第一种：饿汉模式（线程安全）
代码语言：javascript复制public class Single2 {
private static Single2 instance = new Single2();
private Single2(){
System.out.println("Single2: " System.nanoTime());
}
public static Single2 getInstance(){
return instance;
}
}第二种：懒汉模式 （如果方法没有synchronized，则线程不安全）
代码语言：javascript复制public class Single3 {
private static Single3 instance = null;
private Single3(){
System.out.println("Single3: " System.nanoTime());
}
public static synchronized Single3 getInstance(){
if(instance == null){
instance = new Single3();
}
return instance;
}
}第三种：懒汉模式改良版（线程安全，使用了double-check，即check-加锁-check，目的是为了减少同步的开销）
代码语言：javascript复制public class Single4 {
private volatile static Single4 instance = null;
private Single4(){
System.out.println("Single4: " System.nanoTime());
}
public static Single4 getInstance(){
if(instance == null){
synchronized (Single4.class) {
if(instance == null){
instance = new Single4();
}
}
}
return instance;
}
}第四种：利用私有的内部工厂类（线程安全，内部类也可以换成内部接口，不过工厂类变量的作用于要改为public了。）
代码语言：javascript复制public class Singleton {
private Singleton(){
System.out.println("Singleton: " System.nanoTime());
}
public static Singleton getInstance(){
return SingletonFactory.singletonInstance;
}
private static class SingletonFactory{
private static Singleton singletonInstance = new Singleton();
}
}转自：https://www.nowcoder.com/profile/6801164/myFollowings/detail/13699402
