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# ParamNameResolver 源码解析
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- Author: [HuiFer](https://github.com/huifer)
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- Description: 该文介绍 mybatis `@Param` 注解和`ParamNameResolver`
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- 源码阅读工程: [SourceHot-Mybatis](https://github.com/SourceHot/mybatis-read.git)
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## 源码
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- `org.apache.ibatis.reflection.ParamNameResolver`
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```java
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/**
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* {@link Param} 注解的扫描工具和处理工具
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*/
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public class ParamNameResolver {
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public static final String GENERIC_NAME_PREFIX = "param";
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/**
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* <p>
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* The key is the index and the value is the name of the parameter.<br />
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* The name is obtained from {@link Param} if specified. When {@link Param} is not specified,
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* the parameter index is used. Note that this index could be different from the actual index
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* when the method has special parameters (i.e. {@link RowBounds} or {@link ResultHandler}).
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* </p>
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*
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* {@link ParamNameResolver#ParamNameResolver(org.apache.ibatis.session.Configuration, java.lang.reflect.Method)} 中的map 变量值转换而得
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* {参数索引: 参数名称(arg0,Param注解的value)}
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*
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*/
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private final SortedMap<Integer, String> names;
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private boolean hasParamAnnotation;
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public ParamNameResolver(Configuration config, Method method) {
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// 方法参数类型
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final Class<?>[] paramTypes = method.getParameterTypes();
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// 参数上的注解
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final Annotation[][] paramAnnotations = method.getParameterAnnotations();
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// 参数索引和参数名称
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// {参数索引:参数名称}
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final SortedMap<Integer, String> map = new TreeMap<>();
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int paramCount = paramAnnotations.length;
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// get names from @Param annotations
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for (int paramIndex = 0; paramIndex < paramCount; paramIndex++) {
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if (isSpecialParameter(paramTypes[paramIndex])) {
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// skip special parameters
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// 如果是特殊类型跳过
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continue;
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}
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String name = null;
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// 注解扫描@Param
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for (Annotation annotation : paramAnnotations[paramIndex]) {
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// 是否为 Param 注解的下级
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if (annotation instanceof Param) {
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hasParamAnnotation = true;
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// 获取 value 属性值
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name = ((Param) annotation).value();
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break;
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}
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}
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if (name == null) {
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// 如果没有写 @param 处理方式如下
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// @Param was not specified.
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if (config.isUseActualParamName()) {
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name = getActualParamName(method, paramIndex);
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}
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if (name == null) {
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// use the parameter index as the name ("0", "1", ...)
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// gcode issue #71
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name = String.valueOf(map.size());
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}
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}
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// 循环参数列表 放入map 对象
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map.put(paramIndex, name);
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}
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names = Collections.unmodifiableSortedMap(map);
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}
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/**
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* 是否为特殊参数 , 依据 是否是 {@link RowBounds} 或者 {@link ResultHandler}
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* @param clazz
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* @return
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*/
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private static boolean isSpecialParameter(Class<?> clazz) {
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return RowBounds.class.isAssignableFrom(clazz) || ResultHandler.class.isAssignableFrom(clazz);
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}
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/**
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* 返回方法名 参数索引
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* @param method
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* @param paramIndex
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* @return
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*/
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private String getActualParamName(Method method, int paramIndex) {
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return ParamNameUtil.getParamNames(method).get(paramIndex);
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}
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/**
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* Returns parameter names referenced by SQL providers.
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*/
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public String[] getNames() {
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return names.values().toArray(new String[0]);
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}
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/**
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* <p>
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* A single non-special parameter is returned without a name.
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* Multiple parameters are named using the naming rule.
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* In addition to the default names, this method also adds the generic names (param1, param2,
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* ...).
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* </p>
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* <p>
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* 通常参数异常在这个地方抛出 param ... 异常
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* 获取参数名称,和参数传递的真实数据
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*/
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public Object getNamedParams(Object[] args) {
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final int paramCount = names.size();
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if (args == null || paramCount == 0) {
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// 是否有参数
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return null;
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} else if (!hasParamAnnotation && paramCount == 1) {
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// 没有使用 @param 注解 参数只有一个
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return args[names.firstKey()];
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} else {
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// 根据索引创建
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final Map<String, Object> param = new ParamMap<>();
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int i = 0;
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for (Map.Entry<Integer, String> entry : names.entrySet()) {
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param.put(entry.getValue(), args[entry.getKey()]);
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// add generic param names (param1, param2, ...)
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// param + 当前索引位置
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final String genericParamName = GENERIC_NAME_PREFIX + (i + 1);
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// ensure not to overwrite parameter named with @Param
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if (!names.containsValue(genericParamName)) {
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param.put(genericParamName, args[entry.getKey()]);
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}
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i++;
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}
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return param;
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}
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}
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}
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```
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## debug 阶段
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- 测试用例为同一个 ， 每次修改 mapper 方法参数来进行 debug
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```java
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@Test
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void testXmlConfigurationLoad() throws IOException {
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Reader reader = Resources.getResourceAsReader("mybatis-config-demo.xml");
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SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(reader);
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Configuration configuration = factory.getConfiguration();
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SqlSession sqlSession = factory.openSession();
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HsSellMapper mapper = sqlSession.getMapper(HsSellMapper.class);
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List<HsSell> list = mapper.list(2);
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List<Object> objects = sqlSession.selectList("com.huifer.mybatis.mapper.HsSellMapper.list");
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assertEquals(list.size(), objects.size());
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}
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```
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```java
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List<HsSell> list( Integer id);
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```
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如果不写`@Param`称则返回
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```java
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List<HsSell> list(@Param("ID") Integer id);
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```
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- 写`@Param`返回
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- `org.apache.ibatis.reflection.ParamNameResolver#getNamedParams`
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```java
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List<HsSell> list( Integer id);
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```
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```java
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List<HsSell> list(@Param("ID") Integer id);
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```
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写上`@Param`
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