单例模式

public class Test02 {  
    /** 
     * 单例模式，懒汉式，线程安全 
     */  
    public static class Singleton {  
        private final static Singleton INSTANCE = new Singleton();  
        private Singleton() {  
        }  
        public static Singleton getInstance() {  
            return INSTANCE;  
        }  
    }    
    /** 
     * 单例模式，饿汉式，线程不安全 
     */  
    public static class Singleton2 {  
        private static Singleton2 instance = null;  
        private Singleton2() {  
        }  
        public static Singleton2 getInstance() {  
            if (instance == null) {  
                instance = new Singleton2();  
            }  
            return instance;  
        }  
    }  
    /** 
     * 单例模式，饿汉式，线程安全，多线程环境下效率不高 
     */  
    public static class Singleton3 {  
        private static Singleton3 instance = null;  
        private Singleton3() {  
        }  
        public static synchronized Singleton3 getInstance() {  
            if (instance == null) {  
                instance = new Singleton3();  
            }  
            return instance;  
        }  
    }  
    /** 
     * 单例模式，懒汉式，变种，线程安全 
     */  
    public static class Singleton4 {  
        private static Singleton4 instance = null;  
        static {  
            instance = new Singleton4();  
        }  
        private Singleton4() {  
        }  
        public static Singleton4 getInstance() {  
            return instance;  
        }  
    }  
    /** 
     * 单例模式，使用静态内部类，线程安全【推荐】 
     */  
    public static class Singleton5 {  
        private final static class SingletonHolder {  
            private static final Singleton5 INSTANCE = new Singleton5();  
        }  
        private Singleton5() {  
        }  
        public static Singleton5 getInstance() {  
            return SingletonHolder.INSTANCE;  
        }  
    }  
    /** 
     * 静态内部类，使用枚举方式，线程安全【推荐】 
     */  
    public enum Singleton6 {  
        INSTANCE;  
        public void whateverMethod() {  
        }  
    }  
    /** 
     * 静态内部类，使用双重校验锁，线程安全【推荐】 
     */  
    public static class Singleton7 {  
        private volatile static Singleton7 instance = null;  
        private Singleton7() {  
        }  
        public static Singleton7 getInstance() {  
            if (instance == null) {  
                synchronized (Singleton7.class) {  
                    if (instance == null) {  
                        instance = new Singleton7();  
                    }  
                }  
            }  
            return instance;  
        }  
    }  
    public static void main(String[] args) {  
        System.out.println(Singleton.getInstance() == Singleton.getInstance());  
        System.out.println(Singleton2.getInstance() == Singleton2.getInstance());  
        System.out.println(Singleton3.getInstance() == Singleton3.getInstance());  
        System.out.println(Singleton4.getInstance() == Singleton4.getInstance());  
        System.out.println(Singleton5.getInstance() == Singleton5.getInstance());  
        System.out.println(Singleton6.INSTANCE == Singleton6.INSTANCE);  
        System.out.println(Singleton7.getInstance() == Singleton7.getInstance());  
    }  
}  

参考1
参考2