377 lines
14 KiB
XML
377 lines
14 KiB
XML
<chapter id="querycriteria">
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<title>
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条件查询(Criteria Queries)
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</title>
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<para>
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具有一个直观的、可扩展的条件查询API是Hibernate的特色。
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</para>
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<sect1 id="querycriteria-creating">
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<title>创建一个<literal>Criteria</literal> 实例</title>
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<para>
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<literal>org.hibernate.Criteria</literal>接口表示特定持久类的一个查询。<literal>Session</literal>是
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<literal>Criteria</literal>实例的工厂。
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</para>
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<programlisting><![CDATA[Criteria crit = sess.createCriteria(Cat.class);
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crit.setMaxResults(50);
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List cats = crit.list();]]></programlisting>
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</sect1>
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<sect1 id="querycriteria-narrowing">
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<title>限制结果集内容</title>
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<para>
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一个单独的查询条件是<literal>org.hibernate.criterion.Criterion</literal>
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接口的一个实例。<literal>org.hibernate.criterion.Restrictions</literal>类
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定义了获得某些内置<literal>Criterion</literal>类型的工厂方法。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.like("name", "Fritz%") )
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.add( Restrictions.between("weight", minWeight, maxWeight) )
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.list();]]></programlisting>
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<para>
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约束可以按逻辑分组。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.like("name", "Fritz%") )
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.add( Restrictions.or(
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Restrictions.eq( "age", new Integer(0) ),
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Restrictions.isNull("age")
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) )
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.list();]]></programlisting>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.in( "name", new String[] { "Fritz", "Izi", "Pk" } ) )
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.add( Restrictions.disjunction()
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.add( Restrictions.isNull("age") )
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.add( Restrictions.eq("age", new Integer(0) ) )
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.add( Restrictions.eq("age", new Integer(1) ) )
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.add( Restrictions.eq("age", new Integer(2) ) )
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) )
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.list();]]></programlisting>
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<para>
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Hibernate提供了相当多的内置criterion类型(<literal>Restrictions</literal>
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子类), 但是尤其有用的是可以允许你直接使用SQL。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.sql("lower({alias}.name) like lower(?)", "Fritz%", Hibernate.STRING) )
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.list();]]></programlisting>
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<para>
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<literal>{alias}</literal>占位符应当被替换为被查询实体的列别名。
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</para>
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<para>
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<literal>Property</literal>实例是获得一个条件的另外一种途径。你可以通过调用<literal>Property.forName()</literal>
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创建一个<literal>Property</literal>。
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</para>
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<programlisting><![CDATA[
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Property age = Property.forName("age");
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List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.disjunction()
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.add( age.isNull() )
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.add( age.eq( new Integer(0) ) )
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.add( age.eq( new Integer(1) ) )
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.add( age.eq( new Integer(2) ) )
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) )
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.add( Property.forName("name").in( new String[] { "Fritz", "Izi", "Pk" } ) )
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.list();]]></programlisting>
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</sect1>
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<sect1 id="querycriteria-ordering">
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<title>结果集排序</title>
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<para>
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你可以使用<literal>org.hibernate.criterion.Order</literal>来为查询结果排序。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.like("name", "F%")
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.addOrder( Order.asc("name") )
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.addOrder( Order.desc("age") )
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.setMaxResults(50)
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.list();]]></programlisting>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Property.forName("name").like("F%") )
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.addOrder( Property.forName("name").asc() )
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.addOrder( Property.forName("age").desc() )
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.setMaxResults(50)
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.list();]]></programlisting>
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</sect1>
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<sect1 id="querycriteria-associations">
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<title>关联</title>
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<para>
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你可以使用<literal>createCriteria()</literal>非常容易的在互相关联的实体间建立
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约束。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.like("name", "F%")
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.createCriteria("kittens")
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.add( Restrictions.like("name", "F%")
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.list();]]></programlisting>
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<para>
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注意第二个 <literal>createCriteria()</literal>返回一个新的
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<literal>Criteria</literal>实例，该实例引用<literal>kittens</literal> 集合中的元素。
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</para>
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<para>
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接下来，替换形态在某些情况下也是很有用的。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.createAlias("kittens", "kt")
