MyBatis参数获取和传递

1、参数获取方式

MyBatis可以通过以下两种方式获取参数值:

#{变量名}
本质是占位符赋值
${变量名}

本质是字符串拼接，如果拼接的是字符串类型或日期类型，则需要手动添加单引号

2、参数获取的几种情况：

2.1 mapper接口方法的参数为单个字面量类型

可以通过${}和#{}以任意名称获取变量值，需注意${}的单引号问题

如下：

    public interface UserMapper {User selectUserByUserName(String username);}

    <select id="selectUserByUserName" resultType="User"><!-- select * from t_user where username = '${username}' -->select * from t_user where username = #{username}</select>

2.2 mapper接口方法的参数为多个时

mybatis会将多个参数封装到map集合中，并以两种方式存储：

方式一：以arg0,arg1,arg2,…为键，以参数为值
方式二：以param1,param2,param3,…为键，以参数为值

可以通过#{}和${}两种方式以键的方式获取值即可，需注意${}的单引号问题

如下：

    List<User> selectUserByUserNameAndAge(String username, Integer age, String sex);

    <select id="selectUserByUserNameAndAge" resultType="User"><!-- select * from t_user where username = '${arg0}' and age = ${arg1} and sex = '${arg2}' -->select * from t_user where username = #{arg0} and age = #{arg1} and sex = #{arg2}</select>

2.3 mapper接口方法有多个参数时，手动封装map参数

可以通过#{}和${}两种方式以键的方式获取值即可，需注意${}的单引号问题

如下：

    @Testpublic void testQueryByMapParam(){SqlSession sqlSession = SqlSessionUtil.getSqlSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);Map<String, Object> map = new HashMap<>();map.put("username", "李斯");map.put("age", 23);map.put("sex", "女");List<User> admin = userMapper.selectUserByMap(map);admin.stream().forEach(System.out::println);}

    List<User> selectUserByMap(Map<String, Object> map);

    <select id="selectUserByMap" resultType="User">select * from t_user where username = #{username} and age = #{age} and sex = #{sex}</select>

2.4 mapper接口方法的参数为实体类类型的参数

可以通过${}和#{}以任意名称获取变量值，需注意${}的单引号问题

如下：

    @Testpublic void testInsertUser(){SqlSession sqlSession = SqlSessionUtil.getSqlSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUsername("中华");user.setPassword("123456");user.setAge(32);user.setSex("男");int i = userMapper.insertUser(user);System.out.println(i);}

    int insertUser(User user);

    <insert id="insertUser">insert into t_user values(null, #{username}, #{password}, #{age}, #{sex})</insert>

2.5 使用@Param注解命名参数

mybatis会将多个参数封装到map集合中，并以两种方式存储：

方式一：以@Param注解的值为键，以参数为值
方式二：以param1,param2,param3,…为键，以参数为值
因此可以通过${}和#{}以任意名称获取变量值，需注意${}的单引号问题

    @Testpublic void testQueryByParam(){SqlSession sqlSession = SqlSessionUtil.getSqlSession();UserMapper userMapper = sqlSession.getMapper(UserMapper.class);List<User> admin = userMapper.selectUserByParam("李斯", 23, "女");admin.stream().forEach(System.out::println);}

    List<User> selectUserByParam(@Param("username") String username, @Param("age") Integer age, @Param("sex") String sex);

    <select id="selectUserByParam" resultType="User">select * from t_user where username = #{username} and age = #{age} and sex = #{sex}</select>

3、MyBatis参数解析原理：

3.1 参数封装

主要分析MyBatis对接口参数的处理，如参数的封装、参数值的获取原理。

org.apache.ibatis.reflection.ParamNameResolver#ParamNameResolverorg.apache.ibatis.reflection.ParamNameResolver#getNamedParams

