Hibernate的优化方案

IamHero 4年前
   <h2>HQL优化</h2>    <ul>     <li><strong>使用参数绑定</strong>      <ol>       <li>使用绑定参数的原因是让数据库一次解析SQL,对后续的重复请求可以使用生成好的执行计划,这样做节省CPU时间和内存。</li>       <li>避免SQL注入。</li>      </ol> </li>     <li><strong>尽量少使用NOT</strong> 如果where子句中包含not关键字,那么执行时该字段的索引失效。</li>     <li><strong>尽量使用where来替换having</strong> having在检索出所有记录后才对结果集进行过滤,这个处理需要一定的开销,而where子句限制记录的数目,能减少这方面的开销。</li>     <li><strong>减少对表的查询</strong> 在含有子查询的HQL中,尽量减少对表的查询,降低开销。</li>     <li><strong>使用表的别名</strong> 当在HQL语句中连接多个表时,使用别名,提高程序阅读性,并把别名前缀与每个列连接上,这样一来,可以减少解析时间并减少列歧义引起的语法错误。</li>     <li><strong>实体的更新与删除</strong> 在Hibernate3以后支持hql的update与delete操作。可参考度娘。</li>    </ul>    <h2>一级缓存优化</h2>    <p>一级缓存也叫做session缓存,在一个hibernate session有效,这级缓存的可干预性不强,大多于hibernate自动管理,但它提供清除缓存的方法,这在大批量增加(更新)操作是有效果的,例如,同时增加十万条记录,按常规进行,很可能会出现异常,这时可能需要手动清除一级缓存, session.evict 以及 session.clear 。</p>    <h2>检索策略(抓取策略)</h2>    <h2>延迟加载</h2>    <p>延迟加载是Hibernate为提高程序执行的效率而提供的一种机制,即只有真正使用该对象的数据时才会创建。load方法采用的策略是延迟加载;get方法采用的策略是立即加载。</p>    <p>检索策略分为两种:</p>    <ol>     <li>类级别检索</li>     <li>关联级别检索</li>    </ol>    <h2>类级别检索</h2>    <p>类级别检索是通过session直接检索某一类对应的数据,例如:</p>    <pre>  <code class="language-java">Customer c = session.load(Customer.class, 1);</code></pre>    <p>或</p>    <pre>  <code class="language-java">session.createQuery("from Order");</code></pre>    <p>类级别检索策略分为立即检索与延迟检索,默认是延迟检索,类级别的检索策略可以通过 <class> 元素的lazy属性来设置,默认值是true。所以我们可在hbm映射配置文件中设置如下:</p>    <pre>  <code class="language-java"><class name="Customer" table="t_customer" catalog="hibernateTest" lazy="true">      ...  </class></code></pre>    <p>除此之外,我们也可在PO类中使用 @Proxy 注解,例如:</p>    <pre>  <code class="language-java">@Proxy(lazy = true)  public class Customer {      ...  }</code></pre>    <p>现在我着重来讲一下在PO类中 @Proxy 注解的使用。提示,以下所有案例代码的编写都是建立在 <strong> Hibernate检索方式概述 </strong> 一文案例基础之上的。首先将 @Proxy(lazy = true) 这样的注解加在PO类——Customer类上,这样Customer类的代码就变成:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  @NamedQuery(name="myHql", query="from Customer")  // @SqlResultSetMapping注解才真正帮我们去规定执行sql语句如何将结果封装到Customer对象  @SqlResultSetMapping(name="customerSetMapping",entities={ @EntityResult(entityClass=Customer.class,fields={           @FieldResult(name="id",column="id"),@FieldResult(name="name",column="name") }) })          // fields指定类里面的每一个属性跟表中的列是如何对应的  @NamedNativeQuery(name="findCustomer",query="select * from t_customer",resultSetMapping="customerSetMapping")                                                                     // resultSetMapping需要指定一个名字,它用来指定结果如何封装的操作  @Proxy(lazy = true)  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      /*       * targetEntity="...":相当于<one-to-many class="...">       * mappedBy="...":相当于inverse=true,即放弃关联关系的维护,不然会生成一个中间表       */      @OneToMany(targetEntity=Order.class,mappedBy="c")      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>为了便于演示延迟加载,在cn.itheima.test包下编写一个LoadTest单元测试类,并在该类中编写如下测试方法:</p>    <pre>  <code class="language-java">// 演示延迟加载  public class LoadTest {        @Test      public void test1() {          Session session = HibernateUtils.openSession();          session.beginTransaction();            Customer c1 = session.load(Customer.class, 1);          // Customer c1 = session.get(Customer.class, 1);          String name = c1.getName();            System.out.println(name);            session.getTransaction().commit();          session.close();      }    }</code></pre>    <p>在 String name = c1.getName(); 这句代码上加上一个断点,然后以Debug的方式调试该程序,就能得到你想要的东西哟!!!</p>    <p>如果将lazy设置为false,代表类级别检索也使用立即检索,这时load与get方法就完全一样了,都是立即检索。</p>    <p>虽然我们是知道了load方法采用的策略是延迟加载;get方法采用的策略是立即加载,但是什么时候用get方法,什么时候用load方法呢?