1、饿汉式，JVM保证线程安全（构造器私有化）

public class HongYan01 {

    private static final HongYan01 INSTANCE = new HongYan01();

    private HongYan01() {};

    public static HongYan01 getInstance() {
        return INSTANCE;
    }

    public static void main(String[] args) {
        System.out.println(HongYan01.getInstance() == HongYan01.getInstance());
    }
}

2、双重校验机制，线程安全，懒加载（构造器私有化）

public class HongYan02 {

    // 懒加载，双重校验机制
    private static volatile HongYan02 singleton;

    private HongYan02() {
    }

    public static HongYan02 getInstance() {
        if (singleton == null) {
            // 双重检查
            synchronized (HongYan02.class) {
                if(singleton == null) {
                    singleton = new HongYan02();
                }
            }
        }
        return singleton;
    }

    public static void main(String[] args) {
        for(int i=0; i<666; i++) {
            new Thread(()-> System.out.println(HongYan02.getInstance().hashCode())).start();
        }
    }
}

3、静态内部类单例，jvm加载类保证单例，线程安全

public class HongYan03 {

    private HongYan03() {
    }

    private static class HongYan03InnerClass {
        private final static HongYan03 INSTANCE = new HongYan03();
    }

    public static HongYan03 getInstance() {
        return HongYan03InnerClass.INSTANCE;
    }

    public static void main(String[] args) {
        for(int i=0; i<666; i++) {
            new Thread(()-> System.out.println(HongYan03.getInstance().hashCode())).start();
        }
    }
}

4、枚举方式，实现单例，线程安全，还可以防止反序列化

public enum HongYan04 {

    INSTANCE;

    public static void main(String[] args) {
        for (int i = 0; i < 666; i++) {
            new Thread(() -> System.out.println(HongYan04.INSTANCE.hashCode())).start();
        }
    }
}

其他方式可能会导致线程不安全，这里不进行展示错误示例了。