Mybatis——TypeHandler 類型轉換器

jdbcType：定義了數據庫中的數據類型。
javaType：定義了java中的數據類型。
TypeHandler：承擔了jdbcType和javaType之間的互相轉換。

一、TypeHandler 接口源碼

/*
 * TypeHandler接口
 * @ BaseTypeHandler<T>
 * */
public interface TypeHandler<T> {
	//通過PreparedStatement設置SQL參數
    void setParameter(PreparedStatement var1, int var2, T var3, JdbcType var4) throws SQLException;
	//根據列名獲取結果集
    T getResult(ResultSet var1, String var2) throws SQLException;
	//根據下標獲取結果集
    T getResult(ResultSet var1, int var2) throws SQLException;
	//存儲過程
    T getResult(CallableStatement var1, int var2) throws SQLException;
}

在mybatis中typeHandler都要（BaseTypeHander）實現org.apache.ibatis.type.TypeHandler接口，或接繼承BaseTypeHander。

二、BaseTypeHandler 抽象類源碼

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package org.apache.ibatis.type;

import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import org.apache.ibatis.executor.result.ResultMapException;
import org.apache.ibatis.session.Configuration;

public abstract class BaseTypeHandler<T> extends TypeReference<T> implements TypeHandler<T> {
    protected Configuration configuration;

    public BaseTypeHandler() {}

    public void setConfiguration(Configuration c) {
        this.configuration = c;
    }
	
    public void setParameter(PreparedStatement ps, int i, T parameter, JdbcType jdbcType) throws SQLException {
    	
        if (parameter == null) {
            if (jdbcType == null) {
            	// jdbcType 爲null是拋出異常，jdbcType不能爲空
                throw new TypeException("JDBC requires that the JdbcType must be specified for all nullable parameters.");
            }
            try {
            	// 設置空參
                ps.setNull(i, jdbcType.TYPE_CODE);
            } catch (SQLException var7) {
                throw new TypeException("Error setting null for parameter #" + i + " with JdbcType " + jdbcType + " . Try setting a different JdbcType for this parameter or a different jdbcTypeForNull configuration property. Cause: " + var7, var7);
            }
        } else {
            try {
            	//setNonNullParameter是一個虛方法，需要BaseTypeHandler子類去實現
                this.setNonNullParameter(ps, i, parameter, jdbcType);
            } catch (Exception var6) {
                throw new TypeException("Error setting non null for parameter #" + i + " with JdbcType " + jdbcType + " . Try setting a different JdbcType for this parameter or a different configuration property. Cause: " + var6, var6);
            }
        }

    }
	
    public T getResult(ResultSet rs, String columnName) throws SQLException {
        Object result;
        try {
        	//getNullableResult是一個虛方法，需要BaseTypeHandler子類去實現
            result = this.getNullableResult(rs, columnName);
        } catch (Exception var5) {
            throw new ResultMapException("Error attempting to get column '" + columnName + "' from result set.  Cause: " + var5, var5);
        }

        return rs.wasNull() ? null : result;
    }

    public T getResult(ResultSet rs, int columnIndex) throws SQLException {
        Object result;
        try {
        	是一個虛方法，需要BaseTypeHandler子類去實現
            result = this.getNullableResult(rs, columnIndex);
        } catch (Exception var5) {
            throw new ResultMapException("Error attempting to get column #" + columnIndex + " from result set.  Cause: " + var5, var5);
        }

        return rs.wasNull() ? null : result;
    }

    public T getResult(CallableStatement cs, int columnIndex) throws SQLException {
        Object result;
        try {
        	//getNullableResult是一個虛方法，需要BaseTypeHandler子類去實現
            result = this.getNullableResult(cs, columnIndex);
        } catch (Exception var5) {
            throw new ResultMapException("Error attempting to get column #" + columnIndex + " from callable statement.  Cause: " + var5, var5);
        }

        return cs.wasNull() ? null : result;
    }
	
	//4個子類需要實現的虛方法
    public abstract void setNonNullParameter(PreparedStatement var1, int var2, T var3, JdbcType var4) throws SQLException;

    public abstract T getNullableResult(ResultSet var1, String var2) throws SQLException;

    public abstract T getNullableResult(ResultSet var1, int var2) throws SQLException;

    public abstract T getNullableResult(CallableStatement var1, int var2) throws SQLException;
}

三、StringTypeHandler源碼

public class StringTypeHandler extends BaseTypeHandler<String> {
    public StringTypeHandler() {
    }

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

    public String getNullableResult(ResultSet rs, String columnName) throws SQLException {
        return rs.getString(columnName);
    }

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

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

mybatis內部實現的TypeHandler

typeHandler	javaType
NClobTypeHandler	string
ClobReaderTypeHandler	Reader
OffsetTimeTypeHandler	OffsetTime
ByteObjectArrayTypeHandler	Byte[]
DateOnlyTypeHandler	Date
BlobTypeHandler	byte[]
DateTypeHandler	Date
IntegerTypeHandler	Integer
SqlTimeTypeHandler	Time
NStringTypeHandler	String
CharacterTypeHandler	Character
ArrayTypeHandler	Object
EnumOrdinalTypeHandler	E extends Enum < E >
StringTypeHandler	String
BigDecimalTypeHandler	BigDecimal
SqlTimestampTypeHandler	Timestamp
BooleanTypeHandler	Boolean
BlobInputStreamTypeHandler	InputStream
BlobByteObjectArrayTypeHandler	Byte[]
EnumTypeHandler	E extends Enum< E >
MonthTypeHandler	Month
FloatTypeHandler	Float
ByteTypeHandler	Byte
TimeOnlyTypeHandler	Date
YearMonthTypeHandler	YearMonth
InstantTypeHandler	Instant
ObjectTypeHandler	Object
ClobTypeHandler	String
DoubleTypeHandler	Double
ShortTypeHandler	Short
LongTypeHandler	Long
LocalDateTypeHandler	LocalDate
UnknownTypeHandler	Object
BigIntegerTypeHandler	BigInteger
ByteArrayTypeHandler	byte[]
OffsetDateTimeTypeHandler	OffsetDateTime
JapaneseDateTypeHandler	JapaneseDate
LocalDateTimeTypeHandler	LocalDateTime
ZonedDateTimeTypeHandler	ZonedDateTime
SqlDateTypeHandler	Date
YearTypeHandler	Year
LocalTimeTypeHandler	LocalTime