1、单例模式

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单例模式

一个类只能有一个实例。比如打印机这种东西。

trick在于：变量，构造方法都必须为private，所以其他类new它的构造方法是不行的，直接调用变量也不行。只能调用getInstance()方法&其他被public修饰的方法。getInstance()方法记得用public修饰。

原理：

public class Singleton {

private static Singleton uniqueInstance = null;

private Singleton(){

}

public static Singleton getInstance(){
if(uniqueInstance == null){
uniqueInstance = new Singleton();
}
return uniqueInstance;
}

}

例子：

public class ChocolateFactory {

private boolean empty;//是否为空
private boolean boiled;//是否加热过了
private static ChocolateFactory uniqueInstance = null;

private ChocolateFactory(){
empty = true;
boiled = false;
}

public static ChocolateFactory getInstance(){
if(uniqueInstance == null){
uniqueInstance = new ChocolateFactory();
}
return uniqueInstance;
}

public void fill(){
if(empty && (!boiled)){
empty = false;
}
}

public void boil(){
if((!empty) && (!boiled)){
boiled = true;
}
}

public void pour(){
if((!empty) && boiled){
empty = true;
}
}
}

bug：多线程不安全。 
解决办法(优化)： 
vs1：放一个同步锁（一个线程执行完后，另一个线程才可以调用），但是比较耗资源。

public class ChocolateFactory {

private boolean empty;
private boolean boiled;
private static ChocolateFactory uniqueInstance = null;

private ChocolateFactory(){
empty = true;
boiled = false;
}

//1、加入一个synchronized关键字
public static synchronized ChocolateFactory getInstance(){
if(uniqueInstance == null){
uniqueInstance = new ChocolateFactory();
}
return uniqueInstance;
}

public void fill(){
if(empty && (!boiled)){
empty = false;
}
}

public void boil(){
if((!empty) && (!boiled)){
boiled = true;
}
}

public void pour(){
if((!empty) && boiled){
empty = true;
}
}
}

vs2：急切创建实例，但是也有点耗资源。

public class ChocolateFactory {

private boolean empty;
private boolean boiled;
//1、快速创建一个实例
private static ChocolateFactory uniqueInstance = new ChocolateFactory();

private ChocolateFactory(){
empty = true;
boiled = false;
}

public static ChocolateFactory getInstance(){
if(uniqueInstance == null){
uniqueInstance = new ChocolateFactory();
}
return uniqueInstance;
}

public void fill(){
if(empty && (!boiled)){
empty = false;
}
}

public void boil(){
if((!empty) && (!boiled)){
boiled = true;
}
}

public void pour(){
if((!empty) && boiled){
empty = true;
}
}
}

vs3：双重检查加锁。

public class ChocolateFactory {

private boolean empty;
private boolean boiled;
//1、volatile是为了处理多线程安全而加的关键字
private volatile static ChocolateFactory uniqueInstance = null;

private ChocolateFactory(){
empty = true;
boiled = false;
}

public static ChocolateFactory getInstance(){
if(uniqueInstance == null){
//2、增多了一个锁
synchronized(ChocolateFactory.class){
if(uniqueInstance == null){
uniqueInstance = new ChocolateFactory();
}
}
}
return uniqueInstance;
}

public void fill(){
if(empty && (!boiled)){
empty = false;
}
}

public void boil(){
if((!empty) && (!boiled)){
boiled = true;
}
}

public void pour(){
if((!empty) && boiled){
empty = true;
}
}
}

有一个方法，适用于看代码是否应该使用当前这种设计模式： 
要在原来代码基础上，多加一个什么功能或什么改动时，改动会复杂吗？如果会变复杂，则考虑采用其他的设计模式。

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