常规方式

懒汉式 DCL「双重检测锁:Double Checked Lock」

public class Singleton {
    private static volatile Singleton instance = null;
    private Singleton() {
    }
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    // 延迟初始化
                    instance  = new Singleton();
                }
            }
        }
        return instance;
    }
}

优点：线程安全；延迟加载；效率较高。
缺点：由于volatile关键字会屏蔽Java虚拟机所做的一些代码优化，略微的性能降低。

饿汉式（静态常量）

public class Singleton {
    private final static Singleton INSTANCE = new Singleton();
    private Singleton() {
    }
    public static Singleton getInstance() {
        return INSTANCE;
    }
}

缺点:使用静态final的实例对象或者使用静态代码块依旧不能解决在反序列化、反射、克隆时重新生成实例对象的问题(见参考1)

枚举方式

饿汉式枚举单例

public enum T {
    SPRING,SUMMER,AUTUMN,WINTER;
}
// 反编译之后可知枚举中的各个枚举项是通过static来定义的
public final class T extends Enum
{
    //省略部分内容
    public static final T SPRING;
    public static final T SUMMER;
    public static final T AUTUMN;
    public static final T WINTER;
    private static final T ENUM$VALUES[];
    static
    {
        SPRING = new T("SPRING", 0);
        SUMMER = new T("SUMMER", 1);
        AUTUMN = new T("AUTUMN", 2);
        WINTER = new T("WINTER", 3);
        ENUM$VALUES = (new T[] {
            SPRING, SUMMER, AUTUMN, WINTER
        });
    }
}

懒汉式枚举单例(枚举+内部类)

enum LanguageEnum {
    INSTANCE;
    private static class Language {
        public String realName(String name) {
            return "Language " + name;
        }
    }

    private Language instance = null;
    private LanguageEnum() {
        instance = new Language();
    }
    
    // 实际对外开放的两个方法
    public Language getLanguage() {
        return instance;
    }
    
    public void printlnLanguageName(String name) {
        String str = getLanguage().realName(name);
        System.out.println(str);
    }
}