Home > Articles > Programming > Java

Practical Java Praxis 67: Do Not Rely on finalize Methods for Nonmemory Resource Cleanup

Because the garbage collector in Java frees only object mem-ory, the finalize method provides a way to free other resources. However, you can't rely on finalize methods for nonmemory resource cleanup. You should implement your own nonmemory resource cleanup mechanism that is used in conjunction with the class finalize method. Java expert Peter Haggar explains why.
Like this article? We recommend

The finalize method of a class is invoked by the JVM for an object of the class before the garbage collector reclaims its memory. This method is sometimes advertised as a way to ensure that nonmemory resources are freed before the memory for an object is reclaimed. Because the garbage collector frees only object memory, the finalize method provides a way to free other resources.

Programmers might place calls to close open sockets or file handles inside of a finalize method. The thinking is that this ensures that the program never runs out of these resources. In fact, the program can still run out of these resources regardless of the code in a finalize method.

The issue is that an object's finalize method is called before the garbage collector runs to free the storage for that object. Depending on the garbage collection algorithm used, possibly not all eligible objects will be collected on a given execution of the garbage collector. In addition, finalize methods are not guaranteed to run at predictable times. This is because of the likely asynchronous nature of finalization and garbage collection. Consequently, the finalize methods of objects are not guaranteed to run before the program terminates. This means that even though you have coded the finalize methods properly to free nonmemory resources, you can still deplete these resources before your program terminates.

One way to potentially avoid this problem is to use the System.runFinalization method. This method requests that the JVM invoke the finalize methods on all objects that have been identified as "finalizable" by a previous run of the garbage collector. Whether it does depends on the algorithms used in both the garbage collector and this method. However, there is no guarantee that invoking this method will run the finalize methods.

Another method previously available was the System.runFinalizersOnExit method, now deprecated. This method guaranteed only that the finalize method of all objects would run before the JVM exited. This meant that by the time the finalize method executed, you could already be out of resources. You typically want the finalize method to run during the execution of your program, not at the end, when the JVM is exiting. This method was deemed unsafe and is deprecated in Java 2.

Therefore, you cannot rely on the finalize method for an object being called. You should implement your own nonmemory resource cleanup mechanism that is used in conjunction with the class finalize method. You must ensure that classes that require such cleanup contain a public method that can be called to free the resources. This method should be called from the class finalize method to ensure that the nonmemory resources are freed if the finalize method is invoked. If the finalize method is not invoked, the user can call the public method provided by the class as a way to free these resources. Consider the following class:

class Communication
  private ServerSocket ss;
  private FileInputStream fileIn;
  public synchronized void cleanup() throws IOException
    if (ss != null)  //Check for null, so we don’t call close on an
{                //already closed socket.
      ss = null;
    if (fileIn != null)  //Ditto
      fileIn = null;

  protected void finalize() throws Throwable
    try {
    finally {
      super.finalize();  //Always call super.finalize in a finally

This class provides a finalize method that invokes the cleanup method. If the finalize method is called by the JVM, the nonmemory resources are freed. Because the cleanup method is public, it also can be called by other code at any time. For example, you might implement a resource pool management framework that includes a cleanup method.

Depending on the design of your system, you should call the cleanup method of an object at regular intervals to ensure that you do not run out of the resources that the class manages. Because you might call it frequently, you must code it to handle multiple invocations properly. For example, notice that the cleanup method code is careful to check that the object references are not null before it calls their close methods. Furthermore, after the close methods are called, the object references are set to null. This ensures that multiple invocations of the cleanup method do not result in multiple calls to the close method. In addition, the cleanup method is declared synchronized. This guarantees that multiple threads do not enter this method concurrently for the same object.

The finalize method is also careful to call super.finalize from a finally block. All finalize methods should call super.finalize to ensure that any superclass finalize methods are invoked. Unlike superclass constructors that are invoked automatically, finalize methods must be chained manually. The super.finalize call is made from a finally block to ensure that it is called regardless of whether the call to the cleanup method generates an exception.

Because you cannot guarantee if or when a finalize method is called, you should provide a public method to perform nonmemory resource cleanup. This method should also be called by the class finalize method.

About the Author

Peter Haggar is a senior software engineer with IBM in Research Triangle Park, North Carolina, and author of the best-selling book Practical Java, published by Addison-Wesley. Having worked on development tools, class libraries, and operating systems, he has a broad range of programming experience. At IBM, Peter works on emerging Java technology and, most recently, on real-time Java. He is also a frequent technical speaker on Java technology at numerous industry conferences. Peter received his bachelor of science degree in computer science from Clarkson University. He can be contacted at haggar@us.ibm.com.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020