Singleton mode
There are many ways to implement singleton mode: hungry man, lazy man, enumeration, static inner class, etc.
Reposted from: https://www.jianshu.com/p/b8c578b07fbc
Why use singleton mode?
Scenario For some classes (mostly tool classes), the entire system only needs one instance to complete the task, such as: log printing class, obtaining thread pool class, etc.
advantage
- There is only one object in the system, which saves system resources, reduces the overhead of frequently creating and destroying objects, and improves system performance.
Disadvantage
- If the instantiated object is not used for a long time, it will be considered as garbage by the system and will be recycled, which will result in the loss of the object state.
Method to realize
- Lazy man mode
/**
* <饿汉模式>
* <p>
* 类一旦加载就创建instance实例，getInstance时实例已经存在
*
* @author dkangel
*/
public class SingletonHungry {
private static final SingletonHungry instance = new SingletonHungry();
/**
* 私有构造方法，避免其他类创建实例
*/
private SingletonHungry() {
}
public static SingletonHungry getInstance() {
return instance;
}
}
- Hungry man mode
/**
* <懒汉模式>
* 只有在getInstance时才去实例化instance
* <p>
* <应用场景>
* 除了初始化单例类时 即 创建单例外，继续延伸出来的是：单例对象 要求初始化速度快 & 占用内存小
*
* @author dkangel
*/
public class SingletonLazy {
/**
* volatile 保证指令重排序的正确性
*/
private volatile static SingletonLazy instance;
/**
* 私有构造方法，避免其他类创建实例
*/
private SingletonLazy() {
}
/**
* <双重校验锁机制>
* <p>
* 第一个if：若实例已经创建，则直接返回，无需加锁
* <p>
* 第二个if：避免多次创建实例
* 如果有两个线程同时调用getInstance()方法，线程A先获取到锁，线程B等待线程A释放锁
* 当线程A执行instance = new SingletonLazy();完毕，释放了锁，此时线程B获取到锁
* 假如没有第二个if判空，则线程B也会执行instance = new SingletonLazy();
* 这样就导致创建了多余的实例
*
* @return SingletonLazy
*/
public static SingletonLazy getInstance() {
if (instance == null) {
synchronized (SingletonLazy.class) {
if (instance == null) {
instance = new SingletonLazy();
}
}
}
return instance;
}
/**
* <同步锁>
* <p>
* 每次访问都要进行线程同步（即 调用synchronized锁)
* 造成过多的同步开销（加锁 = 耗时、耗能）
*
* @return SingletonLazy
*/
public static synchronized SingletonLazy getInstance1() {
if (instance == null) {
instance = new SingletonLazy();
}
return instance;
}
}