在Java中，如果需要进行嵌套查询映射，可以使用ResultMap来实现。以下是一个示例代码：
<resultMap id="orderResultMap" type="Order">
<id property="id" column="order_id"/>
<result property="orderDate" column="order_date"/>
<result property="totalAmount" column="total_amount"/>
<association property="customer" column="customer_id" javaType="Customer" select="selectCustomerById"/>
</resultMap>
<resultMap id="customerResultMap" type="Customer">
<id property="id" column="customer_id"/>
<result property="name" column="customer_name"/>
<result property="email" column="customer_email"/>
</resultMap>
<select id="selectOrderWithCustomer" resultMap="orderResultMap">
SELECT o.id as order_id, o.order_date, o.total_amount, o.customer_id, c.id as customer_id, c.name as customer_name, c.email as customer_email
FROM orders o
JOIN customers c ON o.customer_id = c.id
WHERE o.id = #{orderId}
</select>
<select id="selectCustomerById" resultMap="customerResultMap">
SELECT id as customer_id, name as customer_name, email as customer_email
FROM customers
WHERE id = #{customerId}
</select>
在上面的示例中，我们定义了两个ResultMap，分别用于映射Order和Customer对象。在Order对象的ResultMap中，我们使用了association标签来指定在查询Order对象时需要查询关联的Customer对象，并通过select属性指定了selectCustomerById查询语句的映射。在selectOrderWithCustomer查询语句中使用了orderResultMap来映射查询结果，从而实现了嵌套查询映射。
