How to determine which object in Java is garbage?

An object is considered garbage in Java when there are no more references to it from active parts of the program. This occurs in the following cases:

• No references from root objects: Root objects include references from:

  • Local variables in active threads.
  • Static variables.
  • JNI references (objects accessible from native code).

• References only from other "garbage" objects: Even if object O1 references object O2, but neither O1 nor O2 have references from root objects, both objects are considered garbage.

Example:

// Root reference from a static variable
static Object staticObject = new Object();

// Root reference from a local variable of an active method
public void someMethod() {
    Object localObject = new Object(); // Reference from a local variable
    // After exiting SomeMethod, localObject becomes inaccessible
}

// Example of a "garbage" object
public Object createGarbage() {
    Object obj1 = new Object();
    Object obj2 = new Object();
    obj1 = obj2; // obj1 now references the same object as obj2
    // The original object that obj1 referenced is now inaccessible and considered garbage
    return obj2; // Returning a reference to a live object
}

The garbage collector in Java determines unreachable objects using the "reachability analysis" algorithm, starting from root objects. Objects that cannot be reached through a chain of references from root objects are considered unreachable and are eligible for collection.