Java中的Flyweight设计模式

Today we will look into Flyweight design pattern.

今天，我们将探讨Flyweight设计模式。

轻量化设计模式 (Flyweight Design Pattern)

According to GoF, flyweight design pattern intent is:

根据GoF， flyweight设计模式的意图是：

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

使用共享有效支持大量细粒度对象

Flyweight design pattern is a Structural design pattern like Facade pattern, Adapter Pattern and Decorator pattern.

Flyweight设计模式是一种结构设计模式，例如Facade模式， Adapter模式和Decorator模式。

Flyweight design pattern is used when we need to create a lot of Objects of a class. Since every object consumes memory space that can be crucial for low memory devices, such as mobile devices or embedded systems, flyweight design pattern can be applied to reduce the load on memory by sharing objects.

当我们需要创建一个类的许多对象时，可以使用Flyweight设计模式。由于每个对象都会占用对于低内存设备（例如移动设备或嵌入式系统）至关重要的内存空间，因此可以应用轻量级设计模式来通过共享对象来减少内存负载。

Before we apply flyweight design pattern, we need to consider following factors:

在应用flyweight设计模式之前，我们需要考虑以下因素：

The number of Objects to be created by application should be huge.
应用程序要创建的对象数量应该很大。
The object creation is heavy on memory and it can be time consuming too.
对象的创建占用大量内存，并且也很耗时。
The object properties can be divided into intrinsic and extrinsic properties, extrinsic properties of an Object should be defined by the client program.
对象属性可以分为内部属性和外部属性，对象的外部属性应由客户端程序定义。

To apply flyweight pattern, we need to divide Object property into intrinsic and extrinsic properties. Intrinsic properties make the Object unique whereas extrinsic properties are set by client code and used to perform different operations. For example, an Object Circle can have extrinsic properties such as color and width.

要应用flyweight模式，我们需要将Object属性分为内部属性和外部属性。内部属性使对象唯一，而外部属性由客户端代码设置并用于执行不同的操作。例如，对象圆可以具有外部属性，例如颜色和宽度。

For applying flyweight pattern, we need to create a Flyweight factory that returns the shared objects. For our example, lets say we need to create a drawing with lines and Ovals. So we will have an interface Shape and its concrete implementations as Line and Oval. Oval class will have intrinsic property to determine whether to fill the Oval with given color or not whereas Line will not have any intrinsic property.

为了应用flyweight模式，我们需要创建一个Flyweight工厂来返回共享对象。对于我们的示例，假设我们需要使用线条和椭圆形创建图形。因此，我们将有一个Shape界面及其具体实现，如Line和Oval 。椭圆类将具有固有属性，以确定是否用给定的颜色填充椭圆，而Line将不具有任何固有属性。

Flyweight设计模式界面和具体类 (Flyweight Design Pattern Interface and Concrete Classes)

Shape.java

package com.journaldev.design.flyweight;import java.awt.Color;
import java.awt.Graphics;public interface Shape {public void draw(Graphics g, int x, int y, int width, int height,Color color);
}

Line.java

package com.journaldev.design.flyweight;import java.awt.Color;
import java.awt.Graphics;public class Line implements Shape {public Line(){System.out.println("Creating Line object");//adding time delaytry {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}}@Overridepublic void draw(Graphics line, int x1, int y1, int x2, int y2,Color color) {line.setColor(color);line.drawLine(x1, y1, x2, y2);}}

Oval.java

package com.journaldev.design.flyweight;import java.awt.Color;
import java.awt.Graphics;public class Oval implements Shape {//intrinsic propertyprivate boolean fill;public Oval(boolean f){this.fill=f;System.out.println("Creating Oval object with fill="+f);//adding time delaytry {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}}@Overridepublic void draw(Graphics circle, int x, int y, int width, int height,Color color) {circle.setColor(color);circle.drawOval(x, y, width, height);if(fill){circle.fillOval(x, y, width, height);}}}

Notice that I have intensionally introduced delay in creating the Object of concrete classes to make the point that flyweight pattern can be used for Objects that takes a lot of time while instantiated.

注意，我在创建具体类的对象时故意引入了延迟，以使flyweight模式可用于实例化时花费大量时间的对象。

轻量化工厂 (Flyweight Factory)

The flyweight factory will be used by client programs to instantiate the Object, so we need to keep a map of Objects in the factory that should not be accessible by client application.

客户端程序将使用flyweight工厂实例化对象，因此我们需要在工厂中保留一个对象映射，客户端应用程序不应访问该映射。

Whenever client program makes a call to get an instance of Object, it should be returned from the HashMap, if not found then create a new Object and put in the Map and then return it. We need to make sure that all the intrinsic properties are considered while creating the Object.

每当客户端程序调用以获取Object的实例时，都应从HashMap返回它，如果找不到，则创建一个新的Object并放入Map中，然后返回它。我们需要确保在创建对象时考虑所有固有属性。

Our flyweight factory class looks like below code.

