Advanced Java Class

Java Database Connectivity

(JDBC)

Question from yesterday

• Please talk a little about “security.policy”– java –Djava.security.policy=policyfilename– It will look for a default file in

java.home/lib/security/java.policy– Or you can supply your own:

• java –Djava.security.policy=policyfilename• Example: my RMISecurity.policy file:

grant { permission java.net.SocketPermission "*:1024-65535","connect,accept";};

• Also, you can use the policytool program that comes with the JDK to write your own security file.

• See java.security.Permissions class for more documentation

Policy for Assignment 1a

• As written in an email yesterday, your WebCrawler will only be tested on html pages that are xhtml pages.

• In other words, you may assume that the links will be well-formed.

PostGresQL Syntax

• You need to know a little PostGresQL syntax to set up your DB and use JDBC

• Types: see p. 623, figure 9-6– Also in postgresQL: money, text (among others)

• Documentation: http://www.postgresql.org• Look at \h and \? for command syntax• At the command line:

– psql– pg_dump <database_name>– man psql

Common Table Commands

• CREATE TABLE table_name (column_name1 column_type1,

column_name2 column_type2, etc.);

• Can also specify a DEFAULT value, or other constraints like NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, etc.

• \dt (shows tables)

• \d table_name (describes that table)

• DROP TABLE table_name;

• ALTER TABLE table_name RENAME TO new_table_name;

Common Column Commands

• ALTER TABLE table_name

– ADD column_name column_type [constraints];

– ALTER column_name SET DEFAULT value;

– ALTER column_name DROP DEFAULT;

– RENAME old_column_name TO new_column_name;

Common Row Commands

• INSERT INTO table_name values (42, 3, ‘foo’)

• INSERT INTO table_name (col2_name, col3_name) values (3, ‘foo’);

• UPDATE table_name SET col = expression [WHERE condition];

• DELETE FROM table_name [WHERE condition];

Common “\” Commands

• \? Shows all “\” commands

• \h shows help menu

• \h COMMAND_NAME shows help for a

specific command

• \q quits psql program

Driver Types

1. JDBC-ODBC bridge (inefficient)2. Most efficient driver if all classes using db are

on the db host3. Most efficient driver if not all classes using db

are on the db host4. pure Java, and therefore platform independent

• A driver for your first project is linked from the assignment page.

• For many databases, there may be only one choice, not all four.

Basic use of java.sql

1. Load driver class

2. Get connection

3. Get statement

4. Ask statement to execute sql string

5. Process results if needed

6. Close Statement and Connection

Optional arguments for getting Statement

• Scrollable?

– TYPE_FORWARD_ONLY [Default]

(note error in text:

TYPE_READ_ONLY)

– TYPE_SCROLL_INSENSITIVE [ignores changes made to

ResultSet by other code]

– TYPE_SCROLL_SENSITIVE

• Concurrency?

– CONCUR_READ_ONLY [rs can’t change while processing it]

– CONCUR_UPDATABLE [not allowed by some DB drivers]

Execution of SQL by Statement• int Statement.executeUpdate(String sql);

– Returns number rows affected– Good for INSERT, UPDATE, and DELETE

• ResultSet Statement.executeQuery(String sql);

– Good for SELECT– Good if only one ResultSet is returned

• boolean execute(String sql);

– Returns true if a ResultSet(s) was returned– Good if more than one ResultSet might be returned– Very rare to do this.

Processing Result Sets

• Move to appropriate row – beforeFirst(), next())

• Process values at that row– get values (methods are named by type) – update values [local], then updateRow() [db]– deleteRow()– insertRow()

Advanced Efficiency Options

1. Prepared Statements [use if many sql statements vary only in the literal values] – SEE NEXT SLIDE

2. Stored Procedures– Creating them varies from db to db – not covered in this class– Can use them in Java via CallableStatement– Can produce more than one ResultSet

3. Use fewer Connections– If single threaded program, open a Connection and keep it,

rather than closing it and opening a new one constantly– Connection Pooling is provided by DataSource objects for

J2EE

Prepared Statements – Very Useful!

• PreparedStatement ps = Connection.prepareStatement(“INSERT INTO my_table (col_a, col_b) VALUES (?, ?)”

);• ps.setString(1, “foo”);

ps.setInt(2, 512);• ps.executeUpdate() or

ps.executeQuery();• (note: indexing starts from 1)

Good Database Programming

• Program layers (i.e. Model-View-Persistence)

• Define mapping between Java Objects and Data Elements

• Utilize optimizations and special powers of db

• Program transactions

Program Layers (and not just in JDBC)

• See p. 613, figure 9-1

• Conceptually clearer

• Layers of security

• Scalability

• Abstraction of implementations

Define mapping between Java Objects and Data Elements

• Relationships between Relational Database Concepts and Object Oriented Programming Concepts

– See figure 9-2

• Mappings (see 9-4 and 9-5)– One-to-one (use same ids)– One-to-many (give each of the many a reference to

the one, using a foreign key)– Many-to-many (make another table just for the

mappings – this table will have 2 foreign keys)

Utilize optimizations and special powers of DB

• serial “types”

• DB can check for uniqueness of primary key

• Foreign key constraints signal error for impossible values

• Set up dependency rules for deletes and modifications

(i.e. DB will automatically nix employee records when a

department is deleted if the foreign key is defined with

“cascade delete”

• WARNING: if you depend on these too much, you may

have difficulty switching Databases.

Program For Transactions

• Goal:

– Atomic [all or nothing]

– Consistent [same each time you run it under same conditions]

– Isolated [independent of other transactions]• (several possible levels of isolation)

– Durable [permanent once commited]

Program For Transactions

• Implementation

– Connection.setAutoCommit(false) [true by default]

– Connection.commit() or Connection.rollback()

– Note: can also rollback to Savepoints within a transaction, as of Java 1.4

Group Database Task• Draw and Entity Relationship diagram (see example on board) to

show the structure of a DB for an online shopping application.– Show Foreign and Primary Keys– Show column names, SQL types, any restraints

• Classes in Application:– Customer: Has name and address. Has one or more accounts.– Account: has owner, balance, credit rating, value of last

statement, and date of last payment.– Order: Knows what account it’s owned by. Also has date

opened, date closed, status, and total value. Has one or more Items, and associates them with the quantity ordered.

– Item: Has a part number, picture (image file), unit cost.– Inventory: list of the items available for sale. Associates each

Item with a quantity in stock.