单例模式

public class Singleton {
    //静态变量
    private static Singleton instance;

    //私有化构造器, 防止外部进行new
    private Singleton(){}

    //synchronized保证线程安全性, 但是会影响效率,存在实例时也要进行同步, 这是不必要的
    public static synchronized Singleton getInstance(){
        if (instance!=null){
            return instance;
        }else{
            return instance=new Singleton();
        }
    }
}

public class DoubleCheck {
    // 保证可见性,一但有实例时,所有线程都会知道
    private static volatile DoubleCheck instance;

    private DoubleCheck(){}

    private static DoubleCheck getInstance(){
        if (instance==null){
            synchronized (DoubleCheck.class){
                if (instance==null){
                    return instance=new DoubleCheck();
                }
            }
        }
        return instance;
    }
}

public class StaticInnerClass {
    private StaticInnerClass() {
    }
    // 类装载时,静态内部类不会被加载, 保证了懒加载
    // 静态内部类装载时, JVM提供了线程安全, 保证了线程安全
    // 类的静态属性,只在类装载时被加载,保证了单例
    private static class StaticInnerClassIntance {
        private static final StaticInnerClass INTANCE = new StaticInnerClass();
    }

    public static StaticInnerClass getInstance() {
        return StaticInnerClassIntance.INTANCE;
    }
}

public enum EnumSingleton {
    INSTACNE;

    public static EnumSingleton getIntance(){
        return INSTACNE;
    }
}