Garbage collector in java – set an object null

Garbage collection in Java is performed on the basis of “reachability”. The JLS defines the term as follows:

“A reachable object is any object that can be accessed in any potential continuing computation from any live thread.”

So long as an object is reachable¹, it is not eligible for garbage collection.

The JLS leaves it up to the Java implementation to figure out how to determine whether an object could be accessible. If the implementation cannot be sure, it is free to treat a theoretically unreachable object as reachable … and not collect it. (Indeed, the JLS allows an implementation to not collect anything, ever! No practical implementation would do that though².)

In practice, (conservative) reachability is calculated by tracing; looking at what can be reached by following references starting with the class (static) variables, and local variables on thread stacks.

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Here’s what this means for your question:

If i call: myTree = null; what really happens with the related TreeNode objects inside the tree? Will be garbage collected as well, or i have to set null all the related objects inside the tree object??

Let’s assume that myTree contains the last remaining reachable reference to the tree root.

1. Nothing happens immediately.
2. If the internal nodes were previously only reachable via the root node, then they are now unreachable, and eligible for garbage collection. (In this case, assigning null to references to internal nodes is unnecessary.)
3. However, if the internal nodes were reachable via other paths, they are presumably still reachable, and therefore NOT eligible for garbage collection. (In this case, assigning null to references to internal nodes is a mistake. You are dismantling a data structure that something else might later try to use.)

If myTree does not contain the last remaining reachable reference to the tree root, then nulling the internal reference is a mistake for the same reason as in 3. above.

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So when should you null things to help the garbage collector?

The cases where you need to worry are when you can figure out that that the reference in some cell (local, instance or class variable, or array element) won’t be used again, but the compiler and runtime can’t! The cases fall into roughly three categories:

1. Object references in class variables … which (by definition) never go out of scope.

2. Object references in local variables that are still in scope … but won’t be used. For example:

    public List<Pig> pigSquadron(boolean pigsMightFly) {
      List<Pig> airbornePigs = new ArrayList<Pig>();
      while (...) {
        Pig piggy = new Pig();
        ...
        if (pigsMightFly) {
          airbornePigs.add(piggy);
        }
        ...
      }
      return airbornePigs.size() > 0 ? airbornePigs : null;
    }
   

   In the above, we know that if pigsMightFly is false, that the list object won’t be used. But no mainstream Java compiler could be expected to figure this out.

3. Object references in instance variables or in array cells where the data structure invariants mean that they won’t be used. @edalorzo’s stack example is an example of this.

It should be noted that the compiler / runtime can sometimes figure out that an in-scope variable is effectively dead. For example:

public void method(...) {
    Object o = ...
    Object p = ...
    while (...) {
        // Do things to 'o' and 'p'
    }
    // No further references to 'o'
    // Do lots more things to 'p'
}

Some Java compilers / runtimes may be able to detect that ‘o’ is not needed after the loop ends, and treat the variable as dead.

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^{1 – In fact, what we are talking about here is strong reachability. The GC reachability model is more complicated when you consider soft, weak and phantom references. However, these are not relevant to the OP’s use-case.}

^{2 – In Java 11 there is an experimental GC called the Epsilon GC that explicitly doesn’t collect anything.}