在Java的JDBC使用中设置事务回滚的保存点的方法
新的JDBC3.0保存点的接口提供了额外的事务控制。他们的环境中，如Oracle的PL/SQL中的大多数现代的DBMS支持保存点。
当设置一个保存点在事务中定义一个逻辑回滚点。如果发生错误，过去一个保存点，则可以使用rollback方法来撤消要么所有的改变或仅保存点之后所做的更改。
Connection对象有两个新的方法，可帮助管理保存点：
setSavepoint(StringsavepointName):定义了一个新的保存点。它也返回一个Savepoint对象。
releaseSavepoint(SavepointsavepointName):删除一个保存点。请注意，它需要一个Savepoint对象作为参数。这个对象通常是由setSavepoint()方法生成一个保存点。
有一个rollback(StringsavepointName)方法回滚工作到指定的保存点。
下面的例子演示如何使用Savepoint对象：
try{
//Assumeavalidconnectionobjectconn
conn.setAutoCommit(false);
Statementstmt=conn.createStatement();
//setaSavepoint
Savepointsavepoint1=conn.setSavepoint("Savepoint1");
StringSQL="INSERTINTOEmployees"+
"VALUES(106,20,'Rita','Tez')";
stmt.executeUpdate(SQL);
//SubmitamalformedSQLstatementthatbreaks
StringSQL="INSERTEDINEmployees"+
"VALUES(107,22,'Sita','Tez')";
stmt.executeUpdate(SQL);
//Ifthereisnoerror,committhechanges.
conn.commit();
}catch(SQLExceptionse){
//Ifthereisanyerror.
conn.rollback(savepoint1);
}
在这种情况下没有上述INSERT语句会成功，一切都将被回滚。
下面是利用setSavepoint和事务教程描述回滚的例子。
基于对环境和数据库安装在前面的章节中做此示例代码已经解释。
复制下面的例子JDBCExample.java，编译并运行，如下所示：
//STEP1.Importrequiredpackages
importjava.sql.*;
publicclassJDBCExample{
//JDBCdrivernameanddatabaseURL
staticfinalStringJDBC_DRIVER="com.mysql.jdbc.Driver";
staticfinalStringDB_URL="jdbc:mysql://localhost/EMP";
//Databasecredentials
staticfinalStringUSER="username";
staticfinalStringPASS="password";
publicstaticvoidmain(String[]args){
Connectionconn=null;
Statementstmt=null;
try{
//STEP2:RegisterJDBCdriver
Class.forName("com.mysql.jdbc.Driver");
//STEP3:Openaconnection
System.out.println("Connectingtodatabase...");
conn=DriverManager.getConnection(DB_URL,USER,PASS);
//STEP4:Setautocommitasfalse.
conn.setAutoCommit(false);
//STEP5:Executeaquerytodeletestatmentwith
//requiredargumentsforRSexample.
System.out.println("Creatingstatement...");
stmt=conn.createStatement();
//STEP6:Nowlistalltheavailablerecords.
Stringsql="SELECTid,first,last,ageFROMEmployees";
ResultSetrs=stmt.executeQuery(sql);
System.out.println("Listresultsetforreference....");
printRs(rs);
//STEP7:deleterowshavingIDgraterthan104
//Butsavepointbeforedoingso.
Savepointsavepoint1=conn.setSavepoint("ROWS_DELETED_1");
System.out.println("Deletingrow....");
StringSQL="DELETEFROMEmployees"+
"WHEREID=110";
stmt.executeUpdate(SQL);
//oops...wedeletedtoowrongemployees!
//STEP8:Rollbackthechangesafetrsavepoint2.
conn.rollback(savepoint1);
//STEP9:deleterowshavingIDgraterthan104
//Butsavepointbeforedoingso.
Savepointsavepoint2=conn.setSavepoint("ROWS_DELETED_2");
System.out.println("Deletingrow....");
SQL="DELETEFROMEmployees"+
"WHEREID=95";
stmt.executeUpdate(SQL);
//STEP10:Nowlistalltheavailablerecords.
sql="SELECTid,first,last,ageFROMEmployees";
rs=stmt.executeQuery(sql);
System.out.println("Listresultsetforreference....");
printRs(rs);
//STEP10:Clean-upenvironment
rs.close();
stmt.close();
conn.close();
}catch(SQLExceptionse){
//HandleerrorsforJDBC
se.printStackTrace();
//Ifthereisanerrorthenrollbackthechanges.
System.out.println("Rollingbackdatahere....");
try{
if(conn!=null)
conn.rollback();
}catch(SQLExceptionse2){
se2.printStackTrace();
}//endtry
}catch(Exceptione){
//HandleerrorsforClass.forName
e.printStackTrace();
}finally{
//finallyblockusedtocloseresources
try{
if(stmt!=null)
stmt.close();
}catch(SQLExceptionse2){
}//nothingwecando
try{
if(conn!=null)
conn.close();
}catch(SQLExceptionse){
se.printStackTrace();
}//endfinallytry
}//endtry
System.out.println("Goodbye!");
}//endmain
publicstaticvoidprintRs(ResultSetrs)throwsSQLException{
//Ensurewestartwithfirstrow
rs.beforeFirst();
while(rs.next()){
//Retrievebycolumnname
intid=rs.getInt("id");
intage=rs.getInt("age");
Stringfirst=rs.getString("first");
Stringlast=rs.getString("last");
//Displayvalues
System.out.print("ID:"+id);
System.out.print(",Age:"+age);
System.out.print(",First:"+first);
System.out.println(",Last:"+last);
}
System.out.println();
}//endprintRs()
}//endJDBCExample
现在让我们来编译上面的例子如下：
C:>javacJDBCExample.java
当运行JDBCExample，它会产生以下结果：
C:>javaJDBCExample
Connectingtodatabase... Creatingstatement... Listresultsetforreference.... ID:95,Age:20,First:Sima,Last:Chug ID:100,Age:18,First:Zara,Last:Ali ID:101,Age:25,First:Mahnaz,Last:Fatma ID:102,Age:30,First:Zaid,Last:Khan ID:103,Age:30,First:Sumit,Last:Mittal ID:110,Age:20,First:Sima,Last:Chug Deletingrow.... Deletingrow.... Listresultsetforreference.... ID:100,Age:18,First:Zara,Last:Ali ID:101,Age:25,First:Mahnaz,Last:Fatma ID:102,Age:30,First:Zaid,Last:Khan ID:103,Age:30,First:Sumit,Last:Mittal ID:110,Age:20,First:Sima,Last:Chug Goodbye!