无法从Java的ResultSet中加载对象

Question

I have a problem with loading objects from a SQLite database.

First of all, this is my table definition:

CREATE TABLE MyTable (
  rowid INTEGER PRIMARY KEY,
  data BLOB
);

This is the simple class which I want to store and reload:

public class MyHashMap extends HashMap<String, Integer> {
    private static final long serialVersionUID = 0L;
}

Then I'm filling the map with some data and store it with an SQL INSERT statement in the database. Everything works fine and if I execute a SELECT (with the sqlite3 command-line client) I will see the correct information.

Now I'm using the java.sql package to load the object:

String sql = "SELECT data FROM MyTable WHERE rowid = 1";
MyHashMap map = null;

try {
  try (Statement stmt = db.createStatement()) {
    try (ResultSet rs = stmt.executeQuery(sql)) {
      if (rs.next()) {
        map = rs.getObject("data", MyHashMap.class);
      }
    }
  }
} catch (Exception e) {
  e.printStackTrace();
}

There's no exception thrown but my map variable is null. I debugged the program and I can say that the getObject method is called as expected.

Answer 1

First, you definition of MyHashMap is incorrect

public class MyHashMap extends HashMap<Integer, String> {
    private static final long serialVersionUID = 0L;
}

The main issue, though, is that SQL doesn't store Java objects; it merely stores rows of records, which consist of fields. You need to read these records one by one, and store them in your map. Roughly as follows:

MyHashMap map = new MyHashMap();
final String sql = "SELECT rowid, data FROM MyTable";
try (final Statement stmt = connection.createStatement;
        final ResultSet rs = stmt.executeQuery(sql)) {
    while (rs.next()) {
        map.put(rs.getInt(1), rs.getString(2));
    }
 }

Please note that there's a good chance that reading a Blob into a String will fail. Usually, JDBC drivers are clever enough to convert data types, but if you have raw binary data in your blob, you cannot read it into a string. You would need the getBlob method instead, and deal with the resulting object. But I can't tell from your code what you'll be storing in that blob.

Answer 2

Ok, I found a solution with the following method:

private Object getObjectFromBlob(ResultSet rs, String columnName)
    throws ClassNotFoundException, IOException, SQLException {
  InputStream binaryInput = rs.getBinaryStream(columnName);
  if (binaryInput == null || binaryInput.available() == 0) {
    return null;
  }

  ObjectInputStream in = new ObjectInputStream(binaryInput);
  try {
    return in.readObject();
  } finally {
    in.close();
  }
}