JDBC数据库开发-研究生教学

JDBC与Java数据库开发姚羽编写yaoyu@Module 1 JDBC Overview东北大学姚羽yaoyu@ 3N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Tiered Database ModelOne-tier databases:•Not readily extensible•Not easily accessible through anetwork东北大学姚羽yaoyu@ 4N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Tiered Database ModelTwo-tier databases:•Mirroring•Caching•Proxy services•Secure transactions东北大学姚羽yaoyu@ 5N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Tiered Database Model Three-tier databases:东北大学姚羽yaoyu@ 6N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发What Is the JDBC API?JDBC: Java Data Base ConnectivityThe JDBC API is:A standard data access interface to a widerange of relational databasesA set of classes and interfaces that are partof the Java programming language东北大学姚羽yaoyu@ 7N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发What Is the JDBC API?JDBC enables:Connecting to a databaseSending a String SQL query to thedatabaseProcessing the results东北大学姚羽yaoyu@ 8N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发What Is the JDBC API?The JDBC API has two main parts: Java application developers’interface forJava programming language developersJDBC driver developers’implementationinterfaceThis course concentrates on thedeveloper interface.东北大学姚羽yaoyu@ 9N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发JDBC Driver Interfacejava.sql.Connectionjava.sql.Statementjava.sql.PreparedStatement java.sql.CallableStatementjava.sql.ResultSetjava.sql.Driver东北大学姚羽yaoyu@ 10N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发DriversAre a collection of classes implementingJDBC classes and interfacesProvide a class that has implementedthe java.sql.Driver interface东北大学姚羽yaoyu@ 11N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Drivers东北大学姚羽yaoyu@ 12N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Drivers东北大学姚羽yaoyu@ 13N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发JDBC-ODBC Bridge Plus ODBC DriverUses existing technologyProvides JDBC access to the databaseusing an existing ODBC driverRequires code to be installed on eachclientIs most appropriate with fewer clientinstallationsHas several potential failure points东北大学姚羽yaoyu@ 14N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发JDBC-ODBC Bridge Plus ODBC Driver东北大学姚羽yaoyu@ 15N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Native API DriverThis type is built on top of a native databaseclient library.The driver translates the JDBC calls into callsto the API of the database client library.The library is distributed by the provider ofthe database management system (DBMS).It does not accomplish the goal of the JDBCAPI, which is cross-platform portability.东北大学姚羽yaoyu@ 16N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Native API Driver东北大学姚羽yaoyu@ 17N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发JDBC-Net DriverThis type communicates with anintermediary server seated between theclient and the database.It uses a network protocol specific tothe intermediary.No native code needs to be installed onthe database client machines.It has the most flexibility.东北大学姚羽yaoyu@ 18N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发JDBC-Net Driver东北大学姚羽yaoyu@ 19N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Native Protocol DriverTranslates JDBC calls directly into thenetwork protocol used by the DBMSAllows direct calls from the client to thedatabaseHas good performance, but no flexibilityfor switching DBMSs东北大学姚羽yaoyu@ 20N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Native Protocol Driver东北大学姚羽yaoyu@ 21N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发JDBC Developer InterfaceThe JDBC API developer interface isdefined in two packages:java.sql–Primary features of JDBC inJava™2 Platform, Standard Edition(J2SE™)javax.sql–Extended functionality inJava™2 Platform, Enterprise Edition(J2EE™)东北大学姚羽yaoyu@ 22N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Classes and InterfacesDriverManager: Manages database drivers andsupports new database connection creationDriver: Represents a specific JDBCimplementationConnection: Is a session with the specificdatabaseStatement, PreparedStatement,CallableStatement: