JDBC – Java Database Connectivity Representation and Management of Data on the Internet

Introduction to JDBC JDBC is used for accessing databases from Java applications Information is transferred from relations to objects and vice-versa databases optimized for searching/indexing objects optimized for engineering/flexibility

Overview RMI JDBC CORBA java.net TCP/IP Network OS

Working With Oracle Add to your .cshrc the following: if ($HOST == sol4) then setenv ORACLE_HOME /opt/oracle else setenv ORACLE_HOME /usr/local/oracle8i endif setenv PATH $ORACLE_HOME/bin:$PATH setenv ORACLE_SID stud

sqlplus snoopy/snoopy@stud.cs Using Oracle If a student whose login is Snoopy wants to work directly with Oracle: sqlplus snoopy/snoopy@stud.cs Note: we use the login for a password! (Don’t change your password)

Seven Steps Load the driver Define the Connection URL Establish the Connection Create a Statement object Execute a query Process the result Close the connection

Packages to Import In order to connect to the Oracle database from java, import the following packages: java.sql.*; (usually enough) javax.sql.* (for advanced features, such as scrollable result sets)

JDBC Architecture Java code calls JDBC library JDBC loads a driver Application JDBC Driver Java code calls JDBC library JDBC loads a driver Driver talks to a particular database Can have more than one driver -> more than one database Ideal: can change database engines without changing any application code

Loading the Driver We can register the Driver indirectly using the Java statement: Class.forName(“oracle.jdbc.driver.OracleDriver"); Calling Class.forName, automatically creates an instance of the driver registers the driver with the DriverManager

Another Option Another option is to create an instance of the driver and register it with the Driver Manager: Driver driver = new oracle.jdbc.OracleDriver(); DriverManager.registerDriver(driver);

The DriverManager The DriverManager tries all the drivers Uses the first one that works When a driver class is first loaded, it registers itself with the DriverManager Therefore, to load a driver, just register it!

Connecting to the Database String path = "jdbc:oracle:thin:"; String host = "sol4"; String port = "1521"; String db = "stud"; String login = "snoopy"; String url = path + login + "/" + login + "@" + host +":" + port + ":" + db; Class.forName("oracle.jdbc.driver.OracleDriver"); Connection con = DriverManager.getConnection(conStr); This is actually the password

Connection Methods Statement createStatement() returns a new Statement object PreparedStatement prepareStatement(String sql) returns a new PreparedStatement object CallableStatement prepareCall(String sql) returns a new CallableStatement object Why all these different kinds of statements? Optimization.

Querying with Statement String queryStr = "SELECT * FROM Member " + "WHERE Lower(Name) = 'harry potter'"; Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery(queryStr); Statements are used for queries that are only issued once. The executeQuery method returns a ResultSet object representing the query result.

Changing DB with Statement String deleteStr = “DELETE FROM Member " + "WHERE Lower(Name) = ‘lord voldemort’"; Statement stmt = con.createStatement(); int delnum = stmt.executeUpdate(deleteStr); executeUpdate is used for data manipulation: insert, delete, update, create table, etc. (anything other than querying!) executeUpdate returns the number of rows modified.

About Prepared Statements Prepared Statements are used for queries that are executed many times. They are parsed only once. Using setString(i, value) (setInt(i, value), etc.) the i-th question mark is set to the given value.

Querying with PreparedStatement String queryStr = "SELECT * FROM Program " + "WHERE Name = ? and Cost < ?”; PreparedStatement pstmt = con.prepareStatement(queryStr); pstmt.setString(1, “Unfogging the Future”); pstmt.setInt(2, 1000); ResultSet rs = pstmt.executeQuery();

Changing DB with PreparedStatement String deleteStr = “DELETE FROM Program " + "WHERE Name = ? and Cost < ?”; PreparedStatement pstmt = con.prepareStatement(deleteStr); pstmt.setString(1, “Unfogging the Future”); pstmt.setInt(2, 1000); int delnum = pstmt.executeUpdate();

Timeout Use setQueryTimeOut(int seconds) to set a timeout for the driver to wait for a statement to be completed If the operation is not completed in the given time, an SQLException is thrown What is it good for?

Statements vs. PreparedStatements: Be Careful! Are these the same? What do they do? String val = “abc”; PreparedStatement pstmt = con.prepareStatement(“select * from R where A=?”); pstmt.setString(1, val); ResultSet rs = pstmt.executeQuery(); String val = “abc”; Statement stmt = con.createStatement( ); ResultSet rs = stmt.executeQuery(“select * from R where A=” + val);

Statements vs. PreparedStatements: Be Careful! Will this always work? Statement stmt = con.createStatement( ); ResultSet rs = stmt.executeQuery(“select * from R where A=‘ ” + val + “ ’ ”); The moral: When getting input from the user, always use a PreparedStatement

Statements vs. PreparedStatements: Be Careful! Will this work? PreparedStatement pstmt = con.prepareStatement(“select * from ?”); pstmt.setString(1, myFavoriteTableString);

ResultSet A ResultSet provides access to a table of data generated by executing a Statement. Only one ResultSet per Statement can be open at once. The table rows are retrieved in sequence. A ResultSet maintains a cursor pointing to its current row of data. The 'next' method moves the cursor to the next row.