ParamNameResolver构造
构造方法中处理方法参数的映射，建立参数的索引和参数名的映射关系。

    // 参数名称处理public ParamNameResolver(Configuration config, Method method) {// 是否使用真实的参数名，默认为truethis.useActualParamName = config.isUseActualParamName();// 获取方法的参数类型数组final Class<?>[] paramTypes = method.getParameterTypes();// 获取方法参数注解final Annotation[][] paramAnnotations = method.getParameterAnnotations();// 存储参数信息临时对象final SortedMap<Integer, String> map = new TreeMap<>();int paramCount = paramAnnotations.length;// get names from @Param annotationsfor (int paramIndex = 0; paramIndex < paramCount; paramIndex++) {/**The key is the index and the value is the name of the parameter. The name is obtained from Param if specified. When Param is not specified, the parameter index is used. Note that this index could be different from the actual index when the method has special parameters (i.e. RowBounds or ResultHandler).aMethod(@Param("M") int a, @Param("N") int b) -> {{0, "M"}, {1, "N"}}aMethod(int a, int b) -> {{0, "0"}, {1, "1"}}aMethod(int a, RowBounds rb, int b) -> {{0, "0"}, {2, "1"}}*/if (isSpecialParameter(paramTypes[paramIndex])) {// 跳过特殊参数 ，如RowBounds 或 ResultHandlercontinue;}String name = null;for (Annotation annotation : paramAnnotations[paramIndex]) {if (annotation instanceof Param) {// 当前参数存在@Param注解时，参数名设置为注解的value值hasParamAnnotation = true;name = ((Param) annotation).value();break;}}if (name == null) {// 如果存在@Param注解，且没有指定参数名if (useActualParamName) {// 取出方法参数名，如果时单一普通参数，则返回arg0name = getActualParamName(method, paramIndex);}if (name == null) {// use the parameter index as the name ("0", "1", ...)// gcode issue #71// 此时，将索引作为参数名name = String.valueOf(map.size());}}map.put(paramIndex, name);}names = Collections.unmodifiableSortedMap(map);}

getNamedParams

建立参数名和参数值的映射关系，并返回参数值

      /*** <p>* A single non-special parameter is returned without a name. Multiple parameters are named using the naming rule. In* addition to the default names, this method also adds the generic names (param1, param2, ...).* </p>** @param args*          the args** @return the named params*/public Object getNamedParams(Object[] args) {final int paramCount = names.size();if (args == null || paramCount == 0) {return null;}// 单一参数，且没有@Param注解时if (!hasParamAnnotation && paramCount == 1) {Object value = args[names.firstKey()];// 此处会对集合类型的参数进行包装处理return wrapToMapIfCollection(value, useActualParamName ? names.get(names.firstKey()) : null);} else {// 建立参数名和值的映射，key:参数名 value:参数值final Map<String, Object> param = new ParamMap<>();int i = 0;for (Map.Entry<Integer, String> entry : names.entrySet()) {// 1.添加命名参数和参数值的映射关系param.put(entry.getValue(), args[entry.getKey()]);// add generic param names (param1, param2, ...)final String genericParamName = GENERIC_NAME_PREFIX + (i + 1);// ensure not to overwrite parameter named with @Paramif (!names.containsValue(genericParamName)) {// 2.同时添加一个以param1,param2...为参数名 和 参数值的映射关系param.put(genericParamName, args[entry.getKey()]);}i++;}return param;}}

wrapToMapIfCollection

      /*** Wrap to a {@link ParamMap} if object is {@link Collection} or array.** @param object*          a parameter object* @param actualParamName*          an actual parameter name (If specify a name, set an object to {@link ParamMap} with specified name)** @return a {@link ParamMap}** @since 3.5.5*/public static Object wrapToMapIfCollection(Object object, String actualParamName) {// 如果是集合类型，则包装key为collection、list的map返回if (object instanceof Collection) {ParamMap<Object> map = new ParamMap<>();map.put("collection", object);if (object instanceof List) {map.put("list", object);}Optional.ofNullable(actualParamName).ifPresent(name -> map.put(name, object));return map;}// 如果是数据组类型，则包装key为array的map返回if (object != null && object.getClass().isArray()) {ParamMap<Object> map = new ParamMap<>();map.put("array", object);Optional.ofNullable(actualParamName).ifPresent(name -> map.put(name, object));return map;}// 如果不是集合类型或数组类型，则直接返回当前值return object;}

3.2 参数调用

在org.apache.ibatis.binding.MapperMethod#execute方法中，通过convertArgsToSqlCommandParam获取参数值。

    public Object execute(SqlSession sqlSession, Object[] args) {Object result;switch (command.getType()) {case INSERT: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));break;}case UPDATE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break;}case DELETE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break;}case SELECT:if (method.returnsVoid() && method.hasResultHandler()) {executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {result = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {result = executeForCursor(sqlSession, args);} else {// 获取参数值Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);if (method.returnsOptional() && (result == null || !method.getReturnType().equals(result.getClass()))) {result = Optional.ofNullable(result);}}break;case FLUSH:result = sqlSession.flushStatements();break;default:throw new BindingException("Unknown execution method for: " + command.getName());}if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {throw new BindingException("Mapper method '" + command.getName()+ "' attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");}return result;}