Are Spring singleton beans thread-safe?

Question

I want to know whether the Spring singleton beans are thread-safe, if yes then why, if not then why?

As I am beginner with spring so help would be appreciated.

Answer 1

No. The two concepts are not even related.

Singletons are about creation. This design pattern ensures that only one instance of a class is created.

Thread safety is about execution. To quote Wikipedia:

A piece of code is thread-safe if it only manipulates shared data structures in a manner that guarantees safe execution by multiple threads at the same time.

So eventually thread safety depends on the code and the code only. And this is the reason why Spring beans are not thread safe per se.

Answer 2

I have different perception: Spring singleton beans are created once and there can be only one instance available at any point of time.

Lets say your have an instance variable which is modified in an non-synchronized method. In multi-threaded environment,same class instance will be assigned to all the threads and 2 concurrent threads can update/change the instance variables which may lead unexpected situation. Singleton beans does not provide thread safety and now you know that instance variables usage may lead to unexpected result, you have 2 options to solve the same :

1. Don't use instance variables in multithreaded environment. OR
2. Use synchronized block/keyword on methods wherever the instance variables are modified to avoid unexpected results.

Answer 3

Spring just manage the life cycle of singleton bean and maintains single instance of object. Thread safety has nothing to do with it.

if not then why?

Because singleton and thread safety are two different concepts. You can go for thread safety with synchronized keyword

Answer 4

Spring singleton beans are NOT thread-safe just because Spring instantiates them. Sorry.

Answer 5

Singleton Beans is thread safe or not depends on how the class whose scope is singleton is written. Each calling thread will have its own execution and does not interfere with another thread's execution unless there is some code in the singleton scoped class which is shared by all calling threads.e.g if a class has globally declared variables that is being accessed by its method and the values are modified then this may cause concurrency issue so it is better to have those variables at the method level and not at the class level.

Answer 6

Spring singleton bean define the single instance is created by BeanAwareFactory in spring IOC container. But when we try to use the same bean in multiple threads then the simultaneous access can cause unexpected behaviour as it not thread safe.

Singleton is about creating object instance in IOC container. So when running multiple thread then it’s about runtime behaviour.

And the value of same object can be performed by both threads as there’s no restriction on it for multiple thread accesst.

Answer 7

No , Spring singelton bean is not thread safe , Here is example

public class Emmloyee {

private int id;

private String name;

public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}


}

And here is applicationContext.xml

  <bean id="emp" class="com.manikant.Emmloyee" p:id="26"  p:name="Manikant Gautam">

And here is test class

   public class Test {

public static void main(String[] args) {

    ApplicationContext ctx=new ClassPathXmlApplicationContext("com/manikant/config.xml");
    Emmloyee emp=(Emmloyee)ctx.getBean("emp");
    System.out.println("User "+emp.getName() + " is of age "+emp.getId());
    emp.setName("Changed value");

    Emmloyee emp1=(Emmloyee)ctx.getBean("emp");
    System.out.println("User "+emp1.getName() + " is of age "+emp1.getId());


}

}

Here is output

      User Manikant Gautam is of age 26

      User Changed name is of age 26

change in value by emp.setName("Changed value"); reflects on different bean emp1 also .

Answer 8

According to my view, it totally thread safety of your singleton class totally depends on you have design/written your singleton class.

If there is any global variable defined the singleton class then it will not be thread safe because if multiple thread share the singleton object and execute the method which can updates the global variable it will not be thread safe.

To make any singleton class thread safe it is advised that make that class stateless means you should avoid defining the global variable in the singleton class.

Answer 9

Spring doesn't guarantee thread-safety. It will be your responsibility . Spring will create a Singleton , but if its mutable then it might not be thread safe. IT'S programmer responsibility to take care the spring bean class such way it should be thread safe.

Answer 10

In Spring, singleton beans will not have any state (stateless). Singleton bean scope ensures that single instance per BeanFactory. So in multi threading environment it will not assure the single instance even with singleton bean scope.

So to fix this, we need to change the bean scope from singleton scope to prototype scope as it is the developers responsibility to ensure the thread safety.

Answer 11

My service was called by upstream using parallel processing and it crashed my application. While thread 1 was working on beanA, another request arrived and it set some parameters of beanA. When my beanA was stored in database, I verified that it was a complete mess. I fixed it by using prototype.

Answer 12

if not then why?

becuase you could have a reference to non-threadsafe objects in your singleton.

But if you don't, and you use spring to inject any instance variables then yes they are thread safe.