Java 享元模式（Flyweight）

當一個應用中使用了大量的對象，這些對象造成了很大的存儲開銷，而對象的大部分狀態或參數都是相同（內部狀態）的時候，可以考慮使用享元模式，使用享元模式可以是這些對象引用都共享相同的實例，降低存儲開銷，而對象之間的不同的狀態參數(外部狀態)則使用外部參數傳入來實現。

package flyweight;

public abstract class WebSite {
    protected String type;
    
    public WebSite(String type) {
        this.type = type;
    }
    
    public String getType() {
        return type;
    }

    public abstract void use(User user);
}


package flyweight;

public class ConcurrentWebSite extends WebSite {
    public ConcurrentWebSite(String type) {
        super(type);
    }

    @Override
    public void use(User user) {
        System.out.println("The web site's type is: " + type);
        System.out.println("User: " + user.getUserName());
        System.out.println("Passwd: " + user.getPassWd());
        System.out.println();
    }
}


package flyweight;

import java.util.HashMap;
import java.util.Map;

public class WebSiteFactory {
    private static Map<String, WebSite> webSites = new HashMap<String, WebSite>();
    
    private WebSiteFactory() {}
    
    public static WebSite createWebSite(String type) {
        WebSite webSite = webSites.get(type);
        if (webSite == null) {
            webSite = new ConcurrentWebSite(type);
            webSites.put(type, webSite);
        }
        return webSite;
    }
    
    public static int webSitesCount() {
        return webSites.size();
    }
}


package flyweight;

public class User {
    private String userName;
    private String passWd;
    
    public User(String userName, String passWd) {
        this.userName = userName;
        this.passWd = passWd;
    }
    
    public String getUserName() {
        return userName;
    }
    
    public String getPassWd() {
        return passWd;
    }
}


package flyweight;

public class Test {
    public static void main(String[] args) {
        WebSite wb1 = WebSiteFactory.createWebSite("BusinessWebSite");
        User user1 = new User("root", "root");
        wb1.use(user1);
        
        WebSite wb2 = WebSiteFactory.createWebSite("BusinessWebSite");
        User user2 = new User("admin", "admin");
        wb2.use(user2);
        
        WebSite wb3 = WebSiteFactory.createWebSite("BusinessWebSite");
        User user3 = new User("slave", "slave");
        wb3.use(user3);
        
        WebSite wb4 = WebSiteFactory.createWebSite("PersonalWebSite");
        User user4 = new User("person", "person");
        wb4.use(user4);
        
        WebSite wb5 = WebSiteFactory.createWebSite("PersonalWebSite");
        User user5 = new User("alexis", "alexis");
        wb5.use(user5);
        
        WebSite wb6 = WebSiteFactory.createWebSite("PersonalWebSite");
        User user6 = new User("shadow", "shadow");
        wb6.use(user6);
        
        System.out.println("WebSites Instances Count: " + WebSiteFactory.webSitesCount());
    }
}

輸出

The web site's type is: BusinessWebSite
User: root
Passwd: root

The web site's type is: BusinessWebSite
User: admin
Passwd: admin

The web site's type is: BusinessWebSite
User: slave
Passwd: slave

The web site's type is: PersonalWebSite
User: person
Passwd: person

The web site's type is: PersonalWebSite
User: alexis
Passwd: alexis

The web site's type is: PersonalWebSite
User: shadow
Passwd: shadow

WebSites Instances Count: 2

這里我們共享的實例就是WebSite，type為內部參數，User作為外部參數傳入。