Are containers for executing aSQL statement on a connectionResultSet: Provides access to a table of data东北大学姚羽yaoyu@ 23N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The java.sql Packagejava.sql.Driverjava.sql.Connectionjava.sql.Statementjava.sql.PreparedStatementjava.sql.CallableStatementjava.sql.ResultSetjava.sql.ResultSetMetaDatajava.sql.DatabaseMetaData东北大学姚羽yaoyu@ 24N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发东北大学姚羽yaoyu@ 25N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发JDBC Programming TasksCreate an instance of a JDBC driverLoad JDBC drivers through jdbc.drivers Register a driverSpecify a databaseOpen a database connectionSubmit a queryReceive results东北大学姚羽yaoyu@ 26N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The JDBC Flow东北大学姚羽yaoyu@ 27N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Identifying a Database Using a URLA JDBC uniform resource locator (URL):Identifies a database so that the correctdriver can recognize and establish aconnection with itIs determined by vendorContains encoded informationAllows for a level of indirection东北大学姚羽yaoyu@ 28N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Specifying a DatabaseSpecify a URL string:jdbc:subprotocol:subname东北大学姚羽yaoyu@ 29N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Basic URL SyntaxFull URL syntaxjdbc:subprotocol:subname Examplejdbc:odbc:wombat东北大学姚羽yaoyu@ 30N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Basic URL Syntax:subprotocolIs the name of the driver or the type ofdatabase connectivity mechanismIs often odbcCan be the network naming service东北大学姚羽yaoyu@ 31N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Using a Network Naming Service as the subprotocolBasic syntaxjdbc:naming_service:data_source_name Examplejdbc:dcenaming:accounts-payable东北大学姚羽yaoyu@ 32N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The odbc SubprotocolBasic syntaxjdbc:odbc:dataSrcName[;attributeName=attributeValue] Examplejdbc:odbc:fredjdbc:odbc:wombatjdbc:odbc:wombat;CacheSize=20;ExtensionCase=LOWERjdbc:odbc:qeora;UID=kgh;PWD=fooey东北大学姚羽yaoyu@ 33N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Basic URL Syntax: subname Identifies the database东北大学姚羽yaoyu@ 34N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Connecting to a Remote ServerA database on a remote server needs aURL with more information.Syntaxjdbc:subprotocol://hostname:port/subsubnameExamplejdbc:dbnet://wombat:3560/fredModule 2Using JDBC东北大学姚羽yaoyu@ 36N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Creating a Basic JDBC Application Step 1 –Registering a driverStep 2 –Establishing a connection to thedatabaseStep 3 –Creating a statementStep 4 –Executing the SQLStep 5 –Processing the resultsStep 6 –Closing down JDBC objects东北大学姚羽yaoyu@ 37N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Step 1 –Registering a DriverThe driver is used to connect to the database.The JDBC API uses the first driver it finds thatcan successfully connect to the given URL.You can load multiple drivers at the same time.Register a driver:Using the Class loaderInstantiating a driverUsing the jdbc.drivers property东北大学姚羽yaoyu@ 38N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Using the Class LoaderTo communicate with a databaseengine using the JDBC API, create aninstance of the JDBC driver.SyntaxClass.forName(driverName);ExampleClass.forName("oracle.jdbc.driver.OracleDriver");东北大学姚羽yaoyu@ 39N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Instantiating a DriverIf you need an explicit reference to theDriver object, use the new keyword.SyntaxDriver drvRefVar= new DriverConstructor;ExampleDriver drv= new oracle.jdbc.driver.OracleDriver();东北大学姚羽yaoyu@ 40N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Defining the jdbc.drivers Property Define the jdbc.drivers property as a listof driver class names.Syntaxjdbc.drivers=driverName[:driverName] Examplejdbc.drivers=oracle.jdbc.driver.OracleDriver:acme.cool.Driver东北大学姚羽yaoyu@ 41N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Using the -D Option With the java CommandTo use this method, specify the name=valuepair.Syntaxjava -Djdbc.drivers=driverName[:driverName] Examplejava -Djdbc.drivers=oracle.jdbc.driver.OracleDriver:acme.cool.Driver东北大学姚羽yaoyu@ 42N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Choosing the Best Registration ApproachFor each approach, what are theadvantages and disadvantages, andwhy?