—— 如果你查询的数据非常大,例如说它里面有一些大的字段,这个时候建议你采用load方法,不要一上来就立即加载,把我们的内存占满,这样可以让我们的性能得到一部分的提升;如果你查询的数据非常少,直接get就无所谓了,因为它不会占用我们很多的内存。</p>    <p>还有一个问题:Hibernate这个框架是在Dao层进行操作的,如果说我现在采用了一个load的方案去获取了一个对象,我们最终会把Session关闭再返回,那么我们就要把这个对象返回到Service层,再返回到Web层,这个时候load出来的代理对象其实还没有对数据进行初始化,也即它里面还没有真正有数据,返回的时候就出问题了,那 <strong>如何对一个延迟的代理对象进行初始化呢?</strong> 以码明示,在LoadTest单元测试类中编写如下测试方法:</p>    <pre>  <code class="language-java">public class LoadTest {        // 如果对一个延迟的代理对象进行初始化?      @Test      public void test2() {          Session session = HibernateUtils.openSession();          session.beginTransaction();            Customer c1 = session.load(Customer.class, 1);          Hibernate.initialize(c1);            session.getTransaction().commit();          session.close();            // return c1;      }    }</code></pre>    <p>在 Customer c1 = session.load(Customer.class, 1); 这句代码上加上一个断点,然后以Debug的方式调试该程序,就能得到你想要的东西哟!!!</p>    <h2>关联级别检索</h2>    <p>查询到某个对象,获得其关联的对象或属性,这种就称为关联级别检索,例如:</p>    <pre>  <code class="language-java">c.getOrders().size()  c.getName()</code></pre>    <p>对于关联级别检索我们就要研究其检索策略(抓取策略)了。</p>    <h2>检索策略(抓取策略)</h2>    <h3>抓取策略介绍</h3>    <p>抓取策略指的是查找到某个对象后,通过这个对象去查询关联对象的信息时的一种策略。Hibernate中对象之间的关联关系有:</p>    <ol>     <li>一对一: <one-to-one></li>     <li>一对多(多对一): <set> 下有 <one-to-many> ,与 <many-to-one></li>     <li>多对多: <set> 下有 <many-to-many></li>    </ol>    <p>此处我们主要讲的是在 <set> 与 <many-to-one> 或 <one-to-one> 标签上设置fetch、lazy这两个属性。</p>    <ul>     <li>fetch主要描述的是SQL语句的格式(例如是多条,子查询,多表联查)</li>     <li>lazy用于控制SQL语句何时发送</li>    </ul>    <p>例如,查询一个客户,要关联查询它的订单。客户代表一的一方,在客户中有set集合来描述其订单,在配置中我们是使用的:</p>    <pre>  <code class="language-java"><set>      <one-to-many>  </set></code></pre>    <p>此时就可以在set标签上设置这两个属性fetch、lazy。</p>    <p>再比如,查询一个订单时,要查询关联的客户信息。订单代表多的一方,在订单中有Customer对象来描述其关联的客户,在配置中我们是使用 <many-to-one> 标签,此时也可以在该标签上设置这两个属性fetch、lazy。当然了,也可在标签 <one-to-one> 上设置这两个属性fetch、lazy。</p>    <h3>注解配置抓取策略</h3>    <p>以一个问题来引出该小节的讲解,如何使用注解来配置抓取策略?</p>    <ul>     <li> <p>在 <set> 标签上设置的fetch与lazy可以使用下面注解来描述:</p> <pre>  <code class="language-java">@Fetch(FetchMode.SUBSELECT)  @LazyCollection(LazyCollectionOption.EXTRA)  private Set<Order> orders = new HashSet<Order>();</code></pre> <p>若是在映射配置文件中进行设置,则如下:</p> <img src="https://simg.open-open.com/show/30c779a70e82bb9f21d51d0d358626cb.png"></li>     <li> <p>在 <many-to-one> 或 <one-to-one> 标签上设置的fetch与lazy可以使用下面注解来描述:</p> <pre>  <code class="language-java">@Fetch(FetchMode.SELECT)  @LazyToOne(LazyToOneOption.FALSE)  private Customer c; // 描述订单属于某一个客户</code></pre> <p>若是在映射配置文件中进行设置,则如下:</p> <img src="https://simg.open-open.com/show/5ec08fae78fbc696b662b3450d221eea.png"></li>    </ul>    <h3>set上的fetch与lazy</h3>    <p>set上的fetch与lazy主要是用于设置关联的集合信息的抓取策略。</p>    <p>fetch可取值有:</p>    <ol>     <li>SELECT:多条简单的sql(默认值)</li>     <li>JOIN:采用迫切左外连接</li>     <li>SUBSELECT:将生成子查询的SQL</li>    </ol>    <p>lazy可取值有:</p>    <ol>     <li>TURE:延迟检索(默认值)</li>     <li>FALSE:立即检索</li>     <li>EXTRA:加强延迟检索(及其懒惰)</li>    </ol>    <p>这样说来,fetch与lazy的组合就有九种了,其实不然,fetch与lazy的组合实际上只有七种,且听我娓娓道来。