Java and Databases. COMP514 Distributed Information Systems. Java Database Connectivity. Standards and utilities. Java and Databases

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1 Java and Databases COMP514 Distributed Information Systems Java Database Connectivity One of the problems in writing Java, C, C++,, applications is that the programming languages cannot provide persistence of data (at least on their own) Example: Java (and applets) can be used for developing simple graphical interfaces where an user can input data and retrieve other data. Unfortunately if we stop the Java application all that the data that user has provided are lost. Solution: Persistence is available by storing and accessing data held by a database management system. Java and Databases Standards and utilities Problem: how can my Java program access different kinds of databases? Java programs Relational Database Without standardized database interfaces, integrating diverse data would be extraordinarily difficult. Standardized interfaces include ODBC and JDBC. These interfaces provide drivers that act as intermediary between the application and the DBMS. The interfaces process our Java requests and submit SQL statements to a data source.

2 Java and Databases Database Access in Java - JDBC JDBC Java programs Relational Database JDBC provides: SQL access a driver manager Vendors provide drivers to plug into the managers: Oracle vendors will provide Oracle drivers MySQL vendors will provide MySQL drivers... The programmer (us) has to provide The location of the database The name of a Driver. These two things are provided in the form of a URL Building an Application Connecting to different databases The Driver Manager load an appropriate Driver The Connection is made Queries can then be executed as required Data is returned as objects that can be manipulated by the Java program All the methods that we will use to query/modify a database are the same, regardless of the type of database involved. IE. JDBC provides an homogeneous standard for accessing different kinds of databases But drivers are different. So, first we have to learn how to connect to different databases: Access Oracle

3 Specifying the Connection Specifying the Connection in Oracle We have to provide: The location of the database (URL) Driver name Oracle uses interfaces provided by the Oracle company. There are different kinds of interfaces for different usages. We will see the thin interface. A database URL has the following form jdbc:<subprotocol>:<subname> We need to know: the subprotocol. This enables the driver manager to identify the driver the subname. The subname can vary, depending on the driver. It usually includes information like the name of the database, the location of the database, the port used in the communication, your username and password, etc etc Java program Database connectivity thin Oracle database Specifying the Connection in Oracle Specifying the Connection in Oracle Using the thin driver, the syntax for the URL is jdbc:oracle:thin:@<data-source-name> So, assume I have created an Oracle database called c514. Can I connect to it? <data-source-name> is composed by: the name of the machine where the Oracle server runs; the port used by the JDBC connection; the name of the database schema we want to access In our case: jdbc:oracle:thin:@indian:1521:c514 jdbc:oracle:thin:@indian.csc.liv.ac.uk:1521:c514 No, first we have to tell the java compiler where the thin drivers and the appropriate Java libraries are. It depends, of course, on particular configuration of the system. See details on the next two slides for our particular setting

4 Specifying the Connection in Oracle On a linux machine 1) open your.profile file 2) Add (if it is not already there): # Oracle setup export ORACLE_BASE=/net/oracle/app/oracle export ORACLE_HOME=/net/oracle/app/oracle/product/8.1.7 export PATH=${PATH}:${ORACLE_HOME}/bin export SHLIB_PATH=${ORACLE_HOME}/lib export TWO_TASK=DVLP.csc.liv.ac.uk Specifying the Connection in Oracle # JDBC setup CLASSPATH=$CLASSPATH:$ORACLE_HOME/jdbc/lib/classes111.zip:$ORA CLE_HOME/jdbc/lib/nls_charset11.zip export CLASSPATH export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$ORACLE_HOME/jdbc/lib Log out and log in again Connecting to a database SQL Statement Execution in Java Assume that I have created a database called c512 (Access or Oracle, it doesn t matter) containing the following relations: Beers(name, brewery) Bars(name, addr, licence, barc#) Drinkers(name, gender) Likes (drinker, beer) Sells(beer, bar, price) Frequents(drinker, bar) How can I connect and query it with a Java program? Four steps corresponding to four primary classes from the java.sql package: load a driver (java.sql.drivermanager) connect to the DB (java.sql.connection) create a SQL statement (java.sql.statement) execute a SQL statement (java.sql.resultset)

