如何正确使用Mybatis的@Param注解

本文介绍了如何正确使用Mybatis的@Param注解的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我一开始没有用@Param注解，这是我的mapper.java

I didn't use @Param annotation at first, this is my mapper.java

public void changeUserAuth(Integer userId,int identity);

，这是我的 mapper.xml

, and this is my mapper.xml

<update id="changeUserAuth">
    update user
    <set>
        <if test="identity != 0">identity = #{identity}</if>
    </set>
    <where>
        <if test="userId != 0">userId = #{userId}</if>
    </where>
</update>

然后就正常了！我继续这样写，如下:

then it works correctly!I continue to write like this, as follows:

//this's mapper.java
public void updateUserStatus(Integer userId);

<!--this is mapper.xml>
<update id="changeUserAuth">
    update user
    set deleteFlag= true
    <where>
        <if test="userId != 0">userId = #{userId}</if>
    </where>
</update>

然而，它给了一个错误，消息是

however,it gave an error,the message is

'class.java.lang.Integer' 中没有名为 'userId' 的属性的 getter

我可以理解mybatis无法解析Integer，但是为什么不像我第一次使用那样报错，就因为我有一个int类型的Parameter?在第二种方法中，我必须使用@Param注解

I can understand that mybatis cannot parse the Integer, but why it is not an error like my first use, just because I have an int type Parameter? In the second method, I have to use @Param annotation

使用`@Param`时

如果在参数上添加@Param注解，可以使用指定的名称来引用该参数.这是最简单的情况.

When `@Param` is used

If you add @Param annotation on a parameter, you can use the specified name to reference the parameter. This is the simplest case.

几个例子:

List<USer> select(@Param("id") Integer userId, @Param("name") String userName);

void insert(@Param("record") User user);

<select id="select" resultType="User">
  select * from users
  <where>
    <if test="id != null">and id = #{id}</if>
    <if test="name != null">and name = #{name}</if>
  </where>
</select>

<insert id="insert">
  insert into users (id, name) values
    (#{record.id}, #{record.name})
</insert>

没有`@Param`

如果没有@Param，则取决于几个条件.

... 唯一的参数可分配给 java.util.List，您可以将该参数引用为 list.

... the sole parameter is assignable to java.util.List, you can reference the parameter as list.

List<User> selectByIds(List<Integer> ids);

<select id="select" resultType="User">
  select * from users
  where id in (
    <foreach item="x" collection="list" separator=",">
      #{x}
    </foreach>
  )
</select>

... 唯一的参数可分配给 java.util.Collection，您可以将该参数引用为 collection.

... the sole parameter is assignable to java.util.Collection, you can reference the parameter as collection.

List<User> selectByIds(Set<Integer> ids);

<select id="select" resultType="User">
  select * from users
  where id in (
    <foreach item="x" collection="collection" separator=",">
      #{x}
    </foreach>
  )
</select>

... 有一个映射到唯一参数的类型处理程序(即参数是 String、Integer 等).

在 MyBatis 3.5.2 及更高版本中，您可以使用任何名称引用参数(尽管出于显而易见的原因，您应该使用合理的名称).例如
- With MyBatis 3.5.2 and later, you can reference the parameter using any name (you should use sensible names for obvious reasons, though). e.g.
```
List<User> select(Integer id);
```
```
<select id="select" resultType="User">
  select * from users
  <where>
    <if test="x != null">and id = #{y}</if>
  </where>
</select>
```
- 使用 MyBatis 3.5.1
- With MyBatis 3.5.1
  - 您可以在 #{} 中使用任何名称引用参数.
  - you can reference the parameter with any name in #{}.
  你必须在${}、test属性的中引用参数为_parameter; 和和 value 的属性.这就是您的第二个示例抛出异常的原因.
  you must reference the parameter as _parameter in ${}, test attribute of <if /> and <when /> and value attribute of <bind />. This is why your second example throws exception.
```
List<User> select(Integer id);
```
```
<select id="select" resultType="User">
  select * from users
  <where>
    <if test="_parameter != null">and id = #{z}</if>
  </where>
</select>
```
- 否则，您可以使用 MyBatis 隐式分配的名称来引用参数，即 arg0, arg1, ...(我不建议这样做，因为它是脆弱且容易出错).
- Otherwise, you can reference the parameters using the names implicitly assigned by MyBatis i.e. arg0, arg1, ... (I would not recommend this as it's fragile and error-prone).
```
List<USer> select(Integer userId, String userName);
```
```
<select id="select" resultType="User">
  select * from users
  <where>
    <if test="arg0 != null">and id = #{arg0}</if>
    <if test="arg1 != null">and name = #{arg1}</if>
  </where>
</select>
```