枚举来实现单例模式
前面我们说到序列化和反序列化以及反射对单例都是有破坏的，下面我们介绍一种更加优雅的实现，也是effective java中推荐的实现方式，枚举实现单例模式。话不多说我们直接看代码吧。
public enum EnumInstance {
INSTANCE{
protected void printTest(){
System.out.println("redstar Print SingletonTest");
}
};
protected abstract void printTest();
private Object data;
public Object getData() {
return data;
}
public void setData(Object data) {
this.data = data;
}
public static EnumInstance getInstance(){
return INSTANCE;
}
}
然后我们看看测试类
public class SingletonTest {
public static void main(String[] args) throws IOException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
EnumInstance instance = EnumInstance.getInstance();
instance.setData(new Object());
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream("singleton_file"));
oos.writeObject(instance);
File file = new File("singleton_file");
ObjectInputStream ois = new ObjectInputStream(new FileInputStream(file));
EnumInstance newInstance = (EnumInstance) ois.readObject();
System.out.println(instance.getData());
System.out.println(newInstance.getData());
System.out.println(instance.getData() == newInstance.getData());
}
这里返回的结果是true，然后我们测试一下反射去获取这个对象。
public class SingletonTest {
public static void main(String[] args) throws IOException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
Class objectClass = EnumInstance.class;
Constructor constructor = objectClass.getDeclaredConstructor(String.class,int.class);
constructor.setAccessible(true);
EnumInstance instance = (EnumInstance) constructor.newInstance("redstar",666);
}
运行结果报异常
Exception in thread “main” java.lang.IllegalArgumentException: Cannot reflectively create enum objects at java.lang.reflect.Constructor.newInstance(Constructor.java:417)
我们点击进去查看一下417行的代码
if ((clazz.getModifiers() & Modifier.ENUM) != 0)
throw new IllegalArgumentException("Cannot reflectively create enum objects");
ConstructorAccessor ca = constructorAccessor;
从这里我们可以看到jdk底层就为我们对反射进行处理了。