ResultSet Methods boolean next() void close() activates the next row the first call to next() activates the first row returns false if there are no more rows void close() disposes of the ResultSet allows you to re-use the Statement that created it automatically called by most Statement methods

ResultSet Methods Type getType(int columnIndex) returns the given field as the given type fields indexed starting at 1 (not 0) Type getType(String columnName) same, but uses name of field less efficient int findColumn(String columnName) looks up column index given column name

ResultSet Methods String getString(int columnIndex) boolean getBoolean(int columnIndex) byte getByte(int columnIndex) short getShort(int columnIndex) int getInt(int columnIndex) long getLong(int columnIndex) float getFloat(int columnIndex) double getDouble(int columnIndex) Date getDate(int columnIndex) Time getTime(int columnIndex) Timestamp getTimestamp(int columnIndex)

ResultSet Methods String getString(String columnName) boolean getBoolean(String columnName) byte getByte(String columnName) short getShort(String columnName) int getInt(String columnName) long getLong(String columnName) float getFloat(String columnName) double getDouble(String columnName) Date getDate(String columnName) Time getTime(String columnName) Timestamp getTimestamp(String columnName)

isNull In SQL, NULL means the field is empty Not the same as 0 or “” In JDBC, you must explicitly ask if a field is null by calling ResultSet.isNull(column)

Printing Query Output: Result Set (1) Print Column Headers: ResultSetMetaData rsmd = rs.getMetaData(); int numcols = rsmd.getColumnCount(); for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(","); System.out.print(rsmd.getColumnLabel(i)); }

Printing Query Output: Result Set (2) while (rs.next()) { for (int i = 1 ; i <= numcols; i++) { if (i > 1) System.out.print(","); System.out.print(rs.getString(i)); } System.out.println(""); To get the data in the i-th column: rs.getString(i) To get the data in column Abc: rs.getString(“Abc”)

Mapping Java Types to SQL Types SQL type Java Type CHAR, VARCHAR, LONGVARCHAR String NUMERIC, DECIMAL java.math.BigDecimal BIT boolean TINYINT byte SMALLINT short INTEGER int BIGINT long REAL float FLOAT, DOUBLE double BINARY, VARBINARY, LONGVARBINARY byte[] DATE java.sql.Date TIME java.sql.Time TIMESTAMP java.sql.Timestamp

Database Time Times in SQL are notoriously non-standard Java defines three classes to help java.sql.Date year, month, day java.sql.Time hours, minutes, seconds java.sql.Timestamp year, month, day, hours, minutes, seconds, nanoseconds usually use this one

Cleaning Up After Yourself Remember to close the Connections, Statements, PreparedStatements and ResultSets con.close(); stmt.close(); pstmt.close(); rs.close()

Dealing With Exceptions A exception can have more exceptions in it. catch (SQLException e) { while (e != null) { System.out.println(e.getSQLState()); System.out.println(e.getMessage()); System.out.println(e.getErrorCode()); e = e.getNextException(); }

Advanced Topics

LOBs: Large OBjects Two types: CLOB: Character large object (a lot of characters) BLOB: Binary large object (a lot of bytes) Actual data is not stored in the table with the CLOB/BLOB column. Only a pointer to the data is stored there I will show how to use a CLOB; BLOBs are similar

We can also use getAsciiStream() which returns an InputStream Retrieving a CLOB create table userComments( user varchar(50), comment CLOB) ); We can also use getAsciiStream() which returns an InputStream ResultSet rs = stmt.executeQuery(“select comment from userComments”); while (rs.next) { Clob c = rs.getClob(“comment”); Reader reader = c.getCharacterStream(); doSomething(reader) }

We can also use setAsciiStream() which gets an InputStream Inserting a CLOB PreparedStatement pstmt = con.prepareStatement(“insert into userComments values(‘sara’, ?)”); Reader reader = new FileReader(fileName); pstmt.setCharacterStream(1, reader, Integer.MAX_VALUE); pstmt.executeUpdate(); We can also use setAsciiStream() which gets an InputStream

Transactions Transaction = more than one statement which must all succeed (or all fail) together If one fails, the system must reverse all previous actions Also can’t leave DB in inconsistent state halfway through a transaction COMMIT = complete transaction ROLLBACK = abort

What happens if this update fails? Example Suppose we want to transfer money from bank account 13 to account 72: PreparedStatement pstmt = con.prepareStatement(“update BankAccount set amount = amount + ? where accountId = ?”); pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); What happens if this update fails?

Transaction Management Transactions are not explicitly opened and closed The connection has a state called AutoCommit mode if AutoCommit is true, then every statement is automatically committed if AutoCommit is false, then every statement is added to an ongoing transaction Default: true

AutoCommit Connection.setAutoCommit(boolean val) If you set AutoCommit to false, you must explicitly commit or rollback the transaction using Connection.commit() and Connection.rollback() In order to work with LOBs, you usually have to set AutoCommit to false, while retrieving the data Note: DDL statements in a transaction may be ignored or may cause a commit to occur. The behavior is DBMS dependent

Fixed Example con.setAutoCommit(false); try { PreparedStatement pstmt = con.prepareStatement(“update BankAccount set amount = amount + ? where accountId = ?”); pstmt.setInt(1,-100); pstmt.setInt(2, 13); pstmt.executeUpdate(); pstmt.setInt(1, 100); pstmt.setInt(2, 72); con.commit(); catch (Exception e) { con.rollback(); }

Isolation Levels How do different transactions interact? Do they see what another has written? JDBC supports 4 “isolation modes” Set using: Connection.setTransactionIsolation Oracle only implements: TRANSACTION_SERIALIZABLE TRANSACTION_READ_COMMITED

Isolation Levels (cont.) TRANSACTION_SERIALIZABLE: transactions are equivalent to serial transactions TRANSACTION_READ_COMMITED: A transaction can only read values that have been committed

Level: READ_COMMITED 1 2 Transaction 1: insert into A values(1) select * from A Question: Is it possible for a transaction to see 1 in A, but not 2? Question: Is it possible for the 2 queries to give different answers for level SERIALIZABLE? 1 2 Table: A