Database Application Development

Transcription

1 CS 461: Database Systems Database Application Development supplementary material: Database Management Systems Sec. 6.2, 6.3 DBUtils.java, Student.java, Registrar.java, RegistrarServlet.java, PgRegistrar.sql class notes

2 Goals and challenges Goals do so effectively and efficiently Challenges How do we parse code that uses both host language and SQL syntax? interact with a relational database from a host language Type matching, e.g., varchar(128) = Java.lang.String Sets / bags of tuples vs. collections of instances (referred to as impedance mismatch) What can we know at compile time, and what is deferred to run time? Optimization Error handling 2

3 SQL Programming Models Option 1: Embedded SQL Program in a host language uses special SQL directives Program is interpreted by a preprocessor Preprocessed program is compiled, executable interacts with the DBMS Option 2: ODBC / JDBC APIs A DMBS vendor implements a standard API used in host language program, SQL statements are function calls e.g., ResultSet rs = st.executequery(query); can use regular (unmodified) language compiler A DBMC vendor implements a driver that enacts the API SQL statements are processed at run time 3

4 Embedded SQL vs. ODBC / JDBC What are the pros and cons of each method? Embedded SQL is compiled, ODBC / JDBC is interpreted error handling? platform independence? APIs are implemented in the spirit of the host language, are easier to learn and use for programmers No need to modify the compiler to use the APIs APIs are more popular today, we focus on the JDBC API in the remainder of this lecture 4

5 Database APIs: ODBC / JDBC Each DBMS vendor provides a library for very popular host language Microsoft developed Open DataBase Connectivity (ODBC) on Windows as a standard API to databases that implement SQL Sun developed Java DataBase Connectivity (JDBC) as a set of interfaces in Java A legacy data source may also implement the API. In fact, this is a good idea. Why? 5

6 Using JDBC Two very similar Java interfaces (there are 2 for historical reasons): java.sql and javax.sql Add the jar that implements the API and the driver to your Java class path (already done for us on tux): download/postgresql jdbc4.jar or look for a newer version at download.html 6

7 JDBC: establishing a connection DBUtils.openDBConnection highlights Load a driver by name String driver = org.postgresql.driver ; Class.forName(driver); Open a database connection String url = "jdbc:postgresql://" + host + "/" + dbsid; DriverManager.getConnection(url, username, password); Always close your connections! conn.close(); more on why this is important and how to do it properly later 7

8 Supplying connection information String url = "jdbc:postgresql://" + host + "/" + dbsid; DriverManager.getConnection(url, username, password); Do not hardcode connection parameters! Option 1: read in on the command line Option 2: use Java resource bundle, example in Registrar.java import java.util.resourcebundle; ResourceBundle bundle = ResourceBundle.getBundle( PgBundle"); Connection conn = DBUtils.openDBConnection( bundle.getstring( dbuser ), bundle.getstring("dbpass"), bundle.getstring("dbsid"), bundle.getstring("dbhost"), Integer.parseInt(bundle.getString("dbPort"))); 8

9 JDBC: executing SQL statements Connection conn; open( ) and close() commit() and rollback() will learn about this under transaction processing Statement and PreparedStatement Statement is used to execute a query, which may be dynamic PreparedStatement is used for parametric queries see Registrar.addTermsDynamicSQL() and Registrar.addTermsPreparedStatement() Connections, Statements should be closed when no longer needed 9

10 JDBC: type matching PostgreSQL type CHAR(n), VARCHAR(n) INTEGER DATE NUMERIC Java type String Integer Java.sql.Date???? (any guesses?) What will happen on a type mismatch? Important: VARCHAR is the preferred character domain instead of CHAR. Use this in your SQL code, or String.equals() won t work Use java.sql.date rather than any other date / time class 10

11 JDBC: retrieving results SQL operates over multisets of tuples, Java operates over collections of atomic types or of class instances To convert from SQL to Java, we use cursors, implemented by java.sql.resultset Retrieving a single tuple: DBUtils.testConnection() Retrieving multiple tuples: Registrar.getRoster() Access tuples sequentially in what order? There is one cursor per statement Can refer to columns by name or by position (1based) 11

12 JDBC: more about cursors Other ResultSet methods (more in the API) moving the cursor: absolute(row), first(), last() getting the data: getarray(1), getarray( name ) optimizing: setfetchsize(int rows) ResultSetMetaData(selfdescription of the ResultSet) ResultSet rs = st.executequery( select a,b,c from Foo ) ResultSetMetaData md = rs.getmetadata(); int numcolumns = md.getcolumncount(); String colname = md.getcolumnname(2); int size = md.getcolumndisplaysize(colname); Why is this useful? 12

13 JDBC: error handing Error handling is done in the host language try { rs.getint( name ); } catch (SQLException sale) { //handle the error } SQLException has two useful methods getsqlstate(): returns the same value as defined in SQL for the SQLSTATE host variable getnextexception(): used to access the sequence of exceptions that occurred if there were several errors 13

14 Transactions 101 SQL operations are grouped into transactions Transactions are units that must be executed atomically either all steps take effect or none do Think: transfer money between bank accounts This is important for concurrency control and for logging / recovery A transaction is implicit in the block of code between conn.open() and conn.close() Transactions ensure isolation: if you insert data but do not issue a commit statement or close your database connection your team mate will not see this data Remember always to close connections, and to commit as needed 14

15 Opening and (not) closing connections Do not leave open connections around An issue because of transactions An open connection is a resource, particularly in a multiuser environment Use error handling properly: to diagnose runtime errors and to close connections Refer to DBUtils and Registrar for examples Use the singleton design pattern private static Connection _conn = null; 15

16 Takehome messages Think carefully about your software development methodology Read API documentation Look at the code examples, posted on the course website (under schedule and linked from project description) Have fun! :) 16