将数据库类型映射到具体的Java类

Question

背景

将列数据类型映射到其对应的Java类.

问题

查询从数据库返回元信息:

SELECT
  rb.object_schema,
  rb.object_name,
  rb.column_name
FROM
  dictionary.resource_bundle rb

例如,此查询返回(自引用):

dictionary, resource_bundle, column_name

其中'dictionary'是模式名称,'resource_bundle'是object_name,'column_name'是column_name.

做以下事情会很棒:

SELECT
  rb.object_schema,
  rb.object_name,
  rb.column_name,
  rb.column_type
FROM
  dictionary.resource_bundle rb

并让此查询返回:

dictionary, resource_bundle, column_name, varchar

然后使用JDBC来发现varchar被映射到java.lang.String.

问题

1. 在PostgreSQL中,如果给定模式名称,对象名称(保证是表或视图)和列名,您如何确定用于存储数据的类型？
2. 以数据库中立的方式(利用JDBC),如何确定数据库用于给定数据类型的映射？

谢谢!

Answer 1

解

答案比使用该getMetaData方法更复杂,因为没有来自getMetaData方法返回的整数类型和完整类名的直接映射.此解决方案需要两段代码:

• 实现一个方法来获取java.sql.Types常量整数值.
• 创建将该值转换为类名的方法.

Java类型方法

以下方法检索元信息:

  public String getJavaType( String schema, String object, String column )
    throws Exception {
    String fullName = schema + '.' + object + '.' + column;
    DatabaseMetaData metaData = getConnection().getMetaData();
    ResultSet columnMeta = metaData.getColumns( null, schema, object, column );
    String javaType = null;

    if( columnMeta.first() ) {
      int dataType = columnMeta.getInt( "DATA_TYPE" );
      javaType = SQLTypeMap.convert( dataType );
    }
    else {
      throw new Exception( "Unknown database column " + fullName + '.' );
    }

    return javaType;
  }

静态转换方法

必须将常量整数值转换为类名.这可以通过以下方式完成:

import java.sql.Types;

/**
 * Converts database types to Java class types.
 */
public class SQLTypeMap {
    /**
     * Translates a data type from an integer (java.sql.Types value) to a string
     * that represents the corresponding class.
     * 
     * @param type
     *            The java.sql.Types value to convert to its corresponding class.
     * @return The class that corresponds to the given java.sql.Types
     *         value, or Object.class if the type has no known mapping.
     */
    public static Class<?> toClass(int type) {
        Class<?> result = Object.class;

        switch (type) {
            case Types.CHAR:
            case Types.VARCHAR:
            case Types.LONGVARCHAR:
                result = String.class;
                break;

            case Types.NUMERIC:
            case Types.DECIMAL:
                result = java.math.BigDecimal.class;
                break;

            case Types.BIT:
                result = Boolean.class;
                break;

            case Types.TINYINT:
                result = Byte.class;
                break;

            case Types.SMALLINT:
                result = Short.class;
                break;

            case Types.INTEGER:
                result = Integer.class;
                break;

            case Types.BIGINT:
                result = Long.class;
                break;

            case Types.REAL:
            case Types.FLOAT:
                result = Float.class;
                break;

            case Types.DOUBLE:
                result = Double.class;
                break;

            case Types.BINARY:
            case Types.VARBINARY:
            case Types.LONGVARBINARY:
                result = Byte[].class;
                break;

            case Types.DATE:
                result = java.sql.Date.class;
                break;

            case Types.TIME:
                result = java.sql.Time.class;
                break;

            case Types.TIMESTAMP:
                result = java.sql.Timestamp.class;
                break;
        }

        return result;
    }
}

请注意,不同的数据库可以在映射上具有不同的变体.