MyBatis通用Mapper实现原理及相关内容

public interface SelectMapper<T> {
  /**
   * 根据实体中的属性值进行查询，查询条件使用等号
   */
  @SelectProvider(type = BaseSelectProvider.class, method = "dynamicSQL")
  List<T> select(T record);
}

Type[] types = mapperClass.getGenericInterfaces();
Class<?> entityClass = null;
for (Type type : types) {
  if (type instanceof ParameterizedType) {
    ParameterizedType t = (ParameterizedType) type;
    //判断父接口是否为 SelectMapper.class
    if (t.getRawType() == SelectMapper.class) {
      //得到泛型类型
      entityClass = (Class<?>) t.getActualTypeArguments()[0];
      break;
    }
  }
}

/**
 * 查询全部结果
 * @param ms
 * @return
 */
public String selectAll(MappedStatement ms) {
  final Class<?> entityClass = getEntityClass(ms);
  //修改返回值类型为实体类型
  setResultType(ms, entityClass);
  StringBuilder sql = new StringBuilder();
  sql.append(SqlHelper.selectAllColumns(entityClass));
  sql.append(SqlHelper.fromTable(entityClass, tableName(entityClass)));
  sql.append(SqlHelper.orderByDefault(entityClass));
  return sql.toString();
}

/**
 * 重新设置SqlSource
 * @param ms
 * @param sqlSource
 */
protected void setSqlSource(MappedStatement ms, SqlSource sqlSource) {
  MetaObject msObject = SystemMetaObject.forObject(ms);
  msObject.setValue("sqlSource", sqlSource);
}

public interface BaseMapper<T> {
  @SelectProvider(type = SelectMethodProvider.class, method = "select")
  List<T> select(T entity);
}

public class SelectMethodProvider {
  public String select(Object params) {
    return "什么都不是!";
  }
}

public class SimpleMapperHelper {
  public static final XMLLanguageDriver XML_LANGUAGE_DRIVER
      = new XMLLanguageDriver();
  /**
   * 获取泛型类型
   */
  public static Class getEntityClass(Class<?> mapperClass){
    Type[] types = mapperClass.getGenericInterfaces();
    Class<?> entityClass = null;
    for (Type type : types) {
      if (type instanceof ParameterizedType) {
        ParameterizedType t = (ParameterizedType) type;
        //判断父接口是否为 BaseMapper.class
        if (t.getRawType() == BaseMapper.class) {
          //得到泛型类型
          entityClass = (Class<?>) t.getActualTypeArguments()[0];
          break;
        }
      }
    }
    return entityClass;
  }
  /**
   * 替换 SqlSource
   */
  public static void changeMs(MappedStatement ms) throws Exception {
    String msId = ms.getId();
    //标准msId为 包名.接口名.方法名
    int lastIndex = msId.lastIndexOf(".");
    String methodName = msId.substring(lastIndex + 1);
    String interfaceName = msId.substring(0, lastIndex);
    Class<?> mapperClass = Class.forName(interfaceName);
    //判断是否继承了通用接口
    if(BaseMapper.class.isAssignableFrom(mapperClass)){
      //判断当前方法是否为通用 select 方法
      if (methodName.equals("select")) {
        Class entityClass = getEntityClass(mapperClass);
        //必须使用<script>标签包裹代码
        StringBuffer sqlBuilder = new StringBuffer("<script>");
        //简单使用类名作为包名
        sqlBuilder.append("select * from ").append(entityClass.getSimpleName());
        Field[] fields = entityClass.getDeclaredFields();
        sqlBuilder.append(" <where> ");
        for (Field field : fields) {
          sqlBuilder.append("<if test=\"")
              .append(field.getName()).append("!=null\">");
          //字段名直接作为列名
          sqlBuilder.append(" and ").append(field.getName())
               .append(" = #{").append(field.getName()).append("}");
          sqlBuilder.append("</if>");
        }
        sqlBuilder.append("</where>");
        sqlBuilder.append("</script>");
        //解析 sqlSource
        SqlSource sqlSource = XML_LANGUAGE_DRIVER.createSqlSource(
            ms.getConfiguration(), sqlBuilder.toString(), entityClass);
        //替换
        MetaObject msObject = SystemMetaObject.forObject(ms);
        msObject.setValue("sqlSource", sqlSource);
      }
    }
  }
}

public class Country {
 private Long  id;
 private String countryname;
 private String countrycode;
 //省略 getter,setter
}

