java单例模式

1、什么时候使用单例模式？

2、java单例写法

/**

 * Returns the single instance of this class, creating it if necessary.
*
* @return the {@link PeopleInfoService} instance
*/
public static PeopleInfoService getInstance() {
if (INSTANCE == null) {
INSTANCE = new PeopleInfoService();
}
return INSTANCE;
}
/*** Used to force {@link #getInstance()} to create a new instance
* next time it's called.
*/
public static void destroyInstance() {
INSTANCE = null;
}