When is the finalize() method called in Java?

Question

The finalize method is called when an object is about to get garbage collected. That can be at any time after it has become eligible for garbage collection.

Note that it's entirely possible that an object never gets garbage collected (and thus finalize is never called). This can happen when the object never becomes eligible for gc (because it's reachable through the entire lifetime of the JVM) or when no garbage collection actually runs between the time the object become eligible and the time the JVM stops running (this often occurs with simple test programs).

There are ways to tell the JVM to run finalize on objects that it wasn't called on yet, but using them isn't a good idea either (the guarantees of that method aren't very strong either).

If you rely on finalize for the correct operation of your application, then you're doing something wrong. finalize should only be used for cleanup of (usually non-Java) resources. And that's exactly because the JVM doesn't guarantee that finalize is ever called on any object.

In general it's best not to rely on finalize() to do any cleaning up etc.

According to the Javadoc (which it would be worth reading), it is:

Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.

As Joachim pointed out, this may never happen in the life of a program if the object is always accessible.

Also, the garbage collector is not guaranteed to run at any specific time. In general, what I'm trying to say is finalize() is probably not the best method to use in general unless there's something specific you need it for.

protected void finalize() throws Throwable {}

• every class inherits the finalize() method from java.lang.Object
• the method is called by the garbage collector when it determines no more references to the object exist
• the Object finalize method performs no actions but it may be overridden by any class
• normally it should be overridden to clean-up non-Java resources ie closing a file

• if overridding finalize() it is good programming practice to use a try-catch-finally statement and to always call super.finalize(). This is a safety measure to ensure you do not inadvertently miss closing a resource used by the objects calling class

  protected void finalize() throws Throwable {
       try {
           close();        // close open files
       } finally {
           super.finalize();
       }
   }
  

• any exception thrown by finalize() during garbage collection halts the finalization but is otherwise ignored

• finalize() is never run more than once on any object

quoted from: http://www.janeg.ca/scjp/gc/finalize.html

You could also check this article:

• Object finalization and cleanup

The Java finalize() method is not a destructor and should not be used to handle logic that your application depends on. The Java spec states there is no guarantee that the finalize method is called at all during the livetime of the application.

What you problably want is a combination of finally and a cleanup method, as in:

MyClass myObj;

try {
    myObj = new MyClass();

    // ...
} finally {
    if (null != myObj) {
        myObj.cleanup();
    }
}

This will correctly handle the situation when the MyClass() constructor throws an exception.