代理模式

abstract public class Subject {
    
    /**
     * 生命一个抽象的请求发放
     */
    abstract public void request();
}

public class RealSubject extends Subject{
    
    /**
     * 构造子
     */
    public RealSubject(){}
    
    /**
     * 实现请求求方法
     */
    public void request() {
        System.out.println("From real subject");
    }

}

public class ProxySubject extends Subject{
    
    private RealSubject realSubject;
    
    /**
     * 构造子
     */
    public ProxySubject(){}
    
    /**
     * 实现请求方法
     */
    public void request(){
        preRequest();
        if(realSubject == null){
            realSubject = new RealSubject();
        }
        realSubject.request();
        postRequest();
    }
    
    /**
     * 请求前的操作
     */
    private void preRequest(){
        //something you want to do before requesting.
    }

    /**
     * 请求后的操作
     */
    private void postRequest(){
        //something you want to do after requesting.
    }
}