什麼是代理模式
爲其他對象提供一種代理,並由代理對象控制對原對象的引用,以間接控制對原對象的訪問。
實現一個代理模式
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
}
}