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.createAlias("mate", "mt")
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.add( Restrictions.eqProperty("kt.name", "mt.name") )
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.list();]]></programlisting>
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<para>
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(<literal>createAlias()</literal>并不创建一个新的
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<literal>Criteria</literal>实例。)
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</para>
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<para>
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<literal>Cat</literal>实例所保存的之前两次查询所返回的kittens集合是
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<emphasis>没有</emphasis>被条件预过滤的。如果你希望只获得符合条件的kittens，
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你必须使用<literal>returnMaps()</literal>。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.createCriteria("kittens", "kt")
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.add( Restrictions.eq("name", "F%") )
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.returnMaps()
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.list();
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Iterator iter = cats.iterator();
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while ( iter.hasNext() ) {
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Map map = (Map) iter.next();
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Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS);
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Cat kitten = (Cat) map.get("kt");
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}]]></programlisting>
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</sect1>
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<sect1 id="querycriteria-dynamicfetching" revision="1">
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<title>动态关联抓取</title>
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<para>
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你可以使用<literal>setFetchMode()</literal>在运行时定义动态关联抓取的语义。
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</para>
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<programlisting><![CDATA[List cats = sess.createCriteria(Cat.class)
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.add( Restrictions.like("name", "Fritz%") )
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.setFetchMode("mate", FetchMode.EAGER)
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.setFetchMode("kittens", FetchMode.EAGER)
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.list();]]></programlisting>
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<para>
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这个查询可以通过外连接抓取<literal>mate</literal>和<literal>kittens</literal>。
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查看<xref linkend="performance-fetching"/>可以获得更多信息。
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</para>
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</sect1>
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<sect1 id="querycriteria-examples">
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<title>查询示例</title>
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<para>
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<literal>org.hibernate.criterion.Example</literal>类允许你通过一个给定实例
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构建一个条件查询。
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</para>
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<programlisting><![CDATA[Cat cat = new Cat();
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cat.setSex('F');
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cat.setColor(Color.BLACK);
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List results = session.createCriteria(Cat.class)
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.add( Example.create(cat) )
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.list();]]></programlisting>
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<para>
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版本属性、标识符和关联被忽略。默认情况下值为null的属性将被排除。
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</para>
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<para>
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你可以自行调整<literal>Example</literal>使之更实用。
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</para>
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<programlisting><![CDATA[Example example = Example.create(cat)
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.excludeZeroes() //exclude zero valued properties
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.excludeProperty("color") //exclude the property named "color"
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.ignoreCase() //perform case insensitive string comparisons
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.enableLike(); //use like for string comparisons
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List results = session.createCriteria(Cat.class)
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.add(example)
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.list();]]></programlisting>
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<para>
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你甚至可以使用examples在关联对象上放置条件。
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</para>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.add( Example.create(cat) )
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.createCriteria("mate")
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.add( Example.create( cat.getMate() ) )
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.list();]]></programlisting>
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</sect1>
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<sect1 id="querycriteria-projection">
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<title>投影(Projections)、聚合（aggregation）和分组（grouping）</title>
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<para>
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<literal>org.hibernate.criterion.Projections</literal>是
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<literal>Projection</literal> 的实例工厂。我们通过调用
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<literal>setProjection()</literal>应用投影到一个查询。
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</para>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.setProjection( Projections.rowCount() )
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.add( Restrictions.eq("color", Color.BLACK) )
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.list();]]></programlisting>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.setProjection( Projections.projectionList()
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.add( Projections.rowCount() )
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.add( Projections.avg("weight") )
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.add( Projections.max("weight") )
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.add( Projections.groupProperty("color") )
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)
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.list();]]></programlisting>
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<para>
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在一个条件查询中没有必要显式的使用 "group by" 。某些投影类型就是被定义为<emphasis>
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分组投影</emphasis>，他们也出现在SQL的<literal>group by</literal>子句中。
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</para>
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<para>
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你可以选择把一个别名指派给一个投影，这样可以使投影值被约束或排序所引用。下面是两种不同的实现方式：
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</para>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.setProjection( Projections.alias( Projections.groupProperty("color"), "colr" ) )
