单例模式
描述
控制一个类仅能产生一个实例。
- 懒汉式：类加载时不会生成实例，只有在调用时才会生成实例。
- 饿汉式：类加载时便会生成实例。
类名一般为：xxxSingleton
代码实现示例
/**
* 最优实现：饿汉式-枚举（在Spring环境下）
*/
public enum MySingletonEnum{
INSTANCE;
private MyService myService;
private MySingletonEnum(){
myService = ApplicationContextUtil.getContext().getBean(MyService.class);
}
public String answer(){
return myService.answer();
}
//调用：MySingletonEnum.INSTANCE.answer();
}
/**
* 推荐实现：懒汉式-静态内部类
**/
public class Singleton {
private Singleton() {
}
private static class SingletonHolder {
private static final Singleton INSTANCE = new Singleton();
}
public static Singleton getUniqueInstance() {
return SingletonHolder.INSTANCE;
}
}
/**
* 饿汉式-线程安全
**/
public class Singleton {
private static Singleton uniqueInstance = new Singleton();
private Singleton() {
}
public static Singleton getUniqueInstance() {
return uniqueInstance;
}
}
/**
* 懒汉式-线程安全-双重校验锁
**/
public class Singleton {
private volatile static Singleton uniqueInstance;
private Singleton() {
}
public static Singleton getUniqueInstance() {
if (uniqueInstance == null) {
synchronized (Singleton.class) {
if (uniqueInstance == null) {
uniqueInstance = new Singleton();
}
}
}
return uniqueInstance;
}
}
/**
* 懒汉式-线程不安全
**/
public class Singleton {
private static Singleton uniqueInstance;
private Singleton() {
}
public static Singleton getUniqueInstance() {
if (uniqueInstance == null) {
uniqueInstance = new Singleton();
}
return uniqueInstance;
}
}