java Singleton - 通过反射防止多次创建

java Singleton - prevent multiple creation through reflection

提问by user826323

I have a singleton like this.

我有一个这样的单身人士。

public class BookingFactory {

    private final static BookingFactory instance;

    static {
        instance = new BookingFactory();
    }

    public static BookingFactory getInstance() {
        return instance;
    }

    private BookingFactory() {
        System.out.println("Object is created.");
    }
}

public class Test  {
    BookingFactory instance = BookingFactory.getInstance();
    instance = BookingFactory.getInstance();

    Class<?> clazz = Class.forName("com.test.BookingFactory");

    Constructor pvtConstructor = clazz.getDeclaredConstructors()[0];

    // Set its access control
    pvtConstructor.setAccessible(true);

    // Invoke Private Constructor
    BookingFactory notSingleton = (BookingFactory) pvtConstructor.newInstance(null);
}

When I run this, I saw more than one printout message. Is there any way to prevent this singleton from being instantiated more than once from this reflection?

当我运行它时，我看到不止一条打印输出消息。有没有办法防止这个单例从这个反射中被实例化不止一次？

Thanks.

谢谢。

回答by laher

Try using an enum. enums make for good Singletons.

尝试使用enum. 枚举是好的单身人士。

public static enum BookingFactory {
    INSTANCE;
    public static BookingFactory getInstance() {
        return INSTANCE;
    }
}

You can't create an enum via reflection.

您不能通过反射创建枚举。

The getInstance() method is superfluous but makes it easier to run your test, throwing the following exception:

getInstance() 方法是多余的，但可以更轻松地运行测试，抛出以下异常：

java.lang.IllegalArgumentException: Cannot reflectively create enum objects
    at java.lang.reflect.Constructor.newInstance(Constructor.java:530)
    at MultiSingletonTest.main(MultiSingletonTest.java:40)

Oh look, someone already gave the enum answer. Posting anyway for more completeness.

哦，看，有人已经给出了枚举答案。无论如何发布以获得更多完整性。

回答by Bohemian

Make the assertion in the constructor:

在构造函数中进行断言：

private BookingFactory() {
    if (instance != null)
        throw new IllegalStateException("Only one instance may be created");
    System.out.println("Object is created.");
}

回答by simpatico

Adapted from Making the Java Singleton Reflection Proof when using Lazy Loading:

改编自Make the Java Singleton Reflection Proof when using Lazy Loading：

package server;

import java.lang.reflect.ReflectPermission;
import java.security.*;


public class JavaSingleton {

  private static JavaSingleton INSTANCE = null;

  private JavaSingleton() {
    ReflectPermission perm = new ReflectPermission("suppressAccessChecks", "");
    AccessController.checkPermission(perm); 
  }


  synchronized public static final JavaSingleton getInstance() {
    if (INSTANCE == null) {
      AccessController.doPrivileged(new PrivilegedAction<Object>() {
        public Object run() {
          INSTANCE= new JavaSingleton();
          return null;
        }
      });
    }
    return INSTANCE;
  }

The constructor has a check to see if the caller has access to it. As the link explains, a policy file that permits the Singleton class itself to call the constructor will need to be created.

构造函数检查调用者是否可以访问它。正如链接所解释的，需要创建一个允许 Singleton 类本身调用构造函数的策略文件。

Bohemian's method of throwing an exception does not prevent a client from reflectively calling the constructor before getInstance() is called. Even though it ensures that only one instance gets created, there's no guarantee that this is done by the Singleton class' getInstance()method.

Bohemian 抛出异常的方法不会阻止客户端在调用 getInstance() 之前反射性地调用构造函数。即使它确保只创建一个实例，也不能保证这是由 Singleton 类的getInstance()方法完成的。

The access control check will prevent this unwanted instantiation.

访问控制检查将防止这种不需要的实例化。

回答by Brian Roach

I highly recommend reading What is an efficient way to implement a singleton pattern in Java?- using an enum prevents what you're describing and is the recommended way to implement a singleton in java.

我强烈推荐阅读在 Java 中实现单例模式的有效方法是什么？- 使用枚举可以防止您所描述的内容，并且是在 java 中实现单例的推荐方法。

回答by Deepak

import java.io.Serializable;

public class Singleton implements Serializable,Cloneable{

private static final long serialVersionUID = 1L;
private static  Singleton singleton=null;
//private static volatile Singleton singleton=null;
private Singleton() {
    if(singleton!=null){
        throw new RuntimeException("Its Singleton Class use getInstance method for object creation");
    }
}

public static Singleton getInstance(){
    return Holder.singleton;

}

/****
 * good way for getting the instance. No need to worry about 
 * BillPughSingleton
 */
private static class Holder{
    private static final Singleton singleton=new Singleton();
}

/***
/*
 * Use this code for preventing Singleton breakage in multi threading scenario and comment above getInstance method
 * As this is the efficient way
 * If we put synchronized at method level level then will impact performance and will executed every time when getInstance is called
 * But if once the instance is created then there is no need for synchronized.
 */

/*  public static Singleton getInstance(){
    if(singleton==null){
        synchronized (Singleton.class) {
            if(singleton==null){
                singleton=new Singleton();
            }
        }

    }
    return singleton;

