黑馬程式員-單例模式

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單例模式：

特點：1、單例類隻能有一個執行個體

  2、單例類必須自己建立自己的唯一執行個體

  3、單例類必須給所有其他對象提供這一執行個體

好處：1、節省系統開銷

  2、儲存該執行個體的狀态

分類：

懶漢式

核心代碼：

public class LazySingleton {
	public static LazySingleton lazySingleton = null;

	private LazySingleton() {
	}

	synchronized public static LazySingleton getInstance() {
		if (lazySingleton == null) {
			lazySingleton = new LazySingleton();
		}
		return lazySingleton;
	}

}
           

餓漢式

核心代碼

public class HungerSingleton {
	private static final HungerSingleton HUNGERSINGLETON = new HungerSingleton();

	private HungerSingleton() {
	}

	public HungerSingleton getInstance() {
		return HUNGERSINGLETON;
	}
}
           

推薦使用餓漢式

因為在懶漢式中，由于存在多線程通路時，懶漢式可能會出現建立出多個執行個體，而若對其使用synchronized的話，則又會降低程式性能，是以推薦使用餓漢式。

登記式

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

public class RegistrationSingleton {
	public static Map<String, RegistrationSingleton> map = new HashMap<String, RegistrationSingleton>();
	static {
		RegistrationSingleton rs = new RegistrationSingleton();
		map.put(rs.getClass().getName(), rs);
	}

	protected RegistrationSingleton()// 此處為public或者protected都可，因為外界都無法通路
	{
	}

	public static RegistrationSingleton getInstance(String name) {
		if (name == null) {
			name = "RegistrationSingleton";
		}
		if (map.get(name) == null) {
			try {
				map.put(name, (RegistrationSingleton) Class.forName(name)
						.newInstance());
			} catch (InstantiationException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IllegalAccessException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (ClassNotFoundException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
		return map.get(name);
	}

}
           

如果有個學生類不是單例，但是我們想要把他當成單例模式，就是也隻能建立出一個學生執行個體，我們就可以用到這個，

Student s1=RegistrationSingleton.getInstance(“Student”);//注意此處Student必須為完整包名。注意學習這種思想，當然public static RegistrationSingleton getInstance(String name)  此時RegistrationSingleton必須寫成Object，裡面好多東西也需要改改。

修改如下：

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

public class RegistrationSingleton {
	public static Map<String, Object> map = new HashMap<String, Object>();
	static {
		RegistrationSingleton rs = new RegistrationSingleton();
		map.put(rs.getClass().getName(), rs);
	}

	protected RegistrationSingleton()// 此處為public或者protected都可，因為外界都無法通路
	{
	}

	public static Object getInstance(String name) {
		if (map.get(name) == null) {
			try {
				map.put(name, Class.forName(name).newInstance());
			} catch (InstantiationException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IllegalAccessException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (ClassNotFoundException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
		return map.get(name);
	}

}
           

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