Singleton:如何通过Reflection停止创建实例

Question

我知道在Java中,我们可以通过创建一个类的实例new,clone(),Reflection和serializing and de-serializing.

我创建了一个实现Singleton的简单类.

我需要一直停止创建我的类的实例.

public class Singleton implements Serializable{
    private static final long serialVersionUID = 3119105548371608200L;
    private static final Singleton singleton = new Singleton();
    private Singleton() { }
    public static Singleton getInstance(){
        return singleton;
    }
    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException("Cloning of this class is not allowed"); 
    }
    protected Object readResolve() {
        return singleton;
    }
    //-----> This is my implementation to stop it but Its not working. :(
    public Object newInstance() throws InstantiationException {
        throw new InstantiationError( "Creating of this object is not allowed." );
    }
}

在这个类我已经设法停止了类实例new,clone()并且serialization,但是我无法通过Reflection来阻止它.

我创建对象的代码是

try {
    Class<Singleton> singletonClass = (Class<Singleton>) Class.forName("test.singleton.Singleton");
    Singleton singletonReflection = singletonClass.newInstance();
} catch (ClassNotFoundException e) {
    e.printStackTrace();
} catch (InstantiationException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
}

Answer 1

尝试创建公共构造函数

private Singleton() {
    if( Singleton.singleton != null ) {
        throw new InstantiationError( "Creating of this object is not allowed." );
    }
}

Answer 2

像这样定义单例:

public enum Singleton {
    INSTANCE
}

Answer 3

检查构造函数怎么样:

private Singleton() {
    if (singleton != null) {
        throw new IllegalStateException("Singleton already constructed");
    }
}

当然,这可能并没有真正阻止它 - 如果有人正在使用反射来访问私有成员,他们可能能够将字段设置为null.你必须问自己,你想要阻止什么,以及它是多么有价值.

(编辑:正如Bozho提到的那样,即使通过反思也可能无法设置最终字段.如果有一些方法可以通过JNI等进行,我不会感到惊讶,但是......如果你给人们足够的访问权限,他们就能够做几乎任何事......)

Answer 4

private Singleton() { 
    if (Singleton.singleton != null) {
        throw new RuntimeException("Can't instantiate singleton twice");
    }
}

你应该注意的另一件事是readResolve(..)方法,因为你的类实现了Serialiable.你应该返回现有的实例.

但使用单身人士的最简单方法是通过枚举 - 你不必担心这些事情.