The role and description of the TypeHandler of MyBatis type processor

TypeHandlerIt is the interface in MyBatis for processing conversion between Java types and JDBC types. It plays a crucial role in the parameter binding and result mapping of MyBatis.

Why do I need a TypeHandler?

Java and databases (JDBC) use different type systems. For example:

• There is in JavaString、Integer、Date、Boolean、enumetc.
• There are in JDBCVARCHAR、INTEGER、DATE、BOOLEAN、TIMESTAMP、BLOBetc.

When MyBatis executes SQL statements, it is necessary to set the data in the Java object to the JDBC'sPreparedStatement(parameter binding), and JDBC'sResultSetThe data in it is converted to a Java object (result map).TypeHandlerIt is responsible for handling type conversion in both cases.

The role of TypeHandler

Parameter binding (Java type -> JDBC type):

• When MyBatis sets the data in a Java object toPreparedStatementplaceholder (#{})hour,TypeHandlerThe Java type will be converted to the corresponding JDBC type.
• For example, to use JavaStringType conversion to JDBCVARCHARType, Java'sDateType conversion to JDBCTIMESTAMPtype.

Result Map (JDBC Type -> Java Type):

• When MyBatis fromResultSetWhen reading data and mapping it into a Java object,TypeHandlerThe JDBC type will be converted to the corresponding Java type.
• For example, to JDBC'sVARCHARType conversion to JavaStringType, will JDBCINTEGERType conversion to JavaIntegertype.

TypeHandler interface definition

public interface TypeHandler<T> {

  // Parameter binding: Convert Java type to JDBC type and set it to PreparedStatement  void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException;

  // Result Map: Get the data of the specified column from the ResultSet and convert it to Java type  T getResult(ResultSet rs, String columnName) throws SQLException;

  // Result Mapping: Get the data of the specified column index from the ResultSet and convert it to Java type  T getResult(ResultSet rs, int columnIndex) throws SQLException;
  
    // Result map: Get the data of the specified column index from CallableStatement and convert it to java type  T getResult(CallableStatement cs, int columnIndex) throws SQLException;
}

• setParameter(): Set Java type parametersparameterSet toPreparedStatementThe firstiplaceholders and specify JDBC typejdbcType（jdbcTypeCan be empty, MyBatis will try to automatically infer).
• getResult(): There are three overloaded methods, respectively, based on the column name and column indexResultSetget data fromCallableStatementGets the data and converts it to Java type.

MyBatis built-in TypeHandler

MyBatis has many commonly used ones built inTypeHandler, used to handle common type conversions. For example:

• StringTypeHandler: deal withStringtype.
• IntegerTypeHandler: deal withIntegertype.
• LongTypeHandler: deal withLongtype.
• DateTypeHandler: deal withtype.
• BooleanTypeHandler: deal withBooleantype.
• EnumTypeHandler: Process the enum type (the name of the enum is used by default).
• EnumOrdinalTypeHandler: Process the enum type (using ordinal numbers of enums).
• BlobTypeHandler: Processing binary large objects (BLOBs).
• ClobTypeHandler: Process large characters (CLOB).
• SqlDateTypeHandler:deal withtype
• SqlTimeTypeHandler:deal withtype
• SqlTimestampTypeHandler:deal withtype
• …

Custom TypeHandler

When MyBatis is built-inTypeHandlerWhen the requirements cannot be met, we can customize themTypeHandler. For example:

• Handle special types:For example, convert a JSON string in a database to a Java object, or convert a Java object to a JSON string to store it in the database.
• Custom type conversion logic:For example, make special formatting of date types, or use custom conversion rules for enum types.

Steps to customize TypeHandler

1. ImplementationTypeHandler interface:Create a class to implementTypeHandlerinterface and implement methods in the interface.

2. Register TypeHandler:There are two ways to register custom onesTypeHandler：

• XML configuration:existIn the file, use<typeHandlers>Tag registrationTypeHandler。

<typeHandlers>
  <typeHandler handler="" javaType="" jdbcType="VARCHAR"/>
</typeHandlers>

• Or use package scanning:

 <typeHandlers>
     <package name=""/>
 </typeHandlers>

• Annotation configuration:In customTypeHandlerUsed on the class@MappedTypesand@MappedJdbcTypesannotation.

@MappedTypes() // Specify Java type@MappedJdbcTypes() // Specify the JDBC typepublic class MyTypeHandler implements TypeHandler<MyType> {
    // ... Methods to implement TypeHandler interface ...}

3. Specify jdbcType (optional):If MyBatis cannot infer, it must passjdbcTypeThe attribute isnullValue specifies the type of database column

• Example:
• Customize a TypeHandler that converts the VARCHAR type in the database to an enum type in Java:

// Suppose there is an enumeration typepublic enum Status {
    ACTIVE, INACTIVE, PENDING
}

// Custom TypeHandler@MappedTypes()
@MappedJdbcTypes()
public class StatusTypeHandler implements TypeHandler<Status> {

    @Override
    public void setParameter(PreparedStatement ps, int i, Status parameter, JdbcType jdbcType) throws SQLException {
        if (parameter == null) {
            (i, null);
        } else {
            (i, ()); //Storing the enum name to the database        }
    }

    @Override
    public Status getResult(ResultSet rs, String columnName) throws SQLException {
        String value = (columnName);
        return getStatus(value);
    }

    @Override
    public Status getResult(ResultSet rs, int columnIndex) throws SQLException {
        String value = (columnIndex);
         return getStatus(value);
    }

    @Override
    public Status getResult(CallableStatement cs, int columnIndex) throws SQLException {
        String value = (columnIndex);
        return getStatus(value);
    }
    
    private Status getStatus(String value){
         if (value == null) {
            return null;
        }
        try {
            return (value); // Get enum value from database according to name        } catch (IllegalArgumentException e) {
            return null; // Or throw an exception        }
    }
}

// Register TypeHandler in<typeHandlers>
  <typeHandler handler=""/>
</typeHandlers>

// Or use it in a Mapper XML file<resultMap  type="User">
    <result property="status" column="status" javaType="" jdbcType="VARCHAR" typeHandler=""/>
</resultMap>

Summarize

TypeHandlerIt is a key component in MyBatis for handling conversion between Java types and JDBC types.

MyBatis has many commonly used ones built inTypeHandler, in most cases we can directly use the built-inTypeHandler。

When it is necessary to deal with special types or custom type conversion logic, we can customizeTypeHandler, and register customTypeHandler。

The above is personal experience. I hope you can give you a reference and I hope you can support me more.