</p>    <p>第一种组合</p>    <p>首先修改cn.itheima.domain包下的两个PO类,如下:</p>    <ul>     <li> <p>客户类</p> <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SELECT)      @LazyCollection(LazyCollectionOption.TRUE)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre> </li>     <li> <p>订单类</p> <pre>  <code class="language-java">// 订单 ---- 多的一方  @Entity  @Table(name="t_order")  public class Order {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id;      private Double money;      private String receiverInfo; // 收货地址        // 订单与客户关联      @ManyToOne(targetEntity=Customer.class)      @JoinColumn(name="c_customer_id") // 指定外键列      @Cascade(CascadeType.SAVE_UPDATE)      private Customer c; // 描述订单属于某一个客户        public Customer getC() {          return c;      }      public void setC(Customer c) {          this.c = c;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public Double getMoney() {          return money;      }      public void setMoney(Double money) {          this.money = money;      }      public String getReceiverInfo() {          return receiverInfo;      }      public void setReceiverInfo(String receiverInfo) {          this.receiverInfo = receiverInfo;      }        @Override      public String toString() {          return "Order [id=" + id + ", money=" + money + ", receiverInfo=" + receiverInfo + "]";      }    }</code></pre> </li>    </ul>    <p>接着在cn.itheima.test包下编写一个SetFetchTest单元测试类,并在该类中编写如下测试方法:</p>    <pre>  <code class="language-java">public class SetFetchTest {        @Test      public void test1() {          Session session = HibernateUtils.openSession();          session.beginTransaction();            // 1.得到id=1的Customer          Customer customer = session.get(Customer.class, 1);            // 2.得到id=1的Customer关联的Order信息          int size = customer.getOrders().size();            System.out.println(size);            session.getTransaction().commit();          session.close();      }    }</code></pre>    <p>在 int size = customer.getOrders().size(); 这句代码上加上一个断点,然后以Debug的方式调试该程序,就能得出结论:会首先查询客户信息,当需要订单信息时,才会关联查询订单信息,并在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?  Hibernate:       select          orders0_.c_customer_id as c_custom4_1_0_,          orders0_.id as id1_1_0_,          orders0_.id as id1_1_1_,          orders0_.c_customer_id as c_custom4_1_1_,          orders0_.money as money2_1_1_,          orders0_.receiverInfo as receiver3_1_1_       from          t_order orders0_       where          orders0_.c_customer_id=?</code></pre>    <p>第二种组合</p>    <p>首先将客户类的代码改为:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SELECT)      @LazyCollection(LazyCollectionOption.FALSE)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>然后以Debug的方式运行SetFetchTest单元测试类中的test1方法,就能得出结论:当查询客户信息时,就会将订单信息也查询,也就是说订单信息没有进行延迟查询。并在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?  Hibernate:       select          orders0_.c_customer_id as c_custom4_1_0_,          orders0_.id as id1_1_0_,          orders0_.id as id1_1_1_,          orders0_.c_customer_id as c_custom4_1_1_,          orders0_.money as money2_1_1_,          orders0_.receiverInfo as receiver3_1_1_       from          t_order orders0_       where          orders0_.c_customer_id=?</code></pre>    <p>第三种组合</p>    <p>首先将客户类的代码改为:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SELECT)      @LazyCollection(LazyCollectionOption.EXTRA)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>然后以Debug的方式运行SetFetchTest单元测试类中的test1方法,就能得出结论:当查询客户信息时,不会查询订单信息,当需要订单的个数时,也不会查询订单信息,只会通过count来统计订单个数,当我们使用size()、contains()或isEmpty()方法时也不会查询订单信息。并在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?  Hibernate:       select          count(id)       from          t_order       where          c_customer_id =?</code></pre>    <p>第四种组合</p>    <p>如果fetch选择的是join方案,那么lazy它就会失效。生成SQl语句采用的是迫切左外连接(left outer join fetch),也即这个时候会多表联查,既然是多表联查,就会把信息都查询出来,它既然是一个迫切左外连接,会根据你的需求把信息封装到你指定的对象里面,所以lazy它就会失效。