设计模式之单例模式

1. 单例模式是一种创建模式，用来创建类的单个实例。
    单例模式的实现一般有以下规则：
a. 私有化构造方法
b. 类持有对自身的引用
c. 通过一个静态方法向外界提供这个实例

 

2. 直接创建方式

public class Manager {
	
	private static final Manager mgr = new Manager();
	
	private Manager(){}
	
	public static Manager getInstance()
	{
		return mgr;
	}
}

 

 3. 延迟创建方式

public class Manager {
	
	private static Manager mgr = null;
	
	private Manager(){}
	
	public synchronized static Manager getInstance() {
		if (mgr == null)
		{
			mgr = new Manager();
		}
		return mgr;
	}
}

 

4. 检查多次获取的对象是否相同

 

	public static void main(String[] args)
	{
		Manager mgr1 = Manager.getInstance();
		Manager mgr2 = Manager.getInstance();
		if (mgr1 == mgr2)
		{
			System.out.println("mgr1==mgr2");
		} else {
			System.out.println("mgr1!=mgr2");
		}
	}

 5. 源码实例

public final class Token
{
  public static final String TOKEN_KEY = "com.os.util.TOKEN";
  private int incNumber = 0;

  private static Token token = null;

  public static Token getInstance()
  {
    if (token == null)
    {
      token = new Token();
    }
    return token;
  }

  public synchronized String generate()
  {
    if (this.incNumber >= 1000000)
    {
      this.incNumber = 0;
    }
    else {
      this.incNumber += 1;
    }
    return MD5.encrypt(new Date().toString(), String.valueOf(this.incNumber));
  }

  public synchronized void saveToken(HttpServletRequest request)
  {
    saveToken(request, generate());
  }

  public synchronized void removeToken(HttpServletRequest request)
  {
    HttpSession session = request.getSession(false);
    if (session == null)
    {
      return;
    }
    session.removeAttribute("com.os.util.TOKEN");
  }

  public synchronized void saveToken(HttpServletRequest request, String token)
  {
    HttpSession session = request.getSession(false);
    if (session == null)
    {
      return;
    }
    session.setAttribute("com.os.util.TOKEN", token);
  }
}