Java静态类生命周期

Question

How is a Java static class garbage collected?

I have a static class, which contains a static object. If I set the object to null then it is garbage collected. I don't understand the lifecycle of a static class. Is it garbage collected when the program closes?

Answer 1

Static objects are tied with the class definition and not with the class instance.

They come into effect as soon as your class is loaded through the ClassLoader and destroyed only when class is unloaded.

Answer 2

Static classes are always accessible and therefore never garbage collected.

Now, if your static class has as it's field and instance of a non-static class and that gets set to null, than that memory will be freed up because that object is no longer referenced.

for instance

public static StaticClass
{
    public static InstanceClass myInstanceClass;
}

if you set myInstanceClass to null, then it will be garbage collected.

Answer 3

- static class can only be at class level not at package level , we can say static inner classes or Top level classes .

- A static class is then loaded by the class loader.

- Now the static class will be into the memory till the loader that has loaded it is still running or unless its not unloaded by the loader.

Answer 4

Think of this way:

Every object has a reference to its class.

Every class has a reference to its class loader.

Every class loader has a reference to every class it has ever loaded .

It is possible that this whole ball of mess becomes garbage. This is the basis for app unloading/reloading (in a servlet container). However if there's even one object still referenced, the class loader and all its classes cannot be unloaded, therefore it is not an easy task to achieve app unloading.