设计模式_12_单例模式

文章目录

• • • 单例模式
    • • 1. 分析
      • 2. 示例
      • • 2.1 懒汉式
        • 2.2 懒汉式双重锁检测
        • 2.3 静态内部类单例模式
        • 2.4 饿汉式
        • 2.5 容器式
        • 2.6 ThreadLocal式

单例模式

单例模式的7种写法

1. 分析

• 定义：包证一个类仅有一个实例，并提供一个全局访问点
• 类型：

  创建型

适用场景：

• 想确保任何情况下都绝对只有一个实例

优点：

• 在内存中只有一个实例，减少了内存开销
• 可以避免对资源的多重占用
• 设置全局访问点，严格控制访问

缺点：

• 没有接口，扩展困难

重点：

• 私有构造器
• 线程安全
• 延迟加载
• 序列化和反序列化安全
• 反射

2. 示例

2.1 懒汉式

public class LazySingleton {

    private static LazySingleton lazySingleton = null;

    private LazySingleton() {
    }

    public synchronized static LazySingleton getInstance() {
        if (lazySingleton == null) {
            lazySingleton = new LazySingleton();
        }
        return lazySingleton;
    }
}
           

2.2 懒汉式双重锁检测

public class LazyDoubleCheckSingleton {

    private volatile static LazyDoubleCheckSingleton lazyDoubleCheckSingleton = null;

    private LazyDoubleCheckSingleton() {
    }

    public static LazyDoubleCheckSingleton getInstance() {
        if (lazyDoubleCheckSingleton == null) {
            synchronized (LazyDoubleCheckSingleton.class) {
                if (lazyDoubleCheckSingleton == null) {
                    lazyDoubleCheckSingleton = new LazyDoubleCheckSingleton();
                    //1.分配内存给这个对象
                    //2.初始化对象
                    //3.设置lazyDoubleCheckSingleton 指向刚分配的内存地址
                }
            }
        }
        return lazyDoubleCheckSingleton;
    }
}
           

2.3 静态内部类单例模式

public class StaticInnerClassSingleton {

    private StaticInnerClassSingleton() {
    }

    private static class InnerClass {
        private static StaticInnerClassSingleton staticInnerClassSingleton = new StaticInnerClassSingleton();
    }

    public static StaticInnerClassSingleton getInstance() {
        return InnerClass.staticInnerClassSingleton;
    }
}
           

2.4 饿汉式

• 方式1

public class HungrySingleton {
    private final static HungrySingleton hungrySingleton = new HungrySingleton();

    private HungrySingleton() {

    }

    public static HungrySingleton getInstance() {
        return hungrySingleton;
    }
}
           

• 方式2，使用静态代码块

public class HungrySingleton {
    private final static HungrySingleton hungrySingleton;

    static {
        hungrySingleton = new HungrySingleton();
    }
    private HungrySingleton() {

    }

    public static HungrySingleton getInstance() {
        return hungrySingleton;
    }
}
           

解决序列化与反序列化对象不一致问题

public class HungrySingleton implements Serializable {
    private final static HungrySingleton hungrySingleton;

    static {
        hungrySingleton = new HungrySingleton();
    }

    private HungrySingleton() {

    }

    public static HungrySingleton getInstance() {
        return hungrySingleton;
    }

    private Object readResolve() {
        return hungrySingleton;
    }
}
           

实现

Serializable

接口，添加

readResolve

方法

2.5 容器式

public class ContainerSingleton {

    private ContainerSingleton() {
    }

    private static Map<String, Object> singletonMap = new ConcurrentHashMap<>();

    public static void putInstance(String key, Object instance) {
        if (StringUtils.isNotBlank(key) && instance != null) {
            if (!singletonMap.containsKey(key)) {
                singletonMap.put(key, instance);
            }
        }
    }

    public static Object getInstance(String key) {
        return singletonMap.get(key);
    }
}
           

2.6 ThreadLocal式

public class ThreadLocalInstance {

    private ThreadLocalInstance() {
    }

    private static final ThreadLocal<ThreadLocalInstance> threadLocal = new ThreadLocal<ThreadLocalInstance>() {

        @Override
        protected ThreadLocalInstance initialValue() {
            return new ThreadLocalInstance();
        }
    };

    public ThreadLocalInstance getInstance() {
        return threadLocal.get();
    }

}