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README.md

💾 Chapter 10 - JDBC - Revision Notes ✍️

🟥 10.1 Introducing the interfaces of JDBC

  • The four key interfaces of JDC are:
  1. Driver - knows how to obtain connection to the DB
  2. Connection - knows how to interact with DB
  3. Statement - knows how to run SQL
  4. ResultSet - knows what to return from a SELECT query
  • A database's JAR will have implementations with a name of VendorName+Interface, e.g. MySQLDriver

🟥 10.2 Connecting to a DB

⭐ JDBC URL ⭐

  • The URL consists of three required parts:
  1. jdbc
  2. Vendor name
  3. Database name
  • E.g. jdbc:derby:zoo
  • The location of the database, port number, connection parameters, password are all optional!💡

⭐ Database Connection ⭐

  • DriverManager is one way to obtain a connection - it is contained in the JDK
  • This class implements the singleton pattern and has a .getConnection(String jdbcUrl) method:
public static void main(String[] args) throws SQLException {
	Connection conn = DriverManager.getConnection("jdbc:derby:zoo");
	System.out.println(conn);
	// org.apache.derby.impl.jdbc.EmbedConnection40@1372082959...
}
  • The DriverManager class seeks the classpath for JARs which contain a Driver.
  • JDBC 4 requires that a file called java.sql.Driver is contained a directory META-INF/service - the filename is fully qualified
  • There will also be files for Connection, Statement, and ResultSet

  • Older code may load up a class using Class.forName(String) which throws a checkked ClassNotFoundException

🟥 10.3 Obtaining a Statement

  • We can obtain a Statement easily from our connection:
Statement stmt = conn.createStatement();
  • This will use the default mode of ResultSet.TYPE_FORWARD_ONLY and a concurrency mode of ResultSet.CONCUR_READ_ONLY
  • There is an overload which lets us specify the scroll and concurrency modes:
Statement createStatement(int resultSetType, int resultSetConcurrency);
  • We have the following ResultSet types:
  1. ResultSet.TYPE_FORWARD_ONLY - rows can only be traversed forwards
  2. ResultSet.TYPE_SCROLL_INSENSITIVE - rows can be traversed both forwards and backwards but will NOT change if table is updated
  3. ResultSet.TYPE_SCROLL_SENSITIVE - forwards and backwards scrolling but DOES update if table is updated
  • We have the following concurrency modes:
  1. ResultSet.CONCUR_READ_ONLY - provides a read only view of the data
  2. ResultSet.CONCUR_UPDATABLE - enables you to update the table through the ResultSet
  • JDK will automatically downgrade a mode if the database does not support the mode😅

🟥 10.4 Executing a Statement

  • INSERT/UPDATE/DELETE queries must be done using a int executeUpdate(String) method. Attempting to do a selection will throw an SQLException at runtime!😱
  • SELECT queries must be done using a ResultSet executeQuery(String) method. Attempting to modify the table will throw an SQLException at runtime!😱
  • We have a method which can do both select and update queries: boolean execute(String) which returns whether a ResultSet is returned:
boolean isAResultSet = stmt.execute(sql);
if (isAResultSet)
	ResultSet rs = stmt.getResultSet();
else
	int numberOfRowsUpdated = stmt.getUpdateCount();

🟥 10.5 Getting Data from a ResultSet

⭐ Reading a ResultSet ⭐

  • For a type-forward-only ResultSet, we can look at the results using next() which returns whether the cursor is on a row:
ResultSet rs = stmt.executeQuery("select id, name from species");
while (rs.next()) {
	int id = rs.getInt(1); // rs.getInt("id") also works
	String name = rs.getInt(2); // rs.getString("name") also works
	System.out.println(id+" "+name);
	/* prints the following
	   1 African Elphhant
	   2 Zebra
	*/
}
  • Attempting to obtain data from ResultSet without calling next() will throw a SQLException⚠️
  • Attempting to access a non-existent column with also throw a SQLException

⭐ Getting Data for a Column ⭐

  • The ResultSet interface has many getType() methods.
  • There are two methods which return sql types:
java.sql.Time getTime(); // for TIME
java.sql.Date getDate(); // for TIMESTAMP
java.sql.TimeStamp getTimeStamp(); // for TIMESTAMP
  • We can use getTime() for a TIMESTAMP column, it will only return the time component
ResultSet rs = stmt.executgeQuery("select dateborn from animal");
if(rs.next()) {
  java.sql.Date sqlDate = rs.getDate(1);
  LocalDate date = sqlDate.toLocalDate(); // 2001-05-06
  java.sql.Time sqlTime = rs.getTime(1);
  LocalTime time = sqlTime.toLocalTime(); // 02:15
  java.sql.TimeStamp sqlTimeStamp = rs.getTimeStamp(1);
  LocalDateTime dateTime = sqlTimeStamp.toLocalDateTime(); // 2001―05―06T02:15
}

⭐ Scrolling ResultSet ⭐

  • There are 2 modes which allow you scroll backwards and to any point of the result set:
  1. TYPE_SCROLL_INSENSITIVE
  2. TYPE_SCROLL_SENSITIVE
  • With these modes, we have the following methods for navigating a ResultSet:
boolean next() // can be used for type-forward-only as well
boolean previous()
boolean first()
boolean absolute(int) // giving a negative value will you row in reverse order
boolean relative(int) // can move cursor back and fourth
void afterLast()
void beforeFirst()
  • Attempting to use the above methods on type-forward-only will throw an SQLException

🟥 10.6 Closing Database Resources

  • We can use try-with-resources syntax to automatically close Database Resources:
public static void main(String[] args) throws SQLException {
String url = "jdbc:derby:zoo";
  try (Connection conn = DriverManager.getConnection(url);
    Statement stmt = conn.createStatement();
	ResultSet rs = stmt.executeQuery("select name from animal")) {
	  while(rs.next()) 
	    System.out.println(rs.getString(1));
  }
}
  • The resources will close in reverse order which they are opened
  • But this isn't necessary as closing a Connection closes the Statement and ResultSet, and closing the Statement closes the ResultSet