select cust_id,count(*) from Customer group by cust_name");
List<Object[]> object = query.list();
for (Object[] object2 : object) {
System.out.println(Arrays.toString(object2));
}
transaction.commit();
}QBC:
Query By Criteria，條件查詢。是一種更加面向物件化的查詢的方式。
簡單查詢：
//簡單查詢
public void test(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//獲得Criteria物件
Criteria criteria = session.createCriteria(Customer.class);
List<Customer> list = criteria.list();
for (Customer customer : list) {
System.out.println(customer.toString());
}
transaction.commit();
}排序查詢：
//排序查詢
public void test1(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//獲得Criteria物件
Criteria criteria = session.createCriteria(Customer.class);
//升序查詢，對應降序為desc
criteria.addOrder(Order.asc("cust_id"));
List<Customer> list = criteria.list();
for (Customer customer : list) {
System.out.println(customer.toString());
}
transaction.commit();
}分頁查詢：
//分頁查詢
public void test1(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//獲得Criteria物件
Criteria criteria = session.createCriteria(Customer.class);
criteria.setFirstResult(0);
criteria.setMaxResults(2);
List<Customer> list = criteria.list();
for (Customer customer : list) {
System.out.println(customer);
}
transaction.commit();
}條件查詢：
//條件查詢
public void test1(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//獲得Criteria物件
Criteria criteria = session.createCriteria(Customer.class);
/**
* = eq
* > gt
* >= ge
* < lt
* <= le
* <> ne 不等於
*/
//設定條件
criteria.add(Restrictions.eq("cust_name", "11"));
List<Customer> list = criteria.list();
for (Customer customer : list) {
System.out.println(customer);
}
transaction.commit();
}統計查詢：
//統計查詢
@Test
public void test1(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//獲得Criteria物件
Criteria criteria = session.createCriteria(Customer.class);
/**
* add :普通的條件,where後面事件
* addOrder:排序
* setProjection:聚合函式和group by having
*/
criteria.setProjection(Projections.rowCount());
Long num = (Long)criteria.uniqueResult();
System.out.println(num);
transaction.commit();
}離線查詢：
//離線查詢
public void test1(){
DetachedCriteria detachedCriteria = DetachedCriteria.forClass(Customer.class);
detachedCriteria.add(Restrictions.ilike("cust_name", "1%"));
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
Criteria criteria = detachedCriteria.getExecutableCriteria(session);
List<Customer> list = criteria.list();
for (Customer customer : list) {
System.out.println(customer);
}
}多表查詢：
//多表查詢
public void test1(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
//SQL:SELECT * FROM cst_customer c INNER JOIN cst_linkman l ON c.cust_id = l.lkm_cust_id;
//HQL(內連線):from Customer c inner join c.linkMans
//List<Object[]> list = session.createQuery("from Customer c inner join c.linkMans").list();
//HQL(迫切內連線)：在普通內連線inner join後新增一個關鍵字fetch。它發的SQL語句與
//內連線一樣，作用:通知hibernate將資料封裝到物件裡，因為Customer中有LinkMan的集合，所以把
//LinkMan封裝到Customer中，返回的list集合中放的是Customer(select distincr c 是為了去掉重複)
List<Object[]> list = session.createQuery("select distinct c from Customer c inner join fetch c.linkMans").list();
}SQL:
public void test1(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
SQLQuery sqlQuery = session.createSQLQuery("select * from cst_customer");
sqlQuery.addEntity(Customer.class);
List<Customer> list = sqlQuery.list();
for (Customer customer : list) {
System.out.println(customer);
}
transaction.commit();
} 延遲載入:
延遲載入：lazy（懶載入）。執行到該行程式碼的時候，不會發送語句去進行查詢，在真正使用這個物件的屬性的時候才會傳送SQL語句進行查詢。
類級別延遲載入：
//類級別延遲載入,在<class>標籤上配置lazy=true，這樣load方法就和get方法一樣
//在使用時才傳送查詢語句
public void test1(){
Session session = HibernateUtils.getCurrentSession();
Transaction transaction = session.beginTransaction();
session.load(Customer.class, 1l);
transaction.commit();
}讓lazy失效：
將lazy設定為false
持久化類使用final修飾
Hibernate. Initialize()
關聯級別：
在<set>和<many-to-one>上來配置
抓取策略：
<set>上的fetch和lazy
fetch：抓取策略，控制SQL語句格式
select ：預設值，傳送普通的select語句，查詢關聯物件
join ：傳送一條迫切左外連線查詢關聯物件
subselect ：傳送一條子查詢查詢其關聯物件
lazy：延遲載入，控制查詢關聯物件的時候是否採用延遲
true ：預設值，查詢關聯物件的時候，採用延遲載入
false ：查詢關聯物件的時候，不採用延遲載入
extra ：及其懶惰。
<many-to-one>上的fetch和lazy
fetch ：抓取策略，控制SQL語句格式。
select ：預設值，傳送普通的select語句，查詢關聯物件。
join ：傳送一條迫切左外連線。
lazy ：延遲載入，控制查詢關聯物件的時候是否採用延遲。
proxy ：預設值，proxy具體的取值，取決於另一端的<class>上的lazy的值。
false ：查詢關聯物件，不採用延遲。
no-proxy ：（不會使用）
批量抓取：
一批關聯物件一起抓取，batch-size
<set>上可以配置batch-size,但one-many中不能配置，需要在一的一方的class標籤中配置這個屬性