Mybatis -typeHandlers類處理器

在配置模塊中的parseConfiguration方法中

//解析配置文件中的typeHandlers 節點內容，這裏就是解析自定義的類處理器
 typeHandlerElement(root.evalNode("typeHandlers"));
 //解析方法
  private void typeHandlerElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) {
      //支持包註冊
        if ("package".equals(child.getName())) {
          String typeHandlerPackage = child.getStringAttribute("name");
//註冊包下面的類處理器          typeHandlerRegistry.register(typeHandlerPackage);
        } else {
          String javaTypeName = child.getStringAttribute("javaType");
          String jdbcTypeName = child.getStringAttribute("jdbcType");
          String handlerTypeName = child.getStringAttribute("handler");
          //java類型
          Class<?> javaTypeClass = resolveClass(javaTypeName);
          //jdbc類型
          JdbcType jdbcType = resolveJdbcType(jdbcTypeName);
          //類處理器
          Class<?> typeHandlerClass = resolveClass(handlerTypeName);
          //下面就是根據上面三個參數判斷到底執行什麼方法
          if (javaTypeClass != null) {
            if (jdbcType == null) {
              typeHandlerRegistry.register(javaTypeClass, typeHandlerClass);
            } else {
              typeHandlerRegistry.register(javaTypeClass, jdbcType, typeHandlerClass);
            }
          } else {
//這裏將根據註解類MappedTypes 和 MappedJdbcTypes 自動生成   java類型和jdbc 類型進行註冊，如果沒有註解 則爲null        typeHandlerRegistry.register(typeHandlerClass);
          }
        }
      }
    }
  }

其實在TypeHandlerRegistry中，mybatis已經默認給我們註冊了很多的處理器，給我們解決了大部分的處理所需要的處理器

 public TypeHandlerRegistry() {
    register(Boolean.class, new BooleanTypeHandler());
    register(boolean.class, new BooleanTypeHandler());
    register(JdbcType.BOOLEAN, new BooleanTypeHandler());
    register(JdbcType.BIT, new BooleanTypeHandler());

    register(Byte.class, new ByteTypeHandler());
    register(byte.class, new ByteTypeHandler());
    register(JdbcType.TINYINT, new ByteTypeHandler());

    register(Short.class, new ShortTypeHandler());
    register(short.class, new ShortTypeHandler());
    register(JdbcType.SMALLINT, new ShortTypeHandler());

    register(Integer.class, new IntegerTypeHandler());
    register(int.class, new IntegerTypeHandler());
    register(JdbcType.INTEGER, new IntegerTypeHandler());

    register(Long.class, new LongTypeHandler());
    register(long.class, new LongTypeHandler());

    register(Float.class, new FloatTypeHandler());
    register(float.class, new FloatTypeHandler());
    register(JdbcType.FLOAT, new FloatTypeHandler());

    register(Double.class, new DoubleTypeHandler());
    register(double.class, new DoubleTypeHandler());
    register(JdbcType.DOUBLE, new DoubleTypeHandler());

    register(Reader.class, new ClobReaderTypeHandler());
    register(String.class, new StringTypeHandler());
    register(String.class, JdbcType.CHAR, new StringTypeHandler());
    register(String.class, JdbcType.CLOB, new ClobTypeHandler());
    register(String.class, JdbcType.VARCHAR, new StringTypeHandler());
    register(String.class, JdbcType.LONGVARCHAR, new ClobTypeHandler());
    register(String.class, JdbcType.NVARCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCHAR, new NStringTypeHandler());
    register(String.class, JdbcType.NCLOB, new NClobTypeHandler());
    register(JdbcType.CHAR, new StringTypeHandler());
    register(JdbcType.VARCHAR, new StringTypeHandler());
    register(JdbcType.CLOB, new ClobTypeHandler());
    register(JdbcType.LONGVARCHAR, new ClobTypeHandler());
    register(JdbcType.NVARCHAR, new NStringTypeHandler());
    register(JdbcType.NCHAR, new NStringTypeHandler());
    register(JdbcType.NCLOB, new NClobTypeHandler());

    register(Object.class, JdbcType.ARRAY, new ArrayTypeHandler());
    register(JdbcType.ARRAY, new ArrayTypeHandler());

    register(BigInteger.class, new BigIntegerTypeHandler());
    register(JdbcType.BIGINT, new LongTypeHandler());