</p>    <p>为了测试这第四种组合,首先将客户类的代码改为:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.JOIN)      @LazyCollection(LazyCollectionOption.FALSE)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>然后以Debug的方式运行SetFetchTest单元测试类中的test1方法,在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_,          orders1_.c_customer_id as c_custom4_1_1_,          orders1_.id as id1_1_1_,          orders1_.id as id1_1_2_,          orders1_.c_customer_id as c_custom4_1_2_,          orders1_.money as money2_1_2_,          orders1_.receiverInfo as receiver3_1_2_       from          t_customer customer0_       left outer join          t_order orders1_               on customer0_.id=orders1_.c_customer_id       where          customer0_.id=?</code></pre>    <p>第五种组合</p>    <p>首先将客户类的代码改为:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SUBSELECT)      @LazyCollection(LazyCollectionOption.TRUE)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>接着在SetFetchTest单元测试类中编写如下测试方法:</p>    <pre>  <code class="language-java">public class SetFetchTest {        @SuppressWarnings("unchecked")      @Test      public void test2() {          Session session = HibernateUtils.openSession();          session.beginTransaction();            // 1.查询出所有的客户信息          List<Customer> list = session.createQuery("from Customer").list();            for (Customer customer : list) {              System.out.println(customer.getOrders().size());          }            session.getTransaction().commit();          session.close();      }    }</code></pre>    <p>在 List<Customer> list = session.createQuery("from Customer").list(); 这句代码上加上一个断点,然后以Debug的方式调试该程序,就能得出结论:会生成子查询,但是我们在查询订单时采用的是延迟加载。并在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_,          customer0_.name as name2_0_       from          t_customer customer0_  Hibernate:       select          orders0_.c_customer_id as c_custom4_1_1_,          orders0_.id as id1_1_1_,          orders0_.id as id1_1_0_,          orders0_.c_customer_id as c_custom4_1_0_,          orders0_.money as money2_1_0_,          orders0_.receiverInfo as receiver3_1_0_       from          t_order orders0_       where          orders0_.c_customer_id in (              select                  customer0_.id               from                  t_customer customer0_          )</code></pre>    <p>第六种组合</p>    <p>首先将客户类的代码改为:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SUBSELECT)      @LazyCollection(LazyCollectionOption.FALSE)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>然后以Debug的方式运行SetFetchTest单元测试类中的test2方法,就能得出结论:会生成子查询,在查询客户信息时,就会将订单信息也查询出来。并在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_,          customer0_.name as name2_0_       from          t_customer customer0_  Hibernate:       select          orders0_.c_customer_id as c_custom4_1_1_,          orders0_.id as id1_1_1_,          orders0_.id as id1_1_0_,          orders0_.c_customer_id as c_custom4_1_0_,          orders0_.money as money2_1_0_,          orders0_.receiverInfo as receiver3_1_0_       from          t_order orders0_       where          orders0_.c_customer_id in (              select                  customer0_.id               from                  t_customer customer0_          )</code></pre>    <p>第七种组合</p>    <p>首先将客户类的代码改为:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SUBSELECT)      @LazyCollection(LazyCollectionOption.EXTRA)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>然后以Debug的方式运行SetFetchTest单元测试类中的test2方法,就能得出结论:在查询订单时,只会根据情况来确定是否要订单信息,如果不需要,例如我们程序中的size操作,那么就会发出 select count(*) from Order where c_customer_id=? 这样的语句。这时Eclipse控制台会打印:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_,          customer0_.name as name2_0_       from          t_customer customer0_  Hibernate:       select          count(id)       from          t_order       where          c_customer_id =?  