}*/

@Override
public Object clone() throws CloneNotSupportedException{
    /***
     * We can place below check OR we can remove the exception thrown check and return  singleton instead of super.clone()
     * Use any one way
     */
    if(singleton!=null){
        throw new RuntimeException("Its Singleton Class use getInstance method for object creation");
    }
    return super.clone(); 
}
/***
 * 
 * To Prevent breaking of singleton pattern by using serilization/de serilization
 */
private Object readResolve(){
    System.out.println("Read Resolve executed");
    return singleton;
}
}

** Testing singleton**

** 测试单例**

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/***
 * 
 * Ways to prevent break Singleton
 */
public class Main {

private static ObjectInputStream inputStream;

public static void main(String[] args) throws Exception {
    Singleton orginalSingletonObject = Singleton.getInstance();

    /***
     * Singleton is broken by using Reflection
     * We can prevent that by putting a check in private constructor of Singleton.java
     * 
     */
    breakSingletonByReflection(orginalSingletonObject);

    /***
     * By Serialization/De-Serialization break Singleton We need
     * Serialization interface in a class needs to be serialized like
     * Singleton.java
     * 
     * To prevent breaking of singleton we can add readResolve method in Singleton.java
     * readResolve is the method which returns the instance of the class when a serialized class is de serialized. 
     * So implement the readResolve method to return the same object. 
     *  Hence prevent breaking of Singleton design pattern.
     *  Refer this link for more information on readResolve 
     *  https://docs.oracle.com/javase/6/docs/platform/serialization/spec/input.html#5903
     */
    breakSingletonByserialization(orginalSingletonObject);

    /***
     * By Cloning break Singleton
     * We need to implement Cloneable interface
     * We can prevent that by putting a check in clone method of Singleton.java
     */
    breakSingletonByCloning(orginalSingletonObject);


    /***
     * Break Singleton By thread
     * This scenario is related to multi-threading environment
     * We can do this by putting double lock mechanism in Singleton.java and its good practice to use Volatile
     * We can also prevent this scenario of breaking by creating object eagerly but its not good to create object eagerly
     */

    breakSingletonByThreading(orginalSingletonObject);
}

private static void breakSingletonByThreading(Singleton orginalSingletonObject) {

    ExecutorService executorService=Executors.newFixedThreadPool(2);
    /**
     * Run this code snippet after commenting the other code for better understanding
     * Run it repeatly to create a condition when 2 threads enter the method getInstance() of Singleton class at a same time 
     * When 2 threads enter the getInstance method at same time they will get the singleton object as null (private static Singleton singleton in Singleton.java)
     * Then they will create two different objects ( have different hashcode) in this case singleton pattern will break.
     */
    executorService.submit(Main::useSingleton); // JAVA 8 syntax it will get the singleton instance
    executorService.submit(Main::useSingleton);
    executorService.shutdown();
}

public static void useSingleton(){
    Singleton singleton=Singleton.getInstance();
    printSingletonData("By Threading", singleton);

}




private static void breakSingletonByCloning(Singleton orginalSingletonObject) throws CloneNotSupportedException {
    Singleton clonedSingletonObject=(Singleton) orginalSingletonObject.clone();
    printSingletonData("By Cloning", orginalSingletonObject, clonedSingletonObject);
}

private static void breakSingletonByReflection(Singleton orginalsingleton)
        throws ClassNotFoundException, NoSuchMethodException,
        InstantiationException, IllegalAccessException,
        InvocationTargetException {

    Class<?> singletonClass = Class.forName("SingletonTest.Singleton");
    @SuppressWarnings("unchecked")
    Constructor<Singleton> constructor = (Constructor<Singleton>) singletonClass
            .getDeclaredConstructor();
    constructor.setAccessible(true);
    Singleton s = constructor.newInstance();
    printSingletonData("By Reflection", orginalsingleton, s);
}

private static void breakSingletonByserialization(Singleton orginalsingleton)
        throws FileNotFoundException, IOException, ClassNotFoundException {

    /**
     * Serialization
     */
    ObjectOutputStream outputStream = new ObjectOutputStream(new FileOutputStream("E:\Singleton.ser"));
    outputStream.writeObject(orginalsingleton);
    outputStream.close();

    /**
     * DeSerialization
     */
    inputStream = new ObjectInputStream(new FileInputStream("E:\Singleton.ser"));

    Singleton deserializeObject = (Singleton) inputStream.readObject();
    deserializeObject.hashCode();
    printSingletonData("By Serialization", orginalsingleton, deserializeObject);


}

public static void printSingletonData(String operationName,
        Singleton orginalsingleton, Singleton reflectionSigletonObject) {

    System.out.println("------------------------------------------");
    System.out.println("New Operation");
    System.out.println(operationName);
    System.out.println("orginal Hashcode=" + orginalsingleton.hashCode());
    System.out.println("New Object hashcode="
            + reflectionSigletonObject.hashCode());
    Boolean value = orginalsingleton.hashCode() != reflectionSigletonObject.hashCode();
    System.out.println("These Object have different hascode. They are two different object Right = "
                    + value);
    System.out.println("As these are different Object this means Singleton Pattern is broken");
}


private static void printSingletonData(String operationName,Singleton singleton) {


    System.out.println("------------------------------------------");
    System.out.println("New Operation");
    System.out.println(operationName);
    System.out.println("Object hashcode="   + singleton.hashCode());
    //System.out.println("As these are different Object this means Singleton Pattern is broken");

}

}

回答by DwB

If your singleton does not actually store state, then your best option is to not use a singleton. Instead, implement the factory as a static state-free method.

如果您的单身人士实际上并不存储状态，那么您最好的选择是不使用单身人士。相反，将工厂实现为静态无状态方法。