public interface CountryMapper extends BaseMapper<Country> {
}

public class SqlSessionHelper {
  private static SqlSessionFactory sqlSessionFactory;
  static {
    try {
      Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
      sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
      reader.close();
      //创建数据库
      SqlSession session = null;
      try {
        session = sqlSessionFactory.openSession();
        Connection conn = session.getConnection();
        reader = Resources.getResourceAsReader("hsqldb.sql");
        ScriptRunner runner = new ScriptRunner(conn);
        runner.setLogWriter(null);
        runner.runScript(reader);
        reader.close();
      } finally {
        if (session != null) {
          session.close();
        }
      }
    } catch (IOException ignore) {
      ignore.printStackTrace();
    }
  }
  public static SqlSession getSqlSession() {
    return sqlSessionFactory.openSession();
  }
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
 <environments default="development">
  <environment id="development">
   <transactionManager type="JDBC">
    <property name="" value=""/>
   </transactionManager>
   <dataSource type="UNPOOLED">
    <property name="driver" value="org.hsqldb.jdbcDriver"/>
    <property name="url" value="jdbc:hsqldb:mem:basetest"/>
    <property name="username" value="sa"/>
   </dataSource>
  </environment>
 </environments>
 <mappers>
  <package name="tk.mybatis.simple.mapper"/>
 </mappers>
</configuration>

drop table country if exists;
create table country (
 id integer,
 countryname varchar(32),
 countrycode varchar(2)
);
insert into country (id, countryname, countrycode) values(1,'Angola','AO');
insert into country (id, countryname, countrycode) values(23,'Botswana','BW');
-- 省略部分
insert into country (id, countryname, countrycode) values(34,'Chile','CL');
insert into country (id, countryname, countrycode) values(35,'China','CN');
insert into country (id, countryname, countrycode) values(36,'Colombia','CO');

public class SimpleTest {
  public static void main(String[] args) throws Exception {
    SqlSession sqlSession = SqlSessionHelper.getSqlSession();
    Configuration configuration = sqlSession.getConfiguration();
    HashSet<MappedStatement> mappedStatements
        = new HashSet<MappedStatement>(configuration.getMappedStatements());
    //如果注释下面替换步骤就会出错
    for (MappedStatement ms : mappedStatements) {
      SimpleMapperHelper.changeMs(ms);
    }
    //替换后执行该方法
    CountryMapper mapper = sqlSession.getMapper(CountryMapper.class);
    Country query = new Country();
    //可以修改条件或者注释条件查询全部
    query.setCountrycode("CN");
    List<Country> countryList = mapper.select(query);
    for (Country country : countryList) {
      System.out.printf("%s - %s\n",
          country.getCountryname(),
          country.getCountrycode());
    }
    sqlSession.close();
  }
}

/**
 * The context object for sql provider method.
 * @author Kazuki Shimizu
 * @since 3.4.5
 */
public final class ProviderContext {
 private final Class<?> mapperType;
 private final Method mapperMethod;
 /**
  * Constructor.
  * @param mapperType A mapper interface type that specified provider
  * @param mapperMethod A mapper method that specified provider
  */
 ProviderContext(Class<?> mapperType, Method mapperMethod) {
  this.mapperType = mapperType;
  this.mapperMethod = mapperMethod;
 }
 /**
  * Get a mapper interface type that specified provider.
  * @return A mapper interface type that specified provider
  */
 public Class<?> getMapperType() {
  return mapperType;
 }
 /**
  * Get a mapper method that specified provider.
  * @return A mapper method that specified provider
  */
 public Method getMapperMethod() {
  return mapperMethod;
 }
}

public interface BaseMapper<T> {
 @SelectProvider(type= OurSqlBuilder.class, method= "buildSelectByIdProviderContextOnly")
 @ContainsLogicalDelete
 T selectById(Integer id);
 @Retention(RetentionPolicy.RUNTIME)
 @Target(ElementType.METHOD)
 @interface ContainsLogicalDelete {
  boolean value() default false;
 }
 @Retention(RetentionPolicy.RUNTIME)
 @Target(ElementType.TYPE)
 @interface Meta {
  String tableName();
 }
}

public String buildSelectByIdProviderContextOnly(ProviderContext context) {
 //获取方法上的逻辑删除注解
 final boolean containsLogicalDelete = context.getMapperMethod().
      getAnnotation(BaseMapper.ContainsLogicalDelete.class) != null;
 //获取接口上的元注解（不是实体）
 final String tableName = context.getMapperType().
      getAnnotation(BaseMapper.Meta.class).tableName();
 return new SQL(){{
  SELECT("*");
  FROM(tableName);
  WHERE("id = #{id}");
  if (!containsLogicalDelete){
   WHERE("logical_delete = ${Constants.LOGICAL_DELETE_OFF}");
  }
 }}.toString();
}

@BaseMapper.Meta(tableName = "users")
public interface Mapper extends BaseMapper<User> {
}