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.addOrder( Order.asc("colr") )
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.list();]]></programlisting>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.setProjection( Projections.groupProperty("color").as("colr") )
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.addOrder( Order.asc("colr") )
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.list();]]></programlisting>
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<para>
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<literal>alias()</literal>和<literal>as()</literal>方法简便的将一个投影实例包装到另外一个
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别名的<literal>Projection</literal>实例中。简而言之，当你添加一个投影到一个投影列表中时
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你可以为它指定一个别名：
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</para>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.setProjection( Projections.projectionList()
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.add( Projections.rowCount(), "catCountByColor" )
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.add( Projections.avg("weight"), "avgWeight" )
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.add( Projections.max("weight"), "maxWeight" )
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.add( Projections.groupProperty("color"), "color" )
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)
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.addOrder( Order.desc("catCountByColor") )
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.addOrder( Order.desc("avgWeight") )
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.list();]]></programlisting>
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<programlisting><![CDATA[List results = session.createCriteria(Domestic.class, "cat")
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.createAlias("kittens", "kit")
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.setProjection( Projections.projectionList()
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.add( Projections.property("cat.name"), "catName" )
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.add( Projections.property("kit.name"), "kitName" )
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)
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.addOrder( Order.asc("catName") )
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.addOrder( Order.asc("kitName") )
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.list();]]></programlisting>
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<para>
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你也可以使用<literal>Property.forName()</literal>来表示投影：
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</para>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.setProjection( Property.forName("name") )
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.add( Property.forName("color").eq(Color.BLACK) )
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.list();]]></programlisting>
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<programlisting><![CDATA[List results = session.createCriteria(Cat.class)
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.setProjection( Projections.projectionList()
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.add( Projections.rowCount().as("catCountByColor") )
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.add( Property.forName("weight").avg().as("avgWeight") )
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.add( Property.forName("weight").max().as("maxWeight") )
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.add( Property.forName("color").group().as("color" )
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)
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.addOrder( Order.desc("catCountByColor") )
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.addOrder( Order.desc("avgWeight") )
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.list();]]></programlisting>
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</sect1>
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<sect1 id="querycriteria-detachedqueries">
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<title>离线(detached)查询和子查询</title>
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<para>
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<literal>DetachedCriteria</literal>类使你在一个session范围之外创建一个查询，并且可以使用任意的
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<literal>Session</literal>来执行它。
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</para>
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<programlisting><![CDATA[DetachedCriteria query = DetachedCriteria.forClass(Cat.class)
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.add( Property.forName("sex").eq('F') );
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Session session = ....;
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Transaction txn = session.beginTransaction();
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List results = query.getExecutableCriteria(session).setMaxResults(100).list();
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txn.commit();
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session.close();]]></programlisting>
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<para>
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<literal>DetachedCriteria</literal>也可以用以表示子查询。条件实例包含子查询可以通过
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<literal>Subqueries</literal>或者<literal>Property</literal>获得。
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</para>
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<programlisting><![CDATA[DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class)
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.setProjection( Property.forName("weight").avg() );
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session.createCriteria(Cat.class)
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.add( Property.forName("weight).gt(avgWeight) )
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.list();]]></programlisting>
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<programlisting><![CDATA[DetachedCriteria weights = DetachedCriteria.forClass(Cat.class)
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.setProjection( Property.forName("weight") );
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session.createCriteria(Cat.class)
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.add( Subqueries.geAll("weight", weights) )
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.list();]]></programlisting>
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<para>
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甚至相互关联的子查询也是有可能的：
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</para>
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<programlisting><![CDATA[DetachedCriteria avgWeightForSex = DetachedCriteria.forClass(Cat.class, "cat2")
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.setProjection( Property.forName("weight").avg() )
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.add( Property.forName("cat2.sex").eqProperty("cat.sex") );
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session.createCriteria(Cat.class, "cat")
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.add( Property.forName("weight).gt(avgWeightForSex) )
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.list();]]></programlisting>
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</sect1>
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<!--TODO: ResultSetTransformer + aliasing. AliasToBeanTransformer allow returning arbitrary
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user objects - similar to setResultClass in JDO2. General use of ResultTransformer
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could also be explained. -->
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</chapter>
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