单例模式

定义

一个类只有一个实例，该实例必须自行创建，并且向整个系统提供这个实例

实现考虑因素

• 唯一性
  • 是否防克隆
  • 是否防反射
  • 是否防序列化
• 性能
  • 是否需要懒加载
  • 是否防指令重排（双重锁方式问题）

三种实现方式

双重锁方式（添加了防御功能）

public class Singleton implements Serializable, Cloneable {

    private static volatile Singleton sSingleton; // volatile防止指令重排
    private static boolean sFlag = true;
    private Singleton() {
        if(sFlag){ // 防反射
            sFlag = false;
        }else{
            throw new RuntimeException("对象已经存在");
        }
    }

    public static Singleton getSingleton() {
        if (sSingleton == null) {
            synchronized (Singleton.class) {
                if (sSingleton == null) {
                    sSingleton = new Singleton();
                }
            }
        }
        return sSingleton;
    }

    private Object readResolve() {
        return getSingleton(); // 防序列化
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return getSingleton(); // 防克隆
    }
}

静态内部类方式

public class Instance {
    private Instance() {
    }

    private static class Holder {
        // 类加载机制保证懒加载，static&final确保线程安全
        private static final Instance INSTANCE = new Instance();
    }

    public static Instance getInstance() {
        return Holder.INSTANCE;
    }

}

枚举类方式

public enum Single {
    SINGLE;
    public void whateverMethod(){
    }
}

基本等价于(除enum的防反射、序列化、克隆等属性)

public final class Single{
    public static final Single SINGLE = new Single();
}

测试代码

 public static void main(String args[]) {

        try {
            Singleton singleton = Singleton.getSingleton();
            testCloneSingleton(singleton);
            testReflectSingleton(singleton);
            testSerializableSingleton(singleton);

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static void testCloneSingleton(Singleton singleton) throws CloneNotSupportedException {

        boolean equals = singleton.equals(Singleton.getSingleton().clone());

        System.out.print(equals);
    }

    private static void testReflectSingleton(Singleton singleton) throws Exception {
        Constructor constructor = Singleton.class.getDeclaredConstructor();
        constructor.setAccessible(true);
        constructor.newInstance();

        boolean equals = singleton.equals(constructor.newInstance(););

        System.out.print(equals);

    }

    private static void testSerializableSingleton(Singleton singleton) throws Exception {
        String tempFile = "tempFile";
        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(tempFile));
        oos.writeObject(getSingleton());
        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream(new File(tempFile)));

        boolean equals = singleton.equals(objectInputStream.readObject());

        System.out.print(equals);

    }