10  Hibernate:       select          count(id)       from          t_order       where          c_customer_id =?  10  Hibernate:       select          count(id)       from          t_order       where          c_customer_id =?  0</code></pre>    <h3>many-to-one或one-to-one上的fetch与lazy</h3>    <p>set上的fetch与lazy主要是设置在获取到代表一的一方时,如何去查询代表多的一方。那么在 <many-to-one> 或 <one-to-one> 标签上如何设置fetch与lazy,然后去查询对方。对于我们的程序来说,就是在代表多的一方如何查询代表一的一方的信息。例如,获取到一个订单对象,要查询客户信息。</p>    <p>fetch可取值有:</p>    <ol>     <li>select:默认值,代表发送一条或多条简单的select语句</li>     <li>join:发送一条迫切左外连接</li>    </ol>    <p>lazy可取值有:</p>    <ol>     <li>false:不采用延迟加载</li>     <li>proxy:默认值,是否采用延迟不由本方说了算,而是需要由另一方的类级别延迟策略来决定</li>     <li>no-proxy:在此不讨论</li>    </ol>    <p>第一种组合</p>    <p>首先修改Order类的代码为:</p>    <pre>  <code class="language-java">// 订单 ---- 多的一方  @Entity  @Table(name="t_order")  public class Order {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id;      private Double money;      private String receiverInfo; // 收货地址        // 订单与客户关联      @ManyToOne(targetEntity=Customer.class)      @JoinColumn(name="c_customer_id") // 指定外键列      @Cascade(CascadeType.SAVE_UPDATE)      @Fetch(FetchMode.SELECT)      @LazyToOne(LazyToOneOption.PROXY)      private Customer c; // 描述订单属于某一个客户        public Customer getC() {          return c;      }      public void setC(Customer c) {          this.c = c;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public Double getMoney() {          return money;      }      public void setMoney(Double money) {          this.money = money;      }      public String getReceiverInfo() {          return receiverInfo;      }      public void setReceiverInfo(String receiverInfo) {          this.receiverInfo = receiverInfo;      }        @Override      public String toString() {          return "Order [id=" + id + ", money=" + money + ", receiverInfo=" + receiverInfo + "]";      }    }</code></pre>    <p>然后将Customer类的类级别延迟策略置为lazy=true,此时Customer类变为:</p>    <pre>  <code class="language-java">// 客户 ---- 一的一方  @Entity  @Table(name="t_customer")  @Proxy(lazy=true)  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SUBSELECT)      @LazyCollection(LazyCollectionOption.EXTRA)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre>    <p>接着在cn.itheima.test包下编写一个OneFetchTest单元测试类,并在该类中编写如下测试方法:</p>    <pre>  <code class="language-java">public class OneFetchTest {        @Test      public void test1() {          Session session = HibernateUtils.openSession();          session.beginTransaction();            // 1.得到一个订单          Order order = session.get(Order.class, 1);            // 2.得到订单对应的客户          Customer c = order.getC();            System.out.println(c.getName());          session.getTransaction().commit();          session.close();      }    }</code></pre>    <p>在 Order order = session.get(Order.class, 1); 这句代码上加上一个断点,然后以Debug的方式调试该程序,就能得出结论:会首先发送一条sql只查询订单信息,客户信息会延迟,只有真正需要客户信息时,才会发送sql来查询客户信息。并在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          order0_.id as id1_1_0_,          order0_.c_customer_id as c_custom4_1_0_,          order0_.money as money2_1_0_,          order0_.receiverInfo as receiver3_1_0_       from          t_order order0_       where          order0_.id=?  Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?</code></pre>    <p>第二种组合</p>    <p>首先Order类的代码不用修改,只将Customer类的类级别延迟策略置为lazy=false,即在Customer类上加上 @Proxy(lazy=false) 注解。</p>    <p>然后以Debug的方式运行OneFetchTest单元测试类中的test1方法,就能得出结论:当查询订单时,就会将客户信息也查询到,原因是Customer类的类级别延迟策略为false,也就是立即查询。