单例设计模式

1.懒汉式 线程不安全,当有多个线程并行调用getInstance()的时候，就会创建多个实例。也就是说在多线程下不能正常工作。

public class Singleton{

private static Singleton instance;

private Singleton(){}

public static Singleton getInstance(){

if(instance == null){

instance = new Singleton();

    }

return instance;

}

}

2.懒汉式 线程安全,虽然做到了线程安全，并且解决了多实例的问题，但是它并不高效。因为在任何时候只能有一个线程调用getInstance()方法。但是同步操作只需要在第一次调用时才被需要，即第一次创建单例实例对象时。这就引出了双重检验锁。

public class Singleton{

private static Singleton instance;

private Singleton(){}

public synchronized static Singleton getInstance(){

if(instance == null){

instance = new Singleton();

}

return instance;

}

}

3.双重检验锁

public static Singleton getSingleton() {

if (instance == null) {

synchronized (Singleton.class) {

if (instance == null) {

instance = new Singleton();

}

}

}

return instance ;

}

>改进代码, instance 变量声明成 volatile

public class Singleton {

private volatile static Singleton instance; //声明成 volatile

private Singleton (){}

public static Singleton getSingleton() {

if (instance == null) {

synchronized (Singleton.class) {

if (instance == null) {

instance = new Singleton();

}

}

}

return instance;

}

}

4.饿汉式,在第一次加载类到内存中时就会初始化，所以创建实例本身是线程安全的。

public class Singleton {

//类加载的时候就初始化

private static final Singleton instance =  new Singleton();

private Singleton(){}

public static Singleton getInstance() {

return instance;

}

}

5.静态内部类.

public class Singleton {

private static class SingletonHolder{

private static final INCTANCE = new Singleton();

}

private Singleton(){}

public static final Singleton getInstance(){

return SingletonHolder.INSTANCE;

}

}

6.枚举

public enum EasySingleton {

INSTANCE;

}