Assuming that you have already stored file data into a BLOB column in your database table, you can use JDBC to read the file data from the database using the following steps:

1. Load the JDBC driver and create a connection to the database

// Load the JDBC driver
Class.forName("com.mysql.cj.jdbc.Driver");

// Create a connection to the database
Connection conn = DriverManager.getConnection("jdbc:mysql://localhost/mydatabase", "username", "password");

2. Create a statement and execute a SQL query to retrieve the file data

// Create a statement and execute a SQL query to retrieve the file data
PreparedStatement pstmt = conn.prepareStatement("SELECT filedata FROM files WHERE filename = ?");
pstmt.setString(1, "myfile.txt");
ResultSet rs = pstmt.executeQuery();

if (rs.next()) {
   // Get the BLOB data from the result set
   Blob filedata = rs.getBlob("filedata");

   // Read the file data from the BLOB stream
   InputStream in = filedata.getBinaryStream();
   // ... read from the InputStream here ...
}

3. Clean up resources

// Close the result set, statement, and connection
rs.close();
pstmt.close();
conn.close();

Note that the exact details of the JDBC code may vary depending on your specific database and table schema. Also, be sure to handle exceptions and close resources properly to avoid memory leaks and other issues.