我试图理解代理设计模式。但我无法理解代理设计模式的用法。我从维基百科得到了这个代码示例
interface Image {
public void displayImage();
}
//on System A
class RealImage implements Image {
private String filename = null;
/**
* Constructor
* @param filename
*/
public RealImage(final String filename) {
this.filename = filename;
loadImageFromDisk();
}
/**
* Loads the image from the disk
*/
private void loadImageFromDisk() {
System.out.println("Loading " + filename);
}
/**
* Displays the image
*/
public void displayImage() {
System.out.println("Displaying " + filename);
}
}
//on System B
class ProxyImage implements Image {
private RealImage image = null;
private String filename = null;
/**
* Constructor
* @param filename
*/
public ProxyImage(final String filename) {
this.filename = filename;
}
/**
* Displays the image
*/
public void displayImage() {
if (image == null) {
image = new RealImage(filename);
}
image.displayImage();
}
}
class ProxyExample {
/**
* Test method
*/
public static void main(String[] args) {
final Image IMAGE1 = new ProxyImage("HiRes_10MB_Photo1");
final Image IMAGE2 = new ProxyImage("HiRes_10MB_Photo2");
IMAGE1.displayImage(); // loading necessary
IMAGE1.displayImage(); // loading unnecessary
IMAGE2.displayImage(); // loading necessary
IMAGE2.displayImage(); // loading unnecessary
IMAGE1.displayImage(); // loading unnecessary
}
}
在这个例子中,他们说第二次显示图像时不需要加载。甚至也可以直接访问 RealImage 对象。
final Image IMAGE1 = new RealImage("HiRes_10MB_Photo1");
final Image IMAGE2 = new RealImage("HiRes_10MB_Photo2");
IMAGE1.displayImage(); // loading necessary
IMAGE1.displayImage(); // loading unnecessary
IMAGE2.displayImage(); // loading necessary
IMAGE2.displayImage(); // loading unnecessary
IMAGE1.displayImage(); // loading unnecessary
我需要了解 ProxyImage 类在此模式中的用法。
你知道,我同意你的观点。我感觉有一个much他们可以将更好的例子用于代理模式。这似乎使用相同的示例,但解释得更好。 http://www.oodesign.com/proxy-pattern.html你应该看看那个。
基本上,这一切都归结为这个评论:
// create the Image Object only when the image is required to be shown
这就是好处proxy
在这个例子中给你。如果您不显示图像,则无需支付加载图像的惩罚:
package proxy;
/**
* Image Viewer program
*/
public class ImageViewer {
public static void main(String[] args) {
// assuming that the user selects a folder that has 3 images
//create the 3 images
Image highResolutionImage1 = new ImageProxy("sample/veryHighResPhoto1.jpeg");
Image highResolutionImage2 = new ImageProxy("sample/veryHighResPhoto2.jpeg");
Image highResolutionImage3 = new ImageProxy("sample/veryHighResPhoto3.jpeg");
// assume that the user clicks on Image one item in a list
// this would cause the program to call showImage() for that image only
// note that in this case only image one was loaded into memory
highResolutionImage1.showImage();
// consider using the high resolution image object directly
Image highResolutionImageNoProxy1 = new HighResolutionImage("sample/veryHighResPhoto1.jpeg");
Image highResolutionImageNoProxy2 = new HighResolutionImage("sample/veryHighResPhoto2.jpeg");
Image highResolutionImageBoProxy3 = new HighResolutionImage("sample/veryHighResPhoto3.jpeg");
// assume that the user selects image two item from images list
highResolutionImageNoProxy2.showImage();
// note that in this case all images have been loaded into memory
// and not all have been actually displayed
// this is a waste of memory resources
}
}
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