1 JDBC "Java Database Connectivity". 2 Getting Started Guide: etstart/GettingStartedTOC.fm.html...
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Transcript of 1 JDBC "Java Database Connectivity". 2 Getting Started Guide: etstart/GettingStartedTOC.fm.html...
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• Getting Started Guide:
http://java.sun.com/j2se/1.5.0/docs/guide/jdbc/
getstart/GettingStartedTOC.fm.html
• java.sql Package API:
http://java.sun.com/j2se/1.5.0/docs/api/java/sql/package-summary.html
Useful JDBC LinksUseful JDBC Links
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Introduction to JDBCIntroduction to JDBC
• JDBC allows for convenient database
access from Java applications
• Data is transferred from relations to
objects and vice-versa
– databases optimized for searching/indexing
– objects optimized for engineering/flexibility
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Why Access a Database from Why Access a Database from within a Program?within a Program?
• As we saw last week, some queries can’t be computed in SQL.
• PL/pgSQL includes more programming tools than SQL
• However, sometimes using PL/pgSQL will not be suitable:
– If we require object-oriented programming
– If accessing the database is a small part of a large Java application
– Etc.
• Why not keep all the data in Java objects?
• “Separation of concerns”: DBMSes concentrate on data storage and access; programs concentrate on algorithms, networking, etc.
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Packages to ImportPackages to Import
• In order to connect to a database from
java, import the following packages:
– java.sql.*; (usually enough)
– javax.sql.* (for advanced features, such
as scrollable result sets)
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accessaccess
• Add the following line to
your .classpath file (located in your
home directory):
setenv CLASSPATH ${CLASSPATH}:/usr/share/java/postgresql.jar
• And then open a new shell
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Six StepsSix Steps
• Load the driver
• Establish the Connection
• Create a Statement object
• Execute a query
• Process the result
• Close the connection
JDBC Architecture (1)JDBC Architecture (1)
Application
• DriverManager is provided by Java
Software as part of the Java 2
Platform.
• Drivers are provided by DBMS
vendors.
Driver
DriverManager
DBMS
JDBC Architecture (2)JDBC Architecture (2)
• The application creates a driver instance and registers it with the DriverManager.
• The DriverManager tells the driver to connect to the DB
• The DriverManager keeps track of registered driver instances and their connections to DB’s.
• The Driver “talks” to a particular DB through the connection
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ConnectingConnecting
1. Initializing a driver and registering it with the DriverManager: Class.forName(“org.postgresql.Driver");
2. Getting a connection:
DriverManager.getConnection(URL)
• The URL is:“jdbc:postgresql://dbserver/public”,”YOUR-
LOGIN”,null
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For exampleFor example
Class.forName(“org.postgresql.Driver"); DriverManager.getConnection
(“jdbc:postgresql://dbserver/public”,”gidi”,null);
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Interacting with the DBInteracting with the DB
• Once you have established a connection,
your would like to interact with the DB
• Interaction are done by sending Statements
and PreparedStatements to the DB
• These are used for 2 things (using different
methods):
1. Querying the DB (executeQuery)
2. Changing the DB (executeUpdate)
StatementStatement
Statement createStatement()
– returns a new Statement object
• Used to send SQL commands to the DB
• Created via the connection object
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Statement query methodsStatement query methods
1. stmt.executeQuery(String query): for
queries that return a single ResultSet
object (typically select)
2. stmt.executeUpdate(String query): for
INSERT, UPDATE, DELETE, and SQL DDL
directives
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CompilationCompilation
• When executing an SQL statement via
JDBC, it is not checked for errors until it
is run (Not checked during compilation)
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executeQueryexecuteQuery
String queryStr = "SELECT * FROM Sailors " +"WHERE Name = 'joe smith'";
Statement stmt = con.createStatement();ResultSet rs = stmt.executeQuery(queryStr);
• The executeQuery method returns a ResultSet object representing the query result.
No semi-colon(;)
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executeUpdateexecuteUpdate
String deleteStr = “DELETE FROM Sailors " +"WHERE sid = 15";
Statement stmt = con.createStatement();int delnum = stmt.executeUpdate(deleteStr);
•executeUpdate returns the number of rows modified
No semi-colon(;)
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PreparedStatement PreparedStatement motivationmotivation
• Suppose we would like to run the query
SELECT * FROM Emp
where name=‘moshe’;
• But we would like to run this for all employees (separately), not only ‘moshe’…
• Could we create a variable instead of ‘moshe’ which would get a different name every time??..
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PreparedStatementPreparedStatement
PreparedStatement
prepareStatement(String)
– returns a new PreparedStatement
object
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Prepared StatementsPrepared Statements
• Prepared Statements are used for queries that
are executed many times with possibly
different contents.
• A PreparedStatement object includes the query
and is prepared for execution (precompiled).
