Java DatabaseMetaData.getSchemas()返回空的ResultSet,预期的非空ResultSet

试图了解这里发生了什么. DatabaseMetaData返回一个空结果集,而实际上相同的SQL查询却没有.这不是主要问题,因为我正在使用第二个代码示例来解决.

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

预期为非空结果集.

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

期望得到一个非空的结果集.

解决方法:

据我所知,MySQL JDBC驱动程序考虑的是目录,而不是模式.因此,您应该改用getCatalogs(在所有使用它的地方,都需要使用catalog参数,而不是schema参数).

getSchemas method in Connector/J始终返回空结果集：

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;
}

getCatalogs返回SHOW DATABASES的结果.在DatabaseMetaDataUsingInfoSchema中,您看到信息模式的TABLE_SCHEMA列别名为TABLE_CAT(对于目录),并且在查询中将catalog参数作为TABLE_SCHEMA列的值传递：

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);

标签：jdbc,database-metadata,java,mysql
来源： https://codeday.me/bug/20191120/2042361.html