弹簧注入，全局变量

Question

Probably my question is wrong and I can't do this way, but I can't find a good explanation.

Here are my classes and my injections with Spring:

public class A
{
    @Autowired
    protected B b;

    public void doSomething(Integer i)
    {
        b.doOtherStuff(i);
    }
}

And here is my B class:

public class B
{
    private Integer field;
    private List<SomeOtherClass> list = new ArrayList<>();

    @Autowired
    protected C c;

    public void doOtherStuff(Integer in)
    {
        field = in;
        list = c.getMyList(i);
    }
}

What if I'll have two concurrent calls to a.doSomething(..)? The second call will overwrite the private fields in B and probably the values will not be corret in one of the two executions? Am I wrong?

What if someone calls a.doSomething() and terminates, later B is used (via injection) in another class? Can this last class have an instance of B initialized precedently?

Let me know if my question are unclear, I'm not so good in Java and English too :D

Answer 1

What if I'll have two concurrent calls to a.doSomething(..)? The second call will overwrite the private fields in B and probably the values will not be corret in one of the two executions? Am I wrong?

list property will be assigned twice. And if B#doOtherStuff makes more than just assigning the value of list , then you'll most probably get ConcurrentModificationException or maybe unexpected results.

When you have this scenario when a bean needs to maintain mutable state that depends on execution, then your bean should not be in Singleton scope. The best option would be to have the bean declared as prototype , which is created every time by the Spring IoC container.

It is important to know that if you have a prototype bean injected into a singleton bean, then the IoC container will only create a single instance of the prototype bean, thus making it virtually singleton. To avoid such behavior, you may ask for the ApplicationContext to retrieve a new instance per method execution:

@Component
public class A {
    @Autowired
    private ApplicationContext ctx;

    public void doSomething(Integer i) {
        //ApplicationContext will create the bean using the IoC container
        B b = ctx.getBean(B.class);
        b.doOtherStuff(i);
    } 
}

More info:

• Spring beans scope

Answer 2

You are right in your assumption that the private variables are a problem in your beans. You might want to read up a bit on bean scopes . The default bean scope is singleton, which means only one instance will life for all calls.

This will also mean that your private variables are shared through all calls that you make to the bean. So in class B the variable field would be overwritten by each call to the method doOtherStuff .

In general practice it is bad to have private variables in Spring beans, unless you change the scope. There are some exceptions to this, for example some caching or any other shared and thread safe logic.