使用 RowId 对象

注意:MySQL and Java DB currently do not support the RowId JDBC interface. Consequently, no JDBC tutorial example is available to demonstrate the features described in this section.

A RowId object represents an address to a row in a database table. Note, however, that the ROWID type is not a standard SQL type. ROWID values can be useful because they are typically the fastest way to access a single row and are unique identifies for rows in a table. However, you should not use a ROWID value as the primary key of a table. For example, if you delete a particular row from a table, a database might reassign its ROWID value to a row inserted later.

涵盖以下主题：

• Retrieving RowId Objects
• 使用 RowId 对象
• Lifetime of RowId Validity

Retrieving RowId Objects

Retrieve a java.sql.RowId object by calling the getter methods defined in the interfaces ResultSet and CallableStatement. The RowId object that is returned is an immutable object that you can use for subsequent referrals as a unique identifier to a row. The following is an example of calling the ResultSet.getRowId method:

java.sql.RowId rowId_1 = rs.getRowId(1);

使用 RowId 对象

You can set a RowId object as a parameter in a parameterized PreparedStatement object:

Connection conn = ds.getConnection(username, password);
PreparedStatement ps = conn.prepareStatement(
    "INSERT INTO BOOKLIST" +
    "(ID, AUTHOR, TITLE, ISBN) " +
    "VALUES (?, ?, ?, ?)");
ps.setRowId(1, rowId_1);

You can also update a column with a specific RowId object in an updatable ResultSet object:

ResultSet rs = ...
rs.next();
rs.updateRowId(1, rowId_1);

A RowId object value is typically not portable between data sources and should be considered as specific to the data source when using the set or update method in PreparedStatement and ResultSet objects, respectively. It is therefore inadvisable to get a RowId object from a ResultSet object with a connection to one data source and then attempt to use the same RowId object in a unrelated ResultSet object with a connection to a different data source.

Lifetime of RowId Validity

A RowId object is valid as long as the identified row is not deleted and the lifetime of the RowId object is within the bounds of the lifetime specified by that the data source for the RowId.

To determine the lifetime of RowId objects of your database or data source, call the method DatabaseMetaData.getRowIdLifetime. It returns a value of a RowIdLifetime enumerated data type. The following method JDBCTutorialUtilities.rowIdLifeTime returns the lifetime of RowId objects:

public static void rowIdLifetime(Connection conn)
    throws SQLException {

    DatabaseMetaData dbMetaData = conn.getMetaData();
    RowIdLifetime lifetime = dbMetaData.getRowIdLifetime();

    switch (lifetime) {
        case ROWID_UNSUPPORTED:
            System.out.println("ROWID type not supported");
            break;

        case ROWID_VALID_FOREVER:
            System.out.println("ROWID has unlimited lifetime");
            break;

        case ROWID_VALID_OTHER:
            System.out.println("ROWID has indeterminate lifetime");
            break;

        case ROWID_VALID_SESSION:
            System.out.println(
                "ROWID type has lifetime that " +
                "is valid for at least the " +
                "containing session");
            break;

        case ROWID_VALID_TRANSACTION:
            System.out.println(
                "ROWID type has lifetime that " +
                "is valid for at least the " +
                "containing transaction");
            break;
    }
}