我们的flyweight工厂类看起来像下面的代码。

ShapeFactory.java

package com.journaldev.design.flyweight;import java.util.HashMap;public class ShapeFactory {private static final HashMap<ShapeType,Shape> shapes = new HashMap<ShapeType,Shape>();public static Shape getShape(ShapeType type) {Shape shapeImpl = shapes.get(type);if (shapeImpl == null) {if (type.equals(ShapeType.OVAL_FILL)) {shapeImpl = new Oval(true);} else if (type.equals(ShapeType.OVAL_NOFILL)) {shapeImpl = new Oval(false);} else if (type.equals(ShapeType.LINE)) {shapeImpl = new Line();}shapes.put(type, shapeImpl);}return shapeImpl;}public static enum ShapeType{OVAL_FILL,OVAL_NOFILL,LINE;}
}

Notice the use of Java Enum for type safety, Java Composition (shapes map) and Factory pattern in getShape method.

请注意，在getShape方法中将Java Enum用于类型安全， Java Composition （形状映射）和Factory模式。

Flyweight设计模式客户端示例 (Flyweight Design Pattern Client Example)

Below is a sample program that consumes flyweight pattern implementation.

下面是使用flyweight模式实现的示例程序。

DrawingClient.java

package com.journaldev.design.flyweight;import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;import com.journaldev.design.flyweight.ShapeFactory.ShapeType;public class DrawingClient extends JFrame{private static final long serialVersionUID = -1350200437285282550L;private final int WIDTH;private final int HEIGHT;private static final ShapeType shapes[] = { ShapeType.LINE, ShapeType.OVAL_FILL,ShapeType.OVAL_NOFILL };private static final Color colors[] = { Color.RED, Color.GREEN, Color.YELLOW };public DrawingClient(int width, int height){this.WIDTH=width;this.HEIGHT=height;Container contentPane = getContentPane();JButton startButton = new JButton("Draw");final JPanel panel = new JPanel();contentPane.add(panel, BorderLayout.CENTER);contentPane.add(startButton, BorderLayout.SOUTH);setSize(WIDTH, HEIGHT);setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);setVisible(true);startButton.addActionListener(new ActionListener() {public void actionPerformed(ActionEvent event) {Graphics g = panel.getGraphics();for (int i = 0; i < 20; ++i) {Shape shape = ShapeFactory.getShape(getRandomShape());shape.draw(g, getRandomX(), getRandomY(), getRandomWidth(),getRandomHeight(), getRandomColor());}}});}private ShapeType getRandomShape() {return shapes[(int) (Math.random() * shapes.length)];}private int getRandomX() {return (int) (Math.random() * WIDTH);}private int getRandomY() {return (int) (Math.random() * HEIGHT);}private int getRandomWidth() {return (int) (Math.random() * (WIDTH / 10));}private int getRandomHeight() {return (int) (Math.random() * (HEIGHT / 10));}private Color getRandomColor() {return colors[(int) (Math.random() * colors.length)];}public static void main(String[] args) {DrawingClient drawing = new DrawingClient(500,600);}
}

I have used random number generation to generate different type of Shapes in our frame.

我已经使用随机数生成在框架中生成不同类型的Shape。

If you run above client program, you will notice the delay in creating first Line Object and Oval objects with fill as true and false. After that the program executes quickly since its using the shared objects.

如果在客户端程序上运行，您会注意到创建第一个Line Object和Oval对象时填充为true和false的延迟。之后，由于程序使用共享库，因此程序将快速执行。

After clicking “Draw” button multiple times, the frame looks like below image.

多次单击“绘制”按钮后，框架如下图所示。

And you will see following output in command line confirming that Objects are shared.

并且您将在命令行中看到以下输出，确认已共享对象。

Creating Line object
Creating Oval object with fill=true
Creating Oval object with fill=false

Thats all for flyweight pattern, we will look into more design patterns in future posts. If you liked it, please share your thoughts in comments section and share it with others too.

这就是flyweight模式的全部内容，我们将在以后的文章中探讨更多的设计模式。如果您喜欢它，请在评论部分分享您的想法，并与他人分享。

JDK中的Flyweight设计模式示例 (Flyweight Design Pattern Example in JDK)

All the wrapper classes valueOf() method uses cached objects showing use of Flyweight design pattern. The best example is Java String class String Pool implementation.

所有包装器类的 valueOf()方法都使用缓存的对象，这些对象显示了Flyweight设计模式的使用。最好的示例是Java String类String Pool实现。

举重设计模式要点 (Flyweight Design Pattern Important Points)

In our example, the client code is not forced to create object using Flyweight factory but we can force that to make sure client code uses flyweight pattern implementation but its a complete design decision for particular application.
在我们的示例中，没有强制客户端代码使用Flyweight工厂创建对象，但是我们可以强制这样做以确保客户端代码使用Flyweight模式实现，但这是针对特定应用程序的完整设计决策。
Flyweight pattern introduces complexity and if number of shared objects are huge then there is a trade of between memory and time, so we need to use it judiciously based on our requirements.
Flyweight模式引入了复杂性，如果共享对象的数量巨大，那么内存和时间之间就存在交易关系，因此我们需要根据我们的需求明智地使用它。
Flyweight pattern implementation is not useful when the number of intrinsic properties of Object is huge, making implementation of Factory class complex.
当Object的固有属性数量巨大时，Flyweight模式实现就没有用，这使得Factory类的实现变得复杂。

That’s all for Flyweight design pattern in java.

这就是Java中Flyweight设计模式的全部内容。

翻译自: https://www.journaldev.com/1562/flyweight-design-pattern-java