MyBatis配置文件详解之typeHandler

TypeHandler作用

无论是 MyBatis在预处理语句（PreparedStatement）中设置一个参数时，还是从结果集中取出一个值时， 都会用类型处理器将获取的值以合适的方式转换成 Java 类型。

TypeHandler配置方式

备注：javaType和jdbcType可以在配置文件指定，如果没有使用配置文件指定，则需要在typeHandler中使用注解指定。javaType必须指定，jdbcType可以不指定

自定义TypeHandler将List转换为String(，分割)

@MappedJdbcTypes(JdbcType.VARCHAR)
@MappedTypes(List.class)
public class ListTypeHandler implements TypeHandler {
    private static final String LIST_SPLIT_FLAG = ",";

    public void setParameter(PreparedStatement ps, int i, Object parameter, JdbcType jdbcType) throws SQLException {
        List list = (List) parameter;
        StringBuilder stringBuilder = new StringBuilder();
        if(!CollectionUtils.isEmpty(list)){
            int j = 1;
            for(String str : list){
                stringBuilder.append(str);
                if(j++ < list.size()){
                    stringBuilder.append(",");
                }
            }
        }
        ps.setString(i,stringBuilder.toString());

    }

    public Object getResult(ResultSet rs, String columnName) throws SQLException {
        String str = rs.getString(columnName);
        return CollectionUtils.arrayToList(str.split(LIST_SPLIT_FLAG));
    }

    public Object getResult(ResultSet rs, int columnIndex) throws SQLException {
        String str = rs.getString(columnIndex);
        return CollectionUtils.arrayToList(str.split(LIST_SPLIT_FLAG));
    }

    public Object getResult(CallableStatement cs, int columnIndex) throws SQLException {
        String str = cs.getString(columnIndex);
        return CollectionUtils.arrayToList(str.split(LIST_SPLIT_FLAG));
    }
}


    

源码片段

  private void typeHandlerElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
        if ("package".equals(child.getName())) {
          String typeHandlerPackage = child.getStringAttribute("name");
          typeHandlerRegistry.register(typeHandlerPackage);
        } else {
          String javaTypeName = child.getStringAttribute("javaType");
          String jdbcTypeName = child.getStringAttribute("jdbcType");
          String handlerTypeName = child.getStringAttribute("handler");
          Class javaTypeClass = resolveClass(javaTypeName);
          JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
          Class typeHandlerClass = resolveClass(handlerTypeName);
          if (javaTypeClass != null) {
            //指定jdbcType
            if (jdbcType == null) {
              typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
            } else {
              typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
            }
          } else {
            //未指定jdbcType
            typeHandlerRegistry.register(typeHandlerClass);
          }
        }
      }
    }
  }

TypeHandlerRegistry
  public final class TypeHandlerRegistry {

  private static final Map, Class> reversePrimitiveMap = new HashMap, Class>() {
    private static final long serialVersionUID = 1L;
    {
      put(Byte.class, byte.class);
      put(Short.class, short.class);
      put(Integer.class, int.class);
      put(Long.class, long.class);
      put(Float.class, float.class);
      put(Double.class, double.class);
      put(Boolean.class, boolean.class);
      put(Character.class, char.class);
    }
  };

  private final Map> JDBC_TYPE_HANDLER_MAP = new EnumMap>(JdbcType.class);
  private final Map>> TYPE_HANDLER_MAP = new HashMap>>();
  private final TypeHandler