单例模式

Java下实现线程安全的单例模式有如下4种方式：（直接上代码）

public class Singleton {

    static class SingletonA {
        private static final SingletonA INSTANCE = new SingletonA();
        public static SingletonA getInstance() {
            return INSTANCE;
        }
        private SingletonA() {
            System.out.println("SingletonA ctor");
        }
        public void printName() {
            System.out.println("This is " + getClass().getSimpleName());
        }
    }

    static class SingletonB {
        private static class InstanceHolder {
            private static final SingletonB INSTANCE = new SingletonB();
        }
        public static SingletonB getInstance() {
            return InstanceHolder.INSTANCE;
        }
        private SingletonB() {
            System.out.println("SingletonB ctor");
        }
        public void printName() {
            System.out.println("This is " + getClass().getSimpleName());
        }
    }

    static class SingletonC {
        private volatile static SingletonC INSTANCE;
        public static SingletonC getInstance() {
            if (INSTANCE == null) {
                synchronized (SingletonC.class) {
                    if (INSTANCE == null) {
                        INSTANCE = new SingletonC();
                    }
                }
            }
            return INSTANCE;
        }
        private SingletonC() {
            System.out.println("SingletonC ctor");
        }
        public void printName() {
            System.out.println("This is " + getClass().getSimpleName());
        }
    }

    enum SingletonD {
        INSTANCE;
        private SingletonD() {
            System.out.println("SingletonD ctor");
        }
        public void printName() {
            System.out.println("This is " + getClass().getSimpleName());
        }
    }
    public static void main(String[] argv) {
        SingletonA.getInstance().printName();
        SingletonB.getInstance().printName();
        SingletonC.getInstance().printName();
        SingletonD.INSTANCE.printName();
        SingletonA.getInstance().printName();
        SingletonB.getInstance().printName();
        SingletonC.getInstance().printName();
        SingletonD.INSTANCE.printName();
    }
}

Java中Class的静态代码块是在Class被第一次使用，ClassLoader加载时执行的，只执行一次（如果被多个ClassLoader加载，则会执行多次），并且是线程安全的; Enum的任何一个枚举值都是在第一次使用时构造的，并且构造过程是线程安全的。上面4种方法的优缺点如下：

参考阅读

Java Spec by Oracle
Implementing Singleton with an Enum (in Java)
Are Java static initializers thread safe?
Singleton class design