java - Mybatis 逆关系导致对象重复

Question

我尝试用 MyBatis 3.2.8 映射集合的反向关系。看起来 MyBatis 会复制对象，即使它们具有相同的 id。

public class ObjA {
    private String id;
    private ArrayList<ObjB> objBs;

    // Getters, setters ...

    // Equals based on the id field
}

public class ObjB {
    private String id;
    private ObjA objA;

    // Getters, setters ...

    // Equals based on the id field
}

映射xml文件

<resultMap id="xx" type="ObjA">
    <id column="idA" property="id" />
    <collection property="objBs" javaType="ArrayList" ofType="ObjB">
        <id column="idB" property="id"/>
        <association property="objA" type="ObjA">
            <id column="idA" property="id" />
        </association>
    </collection>
</resultMap>

JUnit 测试

ArrayList<ObjA> result = service.getAllObjA();

for(ObjA objA : result) {
    for(ObjB objB : objA.getObjBs()) {
        assertEquals(objB.getObjA(), objA); // Pass
        assertTrue(objB.getObjA() == objA); // Does not pass
    }
}

我希望 objB.getObjA() 和 objA 是 ObjA 的同一个实例(同一个引用)。

我如何配置我的结果映射才能使其正常工作。

Answer 1

您描述的POJO的ObjA和ObjB是循环依赖。

考虑以下示例:让我们有一个Artist 类和一个Recording 类。它们不需要实现 Comparable 接口(interface)，但我们这样做是因为我们要对这些对象进行排序。或者，可以编写 SQL 查询以指定返回行的所需排序顺序。

这是艺术家类。请注意，艺术家持有其录音的集合。

import java.util.List;

public class Artist implements Comparable {

  private int id;
  private List<Recording> recordings;
  private String name;

  public Artist() {}
  public Artist(String name) { this.name = name; }

  public int compareTo(Object obj) {
    Artist r = (Artist) obj;
    return name.compareTo(r.name);
  }

  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 List<Recording> getRecordings() { return recordings; }
  public void setRecordings(List<Recording> recordings) {
    this.recordings = recordings;
  }
}

这是 Recording 类。请注意，录音包含对其艺术家的引用。

public class Recording implements Comparable {

  private int id;
  private int year;
  private Artist artist;
  private String name;

  public Recording() {}
  public Recording(Artist artist, String name, int year) {
    this.artist = artist;
    this.name = name;
    this.year = year;
  }

  public int compareTo(Object obj) {
    Recording r = (Recording) obj;
    return name.compareTo(r.name);
  }

  public Artist getArtist() { return artist; }
  public void setArtist(Artist artist) { this.artist = artist; }

  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 int getYear() { return year; }
  public void setYear(int year) { this.year = year; }
}

配置:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sqlMapConfig
  PUBLIC "-//ibatis.apache.org//DTD SQL Map Config 2.0//EN"
  "http://ibatis.apache.org/dtd/sql-map-config-2.dtd">

<sqlMapConfig>

  <settings useStatementNamespaces="true"/>

  <transactionManager type="JDBC">
    <dataSource type="SIMPLE">
      <property name="JDBC.Driver" value="com.mysql.jdbc.Driver"/>
      <property name="JDBC.ConnectionURL"
        value="jdbc:mysql://localhost:3306/music"/>
      <property name="JDBC.Username" value="root"/>
      <property name="JDBC.Password" value=""/>
    </dataSource>
  </transactionManager>

  <sqlMap resource="Artist.xml"/>
  <sqlMap resource="Recording.xml"/>

</sqlMapConfig>

映射语句在从 SqlMapConfig.xml 文件引用的 XML 文件中定义。

这是指定与我们的艺术家表相关的语句的文件。

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sqlMap
  PUBLIC "-//ibatis.apache.org//DTD SQL Map 2.0//EN"
  "http://ibatis.apache.org/dtd/sql-map-2.dtd">

<sqlMap namespace="Artist">

  <delete id="deleteAll">
    delete from artists
  </delete>

  <insert id="insert" parameterClass="com.ociweb.music.Artist">
    insert into artists (name) values (#name#)
    <selectKey resultClass="int" keyProperty="id">
      select last_insert_id() as id
    </selectKey>
  </insert>

  <resultMap id="result" class="com.ociweb.music.Artist">
    <result property="id" column="id"/>
    <result property="name" column="name"/>
    <!-- This results in N+1 selects. To avoid this, see page 40
         in the iBATIS Developer Guide. -->
    <result property="recordings" column="id"
      select="Recording.getByArtist"/>
  </resultMap>

  <select id="getById"
    parameterClass="java.lang.Integer" resultMap="result">
    select * from artists where id=#id#
  </select>

  <select id="getAll" resultClass="com.ociweb.music.Artist">
    select * from artists
  </select>

</sqlMap>

这是指定与我们的记录表相关的语句的文件。

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sqlMap
  PUBLIC "-//ibatis.apache.org//DTD SQL Map 2.0//EN"
  "http://ibatis.apache.org/dtd/sql-map-2.dtd">

<sqlMap namespace="Recording">

  <delete id="deleteAll">
    delete from recordings
  </delete>

  <insert id="insert" parameterClass="com.ociweb.music.Recording">
    insert into recordings (name, year, artist_id)
    values (#name#, #year#, #artist.id#)
    <selectKey resultClass="int" keyProperty="id">
      select last_insert_id() as id
    </selectKey>
  </insert>

  <resultMap id="resultWithoutArtist" class="com.ociweb.music.Recording">
    <result property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="year" column="year"/>
  </resultMap>

  <resultMap id="result"
    class="com.ociweb.music.Recording" extends="resultWithoutArtist">
    <result property="artist" column="artist_id" select="Artist.getById"/>
  </resultMap>

  <select id="getAll" resultClass="com.ociweb.music.Recording">
    select * from recordings
  </select>

  <!-- resultWithoutArtist is used here to avoid a circular
       dependency when Artist.result (in Artist.xml) is used. -->
  <select id="getByArtist"
    parameterClass="java.lang.Integer" resultMap="resultWithoutArtist">
    select * from recordings where artist_id=#artistId#
  </select>

  <select id="getByYear"
    parameterClass="java.lang.Integer" resultMap="result">
    select * from recordings where year=#year#
  </select>

</sqlMap>

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