JavaSE面試題---單例模式七種寫法

定義

保證一個類僅有一個實例，並提供一個訪問它的全局訪問點

七種寫法

1.餓漢式（簡潔直觀）

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();

    private Singleton() {
    }

    public static Singleton getInstance() {
        return INSTANCE;
    }
}

2.枚舉（餓漢式的變種，最簡潔）

public enum  Singleton {
    INSTANCE;
}

枚舉類型默認構造是私有的

3.靜態代碼快（餓漢式的變種，適合複雜的實例化（如外部資源導入等））

public class Singleton {
    private static final Singleton INSTANCE;
    
    private Singleton(){
    }

    static {
       INSTANCE = new Singleton();
    }
    
    public static Singleton getInstance() {
        return INSTANCE;
    }
}

4.懶漢式（線程不安全，適用於單線程）

public class Singleton {
    private static Singleton instance = null;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

5.雙重檢測鎖（DCL，懶漢式的改進，實現線程安全（同步），適用於多線程）

public class Singleton {  
	private static volatile Singleton instance = null;  
    
	private Singleton (){
	}   
    
	public static Singleton getInstance() {  
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
		return instance;  
	}  
}  

6.靜態內部類（懶漢式的改進，適用於多線程）

public class Singleton {
    private Singleton() {
    }

    private static class InnerClass {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return InnerClass.INSTANCE;
    }
}

7.容器（懶漢式的一種，適用於多線程）

public class Singleton {
    private static Map<String,Object> map=new HashMap<String, Object>();
    
    private Singleton(){
    }
    
    public static void registerService(String key,Object instance){
        if (!map.containsKey(key)){
            map.put(key,instance);
        }
    }
    
    public static Object getService(String key){
        return map.get(key);
    } 
}