Method finalize in Java

By calling the finalize() method (which inherits from Java.lang.Object), the JVM implements functionality similar to the C++ destructors used to clear memory before returning control to the operating system. This method is called when the object is destroyed by the garbage collector and by overriding finalize(), you can program the actions necessary for the correct deletion of the class instance - for example, closing network connections, database connections, unlocking files, etc.

After executing this method, the object must be re-collected by the garbage collector (and this is considered a serious problem with the finalize() method as it prevents the garbage collector from freeing memory). This method is not guaranteed to be called because the application can be terminated before garbage collection starts.

The object will not necessarily be immediately available for assembly - the finalize() method can store a reference to the object somewhere. Such a situation is called "reviving" an object and is considered an anti-pattern. The main problem with this trick is that you can only "revive" an object once.

Example:

public class MainClass {

    public static void main(String args[]) {
        TestClass a = new TestClass();
        a.a();
        a = null;
        a = new TestClass();
        a.a();
        System.out.println("!!! done");
    }
}
public class TestClass {

    public void a() {
        System.out.println("!!! a() called");
    }

    @Override
    protected void finalize() throws Throwable {
        System.out.println("!!! finalize() called");
        super.finalize();
    }
}

Since in this case the garbage collector may not be called (due to the simplicity of the application), the result of program execution is likely to be the following:

!!! a() called
!!! a() called
!!! done

Now let's complicate the program a little by adding a forced call to Garbage Collector:

public class MainClass {

    public static void main(String args[]) {
        TestClass a = new TestClass();
        a.a();
        a = null;
        System.gc(); // Forcibly call the garbage collector
        a = new TestClass();
        a.a();
        System.out.println("!!! done");
    }

}

As mentioned earlier, the Garbage Collector can run at different times, so the execution result may vary from run to run:

Option a:

!!! a() called
!!! a() called
!!! done
!!! finalize() called

Option b:

!!! a() called
!!! a() called
!!! finalize() called
!!! done

Read also:


Comments

Post a Comment

Popular posts from this blog

Methods for reading XML in Java

Image
DOM (Document Object Model) - object - reads XML, recreating it in memory in the form of an object structure, while the XML document is represented as a set of tags - nodes. Each node can have an unlimited number of child nodes. Each child can also contain several levels of descendants or none at all. Thus, the result is a kind of tree. Pluses: Easy to program. If there are many objects in XML with cross-references to each other, it is enough to go through the document twice: first time create objects without references and fill in the name-object dictionary, the second time - restore links. If an error occurs in XML, a half-created XML structure remains in memory, which will be automatically destroyed. Suitable for both reading and writing. Minuses: Low speed. Wastes a lot of memory. SAX (Simple API for XML) - event-driven - reads an XML document, responding to emerging events (opening or closing tag, string, attribute) by calling the event handlers provided by the appl...
Read more

XML, well-formed XML and valid XML

Image
XML, the eXtensible Markup Language, is a language with a simple formal syntax, well suited for creating and manipulating documents with programs, and at the same time easy to read and create documents by humans. XML is extensible, it does not fix the markup used in documents, and the developer is free to create markup in accordance with the needs of a particular area, being limited only by the syntax rules of the language. DTD A DTD, Document Type Definition, is a predefined set of rules that defines the relationships between elements and attributes. For example, the DTD for HTML says that the DIV tag must be inside the BODY tag and can appear multiple times, TITLE - in HEAD and only once, and SCRIPT - both there and there as many times as you want. A DTD is usually described directly in the document as a formulation line starting with <!DOCTYPE ... > or in a separate file. Difference between well-formed XML and valid XML Depending on the level of compliance, the document...
Read more

ArrayList and LinkedList in Java, memory usage and speed

Image
How much extra memory is needed when calling ArrayList.add()? If there is enough space in the array to accommodate a new element, then no additional memory is required. Otherwise, a new array is created, 1.5 times larger than the existing one (this is true for JDK above 1.7, in earlier versions the size of the increase is different). How much extra memory is allocated when you call LinkedList.add()? One new instance of the nested Node class is created. How much memory to store one byte primitive in LinkedList? Each LinkedList element stores a link to the previous element, the next element, and a data link. private static class Node<E> { E item; Node<E> next; Node<E> prev; //... } For 32-bit systems, each link is 32 bits (4 bytes). The object (header) of the nested Node class itself takes 8 bytes. 4 + 4 + 4 + 8 = 20 bytes, and since the size of each object in Java is a multiple of 8, so we get 24 bytes. A byte primitive occupies 1 by...
Read more