Delphi多线程学习：多线程数据库查询（ADO）

ADO多线程数据库查询通常会出现3个问题：1、CoInitialize 没有调用 （CoInitialize was not called）；所以，在使用任何dbGo对象前，必须手 调用CoInitialize和CoUninitialize。调用CoInitialize失败会产生"CoInitialize was not called"例外。2、画布不允许绘画 （Canvas does not allow drawing）；所以，必须通过Synchronize过程来通知主线程访问主窗体上的任何控件。3、不能使用主ADO连接 （Main TADoConnection cannot be used!）；所以，线程中不能使用主线程中TADOConnection对象，每个线程必须创建自己的数据库连接。     Delphi2007安装后在X:/Program Files/Common Files/CodeGear Shared/Data目录下有一个dbdemos.mdb文件，用来作为测试的例子。dbdemos.mdb中的customer表保存了客户信 息，orders表中保存了订单信息。       测试程序流程大致是这样的：在主窗体上放TADOConnection和TQuery控件，启动时这个TQuery从Customer表中查出客户编码 CustNo和公司名称Company，放到三个Combox框中，分别在三个列表框中选定客户公司名称，按照公司名称所对应的客户代码建立三个线程同时 在orders表中查询销售日期SaleDate分别填入ListBox中unit Main;interfaceuses  Windows,  Messages,  SysUtils,   Variants, Classes, Graphics,   Controls, Forms,   Dialogs, DB,       ADODB,      StdCtrls;type  TForm2 =class (TForm)    ComboBox1:  TComboBox;    ComboBox2:  TComboBox;    ComboBox3:   TComboBox;    ListBox1:    TListBox;    ListBox2:   TListBox;    ListBox3:    TListBox;    Button1:  TButton;    ADOConnection1:  TADOConnection;    ADOQuery1:  TADOQuery;    Label1: TLabel;    Label2:  TLabel;    Label3: TLabel;    procedure  FormCreate(Sender: TObject) ;    procedure  Button1Click(Sender: TObject) ;  private    { Private declarations }  public    { Public declarations }  end    ;var  Form2:    TForm2;implementationuses ADOThread;{$R *.dfm}procedure   TForm2.Button1Click(Sender: TObject);const  SQL_CONST= 'Select SaleDate from orders where CustNo = %d' ;var  c1,c2,c3: Integer  ;  s1, s2,s3: string ;begin  //取得三个选择框客户的编码  c1:= Integer(ComboBox1.Items.Objects[ComboBox1.ItemIndex]);  c2:= Integer(ComboBox2.Items.Objects[ComboBox2.ItemIndex]);  c3:= Integer(ComboBox3.Items.Objects[ComboBox3.ItemIndex]);  //生成SQL 查询语句  s1:=Format(SQL_CONST,[c1]);  s2:=Format(SQL_CONST,[c2]);  s3:=Format(SQL_CONST,[c3]);  //三个线程同时查询  TADOThread.Create(s1,ListBox1,Label1) ;  TADOThread.Create(s2,ListBox2,Label2);  TADOThread.Create(s3,ListBox3,Label3);end ;procedure TForm2.FormCreate(Sender: TObject);var  strSQL:string ;begin  strSQL:='SELECT CustNo,Company FROM customer';  ADOQuery1.Close;  ADOQuery1.SQL.Clear;  ADOQuery1.SQL.Add(strSQL);  ADOQuery1.Open;  ComboBox1.Clear;  ComboBox2.Clear;  ComboBox3.Clear;  //将客户Company和相关CustNo填到ComboBox中  while  not  ADOQuery1.Eof  do  begin    ComboBox1.AddItem(ADOQuery1.Fields[1].asString, TObject(ADOQuery1.Fields[0].AsInteger));    ADOQuery1.Next;  end ;  ComboBox2.Items.Assign(ComboBox1.Items);  ComboBox3.Items.Assign(ComboBox1.Items);  // 默认选中第一个  ComboBox1.ItemIndex := 0;  ComboBox2.ItemIndex := 0;  ComboBox3.ItemIndex := 0;end ;end.{ADO查询多线程单元}unit ADOThread;interfaceuses  Classes,StdCtrls,ADODB;type  TADOThread = class(TThread)  private    { Private declarations }    FListBox:TListBox;    FLabel:TLabel;    ConnString:WideString;    FSQLString:string;    procedure  UpdateCount;  protected    procedure  Execute; override;  public    constructor  Create(SQL:string;LB:TListBox;Lab:TLabel);  end   ;implementationuses  Main,SysUtils,ActiveX;{ TADOThread }constructor  TADOThread.Create(SQL: string; LB: TListBox;Lab:TLabel);begin  ConnString:=Form2.ADOConnection1.ConnectionString;  FListBox:=LB;    FLabel:=Lab;  FSQLString:=SQL;  Inherited  Create(False);end  ;procedure  TADOThread.Execute;var  Qry:TADOQuery;  i:Integer;begin  { Place thread code here }  FreeOnTerminate:=True;  CoInitialize(nil);    //必须调用（需Uses ActiveX）  Qry:=TADOQuery.Create(nil);  try    Qry.ConnectionString:=ConnString;   //必须有自己的连接    Qry.Close;    Qry.SQL.Clear;    Qry.SQL.Add(FSQLString);    Qry.Open;    FListBox.Clear;    for  i := 0 to 100 do     //为了执行久点重复历遍数据集101次      begin        while  not Qry.Eof And  not Terminated do        begin        FListBox.AddItem(Qry.Fields[0].asstring,nil);          //如果不调用Synchronize，会出现Canvas Does NOT Allow Drawing          Synchronize(UpdateCount);          Qry.Next;        end ;        Qry.First;        FListBox.AddItem('*******',nil);      end ;  finally    Qry.Free;  end ;  CoUninitialize;end  ;procedure   TADOThread.UpdateCount;begin  FLabel.Caption:=IntToStr(FListBox.Items.Count);end     ;end.