5 An example The Java code (Oracle version) We want a Java program that executes the query: SELECT name FROM Bars WHERE licence = pub // You need to import the java.sql package to use JDBC import java.sql.*; class myfirstquery { public static void main (String args []) throws SQLException { // Load the ORACLE driver DriverManager.registerDriver(new oracle.jdbc.driver.oracledriver()); String url = "jdbc:oracle:thin:@indian:1521:c514"; String username = yourusername"; String password = yourpassword"; String query = "SELECT name FROM Bars WHERE license = pub " The Java code (Oracle version) The Java code explained // Connect to the database Connection conn = DriverManager.getConnection (url, username, password); // Create a Statement Statement SelectStatement = conn.createstatement (); // Execute the SQL statement ResultSet rset = SelectStatement.executeQuery (query); // Iterate through the result and print the bars names while (rset.next ()) System.out.println (rset.getstring (1)); }} // You need to import the java.sql package to use JDBC import java.sql.*; // Load the ODBC driver Class.forName(sun.jdbc.odbc.JdbcOdbcDriver); // Load the ORACLE driver DriverManager.registerDriver(new oracle.jdbc.driver.oracledriver()); java.sql is the package we have to use in order to access SQL databases. Load the appropriate driver.

6 The Java code explained The Java code explained String url = "jdbc:odbc:mydatabase"; String username = ""; String password = ""; String url = "jdbc:oracle:thin:@indian:1521:c302"; String username = yourusername"; String password = yourpassword"; // Connect to the database Connection conn = DriverManager.getConnection (url, username, password); // Create a Statement Statement SelectStatement = conn.createstatement (); Associate one (or more) statements with this connection; // Execute the SQL statement ResultSet rset = SelectStatement.executeQuery (query); Execute the statement(s); Connect to the database The Java code explained Another example // Iterate through the result and print the bars names while (rset.next ()) System.out.println (rset.getstring (1) ); rset contains the table which is the result of the query. This object contains a reference to the rows of this table. We can use.next() to processe all the rows in rset one by one. Also we can use methods like getstring to process the columns. getstring takes an int argument which represents the column number of the rset table. String query = "SELECT name, address FROM Bars WHERE licence = pub ; // Iterate through the result and print the bars names and addresses while (rset.next ()) System.out.println (rset.getstring (1) + rset.getstring (2) ); OR, if you want to add some extra information: // Iterate through the result and print the bars names and addresses while (rset.next ()) System.out.println (``Name: + rset.getstring (1) + ``Address: + rset.getstring (2) );

7 Another example ResultSet Class String query = "SELECT beer,price FROM Sells WHERE bar = The Polar Bar ; // Iterate through the result and print the beers names and prices while (rset.next ()) System.out.println ``Beer: + rset.getstring (1) + ``Price: + rset.getfloat (2) ); Since price is a Float we need the appropriate method!! next close wasnull getstring getboolean getbyte getshort getint getlong getfloat getdouble getnumeric getbytes getdate gettime gettimestamp getasciistream getunicodestream getbinarystream An example of updating: insert rows An example of updating: modify rows // Create a Statement Statement UpdateStatement = conn.createstatement (); //Create an update string String update = INSERT INTO Sells (bar, beer, price) + VALUES ( The Bar, Bud, 2.0) ; // Execute the SQL statement UpdateStatement.executeUpdate (update); Set the price of Stella to 2.30 in the table Sells The SQL statement is UPDATE Sells SET price = 2.30 WHERE beer = Stella Artois The only difference with the previous code is the use of the executeupdate method. Also we do not need to store any result set!!