• Question marks can be inserted as variables.
-setString(i, value)
-setInt(i, value)The i-th question
mark is set to the
given value.
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PreparedStatement.executeQueryPreparedStatement.executeQuery() ()
String queryStr = "SELECT * FROM Sailors " +"WHERE Name = ? and Rating < ?”;
PreparedStatement pstmt = con.prepareStatement(queryStr);
pstmt.setString(1, “Joe”);pstmt.setInt(2, 8);
ResultSet rs = pstmt.executeQuery();
1st question
mark
Value to insert
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PreparedStatement.executeUpdatePreparedStatement.executeUpdate()()
String deleteStr = “DELETE FROM Boats " +"WHERE Name = ? and Color = ?”;
PreparedStatement pstmt = con.prepareStatement(deleteStr);
pstmt.setString(1, “Fluffy”);pstmt.setString(2, "red");
int delnum = pstmt.executeUpdate();
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• Will this work?
PreparedStatement pstmt = con.prepareStatement(“select * from ?”);
pstmt.setString(1, "Sailors");
No! We may put ? only instead of values
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Why use Why use preparedStatement?preparedStatement?
• In most cases, you can use a regular statement and
just change the string you send to executeQuery
each time
• You have to be careful
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Why use Why use preparedStatement?preparedStatement?
• Suppose google worked without PreparedStatements,
they would implement search queries as something
like:
Statement s;
s.executeQuery(‘select URL,Title from internet where
content like ‘%”+searchString+”%’”);
• What would happen if a hacker searched for:
bla bla’ UNION select company as URL,
CreditCardNumber AS title from advertisingClients
where company like ‘Example taken from dbi course
ResultSet (1)ResultSet (1)• A ResultSet is an object which contains the
result of a query - a “table”.
• At most one ResultSet per Statement can be open at the same time(!!).
• A ResultSet maintains a cursor pointing to its current row of data.
• The 'next' method moves the cursor to the next row
• As of JDBC 2.0, scrollable ResultSets are available, which also include ‘previous’, ’first’, ‘last’, etc..
ResultSet (2)ResultSet (2)
• resultSet methods work on the
current row.
• The cursor is positioned before
the first row upon creation.
ResultSet (3)ResultSet (3)
Statement stmt=
con.createStatement();
ResultSet rs =
stmt.executeQuery (
"SELECT * FROM Table1");
while (rs.next()) {
//something…
}
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ResultSet methodsResultSet methods
• Getting the value in some column (for the current row):– getString(int columnNum);
– getString(String columnName);
– getInt(int columnNum);
– getInt(String columnName);
– Etc…
• To check if NULL was returned, you have to use wasNull() on the ResultSet after getting the value.
String s = rs.getString(“column1");
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Example revisitedExample revisited
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM Table1");
// retrieve and print the values for the current row
while (rs.next()) {
int i = rs.getInt("a");
String s = rs.getString("b");
float f = rs.getFloat("c");
System.out.println("ROW = " + i + " " + s + " " + f);
}
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ResultSetMetaData ResultSetMetaData
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));
}
An object created by the ResultSet which holds information about its columns
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Printing Query Output: Printing Query Output: Result Set (2) 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("");
}
• getString() is allowed to access all simple JDBC types
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Cleaning Up After YourselfCleaning Up After Yourself
• Remember to close the Connections, Statements, PreparedStatements and ResultSets
con.close();stmt.close();pstmt.close();rs.close();
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Dealing With ExceptionsDealing With Exceptions
catch (SQLException e) {//human readable message about the
exceptionSystem.out.println(e.getMessage());
//String describing the reason of the exception
System.out.println(e.getSQLState());//driver-dependent code for the exceptionSystem.out.println(e.getErrorCode());
}
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Mapping SQL and Java Mapping SQL and Java Types Types
• SQL and Java data types are not identical
• There are significant variations between the SQL types supported by different database products
• JDBC defines a set of generic SQL type identifiers in the class java.sql.Types
• The driver is responsible for mapping between the DB SQL types and JDBC SQL types
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TransactionsTransactions
• Transaction = 2 or more statements which
must all succeed (or all fail) together
• If one fails, the system must reverse all
previous actions
• Aim: don’t leave DB in inconsistent state
halfway through a transaction
• COMMIT = complete transaction
• ROLLBACK = abort
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ExampleExample
• 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);pstmt.executeUpdate();
What happens if this update fails?
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Transaction ManagementTransaction Management
• 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
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AutoCommitAutoCommit
• If you set AutoCommit to false, you must
explicitly commit or rollback the transaction
using Connection.commit() and
Connection.rollback()
con.setAutoCommit(boolean val)
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ExampleExamplecon.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); pstmt.executeUpdate(); con.commit();}catch (SQLException e) { con.rollback();}