Java23种设计模式中的单例模式你了解吗

2024-04-02 19:04:59 266人浏览薄情痞子

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摘要

目录1、定义2、适用场景3、常见写法4、如何防止单例被破坏1.多线程破坏单例以及解决方法2.反射破坏单例以及解决方法3.序列化破坏单例以及解决方法5、优缺点6、总结1、定义单例模式

1、定义

单例模式(Singleton Pattern)是指确保一个类在任何情况下都绝对只有一个实例，并提供一个全局访问点。
隐藏其所有的构造方法。
属于创建型模式。

2、适用场景

确保任何情况下都绝对只有一个实例。

3、常见写法

第一种：饿汉式单例：在单例类首次加载时就创建实例


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

饿汉式单例静态代码块写法：


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

第二种：懒汉式单例：被外部类调用时才创建实例


public class LazySingleton {
    private static LazySingleton INSTANCE = null;
    
    private LazySingleton() {
    }
    
    public static LazySingleton getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new LazySingleton();
        }
        return INSTANCE;
    }
}

懒汉式单例静态匿名内部类写法(性能最优)：


public class LazyInnerClazzSingleton implements Serializable {
    
    private LazyInnerClazzSingleton() {
    }
    
    public static final LazyInnerClazzSingleton getInstance() {
        return LazyHolder.INSTANCE;
    }
    private static class LazyHolder {
        private static final LazyInnerClazzSingleton INSTANCE = new LazyInnerClazzSingleton();
    }
}

第三种：注册式单例：将每一个实例都缓存到统一的容器中，使用唯一标识获取实例

注册式单例枚举写法：


public enum EnumSingleton {
    INSTANCE;
    
    private Object data;
    public Object getData() {
        return data;
    }
    public void setData(Object data) {
        this.data = data;
    }
    public static EnumSingleton getInstance() {
        return INSTANCE;
    }
}

Spring中常见的注册式单例写法：


public class ContainerSingleton {
    
    private static Map<String, Object> container = new ConcurrentHashMap<>();
    private ContainerSingleton() {
    }
    public static Object getBean(String clazzName) {
        // 加synchronized代码块保证线程安全
        synchronized (container) {
            if (!container.containsKey(clazzName)) {
                Object object = null;
                try {
                    object = Class.forName(clazzName).newInstance();
                    container.put(clazzName, object);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                return object;
            }
            return container.get(clazzName);
        }
    }
}

第四种：ThreadLocal线程单例：保证线程内部的全局唯一，且天生线程安全


public class ThreadLocalSingleton {
    public static final ThreadLocal<ThreadLocalSingleton> THREAD_LOCAL = new ThreadLocal<ThreadLocalSingleton>() {
        @Override
        protected ThreadLocalSingleton initialValue() {
            return new ThreadLocalSingleton();
        }
    };
    private ThreadLocalSingleton() {
    }
    public static ThreadLocalSingleton getInstance() {
        return THREAD_LOCAL.get();
    }
}

4、如何防止单例被破坏

1.多线程破坏单例以及解决方法

以懒汉式单例LazySingleton为例：

首先写一个线程实现类，如下：

import com.hzg.study.design.pattern.singleton.lazy.LazySingleton;

public class ExecutorThread implements Runnable {
    @Override
    public void run() {
        LazySingleton instance = LazySingleton.getInstance();
        System.out.println(Thread.currentThread().getName() + ":" + instance);
    }
}

main方法测试：

public class LazySingletonTest {
    public static void main(String[] args) {
        Thread thread1 = new Thread(new ExecutorThread());
        thread1.start();
        Thread thread2 = new Thread(new ExecutorThread());
        thread2.start();
        System.out.println("----------------------------------------");
    }
}

测试结果：显然出现了两个不同的实例

解决方法1：加synchronized关键字修饰getInstance方法

public class LazySingleton {
    private static LazySingleton INSTANCE = null;
    
    private LazySingleton() {
    }
    
    public static synchronized LazySingleton getInstance() {
        if (INSTANCE == null) {
            INSTANCE = new LazySingleton();
        }
        return INSTANCE;
    }
}

解决方法2：双重检查锁DoubleCheck


public class LazyDoubleCheckSingleton {
    
    private volatile static LazyDoubleCheckSingleton INSTANCE = null;
    
    private LazyDoubleCheckSingleton() {
    }
    
    public static LazyDoubleCheckSingleton getInstance() {
        if (INSTANCE == null) {
            synchronized (LazyDoubleCheckSingleton.class) {
                if (INSTANCE == null) {
                    INSTANCE = new LazyDoubleCheckSingleton();
                }
            }
        }
        return INSTANCE;
    }
}

2.反射破坏单例以及解决方法

以懒汉式单例静态匿名内部类写法LazyInnerClazzSingleton为例：

main方法测试：

public class LazyInnerClazzSingletonTest {
    public static void main(String[] args) {
        try {
            Class<LazyInnerClazzSingleton> aClazz = LazyInnerClazzSingleton.class;
            Constructor<LazyInnerClazzSingleton> declaredConstructor = aClazz.getDeclaredConstructor(null);
            declaredConstructor.setAccessible(true);
            LazyInnerClazzSingleton instance1 = declaredConstructor.newInstance();
            LazyInnerClazzSingleton instance2 = LazyInnerClazzSingleton.getInstance();
            System.out.println(instance1);
            System.out.println(instance2);
            System.out.println(instance1 == instance2);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

测试结果：构建了两个不同的实例

解决方法：在构造器中增加如下if判断

public class LazyInnerClazzSingleton implements Serializable {
    
    private LazyInnerClazzSingleton() {
        if (null != LazyHolder.INSTANCE) {
            throw new RuntimeException("不允许构建多个实例");
        }
    }
    
    public static final LazyInnerClazzSingleton getInstance() {
        return LazyHolder.INSTANCE;
    }
    private static class LazyHolder {
        private static final LazyInnerClazzSingleton INSTANCE = new LazyInnerClazzSingleton();
    }
}

再次测试：

3.序列化破坏单例以及解决方法

以懒汉式单例静态匿名内部类写法LazyInnerClazzSingleton为例：注意必须先实现序列化接口Serializable

main方法测试：

    public static void main(String[] args) {
        LazyInnerClazzSingleton instance1 = LazyInnerClazzSingleton.getInstance();
        LazyInnerClazzSingleton instance2 = null;
        try (
                FileOutputStream fileOutputStream = new FileOutputStream("LazyInnerClazzSingleton.obj");
                ObjectOutputStream objectOutputStream = new ObjectOutputStream(fileOutputStream);
                FileInputStream fileInputStream = new FileInputStream("LazyInnerClazzSingleton.obj");
                ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);
        ) {
            // 序列化
            objectOutputStream.writeObject(instance1);
            objectOutputStream.flush();
            // 反序列化
            instance2 = (LazyInnerClazzSingleton) objectInputStream.readObject();
            System.out.println(instance1);
            System.out.println(instance2);
            System.out.println(instance1 == instance2);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

测试结果：构建了两个不同的实例

解决方法：新增readResolve方法

public class LazyInnerClazzSingleton implements Serializable {
    
    private LazyInnerClazzSingleton() {
        if (null != LazyHolder.INSTANCE) {
            throw new RuntimeException("不允许构建多个实例");
        }
    }
    
    public static final LazyInnerClazzSingleton getInstance() {
        return LazyHolder.INSTANCE;
    }
    private static class LazyHolder {
        private static final LazyInnerClazzSingleton INSTANCE = new LazyInnerClazzSingleton();
    }
    
    private Object readResolve() {
        return LazyHolder.INSTANCE;
    }
}