8 An example of updating: modify rows Table creation The Java code: // Create a Statement Statement UpdateStatement = conn.createstatement (); //Create an update string String update = UPDATE Sells SET price = WHERE beer = Stella Artois ; // Execute the SQL statement UpdateStatement.executeUpdate (update); The last missing bit: Create and drop objects (tables, views ) Example: we want to create the table Sells SQL statement CREATE TABLE Sells( bar CHAR(20), beer VARCHAR(20), price REAL) Table creation Managing transactions with Java The Java code // Create a Statement Statement UpdateStatement = conn.createstatement (); //Create an update string String update = CREATE TABLE Sells(bar CHAR(20), + beer VARCHAR(20),price REAL) // Execute the SQL statement UpdateStatement.executeUpdate (update); Concurrent updates on the same database can generate problems. This is because by default different SQL statements contained in a Java program are considered different transactions. Consider the following code. This code provides a simple interface for booking seats in a cinema.

9 Managing transactions with Java Managing transactions with Java The database contains a table Cinema Screen Seats a 150 b 40 c 1 The code queries the database for seats in a given screen and book the seat if there is one. The query is executed and the result is put into rs: String query = "SELECT seats FROM Cinema WHERE screen = " + "'"+screenname+"'"; Statement stmt = conn.createstatement(); ResultSet rs = stmt.executequery(query); Managing transactions with Java Managing transactions with Java The number of seat is extracted from rs and assigned to seats: while(rs.next()) { seats = rs.getint(1); System.out.println (seats); } An if-block tests if there are still seats. If yes, a seat is booked and the database is updated. If no, a sorry message is printed. if (seats > 0){ System.out.println ("There are still seats available"); String update = "UPDATE CINEMA SET seats = seats - 1 WHERE screen = " + "'"+screenname+"'"; Statement updatestmt = conn.createstatement (); updatestmt.executeupdate(update); System.out.println ("seat booked"); } else {System.out.println ("There are no seats available. Sorry.");}

10 Managing transactions with Java The setautocommit mode What happens if two people try to book a seat in screen c at the same time? The query "SELECT seats FROM Cinema WHERE screen = " + "'"+screenname+"' will give answer 1 to both, and seats will be assigned to 1 in both cases. Therefore both people will receive (and pay) a ticket for only one seat. The problem in the previous example was due to the fact that the execution of the query, and the execution of the update generated two different transactions. Sometimes we need to group different SQL statements in a single transaction. That is we want that: either all of the statements are executed, or none of the statements is executed. We group statements using the setautocommint method. The setautocommit mode 2-Tier Server applications conn.setautocommit(false); SQL statements conn.commit(); conn.setautocommit(true); Java Application using JDBC interface network JDBC Enabled Database All the SQL statements after the setautocommit(false) are do not commit until a comit() method is used. This enables us to group SQL statements into a single transaction.

11 2-Tier Server applications 3-Tier Server This application architecture is simple, but.. often has to do all the processing has to know everything about the database where it is current access controls access permissions etc. Database has to be visible to the client Databases are not meant to handle large numbers of simultaneous invocations Assume that different clients want to access to the DB. Possibly asking the same queries. Sockets JavaIDL JavaRMI Server JDBC DB 3-Tier Server - advantages (Firewall) Server can manage database connections efficiently can focus on processing data retrieved Running server on high performance hardware can improve client perceived performance Database only has to be visible to the server - not the rest of the machines!! Can be hidden behind a firewall Operational changes only have to be notified to the server - not all the clients Much better security Firewall - a security gateway that protects an organization from unauthorized access via the Internet Consists of software and sometimes hardware components.

12 3-Tier Server - an example 3-Tier Server - an example Write a server that provides a remote method invocation for a method getgeneraltable(string tablename) The example is contained in: Query.java (the RMI interface) QueryImpl.java (the Server) Query.java (the ) Query Query Query JavaRMI Query QueryImpl JDBC DB 3-Tier Server - an example query does not need any knowledge of the database. Only the names of the tables (more in general the queries) that it wants to submit to the database.