单例模式的一些浅显的理解
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2022-06-03 19:07:03
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单例模式
- 单例模式的目的(作用)
单例模式的目的是为了让外界只能得到一个"类的实例对象". - 单例模式的创作思路
由上述单例模式的作用,我们其实可以倒推出单例模式的写法.
1.一般来说,在创建一个类的同时也会提供公开(public)的无参和有参构造方法,这样在外界就能创建类的实例对象,并且每new一次都会创建一个新的实例对象.这就和单例模式是矛盾的,单例模式的实例对象有且只有一个.那么也就是说,在创建类的同时将其构造方法私有化(private).这样外界就无法通过new来创建类的实例对象,这样就不会产生多个不同的实例对象.
结论: 构造方法私有化.
2.既然已经将构造方法私有化了,外界已经无法通过new来创建实例对象了,那么我们就必须在创建类的同时在本类中创建一个本类的实例对象.构造方法私有化之后在外界其他类中不能同过new创建实例对象,但是可以在本类中可以通过new来创建实例对象(这与private关键字的特点有关).
既然在外界不能new实例对象,那么在外界获得实例对象的方式只有可能是通过类名的方式调用.
那么问题来了? 通过类名来调用什么呢?
2.1.可以调用公开的静态的成员变量.
例:public static String s = new String();
这样虽然可以用类名来调用,但是每调用一次,相应的在表达式右边就是new一次,
还是会创建多个String的实例对象.这与我们的本意不相符.
这时就需要final关键字来修饰成员变量,表示最终的不可变的.
总结:public static final String s = new String();
2.2.可以调用公开的静态的成员方法.
例:public static String getInstance(){}
这个方法我们可以得到String的实例对象,在这种情况下,就需要将公开的静态的
成员变量私有化.即:private static String s = new String();
总结:
2.2.1.类的成员变量私有化
private static String s = new String();
2.2.2提供公开的静态的获得实例对象的方法
public static String getInstance(){ return s;}
3.根据上述的思路创建单例模式
单例模式1_1(饿汉式):
package com.Singleton.deom1;
public class Singleton {
public static void main(String[] args) {
//测试
Singleton is1 = Singleton.getInstance();
Singleton is2 = Singleton.getInstance();
System.out.println(is1 == is2);
}
//1.构造方法私有化
private Singleton() {}
//2.私有的Singleton的对象
private static Singleton s = new Singleton();
//3.提供公开的静态的获得Singleton实例对象的方法
public static Singleton getInstance() {
return s;
}
}
程序运行结果
true
单例模式1_2(饿汉式):
属于1_1的变化形式(根据2.1.可以调用公开的静态的成员变量)
package com.Singleton.deom1_2;
public class Singleton {
public static void main(String[] args) throws Exception {
//测试
Singleton s1 = Singleton.s;
Singleton s2 = Singleton.s;
System.out.println(s1 == s2);
}
//1.构造方法私有化
private Singleton() {}
//2.私有的Singleton的对象
public static final Singleton s = new Singleton();
}
程序运行结果
true
单例模式2(懒汉式):
package com.Singleton.deom2;
public class Singleton {
public static void main(String[] args) {
//测试
Singleton is1 = Singleton.getInstance();
Singleton is2 = Singleton.getInstance();
System.out.println(is1 == is2);
}
//1.构造方法私有化
private Singleton() {}
//2.私有的Singleton的对象
private static Singleton s;
//3.提供公开的静态的获得Singleton实例对象的方法
public static Singleton getInstance() {
//4.判断,当s == null时,则创建对象
if(s == null) {
s = new Singleton();
}
return s;
}
}
程序运行结果
true
对于上述懒汉式在单线程情况下没有问题,但是在多线程情况下就会出现问题,即创建多个对象.程序如下:在多线程(以100个线程为例)情况下,创建单例模式.
package com.Singleton.deom2_2;
public class Singleton {
public static void main(String[] args) {
//测试
for (int i = 0; i < 100; i++) {
new Thread() {
public void run() {
System.out.println(Singleton.getInstance());
}
}.start();
}
}
//1.构造方法私有化
private Singleton() {}
//2.私有的Singleton的对象
private static Singleton s;
//3.提供公开的静态的获得Singleton实例对象的方法
public static Singleton getInstance() {
//4.判断,当s == null时,则创建对象
if(s == null) {
s = new Singleton();
}
return s;
}
}
程序运行结果
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严格来说,上述算不上多线程,因为在循环的时候已经产生了时间差,不过即使是这样都已经导致了两个不同的Singleton对象产生,会对数据安全产生隐患.
优化上述懒汉式代码:
package com.Singleton.deom2_3;
public class Singleton {
public static void main(String[] args) throws Exception {
//测试
for (int i = 1; i <= 100; i++) {
new Thread() {
public void run() {
System.out.println(Singleton.getInstance());
}
}.start();
}
}
//1.构造方法私有化
private Singleton() {}
//2.私有的Singleton的对象
private static Singleton s;
//3.提供公开的静态的获得Singleton实例对象的方法
public static Singleton getInstance() {
//6.当在多线程环境下,这段程序被执行多次后必然会产生Singleton对象,因此在最外层加判断是
//为了不再进行synchronized (Singleton.class)操作,因为运行过程比较复杂,效率很低
if(s == null) {
//5.同步都需要的数据
synchronized (Singleton.class) {
//4.判断,当s == null时,则创建对象
if(s == null) {
s = new Singleton();
}
}
}
return s;
}
}
程序运行结果
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4.如何区分"饿汉式"和"懒汉式"?(个人的理解,仅供参考)
观察上述代码,会发现二者的区别主要体现在创建实例对象的时间上.
饿汉式:
在创建类的时候对象已经创建(主动式创建),并随着jvm虚拟机加载类的同时也被加载.
即不管是否通过类名调用,对象本身已经存在了.
懒汉式:
在创建类的时候不会创建实例对象,当外界通过类名调用的时候才创建对象(被动式创建).
本人只是初学者,文中可能有很多错误,在您阅读的过程中若是发现问题,欢迎留言,谢谢!!!
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