Converting Blob to String yields just the Blob object name in Java

Question

I am trying to read a Blob from a MySQL db, containing some random text.

My code looks like this:

Connection conn = DriverManager.getConnection(connection_string);
String message = "Some message";
byte[] messageBytes = message.getBytes("UTF-8");

Blob blob = conn.createBlob();
blob.setBytes(1l,messageBytes);

String sql = "INSERT INTO db.dbname('blob') VALUES ('" + blob + "');"
PreparedStatement pat = conn.prepareStatement(sql);
pat.executeUpdate();

In another class I have the code to read the blob field from database:

//some SQL code to read the blob field here
Blob readBlob = resultSet.getBlob("blob");
byte[] bytes = readBlob.getBytes(1, (int) readBlob.length());
String str = new String(bytes);

System.out.println(str);

The output is: com.mysql.cj.jdbc.Blob@3be81fc2

My code is exactly the same as some of the working solutions on StackOverflow, so I don't know exactly what is wrong.

Wouldn't CLOB be more suitable than BLOB, since CLOB is for character data and BLOB is for non-character data. — Abra
– Abra, Commented Oct 8, 2020 at 18:12
I am gonna try with CLOB too, but then again, I don't see why my code doesn't work. — CristianBastan
– CristianBastan, Commented Oct 8, 2020 at 18:14
Did you check the value entered into the database after your INSERT (and before you ran the java code that reads from the database)? I mean did you check the value with MySQL Workbench or similar? Also, method executeUpdate() returns a count of the number of rows that were changed. In your case, the method should return 1 (one). Did it? — Abra
– Abra, Commented Oct 8, 2020 at 18:27

Oliver Marienfeld · Accepted Answer · 2020-10-08 18:16:01Z

1

Seems like you make a small mistake during insertion. Your code should be something like:

String sql = "INSERT INTO db.dbname('blob') VALUES (?);"
PreparedStatement pat = conn.prepareStatement(sql);
pat.setBlob(1, blob);
pat.executeUpdate();

Please use prepared statements with those placeholders. Otherwise attackers could easily run an SQL injection attack. If you want to see how SQL injection works, there‘s a great Computerphile Video about it on YouTube.

answered Oct 8, 2020 at 18:16

Oliver Marienfeld

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