而且在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          order0_.id as id1_1_0_,          order0_.c_customer_id as c_custom4_1_0_,          order0_.money as money2_1_0_,          order0_.receiverInfo as receiver3_1_0_       from          t_order order0_       where          order0_.id=?  Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?</code></pre>    <p>第三种组合</p>    <p>首先将Order类的代码修改为:</p>    <pre>  <code class="language-java">// 订单 ---- 多的一方  @Entity  @Table(name="t_order")  public class Order {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id;      private Double money;      private String receiverInfo; // 收货地址        // 订单与客户关联      @ManyToOne(targetEntity=Customer.class)      @JoinColumn(name="c_customer_id") // 指定外键列      @Cascade(CascadeType.SAVE_UPDATE)      @Fetch(FetchMode.SELECT)      @LazyToOne(LazyToOneOption.FALSE)      private Customer c; // 描述订单属于某一个客户        public Customer getC() {          return c;      }      public void setC(Customer c) {          this.c = c;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public Double getMoney() {          return money;      }      public void setMoney(Double money) {          this.money = money;      }      public String getReceiverInfo() {          return receiverInfo;      }      public void setReceiverInfo(String receiverInfo) {          this.receiverInfo = receiverInfo;      }        @Override      public String toString() {          return "Order [id=" + id + ", money=" + money + ", receiverInfo=" + receiverInfo + "]";      }    }</code></pre>    <p>然后以Debug的方式运行OneFetchTest单元测试类中的test1方法,就能得出结论:当查询订单时,不会对客户信息进行延迟,会立即查询客户信息。而且在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          order0_.id as id1_1_0_,          order0_.c_customer_id as c_custom4_1_0_,          order0_.money as money2_1_0_,          order0_.receiverInfo as receiver3_1_0_       from          t_order order0_       where          order0_.id=?  Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?</code></pre>    <p>提示:这种组合不用理Customer类的类级别延迟策略。</p>    <p>第四种组合</p>    <p>首先将Order类的代码修改为:</p>    <pre>  <code class="language-java">// 订单 ---- 多的一方  @Entity  @Table(name="t_order")  public class Order {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id;      private Double money;      private String receiverInfo; // 收货地址        // 订单与客户关联      @ManyToOne(targetEntity=Customer.class)      @JoinColumn(name="c_customer_id") // 指定外键列      @Cascade(CascadeType.SAVE_UPDATE)      @Fetch(FetchMode.JOIN)      @LazyToOne(LazyToOneOption.FALSE)      private Customer c; // 描述订单属于某一个客户        public Customer getC() {          return c;      }      public void setC(Customer c) {          this.c = c;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public Double getMoney() {          return money;      }      public void setMoney(Double money) {          this.money = money;      }      public String getReceiverInfo() {          return receiverInfo;      }      public void setReceiverInfo(String receiverInfo) {          this.receiverInfo = receiverInfo;      }        @Override      public String toString() {          return "Order [id=" + id + ", money=" + money + ", receiverInfo=" + receiverInfo + "]";      }    }</code></pre>    <p>然后以Debug的方式运行OneFetchTest单元测试类中的test1方法,就能得出结论:如果fetch的值为join,那么lazy将失效,这时会发送一条迫切左外连接来查询,也就立即查询。而且在Eclipse控制台打印如下sql语句:</p>    <pre>  <code class="language-java">Hibernate:       select          order0_.id as id1_1_0_,          order0_.c_customer_id as c_custom4_1_0_,          order0_.money as money2_1_0_,          order0_.receiverInfo as receiver3_1_0_,          customer1_.id as id1_0_1_,          customer1_.name as name2_0_1_       from          t_order order0_       left outer join          t_customer customer1_               on order0_.c_customer_id=customer1_.id       where          order0_.id=?