在Session的缓存中存放的是互相关联的对象图。在默认情况下，当Hibernate从数据库中加载Customer对象时，会同时加载所有关联的Order对象，下面我们以Order和Customer为例介绍如何配置Hibernate的检索策略，以优化检索性能。
Session有三种检索方法：load，get和find，他们都用来从数据中检索对象。load和get按照参数指定的OID加载一个持久化对象，find按照参数指定的HQL语句加载一个或多个持久化对象，如下：
<code class="hljs vbnet has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Customer customer = (Customer)session.load(Customer.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Long</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>));Customer customer = (Customer)session.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span>(Customer.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Long</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>));List customerList = session.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"from Customer as c where c.id = 1"</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>
当Hibernate执行以上方法时，需要获得以下信息
1.类级别检索策略：Session的load、get或find方法直接指定检索的是Customer对象，对Customer对象到底采用立即检索还是延迟检索。
2.关联检索策略：对与Customer关联的Order对象，即Customer对象orders集合，到底采用立即检索还是延迟检索或者迫切左外连接检索，详细介绍如下：
三种检索策略的运行机制
下面详细介绍各种检索方式
1、类级别的检索策略
立即检索
类级别默认为立即检索，如下两种配置都为立即检索策略：
<code class="hljs applescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><<span class="hljs-type" style="box-sizing: border-box;">class</span> <span class="hljs-property" style="box-sizing: border-box;">name</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"com.Customer"</span> table=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"CUSTOMER"</span> /><<span class="hljs-type" style="box-sizing: border-box;">class</span> <span class="hljs-property" style="box-sizing: border-box;">name</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"com.Customer"</span> table=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"CUSTOMER"</span> lazy=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"false"</span> /></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>
当通过session的load方法检索Customer对象时：
<code class="hljs vbnet has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Customer customer = (Customer)session.load(Customer.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Long</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
Hibernate会立即查询Customer表
<code class="hljs sql has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-operator" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">select</span> * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> CUSTOMER <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">where</span> id = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>和<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">select</span> * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> ORDERS <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">where</span> customer_id = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
延迟检索
修改以上配置如下：
<code class="hljs fsharp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> name=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"com.Customer"</span> table=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"CUSTOMER"</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">lazy</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"true"</span> /></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
当执行session的load方法时，Hibernate不会立即查询CUSTOMER表的select语句，仅仅返回Customer类的代理类的实例，这个代理类具有以下特征：
1.由Hibernate在运行时动态生成，他扩展了Customer类，因此他继承了Customer类的所有属性和方法，但它的实现对于应用程序是透明的。
2.当Hibernate创建Customer代理类实例时，仅仅初始化了它的OID属性，其他属性都为null，因此这个代理类占用内存很少
3.当应用程序第一次访问Customer代理类实例时，Hibernate会初始化代理类实例，在初始化过程中执行select语句，真正从数据库加载Customer对象所有数据。但有个例外，那就是当应用程序访问Customer代理类的getId()方法时，Hibernate不会初始化代理类实例，因为在创建代理类实例的时候OID就有已经存在了，不必去数据库中查询。
2、一对多和多对多关联的检索策略
在映射文件中，用set元素来配置一对多关联多对多关联关系。Customer.hbm.xml，如下：
<code class="hljs vbnet has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">set</span> name=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"orders"</span> inverse=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"true"</span> > <<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">key</span> column=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"customer_id"</span> /> <one-<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span>-many <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"com.Order"</span> /></<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">set</span>></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>
set元素有个lazy和outer-join属性，下面具体说明：
批量延迟检索和批量立即检索
set元素有一个batch-size属性，用于为延迟检索或者立即检索策略设定批量检索的数量。批量检索能减少select语句的数目，提高检索的性能。
session的find方法用于检索所有的Customer对象，在set元素的lazy属性有true和false来控制延迟加载还是立即加载。
3、迫切左外连接检索
如果把set元素的outer-join属性设置为true
<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">set</span> name=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"orders"</span> inverse=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"true"</span> outer-<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">join</span>=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"true"</span> /></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
当检索Customer对象时，采用迫切左外连接策略来检索所有关联的Order对象，对于以下程序：
<code class="hljs vbnet has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">customer = (Customer)session.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span>(Customer.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span>,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Long</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>));</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
当运行session.get()方法时，执行以下以下sql：
<code class="hljs sql has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-operator" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">select</span> * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> customers <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">left</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">outer</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">join</span> orders <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">on</span> customer_id = orders.customer_id <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">where</span> customer_id = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
需要注意的是session的find方法会忽略映射文件中配置的迫切左外连接检索策略，即使以上set的outer-join配置为true，对于以下代码：
<code class="hljs mathematica has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">List</span> customerList = session.find(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"from Customer as c"</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>
仍然采用立即检索策略。
转载地址：http://blog.csdn.net/fuyuwei2015/article/details/48174195?ref=myread
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