Java DatabaseMetaData.getSchemas() returns empty ResultSet, expected non-empty ResultSet

Trying to understand what is going on here. DatabaseMetaData is returning an empty result set, where as an SQL query that is effectively the same does not. Not a major issue as I am using the second code example as a work around.

DatabaseMetaData dmd = this.connection.getMetaData();
ResultSet rs = dmd.getSchemas();
while (rs.next()){
  // empty result set
}

Expected a non-empty result set.

ResultSet rs = this.connection.prepareStatement("SELECT SCHEMA_NAME FROM INFORMATION_SCHEMA.SCHEMATA;").executeQuery();
while (rs.next()){
  // non-empty result set with expected results
}

Expected a non-empty result set and got it.

Solution

As far as I can tell the MySQL JDBC driver considers that a catalog, not a schema. So you should use getCatalogs instead (and everywhere you use it, you need to use the catalog parameter, not the schema parameter).

The getSchemas method in Connector/J always returns an empty result set:

public java.sql.ResultSet getSchemas() throws SQLException {
    Field[] fields = new Field[2];
    fields[0] = new Field("", "TABLE_SCHEM", java.sql.Types.CHAR, 0);
    fields[1] = new Field("", "TABLE_CATALOG", java.sql.Types.CHAR, 0);

    ArrayList<ResultSetRow> tuples = new ArrayList<ResultSetRow>();
    java.sql.ResultSet results = buildResultSet(fields, tuples);

    return results;
}

The getCatalogs returns the result of SHOW DATABASES. And in DatabaseMetaDataUsingInfoSchema you see the TABLE_SCHEMA column of the information schema aliased as TABLE_CAT (for catalog) and the catalog parameter being passed as a value for the TABLE_SCHEMA column in the query:

String sql = "SELECT TABLE_SCHEMA AS TABLE_CAT, NULL AS TABLE_SCHEM, TABLE_NAME,"
        + "COLUMN_NAME, NULL AS GRANTOR, GRANTEE, PRIVILEGE_TYPE AS PRIVILEGE, IS_GRANTABLE FROM INFORMATION_SCHEMA.COLUMN_PRIVILEGES WHERE "
        + "TABLE_SCHEMA LIKE ? AND TABLE_NAME =? AND COLUMN_NAME LIKE ? ORDER BY COLUMN_NAME, PRIVILEGE_TYPE";

java.sql.PreparedStatement pStmt = null;

try {
    pStmt = prepareMetaDataSafeStatement(sql);

    if (catalog != null) {
        pStmt.setString(1, catalog);
    } else {
        pStmt.setString(1, "%");
    }

    pStmt.setString(2, table);
    pStmt.setString(3, columnNamePattern);