剑指Offer学习之面试题2 ： 实现Singleton 模式

题目：设计一个类，我们只能生成该类的一个实例

package com.www.OfferToSword;

//懒汉式单例模式
public class Singleton2 {
private static Singleton2 instance = null;

private Singleton2() {
}

public static Singleton2 getInstance() {
if (instance == null) {
instance = new Singleton2();
}
return instance;
}
}

public class Test02 {

/**
* 单例模式，饿汉式，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，懒汉式，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，懒汉式，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());
}

}
运行结果：