    register(BigDecimal.class, new BigDecimalTypeHandler());
    register(JdbcType.REAL, new BigDecimalTypeHandler());
    register(JdbcType.DECIMAL, new BigDecimalTypeHandler());
    register(JdbcType.NUMERIC, new BigDecimalTypeHandler());

    register(InputStream.class, new BlobInputStreamTypeHandler());
    register(Byte[].class, new ByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.BLOB, new BlobByteObjectArrayTypeHandler());
    register(Byte[].class, JdbcType.LONGVARBINARY, new BlobByteObjectArrayTypeHandler());
    register(byte[].class, new ByteArrayTypeHandler());
    register(byte[].class, JdbcType.BLOB, new BlobTypeHandler());
    register(byte[].class, JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.LONGVARBINARY, new BlobTypeHandler());
    register(JdbcType.BLOB, new BlobTypeHandler());

    register(Object.class, UNKNOWN_TYPE_HANDLER);
    register(Object.class, JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);
    register(JdbcType.OTHER, UNKNOWN_TYPE_HANDLER);

    register(Date.class, new DateTypeHandler());
    register(Date.class, JdbcType.DATE, new DateOnlyTypeHandler());
    register(Date.class, JdbcType.TIME, new TimeOnlyTypeHandler());
    register(JdbcType.TIMESTAMP, new DateTypeHandler());
    register(JdbcType.DATE, new DateOnlyTypeHandler());
    register(JdbcType.TIME, new TimeOnlyTypeHandler());

    register(java.sql.Date.class, new SqlDateTypeHandler());
    register(java.sql.Time.class, new SqlTimeTypeHandler());
    register(java.sql.Timestamp.class, new SqlTimestampTypeHandler());

    // mybatis-typehandlers-jsr310
    try {
      // since 1.0.0
      register("java.time.Instant", "org.apache.ibatis.type.InstantTypeHandler");
      register("java.time.LocalDateTime", "org.apache.ibatis.type.LocalDateTimeTypeHandler");
      register("java.time.LocalDate", "org.apache.ibatis.type.LocalDateTypeHandler");
      register("java.time.LocalTime", "org.apache.ibatis.type.LocalTimeTypeHandler");
      register("java.time.OffsetDateTime", "org.apache.ibatis.type.OffsetDateTimeTypeHandler");
      register("java.time.OffsetTime", "org.apache.ibatis.type.OffsetTimeTypeHandler");
      register("java.time.ZonedDateTime", "org.apache.ibatis.type.ZonedDateTimeTypeHandler");
      // since 1.0.1
      register("java.time.Month", "org.apache.ibatis.type.MonthTypeHandler");
      register("java.time.Year", "org.apache.ibatis.type.YearTypeHandler");
      // since 1.0.2
      register("java.time.YearMonth", "org.apache.ibatis.type.YearMonthTypeHandler");
      register("java.time.chrono.JapaneseDate", "org.apache.ibatis.type.JapaneseDateTypeHandler");

    } catch (ClassNotFoundException e) {
      // no JSR-310 handlers
    }

    // issue #273
    register(Character.class, new CharacterTypeHandler());
    register(char.class, new CharacterTypeHandler());
  }

我們來看下需要自定義的類處理器的情況
比如我們返回的類型時List<Map<String,Object>> 即裏面包含了所有的查詢的數據了，但是裏面包含一個clob字段，然後通過jackson轉換爲json格式字符串的時候，就會報錯了，因爲jackson處理不了clob數據
上面這種情況，就需要我們自己定義一個類處理器了，將clob轉換成string就可以了
解決方式：java類：Object Jdbc類型：clob 返回類型爲 String

@MappedTypes(value = { Object.class })
@MappedJdbcTypes(value = { JdbcType.CLOB })
public class ExampleTypeHandler extends BaseTypeHandler<String> {

    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType)
            throws SQLException {
        ps.setString(i, parameter);
    }

    @Override
    public String getNullableResult(ResultSet rs, String columnName) throws SQLException {
    //獲取string類型的數據，即將clob轉換爲string了
        return rs.getString(columnName);
    }

    @Override
    public String getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        return rs.getString(columnIndex);
    }

    @Override
    public String getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        return cs.getString(columnIndex);
    }
}

其實下面的就是StringTypeHandler的類處理器的內容，所以，也可以使用攔截器 註冊一個 java類型：Object jdbc類型： Clob 和 類處理器：StringTypeHandler 也是可以解決的