</code></pre>    <p>这种组合当然也就不需要搭理Customer类的类级别延迟策略了。</p>    <h3>批量抓取</h3>    <p>我们在查询多个对象的关联对象时,可以采用批量抓取方式来对程序进行优化。要想实现批量抓取,可以在映射配置文件中通过batch-size属性来设置,也可以使用注解 @BatchSize(size=4) 来设置,其中size表示一次抓取的条数。</p>    <p>先查询客户,然后再查询订单</p>    <p>首先检查两个PO类的代码是否如下:</p>    <ul>     <li> <p>客户类</p> <pre>  <code class="language-java">@Entity  @Table(name="t_customer")  @Proxy(lazy=true)  public class Customer {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id; // 主键      private String name; // 姓名        // 描述客户可以有多个订单      @OneToMany(targetEntity=Order.class,mappedBy="c")      @Fetch(FetchMode.SELECT)      @LazyCollection(LazyCollectionOption.TRUE)      private Set<Order> orders = new HashSet<Order>();        public Customer() {        }        public Customer(Integer id, String name) {          super();          this.id = id;          this.name = name;      }        public Set<Order> getOrders() {          return orders;      }      public void setOrders(Set<Order> orders) {          this.orders = orders;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }        @Override      public String toString() {          return "Customer [id=" + id + ", name=" + name + "]";      }    }</code></pre> </li>     <li> <p>订单类</p> <pre>  <code class="language-java">@Entity  @Table(name="t_order")  public class Order {        @Id      @GeneratedValue(strategy=GenerationType.IDENTITY)      private Integer id;      private Double money;      private String receiverInfo; // 收货地址        // 订单与客户关联      @ManyToOne(targetEntity=Customer.class)      @JoinColumn(name="c_customer_id") // 指定外键列      @Cascade(CascadeType.SAVE_UPDATE)      private Customer c; // 描述订单属于某一个客户        public Customer getC() {          return c;      }      public void setC(Customer c) {          this.c = c;      }      public Integer getId() {          return id;      }      public void setId(Integer id) {          this.id = id;      }      public Double getMoney() {          return money;      }      public void setMoney(Double money) {          this.money = money;      }      public String getReceiverInfo() {          return receiverInfo;      }      public void setReceiverInfo(String receiverInfo) {          this.receiverInfo = receiverInfo;      }        @Override      public String toString() {          return "Order [id=" + id + ", money=" + money + ", receiverInfo=" + receiverInfo + "]";      }    }</code></pre> </li>    </ul>    <p>为了查询出所有用户的订单信息,我在cn.itheima.test包下编写一个BatchFetchTest单元测试类,并在该类中编写如下测试方法:</p>    <pre>  <code class="language-java">// 演示批量抓取  public class BatchFetchTest {        // 查询出所有用户的订单信息      @Test      public void test1() {          Session session = HibernateUtils.openSession();          session.beginTransaction();            // 1.得到所有客户          List<Customer> list = session.createQuery("from Customer").list();            // 2.得到客户的订单信息          for (Customer customer : list) {              System.out.println(customer.getOrders().size());          }            session.getTransaction().commit();          session.close();      }    }</code></pre>    <p>运行以上方法,可发现Eclipse控制台打印如下:</p>    <pre>  <code class="language-java">Hibernate:       select          customer0_.id as id1_0_,          customer0_.name as name2_0_       from          t_customer customer0_  Hibernate:       select          orders0_.c_customer_id as c_custom4_1_0_,          orders0_.id as id1_1_0_,          orders0_.id as id1_1_1_,          orders0_.c_customer_id as c_custom4_1_1_,          orders0_.money as money2_1_1_,          orders0_.receiverInfo as receiver3_1_1_       from          t_order orders0_       where          orders0_.c_customer_id=?  