我们这个直接看源码看如何由example映射到sql语句

Stock.java

   private Integer id;

    private String name;

    private Integer count;

    private Integer sale;

    private Integer version;

只有5个属性。
StockMapper.java:

public interface StockMapper {
    long countByExample(StockExample example);
}

我看只看countByExample接口，看名字就知道是返回给定example参数的结果个数。
再看
StockMapper.xml该方法实现：

  <select id="countByExample" parameterType="com.example.model.mapper.StockExample" resultType="java.lang.Long">
    select count(*) from stock
    <if test="_parameter != null">
      <include refid="Example_Where_Clause" />
    </if>
  </select>

很简单给定参数类型StockExample 返回Long类型 select count(*) from stock 返回满足条件的stock表中数据数量
再看<include refid="Example_Where_Clause" />

<sql id="Example_Where_Clause">
    <where>
      <foreach collection="oredCriteria" item="criteria" separator="or">
        <if test="criteria.valid">
          <trim prefix="(" prefixOverrides="and" suffix=")">
            <foreach collection="criteria.criteria" item="criterion">
              <choose>
                <when test="criterion.noValue">
                  and ${criterion.condition}
                </when>
                <when test="criterion.singleValue">
                  and ${criterion.condition} #{criterion.value}
                </when>
                <when test="criterion.betweenValue">
                  and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
                </when>
                <when test="criterion.listValue">
                  and ${criterion.condition}
                  <foreach close=")" collection="criterion.value" item="listItem" open="(" separator=",">
                    #{listItem}
                  </foreach>
                </when>
              </choose>
            </foreach>
          </trim>
        </if>
      </foreach>
    </where>
  </sql>

foreach表示循环，分隔符是or（或者）重点在于数据集oredCriteria,我们去给定参数StockExample中寻找oredCriteria

protected List<Criteria> oredCriteria;

是一个List刚好和上面的集合对应了，看看单个Criteria是什么

 public static class Criteria extends GeneratedCriteria {

        protected Criteria() {
            super();
        }
    }

再看GeneratedCriteria

  protected abstract static class GeneratedCriteria {
        protected List<Criterion> criteria;

        protected GeneratedCriteria() {
            super();
            criteria = new ArrayList<Criterion>();
        }
}

我们发现Criteria里面居然还有一个list,我们翻上去看刚好与上面一个foreach里面还有一个foreach对应，那接下来研究Criterion

public static class Criterion {
        private String condition;

        private Object value;

        private Object secondValue;

        private boolean noValue;

        private boolean singleValue;

        private boolean betweenValue;

        private boolean listValue;
}

这就是该类的声明，里面有这些值，这时候应该结合着之前xml第二层foreach属性看了，知道这些值有啥用

<when test="criterion.noValue">
                 and ${criterion.condition}
               </when>
               <when test="criterion.singleValue">
                 and ${criterion.condition} #{criterion.value}
               </when>
               <when test="criterion.betweenValue">
                 and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
               </when>
               <when test="criterion.listValue">
                 and ${criterion.condition}
                 <foreach close=")" collection="criterion.value" item="listItem" open="(" separator=",">
                   #{listItem}
                 </foreach>
               </when>
             </choose>

一看我们根据when知道有四类情况，noValue,singleValue,betweenValue,listValue,对应着查询中四种情况
1.只是单纯是否非空
2.value=?多少查询
3.两者之间查询
4.在list中查询。
所以我们构建的查询参数就是criterion，看一下上面四种咋构建吧
以下均是lGeneratedCriterial类中方法,addCriterion是把生成的Criterion加到Ctriteria中
1.是否非空：

public Criteria andIdIsNull() {
            addCriterion("id is null");
            return (Criteria) this;
        }

2.值查询
public Criteria andIdEqualTo(Integer value) {
addCriterion("id =", value, "id");
return (Criteria) this;
}
3.两者之间
public Criteria andSaleNotBetween(Integer value1, Integer value2) {
addCriterion("sale not between", value1, value2, "sale");
return (Criteria) this;
}
4.list
public Criteria andVersionNotIn(List<Integer> values) {
addCriterion("version not in", values, "version");
return (Criteria) this;
}
完结，相信通过上面的源码剖析，大家都知道如何构建查询条件了把。
总结
StockExample.java 构造

public class StockExample {
    protected String orderByClause;

    protected boolean distinct;

    protected List<Criteria> oredCriteria;
}

三个参数
orderByClause:返回结果升降排序，字段+空格+acs/dcs(升序/降序）例子count acs
setOrderByClause方法注入

 public void setOrderByClause(String orderByClause) {
        this.orderByClause = orderByClause;
    }

distinct：true/false 返回结果去不去重复,setDistinct注入

 public void setDistinct(boolean distinct) {
        this.distinct = distinct;
    }

oredCriteria：就是查询参数，之前狗讲了啥意思，构造方法

public void or(Criteria criteria) {
        oredCriteria.add(criteria);
    }

构造Criteria

 StockExample.Criteria criteria=stockExample.createCriteria();
        criteria.andCountEqualTo(1);

收工。