JAVA AND DATABASES. Summer 2018

Transcription

1 JAVA AND DATABASES Summer 2018

2 JDBC JDBC (Java Database Connectivity) an API for working with databases in Java (works with any tabular data, but focuses on relational databases) Works with 3 basic actions: Connecting to a data source (e.g. a database) Sending queries and update statements to the database Retrieve and process the results from the database (responses to the queries) Contains four components: The actual JDBC API JDBC Driver Manager connects applications to a JDBC driver JDBC test suite basic tests JDBC-ODBC Bridge Connects the Java application to ODBC (open database connectivity) drivers

3 CONNECTING TO A DATA SOURCE There are actually two APIs for connecting to a data source DriverManager connects to a data source specified by a database URL. This automatically loads any of the most recent JDBC drivers found in the class path DataSource Allows more details about the underlying data source to be viewed inside of the application. More complex than DriverManager, but often preferred because it has access to those properties The actual connection is represented by a Connection object

4 CONNECTING TO A DATA SOURCE To use DriverManager, you simply call the getconnection() method, which has the following three signatures: static Connection getconnection(string url) static Connection getconnection(string url, String user, String password) static Connection getconnection(string url, Properties info) The URL will be in a format which begins with jdbc and is in a format that depends on the DBMS being connected to. Examples: jdbc:mysql://localhost:5520/ connects to a MySQL database where localhost is the name of the server and 5520 is the port number jdbc:derby:testdb;create=true connects to a Java DB database called testdb and tells the DBMS to create the database One version of the method requires a Properties object, which just contains a set of properties stored in a hash table (in this case it, at least, contains a user property and a password property)

5 CONNECTING TO A DATA SOURCE Using a DataSource object is more complicated. DataSource is actually an interface, with each driver creating objects that implement this interface. It can be implemented in three different ways in the driver: A basic implementation does not use connection pooling or in a distributed transaction (accessing two or more DBMS servers in one transaction) A more complex implementation in which the Connection objects can participate in connection pooling (and thus be recycled) The third version almost always requires connection pooling and allows distributed transactions Deploying DataSource objects consists of three parts: Creating an instance of the DataSource class Setting its properties Registering it with a naming service that uses the Java Naming and Directory Interface (JNDI) API

6 DATASOURCE INTERFACE The DataSource interface only requires two methods: Connection getconnection() Connection getconnection(string username, String password) However, actually establishing the DataSource object may require more steps, but how to do it is database driver-specific, so we will not discuss details here The database vendor should provide documentation to help with this and there are tutorials for some of the common systems on the internet

7 SQL STATEMENTS The way to actually execute SQL queries in Java is through Statement objects The normal way of creating a statement is by using the createstatement() method of a Connection. Example (assume you have a Connection named con): Statement st = con.createstatement(); The actual query is represented by a String, which is passed to the appropriate method, depending on the query. For INSERT, UPDATE, or DELETE statements (or some other SQL statement that returns nothing), you use: int executeupdate(string sql); This also has versions that take in additional parameters for signaling the driver For other statements which return information from the database, you use: ResultSet executequery(string sql);

8 SQL STATEMENTS You can call multiple SQL queries that do not return a ResultSet (such as updating, creating a table, etc.) using batches by using the addbatch() and executebatch() methods in Statement There are two interfaces derived from Statement for doing other types of SQL statements: PreparedStatement precompiles the SQL statement on the database so that it can be more efficiently executed multiple times CallableStatement for executing SQL stored procedures

9 SQL RESULTS Results from SQL queries are returned as ResultSet objects A ResultSet is just a table of data which has a cursor, which is just something that points to a particular row in the set (it begins pointing to the first row) The results may have different characteristics indicating what can be done on them: type (how the cursor can be moved), concurrency (what level of update functionality is supported), and holdability (what happens when Connection.commit() is called)

10 CONTROLLING THE CURSOR There are a number of methods available on a ResultSet to control where the cursor moves: next(): Moves the cursor forward one row, returns false if the cursor is positioned after the last row previous(): Moves the cursor backward one row, returns false if the cursor is before the first row first(): Moves the cursor to the first row, returns false if there are no rows last(): Moves the cursor to the last row, returns false if there are no rows beforefirst(): Moves the cursor to before the first row afterlast(): Moves the cursor to after the last row relative(int rows): Moves the cursor relative to its current position absolute(int row): Moves the cursor to the row row

11 GETTING COLUMN VALUES There are many getter methods for retrieving the values of the columns in the current row (these follow the various Java types, i.e. getdouble(), getstring(), getobject()) These all take one parameter which is either an integer giving the column index or a string giving the name/alias of the column (case-insensitive) It is usually more efficient to use the column index Typically columns are read in left-to-right order and each column is only read once

12 GETTING COLUMN VALUES - EXAMPLE String query = "select COF_NAME, SUP_ID, PRICE, " + "SALES, TOTAL from COFFEES"; Statement stmt = con.createstatement(); ResultSet rs = stmt.executequery(query); rs.beforefirst(); while (rs.next()) { String coffeename = rs.getstring(1); int supplierid = rs.getint(2); float price = rs.getfloat(3); int sales = rs.getint(4); int total = rs.getint(5); System.out.println(coffeeName + "\t" + supplierid + "\t" + price + "\t" + sales + "\t" + total); }

13 UPDATING RESULTS If created with the right properties, a ResultSet object can be updated There are a variety of update methods available, depending on the type of column being updated, each of which takes two parameters, the column index as an integer (or the column name as a string) and the updated value. For example, for doubles, you use one of the following: updatedouble(int columnindex, double x) updatedouble(string columnlabel, double x) After calling any updates on a row that you wish to update, you call updaterow(), which actually commits the updates to the underlying table in the ResultSet You can also insert entire rows into a ResultSet by using a special row called the insert row that you move the cursor to by calling movetoinsertrow()

14 SPECIFIC TYPES OF RESULTS There are a number of interfaces that inherit from ResultSet RowSet adds support for the JavaBeans component model (JavaBeans tutorial) CachedRowSet a RowSet that caches its data in memory FilteredRowSet provides filtering capability JoinRowSet Provides methods to combine related data from multiple RowSet objects JdbcRowSet Remains connected to the dataset, can be used in a Swing application WebRowSet Provides methods for reading/writing result sets as XML SyncResolver Provides a framework for handling synchronization conflicts in RowSet objects

15 SQL ERRORS When dealing with JDBC, exceptions thrown are often of the type SQL Exception Contains a SQL State Code, a standardized 5-character code that gives the state of the database Contains an error code which identifies the actual error Also can be chained together, getting all the exceptions may require cycling through the getcause() method until it returns null (if the exceptions have a causal relationship) or cycling through the getnextexception() method similarly (if there are just multiple exceptions) Also SQL ResultSet or Statement objects have a getwarnings() method which can return type SQLWarning for problems that don t halt execution of the application (such as a data truncation warning)

16 JDBC TUTORIAL TRAIL Much of the content was taken from the official Java JDBC Tutorial Trail here. If you want more details, this is a good place to look