Delphi多线程学习（9）：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。

{主窗体代码} unit Main; interface uses 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; implementation uses 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; interface uses 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; implementation uses 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.

程序运行结果如下：

 

可以看到三个线程同时执行。第一第三两个线程条件一样，查询的结果也一样。

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