iBATIS入门常识进阶四

iBATIS入门知识进阶四
一、关联数据的存储查询方式,以班级和学生为例，
在学生多方添加班级一方的引用，班级一端增加对学生集合的引用，建立双向关联

实体类

public class Classes {private int id;private String name;         private List students; //不可以用set否则会出错public List getStudents() {return students;}public void setStudents(List students) {this.students = students;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}}public class Student {private int id;private String name;private String sex;private Classes classes;public Classes getClasses() {return classes;}public void setClasses(Classes classes) {this.classes = classes;}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}}

Classes.xml文件信息：

<typeAlias alias="Classes" type="com.lzh.ibatis.Classes"/><insert id="saveClasses" parameterkeyProperty="id">select @@identity as id//存储时必须将id返回，以备学生所用</selectKey></insert><resultMap id="classes-result"><result property="id"/><result property="name" column="cname"/><result property="students" column="id" select="findStudentByCid"/></resultMap><select id="findClassesById" resultMap="classes-result" parameterresultMap="classes-result" parametername="code"><typeAlias alias="Student" type="com.lzh.ibatis.Student"/>             <insert id="saveStudent"  parameterid="findStudent-result"><result property="id" column="id"/><result property="name" column="name"/><result property="sex" column="sex"/><result property="classes" column="cid" select="findClassesById"/></resultMap><select id="findStudent" resultMap="findStudent-result" parameterresultMap="findStudent-result" parametername="code"> <?xml version="1.0" encoding="UTF-8"?><!DOCTYPE sqlMapConfig SYSTEM "http://ibatis.apache.org/dtd/sql-map-config-2.dtd" ><sqlMapConfig><settings lazyLoadingEnabled="true"/><transactionManager type="JDBC" commitRequired="false"><dataSource type="SIMPLE"><property name="JDBC.Driver" value="com.mysql.jdbc.Driver"/><property name="JDBC.ConnectionURL" value="jdbc:mysql://127.0.0.1/ibatis"/><property name="JDBC.Username" value="root"/>     <property name="JDBC.Password" value="123"/></dataSource></transactionManager>  <sqlMap resource="com/lzh/ibatis/Student.xml"/>  <sqlMap resource="com/lzh/ibatis/Classes.xml"/></sqlMapConfig>

利用上面的方式，是可以实现的，不过需要明白为什么可以，如果在上面的xml配置文件中不增加lazyLoadingEnabled="true"，程序将会陷入死循环，值栈溢出，同时集合映射时也必须指定为list，而无法使用set集合。
因此实现双向关联，需要完成下面两步操作

* 【双向关联】如果没有启用懒加载，将会出现StackOverFlow的异常！在设置lazyLoadingEnabled="true"（上面的xml文件中可以看到）
* 【双向关联】启用懒加载特性，而且将关联的集合定义为Set，无法进行双向关联定义（但定义成List就可以）

测试代码：

public void testSaveStudent()throws Exception{SqlMapClient sqlMapper = IbatisSQLMapConfig.getSqlMapInstance();Classes classes = new Classes();classes.setName("测试班级"+ new Random().nextInt(9999));sqlMapper.insert("saveClasses", classes);Student student = new Student();student.setName("学生"+new Random().nextInt(9999));student.setSex("男");student.setClasses(classes);sqlMapper.insert("saveStudent",student);}public void testFindClasses()throws Exception{SqlMapClient sqlMapper = IbatisSQLMapConfig.getSqlMapInstance();Classes cla = (Classes)sqlMapper.queryForObject("findClasses",1);System.out.println(cla.getName()+"有学生人数："+cla.getStudents().size());}             public void testFindStudent()throws Exception{SqlMapClient sqlMapper = IbatisSQLMapConfig.getSqlMapInstance();Student stu = (Student)sqlMapper.queryForObject("findStudent",1);System.out.println(stu.getName()+"所属班级是："+stu.getClasses().getName());}

下面来贴出sql语句：

CREATE TABLE t_student(id integer not null auto_increment,name varchar(255),sex varchar(255),cid integer,primary key(id));CREATE TABLE t_classes(id integer not null auto_increment,cname varchar(255),primary key(id))

二、多态查询的映射策略

在设计数据库表的时候，需要提供一个辨别字段，来记录对象的类型，在插入数据的时候，我们要自己定义这个辨别字段的值，在查询数据的时候可以利用iBATIS提供的discriminator元素来映射多态的数据。
下面来贴出实体类代码：

/** * 基类 * @author Administrator * */public class Animal {private int id;private String name;private String sex;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}}/** * Bird子类 * @author Administrator */public class Bird extends Animal{private int height;public int getHeight() {return height;}public void setHeight(int height) {this.height = height;}}/** * Pig子类 * @author Administrator */public class Pig extends Animal{private int weight;public int getWeight() {return weight;}public void setWeight(int weight) {this.weight = weight;}}

下面贴出Animal.xml配置文件的信息：

<sqlMap><typeAlias alias="Animal" type="com.lzh.ibatis.Animal"/><typeAlias alias="Bird" type="com.lzh.ibatis.Bird"/><typeAlias alias="Pig" type="com.lzh.ibatis.Pig"/><insert id="insertAnimal" parameterparameterparameterid="animal"><result property="id" column="id"/><result property="name" column="name"/><result property="sex" column="sex"/><discriminator column="animal_type" javaType="string"><subMap value="Bird" resultMap="bird"/><subMap value="Pig" resultMap="pig"/></discriminator></resultMap><resultMap id="bird" extends="animal"><result property="height" column="height"/></resultMap><resultMap id="pig" extends="animal"><result property="weight" column="weight"/></resultMap><select id="findAllAnimals" resultMap="animal">select * from t_animal</select></sqlMap>

在sqlMapConfig.xml文件中增加下面的映射

  <sqlMap resource="com/lzh/ibatis/Animal.xml"/>

测试代码：

public void testSaveAnimal() throws Exception{//从配置文件中得到SqlMapClient对象 SqlMapClient sqlMapper = IbatisSQLMapConfig.getSqlMapInstance();Animal a = new Animal();a.setName("动物");a.setSex("1");sqlMapper.insert("insertAnimal", a);Bird b = new Bird();b.setName("鸟");b.setSex("0");b.setHeight(1000);sqlMapper.insert("insertBird", b);Pig p = new Pig();p.setName("猪");p.setSex("1");p.setWeight(500);sqlMapper.insert("insertPig", p);}public void testFindAllAnimals() throws Exception{//从配置文件中得到SqlMapClient对象 SqlMapClient sqlMapper = IbatisSQLMapConfig.getSqlMapInstance();;List animals = sqlMapper.queryForList("findAllAnimals");for (Iterator iterator = animals.iterator(); iterator.hasNext();) {Animal a = (Animal) iterator.next();System.out.println(a.getName()+","+a);}}

sql建表语句：

create table t_animal(ID integer not null auto_increment,NAME varchar(255),SEX varchar(255),HEIGHT integer,WEIGHT integer,ANIMAL_TYPE varchar(20),primary key(ID));

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