{
之前说到中间层通过向客户端暴露方法的方式提供服务，实现数据库数据的读取和更新。方法调用的方式，其潜在的意义，就是说中间层不保存客户端状态信息，就像WEB服务一样，客户端需要自己保存自己的状态信息。进一步说，就是中间层具体提供方法的业务对象实例，不是也不应该专属于某个客户端，它应该能够为不同的客户端调用提供服务。如果我们把业务对象实例放到对象池中集中存放，调用方法时随用随取，方法结束即放回池中。这样就可以实现业务对象实例服务于不同的客户端调用请求。更重要的是，利用对象池，能够最大化服务器各种资源的使用效率，而且对客户端的响应也更快了，因为业务对象实例早就创建好了，取来即用。
其实，DataSnap构架，已经为我们的这种构想提供了现实支持。简单的说，就是改造工厂类（TDSServerClass），把LifeCycle属性改为Invocation方式；在OnCreateInstance事件中从对象池中取业务类对象实例；在OnDestroyInstance事件中把业务类对象实例放回对象池。
...
procedure TsmMainForm.dssMethodsCreateInstance( DSCreateInstanceEventObject: TDSCreateInstanceEventObject);begin DSCreateInstanceEventObject.ServerClassInstance := ServerMethodsPool.LockPoolObject;end;
procedure TsmMainForm.dssMethodsDestroyInstance(
DSDestroyInstanceEventObject: TDSDestroyInstanceEventObject);begin ServerMethodsPool.UnlockPoolObject(TPersistent(DSDestroyInstanceEventObject.ServerClassInstance));end;...当然，还有对象池类的创建和释放，也很简单，例如：procedure TsmMainForm.FormCreate(Sender: TObject);begin ServerMethodsPool := ObjPoolMgr.TPoolManager.Create; ServerMethodsPool.InstanceClass := uServerMethods.TPooledDM;end;
procedure TsmMainForm.FormDestroy(Sender: TObject);
begin ServerMethodsPool.Free;end;
下面就是如何实现对象池技术的问题。实现对象池并不复杂，另有两个问题需要注意：
１、多线程。中间层TDSTCPServerTransport对象提供的是多线程服务，允许同时有多个客户端请求。所以对象池类的实现，要考虑多线程情况下公共对象或变量的访问冲突问题。
２、内存泄漏。业务类的基类，采用TDataModule、TComponent或者TPersistent都可以，但不要采用TDSServerModule，因为若采用此基类，TDSServerClass在Invocation方式下会产生内存泄漏。
}
unit ObjPoolMgr;
interface
uses
Classes, SyncObjs, SysUtils, DSServer, DateUtils;type
PServerObject = ^TServerObject;
TServerObject = record ServerObject: TPersistent; InUse: Boolean; end;TPoolManager = class
private FCriticalSection: TCriticalSection; FServerObjects: TList; private FMaxCount: Integer; FInstanceClass: TPersistentClass; function CreateNewInstance: TPersistent; inline; procedure SetInstanceClass(const Value: TPersistentClass); public constructor Create(ACapicity:Integer=30); override; destructor Destroy; override; function Lock: TPersistent; procedure Unlock(var Value: TPersistent); public property InstanceClass: TPersistentClass read FInstanceClass write SetInstanceClass; property MaxCount:Integer read FMaxCount; end;var
ObjPool: TPoolManager;implementation
constructor TPoolManager.Create(ACapicity:Integer=30);
begin FMaxCount :=ACapicity; FServerObjects := TList.Create; FCriticalSection := TCriticalSection.Create;end;destructor TPoolManager.Destroy;
var I: Integer;begin for I := 0 to FServerObjects.Count - 1 do begin PServerObject(FServerObjects[i]).ServerObject.Free; FreeMem(PServerObject(FServerObjects[i])); end; FServerObjects.Free; FCriticalSection.Free; inherited Destroy;end;procedure TPoolManager.SetInstanceClass(const Value: TPersistentClass);
begin FInstanceClass := Value;end;function TPoolManager.CreateNewInstance: TPersistent;
var p: PServerObject; Component: TComponent;begin if not Assigned(FInstanceClass) then Raise Exception.Create('Not specify class of instance!');FCriticalSection.Enter;
try if FInstanceClass.InheritsFrom(TComponent) then begin Component := FInstanceClass.NewInstance as TComponent; Component.Create(nil); Result := Component; end else Result := FInstanceClass.Create;New(p);
p.ServerObject := Result; p.InUse := True; FServerObjects.Add(p); finally FCriticalSection.Leave; end;end;function TPoolManager.Lock: TPersistent;
var i: Integer;begin FCriticalSection.Enter; try for i := 0 to FServerObjects.Count - 1 do begin if not PServerObject(FServerObjects[I]).InUse then begin PServerObject(FServerObjects[I]).InUse := True; Result := PServerObject(FServerObjects[i]).ServerObject; Exit; end; end; finally FCriticalSection.Leave; end; if FServerObjects.Count < MaxCount then Result := CreateNewInstance else Result := nil;end;procedure TPoolManager.Unlock(var Value: TPersistent);
var i: Integer;begin FCriticalSection.Enter; try for i := 0 to FServerObjects.Count - 1 do begin if Value = PServerObject(FServerObjects[i]).ServerObject then begin PServerObject(FServerObjects[i]).InUse := False; Value := nil; Break; end; end; finally FCriticalSection.Leave; end;end;initialization
ObjPool := TPoolManager.Create();finalization FreeAndNil(objpool);end.