享元模式 Flyweight Pattern

目的

以共享機制有效地支援一大堆小規模的物件。

Use sharing to support large numbers of fine-grained objects efficiently.

適用

Flyweight模式是否有效，端看如何使用、用在何處。請在以下條件全都成立時才考慮使用：

當應用程式使用了一大堆物件時。
過多物件耗用過多空間時。
物件的大部份狀態都可歸為外在狀態時，即：需要時才顯現出來（On Demand）。
如果將外在狀態拿掉，就能將好幾群物件換成少數幾個共用物件時。
如果應用程式不依賴「物件等同性質」時（ object.equals() ）。因為flyweight物件在觀念上會視為是同個物件。

結構及成員

Collaborations:
Flyweight 運作所需要狀態必須訂為內在（intrinsic）或外在（extrinsic）。內在狀態乃直接存於 ConcreteFlyweight 物件中，外在狀態則由Client物件計算或儲存，並在必要時將它遞給 Flyweight。
Client 不應直接具現出 ConcreteFlyweight，必須透過 FlyweightFactory 物件來取得，以確保有被妥善共用。

Flyweight:
- 宣告存取外在狀態的介面。
ConcreteFlyweight: like the Character
- 具體實作出Flyweight介面。
- 此物件必須是可共用的，所存的狀態必須是內在狀態，並且與所處環境無關。
UnsharedConcreteFlyweight: like the Row, Column
- 並非所有Flyweight子類別都得共用，Flyweight介面只是提供這樣的可能，而非強制。
- 在物件結構的某一層次（如：Row、Column）常會利用UnsharedConcreteFlyweight物件包含其他的ConcreteFlyweight物件。
FlyweightFactory:
- 建立及管理 Flyweight 物件。
- 確保 Flyweight 物件被妥善共用。當 Client 要求一個 Flyweight 物件時，Flyweight物件會給他既存的一個，如果還沒有，就建一個新的。
Client:
- 持有指向一個或多個 Flyweight 物件的指標。
- 計算或儲存 Flyweight 的外在狀態。

影響結果

好處

允許我們能夠建立大量的虛擬物件。
減少記憶體佔用並加快代碼速度。
該Pattern簡化了代碼。
我們可以透過結合Composite、Singleton、Visitor、和其他Pattern來建立非常優雅的代碼。

壞處

搜尋、傳輸和計算外在狀態時，可能會產生Run-time的負擔。

實作

移除外在狀態

如果在共用物件之前早就有許多各式各樣的外在狀態，即使移掉他們也無法省下多少空間。
理想情況下，可用另一個所需空間不大的物件結構來計算外在狀態。

管理共用物件

因為物件需要共用，所以外界不應該直接實體化他們，應該用FlyweightFactory取得特定的物件來用。
FlyweightFactory通常會用關聯式容器（Associative Store）（如：Map）來檢索想要的Flyweight物件。

Flyweight Pattern與Singleton Pattern相反：
Flyweight：需要建立某個Class的多個實例。
Singleton：需要建立某個Class的最多一個實例。

Example: Shape

Class: Shape

package shape;

public interface Shape {
	
	public void draw();
}

Class: Circle

package shape;

public class Circle implements Shape{
	
	private String color;
	private final int x = 10;
	private final int y = 20;
	private final int radius = 30;
	
	public Circle() {
		
	}

	public Circle( String color ) {
	  this.color = color;
	}

	public String getColor() {
	  return color;
	}

	public void setColor( String color ) {
    this.color = color;
  }

  @Override
  public void draw() {
    System.out.println(this+" : Circle: Draw() [Color : " + color + ", x : " + x + ", y :" + y + ", radius :" + radius);
  }
}

Class: ShapeFactory

package shape;
import java.util.*;

public class ShapeFactory {
	
	private static final HashMap<String, Shape> shapeMap = new HashMap<String, Shape>();

	public static Shape getShape( String shapeType ) {
	  Shape shape=null;
	  if( shapeType.equalsIgnoreCase("circle") ) {
	    shape = (Circle) shapeMap.get("circle");

	    if( shape == null ) 
	    {
	      shape = new Circle();
	      shapeMap.put("circle", shape);
	      System.out.println("Creating circle object without any color in shapefactory : " + shape + "\n");
	    }
	  }
	  return shape;
	}
}

Class: FlyweightPatternDemo

package shape;
public class FlyweightPatternDemo {
  private static final String colors[] = { "Red", "Green", "Blue", "Orange", "Black" };

  public static void main( String[] args ) {

    System.out.println("\n################ Red color Circles ####################");
    for( int i = 0; i < 10; ++i ) {
      Circle circle = (Circle) ShapeFactory.getShape("circle");
      circle.setColor(colors[0]);
      circle.draw();
    }
    System.out.println("\n############### Green color Circles ####################");
    for( int i = 0; i < 10; ++i ) {
      Circle circle = (Circle) ShapeFactory.getShape("circle");
      circle.setColor(colors[1]);
      circle.draw();
    }
    System.out.println("\n################ Blue color Circles ####################");
    
    for( int i = 0; i < 10; ++i ) {
      Circle circle = (Circle) ShapeFactory.getShape("circle");
      circle.setColor(colors[2]);
      circle.draw();
    }
    System.out.println("\n################ Orange color Circles ####################");
    for( int i = 0; i < 10; ++i ) {
      Circle circle = (Circle) ShapeFactory.getShape("circle");
      circle.setColor(colors[3]);
      circle.draw();
    }
    System.out.println("\n################ Black color Circles ####################");
    for( int i = 0; i < 10; ++i ) {
      Circle circle = (Circle) ShapeFactory.getShape("circle");
      circle.setColor(colors[4]);
      circle.draw();
    }
  }
}

Result:

################ Red color Circles ####################
Creating circle object without any color in shapefactory : shape.Circle@15db9742

shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Red, x : 10, y :20, radius :30

############### Green color Circles ####################
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Green, x : 10, y :20, radius :30

################ Blue color Circles ####################
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Blue, x : 10, y :20, radius :30

################ Orange color Circles ####################
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Orange, x : 10, y :20, radius :30

################ Black color Circles ####################
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30
shape.Circle@15db9742 : Circle: Draw() [Color : Black, x : 10, y :20, radius :30

目的

適用

結構及成員

影響結果

好處

壞處

實作

移除外在狀態

管理共用物件

Example: Shape

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適用

結構及成員

影響結果

好處

壞處

實作

移除外在狀態

管理共用物件

Example: Shape

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