标签:
原文地址:http://www.oschina.net/code/snippet_111708_25417
线程安全的单例模式实现有几种思路,个人认为第2种方案最优雅:
1、饿汉式
2、借助内部类
3、普通加锁解决
4、双重检测,但要注意写法
如果单体模式继续扩展为N元单体模式,那就是对象池模式了
public
class
Singleton {
private
final
static
Singleton INSTANCE =
new
Singleton();
private
Singleton() { }
public
static
Singleton getInstance() {
return
INSTANCE;
}
}
public
class
Singleton {
private
Singleton() { }
private
static
class
SingletonHolder {
private
final
static
Singleton INSTANCE =
new
Singleton();
}
public
static
Singleton getInstance() {
return
SingletonHolder.INSTANCE;
}
}
属于懒汉式单例,因为Java机制规定,内部类SingletonHolder只有在getInstance()方法第一次调用的时候才会被加载(实现了lazy),而且其加载过程是线程安全的。内部类加载的时候实例化一次instance。
public
class
Singleton {
private
static
Singleton instance =
null
;
private
Singleton() { }
public
static
synchronized
Singleton getInstance() {
if
(instance ==
null
) {
instance =
new
Singleton();
}
return
instance;
}
}
虽然解决了线程安全问题,但是每个线程调用getInstance都要加锁,我们想要只在第一次调用getInstance时加锁,请看下面的双重检测方案
public
class
Singleton {
private
static
Singleton instance =
null
;
private
Singleton() { }
public
static
Singleton getInstance() {
if
(instance ==
null
) {
synchronzied(Singleton.
class
) {
Singleton temp = instance;
if
(temp ==
null
) {
temp =
new
Singleton();
instance = temp
}
}
}
return
instance;
}
}
由于指令重排序问题,所以不可以直接写成下面这样:
public
class
Singleton {
private
static
Singleton instance =
null
;
private
Singleton() { }
public
static
Singleton getInstance() {
if
(instance ==
null
) {
synchronzied(Singleton.
class
) {
if
(instance ==
null
) {
instance =
new
Singleton();
}
}
}
return
instance;
}
}
但是如果instance实例变量用
volatile
修饰就可以了,
volatile
修饰的话就可以确保instance =
new
Singleton();对应的指令不会重排序,如下的单例代码也是线程安全的:
public
class
Singleton {
private
static
volatile
Singleton instance =
null
;
private
Singleton() { }
public
static
Singleton getInstance() {
if
(instance ==
null
) {
synchronzied(Singleton.
class
) {
if
(instance ==
null
) {
instance =
new
Singleton();
}
}
}
return
instance;
}
}
标签:
原文地址:http://www.cnblogs.com/cheng2015/p/5085008.html