懒汉式（线程不安全）

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

懒汉式（线程安全）

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

饿汉式（线程安全）

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

双重校验锁（线程安全，懒加载，JDK 1.5 以上）

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public class Singleton {  
    private volatile static Singleton instance = null;  
    private Singleton() {  
        
    }
      
    public static Singleton getInstance() {
        if (instance == null) { 
            synchronized(Singleton.class) {
                if (instance == null) { // 多个线程同时来到同步块，拿到锁的新建实例后，后续的线程可以做判断是否已创建 
                    instance = new Singleton(); 
                }
            }
          }
        return instance;
    }  
}

静态内部类（线程安全，懒加载）

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public class Singleton {  
    private Singleton() {  
        
    }
    public static Singleton getInstance() {  
        return SingletonHolder.instance;  
    }

    private static class SingletonHolder {
        private static Singleton instance = new Singleton();
    }
}

上面列出的是常用的几种，还有其它方式来构建单例，比如使用枚举或容器类来构建，但我很少用到，便不再列出，需要的可自行搜索。