设计模式-单例模式

确保一个类只有一个实例，并提供全局访问点。

• 饿汉式
  线程安全 非延时初始化

// Java 饿汉式
public class SingletonJava {
    private SingletonJava() {
    }

    private static SingletonJava instance = new SingletonJava();

    public static SingletonJava getInstance() {
        return instance;
    }
}

// Kotlin 饿汉式
object SingletonKotlin

---

• 懒汉式
  线程不安全 延时初始化

// Java 懒汉式
public class SingletonJava {
    private SingletonJava() {
    }

    private static SingletonJava instance;

    public static SingletonJava getInstance() {
        if (instance == null) {
            instance = new SingletonJava();
        }
        return instance;
    }
}

// Kotlin 懒汉式
class SingletonKotlin private constructor() {
    companion object {
        private val instance: SingletonKotlin? = null
            get() = field ?: SingletonKotlin()

        // 此处方法名不能使用getInstance，因为kotlin在伴生对象声明时已有getInstance方法，会冲突
        fun get(): SingletonKotlin {
            return requireNotNull(instance)
        }
    }
}

---

• 线程安全的懒汉式
  线程安全 延时初始化

// Java 线程安全的懒汉式
public class SingletonJava {
    private SingletonJava() {
    }

    private static SingletonJava instance;

    public static synchronized SingletonJava getInstance() {
        if (instance == null) {
            instance = new SingletonJava();
        }
        return instance;
    }
}

// Kotlin 线程安全的懒汉式
class SingletonKotlin private constructor() {
    companion object {
        private val instance: SingletonKotlin? = null
            get() = field ?: SingletonKotlin()

        // 此处方法名不能使用getInstance，因为kotlin在伴生对象声明时已有getInstance方法，会冲突
        @Synchronized
        fun get(): SingletonKotlin {
            return requireNotNull(instance)
        }
    }
}

---

• 双重校验锁
  线程安全 延时初始化

// Java 双重校验锁
public class SingletonJava {
    private SingletonJava() {
    }

    private volatile static SingletonJava instance;

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

// Kotlin 双重校验锁
// Kotlin中有个天然特性可以支持线程安全DCL的单例 Companion Object + lazy属性代理
class SingletonKotlin private constructor() {
    companion object {
        val instance: SingletonKotlin by lazy(LazyThreadSafetyMode.SYNCHRONIZED) {
            SingletonKotlin()
        }
    }
}

带参数的DCL写法，参照郭霖大佬

---

• 静态内部类
  线程安全 延时初始化

// Java 静态内部类
public class SingletonJava {
    private SingletonJava() {
    }

    private static class SingletonHolder {
        private static SingletonJava instance = new SingletonJava();
    }

    public static SingletonJava getInstance() {
        return SingletonHolder.instance;
    }
}

// Kotlin 静态内部类
class SingletonKotlin private constructor() {
    private object SingletonHolder {
        val holder = SingletonKotlin()
    }

    companion object {
        val instance = SingletonHolder.holder
    }
}

---

• 枚举
  线程安全 非延时初始化

// Java 枚举
public enum SingletonJava {
    INSTANCE
}

// Kotlin 枚举
enum class SingletonKotlin {
    INSTANCE
}

---

枚举实现单例，可防止反序列化，若使用其他方式，则需添加readResolve方法，并返回instance即可