10  Hibernate:       select          orders0_.c_customer_id as c_custom4_1_0_,          orders0_.id as id1_1_0_,          orders0_.id as id1_1_1_,          orders0_.c_customer_id as c_custom4_1_1_,          orders0_.money as money2_1_1_,          orders0_.receiverInfo as receiver3_1_1_       from          t_order orders0_       where          orders0_.c_customer_id=?  10  Hibernate:       select          orders0_.c_customer_id as c_custom4_1_0_,          orders0_.id as id1_1_0_,          orders0_.id as id1_1_1_,          orders0_.c_customer_id as c_custom4_1_1_,          orders0_.money as money2_1_1_,          orders0_.receiverInfo as receiver3_1_1_       from          t_order orders0_       where          orders0_.c_customer_id=?  0</code></pre>    <p>上述代码操作,当我们执行时,首先发出一条sql来查询所有客户信息,然后根据客户的id来查询订单信息,因为有三个客户,所以发送了三条sql,完成了查询订单信息的操作。以上一共执行了四条sql语句来完成操作,这就引出了一个N+1的经典问题。这里,就 <strong>可以采用批量抓取来解决N+1问题。</strong></p>    <p>我们不仅可以在客户类映射配置文件中的 <set> 标签上配置batch-size,如下:</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/bc9ae116ccc6256f4b576fe36878ee90.png"></p>    <p>而且也可使用注解 @BatchSize(size=3) 来进行配置,即需要在Customer类中的 orders 属性上加上 @BatchSize(size=3) 注解。</p>    <p>这样再次运行test1方法,就可发现Eclipse控制台打印如下:</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/e3e119faf9238787670f2b8c52ed3fd5.png"></p>    <p>提示:size的值要根据你当前的环境来设置,但是它的值不要太大,最好不要超过50。</p>    <p>先查询订单,然后再查询客户</p>    <p>为了查询出所有的订单,然后根据订单再查询出客户信息,我在BatchFetchTest单元测试类再编写如下测试方法:</p>    <pre>  <code class="language-java">public class BatchFetchTest {        // 查询出所有的订单,然后根据订单再查询出客户信息      @Test      public void test2() {          Session session = HibernateUtils.openSession();          session.beginTransaction();            // 1.得到所有订单          List<Order> list = session.createQuery("from Order").list();            // 2.得到客户信息          for (Order order : list) {              System.out.println(order.getC().getName());          }            session.getTransaction().commit();          session.close();      }    }</code></pre>    <p>运行以上方法,可发现Eclipse控制台打印如下:</p>    <pre>  <code class="language-java">Hibernate:       select          order0_.id as id1_1_,          order0_.c_customer_id as c_custom4_1_,          order0_.money as money2_1_,          order0_.receiverInfo as receiver3_1_       from          t_order order0_  Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?  Hibernate:       select          customer0_.id as id1_0_0_,          customer0_.name as name2_0_0_       from          t_customer customer0_       where          customer0_.id=?</code></pre>    <p>订单一共有两种,在查询时会首先发送一条sql查询出所有订单,然后再根据订单查询出所有客户,一共3条语句完成。这时也出现同样的N+1问题,当然也可以采用批量抓取来解决这个N+1问题。</p>    <p>注意:订单与客户,客户它是一个主表,订单是一个从表。在设置批量抓取时都是在主表中设置。故我们不仅可以在客户类映射配置文件中的 <class> 标签上配置batch-size,如下:</p>    <p><img src="https://simg.open-open.com/show/0e539fc39b6e51f989d85b2a5d89b928.png"></p>    <p>而且也可使用注解 @BatchSize(size=10) 来进行配置,即需要在Customer类上加上 @BatchSize(size=10) 注解。</p>    <p>这样再次运行test2方法,就可发现Eclipse控制台打印如下:</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/715c1bf893c7277ab483a8555748ab81.png"></p>    <p><strong>总结</strong></p>    <p>无论是根据哪一方来查询另一方,在进行批量抓取时,都是在父方设置。如果是要查询子方信息,那么我们是在父方那个映射配置文件的 <set> 标签上来设置batch-size属性,如果是从子方来查询父方,也是在父方那个映射配置文件的 <class> 标签上设置batch-size属性。</p>    <p>父方与子方的区分:有外键的表是子方(从表),关联方就是父方(主表)。</p>    <p> </p>    <p>来自:http://blog.csdn.net/yerenyuan_pku/article/details/70768603</p>    <p> </p>