东北大学姚羽yaoyu@ 43N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Step 2 –Establishing a Connection to the Database1. DriverManager calls getConnection(urlString), whichcalls Driver.connect(urlString).2. The URL is parsed.3. When a driver responds positively to the databaseURL, the DriverManager makes a connection.4. If the driver does not match, null is returned and thenext driver in the vector is checked.5. If a connection is not made, a SQLException isthrown.东北大学姚羽yaoyu@ 44N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Establishing a Connection to the Database东北大学姚羽yaoyu@ 45N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The DriverManager.getConnection MethodMethodsgetConnection(String url)getConnection(String url, java.util.Properties info)getConnection(String url, String user, String password)SyntaxConnectionconRefVar= DriverManager.getConnection(arguments);ExampleConnection con = DriverManager.getConnection("jdbc:oracle:thin:@:1521:accounts");东北大学姚羽yaoyu@ 46N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Driver.connect MethodMakes a direct call to a specific DriverobjectAvoids the problems of standardapproach东北大学姚羽yaoyu@ 47N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Driver.connect MethodDriver drv= new oracle.jdbc.driver.OracleDriver();Connection con = null;try{// Pass the connection request directly to THIS driver.// If the connection attempt fails, the application// will NOT try to connect to any other JDBC drivers.con =drv.connect("jdbc:oracle:thin:@:1521:accounts",null);} catch (SQLException exc){// If unable to obtain a connection, take some kind// of recovery action here. Options might include// connecting to an alternate DB, prompting the user// for action, or exiting the application.}东北大学姚羽yaoyu@ 48N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Step 3 –Creating a StatementStatementPreparedStatementCallableStatement东北大学姚羽yaoyu@ 49N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Statement ObjectGet a Statement object from theConnection.createStatement method东北大学姚羽yaoyu@ 50N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Statement ObjectSyntaxStatement stmtRefVar= conRefVar.createStatement();ExamplesStatement stmt;try {stmt = con.createStatement();} catch (SQLException e) {System.out.println(e.getMessage());}ResultSet rs= stmt.executeQuery("SELECT * FROM customer WHERE ID = ’11’");东北大学姚羽yaoyu@ 51N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Step 4 –Executing the SQL StatementThe SQL statement is passed unalteredto the underlying database connection.The result is a table of data accessiblethrough java.sql.ResultSet.The statement is coupled with thecorresponding ResultSet.东北大学姚羽yaoyu@ 52N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Statement InterfaceexecuteQuery(sqlString)executeUpdate(sqlString)execute(sqlString)东北大学姚羽yaoyu@ 53N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The Statement InterfaceStatement.executeQuery:SyntaxConnection conRefVar=DriverManager.getConnection(urlString);Statement stmtRefVar=conRefVar.createStatement();ResultSet resultsetRefVar=stmtRefVar.executeQuery(sqlString);ExampleConnection con=DriverManager.getConnection("jdbc:odbc:accounts" );Statement stmt = con.createStatement();ResultSet rs= stmt.executeQuery("SELECT* from employees");东北大学姚羽yaoyu@ 54N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Step 5 –Processing the ResultsTo retrieve data from the ResultSetobject, use its assessor methods.Retrieve these values using a columnname or index.A ResultSet:Keeps a cursor pointed to the current rowIs initially positioned before its first row东北大学姚羽yaoyu@ 55N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Step 5 –Processing the ResultsSyntaxwhile (resultsetRefVar.next()) {System.out.println("column: " +resultsetRefVar.getXXXMethod(column));}Examplewhile (rs.next()) {System.out.println();System.out.println("Coffee Name: " + rs.getString(1));System.out.println("Supplier ID: " + rs.getInt(2));System.out.println("Price: " + rs.getFloat(3));System.out.println("Sales: " + rs.getInt(4));}东北大学姚羽yaoyu@ 56N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Step 6 –Closing Down JDBC ObjectsClose down the following:ConnectionStatementResultSettry {rs.close();stmt.close();con.close();}catch (SQLException ex) {ex.printStackTrace();}东北大学姚羽yaoyu@ 58N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The PreparedStatement ObjectA precompiled SQL statement that ismore efficient than repeatedly callingthe same SQL statementExtends the Statement interface东北大学姚羽yaoyu@ 59N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The PreparedStatement Object//1private static final PreparedStatement prepStmnt= con.prepareStatement("SELECT* FROM coffeesWHERE cof_name= ?");public boolean prepStatement(float sales, String name){prepStmnt.setString(1, "Espresso");ResultSet rs= prepStmnt.executeQuery();}//2PreparedStatement ps= con.prepareStatement("UPDATE customer SET order_num= ? WHERE cid = ?");ps.setInt(1, 543);ps.setString(2, "0147");int result = ps.executeUpdate();东北大学姚羽yaoyu@ 60N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发PreparedStatement vs Statment //1int i=9;Statement stmt = con.createStatement();int result = stmt.executeUpdate("UPDATE table3 SET m = ‘100’WHERE x = "+i);//2int i=9;PreparedStatement ps= con.prepareStatement( "UPDATE table3 SET m = ‘100’WHERE x = ? ");ps.setInt(1, i);int result = ps.executeUpdate();东北大学姚羽yaoyu@ 61N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发PreparedStatement vs Statment for (int i=0;i<10;i++){Statement stmt = con.createStatement();int result = stmt.executeUpdate("UPDATE table3 SET m = ‘100’WHERE x = "+i);}PreparedStatement ps= con.prepareStatement( "UPDATE table3 SET m = ‘100’WHERE x = ? ");for (int i=0;i<10;i++){ps.setInt(1, i);int result = ps.executeUpdate();}东北大学姚羽yaoyu@ 62N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The CallableStatement Object Lets you execute non-SQL statementsagainst the databaseExtends PreparedStatement东北大学姚羽yaoyu@ 63N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The CallableStatement Object String coffeeName= "Espresso";CallableStatement querySales=con.prepareCall("{call return_sales[?, ?]}");try {querySales.setString(1, coffeeName);querySales.registerOutParameter(2, Types.REAL);querySales.execute();float sales = querySales.getFloat(2);} catch (SQLException e){System.out.println("Query failed");e.printStackTrace();}东北大学姚羽yaoyu@ 64N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Comparing Statement Interfaces东北大学姚羽yaoyu@ 65N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Exercise: Using JDBCMySQL-JDBC connection and simple operationClass.forName("com.mysql.jdbc.Driver").ne wInstance();Connection conn=DriverManager.getConnection("jdbc:mysql://localhost/test?user=root&password=123");Module 3 JDBC Advanced Topics东北大学姚羽yaoyu@ 67N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发ExceptionsErrors are usually communicated usingSQLException or SQLWarning.东北大学姚羽yaoyu@ 68N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The SQLException ClassSQLException can have a list of the followingexception objects:getNextException returns the next SQLExceptiongetErrorCode returns an integer error codegetMessage returns a String error message SQL exceptions occur when:There is a loss of JDBC-server communicationYou format a command incorrectlyYou use an unsupported functionYou reference a column that does not exist东北大学姚羽yaoyu@ 69N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发The SQLWarning ClassGenerated for non-fatal SQL statesChained togetherHas the same methods as SQLException,but uses getNextWarning instead ofgetNextException东北大学姚羽yaoyu@ 70N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发MetadataInformation about data, such asformatting or sizeAccessible through the JDBC API: Database metadataResult set metadata东北大学姚羽yaoyu@ 71N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Database MetadataUse the Connection.getMetaDatamethod.Method returns DatabaseMetaDatareference.Methods to retrieve the informationinclude isReadOnly.