Question: How Can We Avoid Singleton By Reflection Serialization And Cloning?

How do you break a singleton with reflection?

As it is not possible to call enum constructor from the program, it is not possible for us to access them by Reflection also.

Hence, reflection can’t break singleton property in case of enums.

Enum Singleton cannot be broken using Serialization and Enums are also by default thread-safe..

How do you protect Singleton from serialization?

As you can see, hashCode of both instances is different, hence there are 2 objects of a singleton class. Thus, the class is no more singleton. Overcome serialization issue:- To overcome this issue, we have to implement method readResolve() method. Above both hashcodes are same hence no other instance is created.

Can we override Singleton class?

The only way to override a singleton is to have a singleton that expects to be overridden. The simplest way to do this is to provide Singleton that implements an interface (or is otherwise fully abstract itself) that internally instantiates an injected singleton upon the first use of getInstance() .

How can we prevent Singleton class from cloning?

It is necessary if your Singleton class extends a class that has a visible clone() method defined in its hierarchy. You can avoid cloning by giving already created object by clone method.

Can we extend singleton class in Java?

All you need to extend a singleton class is a constructor with protected or package-default in the singleton class. If there are only private constructors you simply won’t be able to extend it. If there are public constructors then it’s not a singleton class.

Why Singleton is not thread safe?

In the above code, the getInstance() method is not thread-safe. Multiple threads can access it at the same time. For the first few threads when the instance variable is not initialized, multiple threads can enter the if loop and create multiple instances. It will break our singleton implementation.

Where is Singleton pattern used?

In terms of practical use Singleton patterns are used in logging, caches, thread pools, configuration settings, device driver objects. Design pattern is often used in conjunction with Factory design pattern. This pattern is also used in Service Locator JEE pattern.

What is the most common method signature to obtain a singleton?

The most popular approach is to implement a Singleton by creating a regular class and making sure it has: A private constructor. A static field containing its only instance. A static factory method for obtaining the instance.

Which is better singleton or static class?

While a static class allows only static methods and and you cannot pass static class as parameter. A Singleton can implement interfaces, inherit from other classes and allow inheritance. While a static class cannot inherit their instance members. So Singleton is more flexible than static classes and can maintain state.

Can constructor be private?

Yes, we can declare a constructor as private. If we declare a constructor as private we are not able to create an object of a class. We can use this private constructor in the Singleton Design Pattern.

How can we prevent immutable class from serialization?

If an immutable object was serialized, its raw bytes could be modified so that upon deserialization the object is no longer the same. This can’t be prevented completely. Encryption, checksums, and CRC’s will help to prevent this though. You should read Effective Java written by Joshua Bloch.

Why do we need Singleton class?

A singleton class shouldn’t have multiple instances in any case and at any cost. Singleton classes are used for logging, driver objects, caching and thread pool, database connections. … This is done by making the constructor private in java so that no class can access the constructor and hence cannot instantiate it.

Can we clone singleton class in Java?

clone() is protected method and as every class in java by default extends Object class, object of any class including our Singleton class can call clone() method. If somebody will call instance. … So, if we are not implementing Cloneable interface in our Singleton class, then we do not require to prevent cloning.

Why do we need serialization in Java?

Serialization is usually used When the need arises to send your data over network or stored in files. By data I mean objects and not text. … Serialization is the translation of your Java object’s values/states to bytes to send it over network or save it.

Is Singleton class immutable?

A singleton can be mutable or immutable; a non-singleton can be mutable or immutable. … Your Student class is approximately singleton, but not immutable: any class where you have a setter method that mutates a member variable cannot be immutable.

What is singleton class example?

Singleton Class in Java. In object-oriented programming, a singleton class is a class that can have only one object (an instance of the class) at a time. After first time, if we try to instantiate the Singleton class, the new variable also points to the first instance created.

How can we prevent Singleton pattern from reflection serialization and cloning?

There are many ways to prevent Singleton pattern from Reflection API, but one of the best solutions is to throw a run-time exception in the constructor if the instance already exists. In this, we can not able to create a second instance.

How can we avoid serialization methods?

To avoid Java serialization you need to implement writeObject() and readObject() method in your Class and need to throw NotSerializableException from those method.

Is Singleton class thread safe?

Thread Safe Singleton: A thread safe singleton in created so that singleton property is maintained even in multithreaded environment. To make a singleton class thread-safe, getInstance() method is made synchronized so that multiple threads can’t access it simultaneously.

What is Java Singleton pattern?

Singleton pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the java virtual machine. The singleton class must provide a global access point to get the instance of the class. Singleton pattern is used for logging, drivers objects, caching and thread pool.

Can we serialize Singleton class?

Conclusion: Singleton class can also be serialized by keeping readResolve() method in the Singleton class. This may be a familiar solution but just in case for reference. So if we execute above code we will get following behaviour: “it has created two objects and one static reference for INSTANCE.