详解mybatis @SelectProvider 注解

public MapperAnnotationBuilder(Configuration configuration, Class<?> type) {
  ...
  sqlAnnotationTypes.add(Select.class);
  sqlAnnotationTypes.add(Insert.class);
  sqlAnnotationTypes.add(Update.class);
  sqlAnnotationTypes.add(Delete.class);
  // 这四个注解眼生的很
  sqlProviderAnnotationTypes.add(SelectProvider.class);
  sqlProviderAnnotationTypes.add(InsertProvider.class);
  sqlProviderAnnotationTypes.add(UpdateProvider.class);
  sqlProviderAnnotationTypes.add(DeleteProvider.class);
}

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface InsertProvider {
  // 用于指定获取 sql 语句的指定类
  Class<?> type();
  // 指定类中要执行获取 sql 语句的方法
  String method();
}

public interface AutoConstructorMapper {
  @SelectProvider(type = SubjectSqlProvider.class, method = "getSubjectTestProvider")
  PrimitiveSubject getSubjectTestProvider(@Param("id") int id);
}

public class SubjectSqlProvider {
  public String getSubjectTestProvider(Map<String, Object> params) {
    return new SQL()
        .SELECT("*")
        .FROM("subject")
        .WHERE("id = " + params.get("id"))
        .toString();
  }
}

Class<? extends Annotation> sqlAnnotationType = getSqlAnnotationType(method);
Class<? extends Annotation> sqlProviderAnnotationType = getSqlProviderAnnotationType(method);
if (sqlAnnotationType != null) {
  if (sqlProviderAnnotationType != null) {
    throw new BindingException("You cannot supply both a static SQL and SqlProvider to method named " + method.getName());
  }
  Annotation sqlAnnotation = method.getAnnotation(sqlAnnotationType);
  final String[] strings = (String[]) sqlAnnotation.getClass().getMethod("value").invoke(sqlAnnotation);
  return buildSqlSourceFromStrings(strings, parameterType, languageDriver);
} else if (sqlProviderAnnotationType != null) {
  Annotation sqlProviderAnnotation = method.getAnnotation(sqlProviderAnnotationType);
  return new ProviderSqlSource(assistant.getConfiguration(), sqlProviderAnnotation, type, method);
}

SELECT * FROM subject WHERE id = #{id}