mybatis中association和collection标签总结

关于标签association和collection相同点和不同点如图所示：

测试结果集

（一）准备阶段

（1）实体类（User）

package com.jyk.entity;
public class User {
    private Integer id;
    private String name;
    private String password;
    private Address address;

    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    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;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }


    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", password='" + password + '\'' +
                ", address=" + address +
                '}';
    }
}

（2）实体类Address

package com.jyk.entity;

/**
 * @Classname Address
 * @Description TODO
 * @Date 2020/7/11 10:50
 * @Created by JYK
 */
public class Address {
    private Integer addno;
    private String city;
    private String county;
    private String town;

    public Integer getAddno() {
        return addno;
    }

    public void setAddno(Integer addno) {
        this.addno = addno;
    }

    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public String getCounty() {
        return county;
    }

    public void setCounty(String county) {
        this.county = county;
    }

    public String getTown() {
        return town;
    }

    public void setTown(String town) {
        this.town = town;
    }

    @Override
    public String toString() {
        return "Address{" +
                "addno=" + addno +
                ", city='" + city + '\'' +
                ", county='" + county + '\'' +
                ", town='" + town + '\'' +
                '}';
    }
}

（二）测试阶段（xml文件配置）

association

<resultMap id="userid" type="com.jyk.entity.User">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <result property="password" column="password"></result>
        <association property="address" javaType="com.jyk.entity.Address" >
            <id property="addno" column="addno"></id>
            <result property="city" column="city"></result>
            <result property="county" column="country"></result>
            <result property="town" column="town"></result>
        </association>
    </resultMap>
    <select id="selectById" resultMap="userid">
        select * from user left join address on address.addno=user.id where id=#{id}
    </select>

collection

<resultMap id="userid" type="com.jyk.entity.User">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <result property="password" column="password"></result>
        <collection property="address" ofType="com.jyk.entity.Address">
            <id property="addno" column="addno"></id>
            <result property="city" column="city"></result>
            <result property="town" column="town"></result>
            <result property="county" column="county"></result>
        </collection>
    </resultMap>
    <select id="selectById" resultMap="userid">
        select * from user left join address on user.id=address.addno where id=#{id}
    </select>

association测试结果

collection测试结果

因此得出结论：一些场景下，association和collection两者都可以满足需求，达到关联查询的效果，使用两者任意一个标签都可以，具体情况具体分析，以需求为导向实现相应需求。使用任何标签其实都无所谓，只要能满足需求即可。

（四）不同点案例及测试

collection属性值包含：

association标签属性值包含：

因此ofType成为，ofType为collection传入的类型，javaType为返回值类型，例如下图，ofType=Address(完全限定名)，javaType=List（完全限定名）。
实体类对象：表示一个人可以有多个收货地址
BaseDao.xml

<!--<?xml version="1.0" encoding="UTF-8" ?>-->
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jyk.dao.BaseDao">

    <resultMap id="userid" type="com.jyk.entity.User">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <result property="password" column="password"></result>
        <!--另外一个dao层的接口-->
        <collection property="addresses" ofType="com.jyk.entity.Address" column="id" javaType="java.util.List" fetchType="lazy" select="com.jyk.dao.AddressDao.selectaddId">
        </collection>
    </resultMap>
    <select id="selectById" resultMap="userid">
        select * from user where id=#{id}
    </select>

    <select id="selectAll" resultType="com.jyk.entity.User" >
        select * from test
     </select>
    <insert id="insert" parameterType="com.jyk.entity.User" useGeneratedKeys="true" keyProperty="id" keyColumn="id">
        insert into user(name,password) values (#{name},#{password})
    </insert>
    <update id="update" parameterType="com.jyk.entity.User" useGeneratedKeys="true" keyProperty="id">
        update user set name=#{name},password=#{password} where id=#{id}
    </update>

<!--    <delete id="delete" >-->
<!--        delete from test where id=#{id} and name=#{name}-->
<!--    </delete>-->

</mapper>

AddressDao.xml

<!--<?xml version="1.0" encoding="UTF-8" ?>-->
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.jyk.dao.AddressDao">
    <select id="selectaddId" resultType="com.jyk.entity.Address">
        select * from address where addno=#{addno}
    </select>
</mapper>

最终实验结果：

总结：

出错点：
（1）.xml的文件和dao层接口的文件名称需要一致，在mybatis-config.xml中使用的是package扫描。这一块需要注意

算法题：
（1）求一个数中二进制1的个数
思路：使用N&(~(N)+1)求最右侧为1的位数

public class Demo {
    public static void main(String[] args) {
        int count = getCount(15);
        System.out.println(count);
    }
    public static int getCount(int N){
        int count=0;
        while (N!=0){
            count++;
            N^=(N&((~N)+1));
        }
        return count;
    }
}

（2）一个数组中，只有一个数出现奇数，其他数都是偶数，求此数为多少？(思路：异或)

public int number(int[]arr){
	int result = 0;
	for(int i=0;i<arr.length;i++){
		result^=arr[i];
	}
	return result;
}