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分布式数据库客户端数据集的选用


  有两种方法:
  1、用vector:
  > public Enumeration ejbFindBigAccounts(double balanceGreaterThan) {
  > log("ejbFindBigAccounts (balance > " + balanceGreaterThan + ")");
  > Connection con = null;
  > PreparedStatement ps = null;
  >
  > try {
  > con = getConnection();
  > ps = con.prepareStatement("select id from ejbAccounts where bal > ?");
  > ps.setDouble(1, balanceGreaterThan);
  > ps.executeQuery();
  > ResultSet rs = ps.getResultSet();
  > Vector v = new Vector();
  > String pk;
  > while (rs.next()) {
  > pk = rs.getString(1);
  > v.addElement(pk);
  > }
  > return v.elements();
  > } catch (SQLException sqe) {
  > log("SQLException: " + sqe);
  > throw new EJBException (sqe);
  > } finally {
  > cleanup(con, ps);
  > }
  > }
  
  2、RowSet
  RowSet tutorial chapter :
  http://developer.java.sun.com/developer/Books/JDBCTutorial/chapter5.html
  
  rowset是个interface,需要有东西去实现它,sun的规范中给了三个class:cachedrowset,jdbcrowset,webrowset,如果去查jdk1.4 doc和j2skee1.2,有rowset,却没有那三个class,一般的开发工具(至少我的wsad)中也是这样,所以需要下jdbc2.0 opt-pack:
  http://developer.java.sun.com/developer/earlyAccess/crs/
  
  1、解包,得到rowset.jar,放在哪随您的意,别丢了就行。
  2、在您的开发工具中增加一个路径,如:ROWSET_PATH对应:d:\jdk1.4\jre\rowset.jar(和1的路径对应就行)。
  3、右键您的工程文件,出现:property(大多数工具应该都有吧),加上rowset_path。
  4、在您的源文件中:import sun.jdbc.rowset.*;
  
  应该说rowset(其实主要是CachedRowSet)真的是个好东西,和ms ado的resultset和borland的tclientset非常相似,最大的好处是Cache功能!
  
  package example4;
  
  import java.sql.*;
  import javax.sql.*;
  import sun.jdbc.rowset.*;
  import javax.naming.*;
  import javax.ejb.*;
  
  public class CoffeesBean implements SessionBean {
  
  private SessionContext sc = null;
  private Context ctx = null;
  private DataSource ds = null;
  
  public CoffeesBean () {}
  
  public void ejbCreate() throws CreateException {
  
  try {
  ctx = new InitialContext();
  ds = (DataSource)ctx.lookup("jdbc/CoffeesDB");
  }
  catch (Exception e) {
  System.out.println(e.getMessage());
  throw new CreateException();
  }
  }
  
  public RowSet getCoffees() throws SQLException {
  
  Connection con = null;
  ResultSet rs;
  CachedRowSet crs;
  
  try {
  con = ds.getConnection("webCustomer", "webPassword");
  Statement stmt = con.createStatement();
  rs = stmt.executeQuery("select * from coffees");
  
  crs = new CachedRowSet();
  crs.populate(rs);
  // the writer needs this because JDBC drivers
  // don't provide this meta-data.
  crs.setTableName("coffees");
  
  rs.close();
  stmt.close();
  } finally {
  if (con != null)
  con.close();
  }
  return rset;
  }
  
  public updateCoffees(RowSet rs) throws SQLException {
  
  Connection con = null;
  
  try {
  CachedRowSet crs = (CachedRowSet)rs;
  con = ds.getConnection("webCustomer", "webPassword");
  
  moves the changes back to the database
  crs.acceptChanges(con);
  } finally {
  if (con != null)
  con.close();
  }
  }
  
  //
  // Methods inherited from SessionBean
  //
  
  public void setSessionContext(SessionContext sc) {
  this.sc = sc;
  }
  
  public void ejbRemove() {}
  public void ejbPassivate() {}
  public void ejbActivate() {}
  
  
  }
  
  
  //////////////////client端//////////////
  package example4;
  
  import java.sql.*;
  import javax.sql.*;
  import sun.jdbc.rowset.*;
  import javax.naming.*;
  import javax.ejb.*;
  import javax.rmi.*;
  
  class CoffeesClient {
  
  public static void main(String[] args) {
  
  try {
  // init the bean
  Context ctx = new InitialContext();
  Object obj = ctx.lookup("ejb/Coffees");
  CoffeesHome coffeesHome = (CoffeesHome)
  PortableRemoteObject.narrow(obj, CoffeesHome.class);
  Coffees coffees = coffeesHome.create();
  
  // get the rowset from the bean
  CachedRowSet rset = (CachedRowSet)coffees.getCoffees();
  
  // find the Columbian coffee
  while (rset.next()) {
  String coffeeName = rset.getString("COF_NAME");
  if (coffeeName.equalsIgnoreCase(new String("Columbian"))) {
  // columbian coffee has gone up 10%
  rset.updateFloat("PRICE",
  (float)(rset.getFloat("PRICE") * 1.10));
  rset.updateRow();
  }
  }
  
  // finally send the updated back to the bean...
  System.out.println("Calling update method");
  coffees.updateCoffees((RowSet)rset);
  }
  catch (Exception e) {
  System.out.println(e.getMessage());
  }
  }
  }
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