正确书写单例模式

正确的单例模式有四种实现方式：

• 1.提前初始化(饿汉式)

public class Singleton {  
    private static Singleton instance = new Singleton();  
  
    private Singleton() {  
    }  
  
    public static Singleton getInstance() {  
        return instance;  
    }  
} 

• 2.懒汉式（效率低）

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

• 3.双重检查锁定（DCL） + volatile（变异懒汉式）
  懒汉式在在多线程的情况下会失效，所以需要改写，添加同步锁。

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

volatile保证变量修改的可见性。

• 4.延迟初始化占位类模式

public class Singleton {  
    private static class InstanceHolder {  
        public static Singleton instance = new Singleton();  
    }  
  
    private Singleton() {  
    }  
  
    public static Singleton getInstance() {  
        return InstanceHolder.instance;  
    }  
}