东北大学姚羽yaoyu@ 72N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Result Set MetadataUse the ResultSet.getMetaData method.Method returns ResultSetMetaDatareference.Methods to retrieve the informationinclude getColumnCount.东北大学姚羽yaoyu@ 73N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Using TransactionsA transaction is a set of one or more statements that are executed together as a unit.Either all of the statements are executed, or none of the statements is executed.东北大学姚羽yaoyu@ 74N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Using TransactionsDisabling Auto-commit Mode Committing a TransactionCall the Method rollback东北大学姚羽yaoyu@ 75N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Disabling Auto-commit Mode When a connection is created, it is inauto-commit mode.The way to allow two or morestatements to be grouped into atransaction is to disable auto-commitmode.con.setAutoCommit(false);东北大学姚羽yaoyu@ 76N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Committing a TransactionOnce auto-commit mode is disabled, noSQL statements will be committed untilyou call the method commit explicitly.All statements executed after theprevious call to the method commitwill be included in the currenttransaction and will be committedtogether as a unit.mit();东北大学姚羽yaoyu@ 77N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Committing a TransactionmSQLcon.setAutoCommit(false);PreparedStatement stmt = conn.prepareStatement( "UPDATE customer set cust_name=? WHERE ssn= ?");stmt.setString(1, "Hi");for (int i = 0; i < 10; i++) {stmt.setInt(2, i);int j = stmt.executeUpdate();System.out.println(j+" rows affected when i=" +i);}mit();stmt.close();东北大学姚羽yaoyu@ 78N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Call the Method rollbackCalling the method rollback aborts atransaction and returns any values thatwere modified to their previous values.If you are trying to execute one ormore statements in a transaction andget an SQLException, you should callthe method rollback to abort thetransaction and start the transaction allover again.东北大学姚羽yaoyu@ 79N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Transactionstry {con.setAutoCommit(false);Statement stmt = con.createStatement();stmt.executeUpdate("INSERT INTO customer(1011,'XYZ Corporation')");stmt.executeUpdate("INSERT INTO order(21,1011,12.43f)");mit();} catch (SQLException ex) {try {con.rollback();} catch (SQLException ex2){}}东北大学姚羽yaoyu@ 80N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Exercise: JDBC Advanced TopicsA database monitorshow version, go, commit, rollback, quit,reset, …TerminalMonitorFrame.java东北大学姚羽yaoyu@ 81N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发一个监视器java -Djdbc.drivers=com.mysql.jdbc.DriverTerminalMonitor jdbc:mysql://localhost/330insert into student values('1','aa','home1')东北大学姚羽yaoyu@ 82N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Some points//1if( statement.execute(sql) ) {// true means the SQL was a SELECTprocessResults(statement.getResultSet());}//2String.trim();//3How to save the statements: see TerminalMonitorFrame.java东北大学姚羽yaoyu@ 83N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发D:\Java\myjava\330>java -Djdbc.drivers=com.mysql.jdbc.Driver Terminalmonitor jdbc:mysql://localhost/testHOST: localhostuser: rootpassword: 123Connected to jdbc:mysql://localhost/testTM > select * from student2 -> go3 rows selected.----------------------------------------------------------------| sid| sname| address |----------------------------------------------------------------| 1 | aa| home1 || 2 | bb | home2 || 3 | cc | home3 |----------------------------------------------------------------TM > quitConnection closed.东北大学姚羽yaoyu@ 84N o r t he a st e rn Un i ve r si t yY a oY uJDBC与Java数据库开发Concurrency ControlCreating a transaction is handledthrough the Connection interface.In a connection, you can set thetransaction isolation level to determinewhether the following are allowed:Dirty readsNon-repeatable readsPhantom reads东北大学姚羽yaoyu@85N o r t h e a s t e r n U n i v e r s i t y Y aoY u JDBC 与Java 数据库开发Concurrency ControlDirty reads:脏数据事务T1修改某数据，事务T2读取修改后的数据后，T1的操作被撤销，则T1所修改的数据恢复原值。