Home > Articles > Programming > Java

This chapter is from the book

3.7 Custom Metadata

The type information stored in a Java class file is very thorough, as far as it goes. When you first move to Java from a nonreflective programming environment, the new possibilities seem limitless. Knowing the names and types of all methods and fields makes it easy to implement all sorts of runtime services for your Java objects: XML views, object/relational mappings, generic user interfaces, and on and on. Nevertheless, it is possible to imagine wanting even more metadata.

Consider the hypothetical LaunchVehicle interface shown in Listing 3–27. As a human reader, you can infer several important details about how to use this interface. For example, you know to use liters when you addFuel. From your knowledge of the problem domain, you know that you should always countdown before you launch. These are important, contractual elements of the interface, but they do not have a standard language representation and are not part of the class metadata. You cannot count on clients always getting these details correct. Even if you could, some other important details of the design are not obvious to the reader. What units are to be used when calling thrust? Is it acceptable to addFuel during the countdown? This example illustrates the need for two kinds of metadata not available to the Java language: the correct units for numeric arguments, and tables of state transitions allowed by an interface. If these metadata elements were added to Java, the virtual machine could enforce the rules for you, eliminating two more sources of program errors.

Listing 3–27 The LaunchVehicle Interface

public interface LaunchVehicle {
 public void addFuel(int liters);
 public void countdown(int seconds);
 public void launch(int thrust);

It would be unreasonable to expect a Java virtual machine to support every possible flavor of useful class metadata. Virtual machines would need to be far more complex than they are today, and classes would carry around enormous amounts of metadata not useful to their problem domain. It was good design to limit the scope of the virtual machine's responsibilities; the line had to be drawn somewhere.

Fortunately, the virtual machine specification offers a hook for customization by permitting the addition of custom attributes to the class file format. Attributes can be any binary data, and they are housed in a data structure called an attribute_info. The attribute_info structure, shown in Listing 3–28, contains a constant pool index to a string that names the attribute, plus an opaque array of bytes containing the attribute's data. You can attach attributes to classes, methods, fields, or even to the bytecodes that implement a method.

Listing 3–28 The attribute_info Structure

//pseudocode from the JVM spec
attribute_info {
 u2 attribute_name_index; //reference to constant pool
 u4 attribute_length;
 u1 info[attribute_length]; //custom data

Figure 3–6 is an expanded view of the binary class format diagram, originally presented in Figure 3–1, with custom attributes shown below the solid line. The virtual machine spec already defines some standard attributes for its own use. The bytecodes that implement a method are stored as an attribute, which can in turn have custom subattributes. The standard attributes defined by the specification are listed in Table 3–3.

Figure 3–6 Custom metadata in the binary class format

Table 3–3 Attributes Defined by the Virtual Machine Specification

Attribute Name

Attibute Purpose


Holds bytecodes that implement a method


Holds value used to initialize a constant field


Lists checked exceptions a method may throw


Links nested classes and outer classes


Marks methods not present in original source file


Holds name of original source file


Maps bytecode offsets to source line numbers


Maps variables to source variable names


Marks deprecated class, field, or method

The Code, ConstantValue, and Exceptions attributes contribute to the documented semantics of class files and must be understood by conformant virtual machine implementations. The InnerClass and Synthetic attributes contribute to the semantics of the core API libraries and will therefore also be understood by compliant virtual machines.

The remaining standard attributes provide useful information, but they are not essential for the correct functioning of the language. For example, the SourceFile, LineNumberTable, and LocalVariableTable attributes enable source-level debuggers. Because these attributes are optional, they can be silently ignored by virtual machines that do not understand them. Any custom attributes that you develop must also be optional.

If a custom attribute were necessary for a class to function correctly, then a standard virtual machine would be unable to load a class that relied on the custom attribute. Such a custom attribute would encourage developers to write nonportable Java classes, defeating the write-once, run-anywhere nature of the language. It is legal to write custom tools that manipulate or even mandate custom attributes, but virtual machine implementations must silently ignore attributes that they do not recognize. The rules limit custom attributes to optional data that adds value when it is recognized by the virtual machine but does not break functionality when it is not. Even when they are operating within this constraint, custom attributes have many uses. They can store domain-specific metadata, debugging information, profiling information, documentation, and JIT optimization hints or hints that support interoperation with other programming environments.

Standard Java compilers will not emit your custom attributes, and the Reflection API provides no support for accessing them. If you want to define and use custom attributes, you will need to develop a development tool that injects attributes, a custom class loader that tracks attributes as classes are loaded, and extensions to reflection that can access these attributes at runtime.

Custom attributes are not in wide use today because they receive only limited support from the standard Java tools. Java compilers typically do not emit custom attributes, which is not surprising since there is no Java syntax for defining them. Programmers who want to add custom attributes must write their own tools to crack the class file format and inject attributes. Such tools are easy to write, and many are freely available, but none have the blessing of being standardized.

You are also on your own for extracting custom attributes. The Reflection API currently does not define methods for extracting custom attributes, although such methods are easy to design and add. Listing 3–29 shows part of the Java Class File Editor (JCFE). JCFE is an open source library developed by the author for manipulating custom attributes; see [JCFE] for details. The ClassEx augments the standard java.lang.Class class to include access to custom attributes, and the Attribute base class (not shown here) can be used as a subclass for different custom attributes that you design.

Listing 3–29 JCFE Methods for Custom Attributes

package com.develop.reflect;
public class ClassEx {
 public void addAttribute(Attribute ca); 
 public Attribute[] getAttributes(String name);
 public Attribute[] getAttributes();

In order to access custom attributes at runtime, you need a custom class loader to extract them. A class loader sees the class file as a byte array during findClass, which provides a hook for manually parsing the class file and remembering any custom metadata. §5.5 develops a full example of this technique, using custom attributes and a custom class loader to automatically locate the correct version of a Java class.

Because the virtual machine must already parse the binary class format, parsing the class file a second time in a custom loader is inefficient. In an ideal world, access to custom attributes belongs in the platform, not in custom loaders. Hopefully a future version of Java will move custom attribute support into the core API.

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