Home > Articles

  • Print
  • + Share This
This chapter is from the book

8.6 Source-Level Annotation Processing

In the preceding section, you saw how to analyze annotations in a running program. Another use for annotation is the automatic processing of source files to produce more source code, configuration files, scripts, or whatever else one might want to generate.

8.6.1 Annotation Processors

Annotation processing is integrated into the Java compiler. During compilation, you can invoke annotation processors by running

javac -processor ProcessorClassName1,ProcessorClassName2,. . . sourceFiles

The compiler locates the annotations of the source files. Each annotation processor is executed in turn and given the annotations in which it expressed an interest. If an annotation processor creates a new source file, the process is repeated. Once a processing round yields no further source files, all source files are compiled.

An annotation processor implements the Processor interface, generally by extending the AbstractProcessor class. You need to specify which annotations your processor supports. In our case:

public class ToStringAnnotationProcessor extends AbstractProcessor
   public boolean process(Set<? extends TypeElement> annotations,
         RoundEnvironment currentRound)
      . . .

A processor can claim specific annotation types, wildcards such as "com.horstmann.*" (all annotations in the com.horstmann package or any subpackage), or even "*" (all annotations).

The process method is called once for each round, with the set of all annotations that were found in any files during this round, and a RoundEnvironment reference that contains information about the current processing round.

8.6.2 The Language Model API

Use the language model API for analyzing source-level annotations. Unlike the reflection API, which presents the virtual machine representation of classes and methods, the language model API lets you analyze a Java program according to the rules of the Java language.

The compiler produces a tree whose nodes are instances of classes that implement the javax.lang.model.element.Element interface and its subinterfaces: TypeElement, VariableElement, ExecutableElement, and so on. These are the compile-time analogs to the Class, Field/Parameter, Method/Constructor reflection classes.

I do not want to cover the API in detail, but here are the highlights that you need to know for processing annotations:

  • The RoundEnvironment gives you a set of all elements annotated with a particular annotation, by calling the method

    Set<? extends Element> getElementsAnnotatedWith(Class<? extends Annotation> a)
  • The source-level equivalent of the AnnotateElement interface is AnnotatedConstruct. Use the methods

    A getAnnotation(Class<A> annotationType)
    A[] getAnnotationsByType(Class<A> annotationType)

    to get the annotation or repeated annotations for a given annotation class.

  • A TypeElement represents a class or interface. The getEnclosedElements method yields a list of its fields and methods.

  • Calling getSimpleName on an Element or getQualifiedName on a TypeElement yields a Name object that can be converted to a string with toString.

8.6.3 Using Annotations to Generate Source Code

As an example, we will use annotations to reduce the tedium of implementing toString methods. We can’t put these methods into the original classes—annotation processors can only produce new classes, not modify existing ones.

Therefore, we’ll add all methods into a utility class ToStrings:

public class ToStrings
   public static String toString(Point obj)
      Generated code
   public static String toString(Rectangle obj)
      Generated code
   . . .
   public static String toString(Object obj)
      return Objects.toString(obj);

We don’t want to use reflection, so we annotate accessor methods, not fields:

public class Rectangle
   . . .
   @ToString(includeName=false) public Point getTopLeft() { return topLeft; }
   @ToString public int getWidth() { return width; }
   @ToString public int getHeight() { return height; }

The annotation processor should then generate the following source code:

public static String toString(Rectangle obj)
   StringBuilder result = new StringBuilder();
   return result.toString();

The “boilerplate” code is in gray. Here is an outline of the method that produces the toString method for a class with given TypeElement:

private void writeToStringMethod(PrintWriter out, TypeElement te)
   String className = te.getQualifiedName().toString();
   Print method header and declaration of string builder
   ToString ann = te.getAnnotation(ToString.class);
   if (ann.includeName())
      Print code to add class name
   for (Element c : te.getEnclosedElements())
      ann = c.getAnnotation(ToString.class);
      if (ann != null)
         if (ann.includeName())  Print code to add field name
         Print code to append toString(obj.methodName ())
   Print code to return string

And here is an outline of the process method of the annotation processor. It creates a source file for the helper class and writes the class header and one method for each annotated class.

public boolean process(Set<? extends TypeElement> annotations,
      RoundEnvironment currentRound)
   if (annotations.size() == 0) return true;
      JavaFileObject sourceFile = processingEnv.getFiler().createSourceFile(
      try (PrintWriter out = new PrintWriter(sourceFile.openWriter()))
         Print code for package and class
         for (Element e : currentRound.getElementsAnnotatedWith(ToString.class))
            if (e instanceof TypeElement)
               TypeElement te = (TypeElement) e;
               writeToStringMethod(out, te);
         Print code for toString(Object)
      catch (IOException ex)
               Kind.ERROR, ex.getMessage());
   return true;

For the tedious details, check the book’s companion code.

Note that the process method is called in subsequent rounds with an empty list of annotations. It then returns immediately so it doesn’t create the source file twice.

First compile the annotation processor, and then compile and run the test program as follows:

javac sourceAnnotations/ToStringAnnotationProcessor.java
javac -processor sourceAnnotations.ToStringAnnotationProcessor rect/*.java
java rect.SourceLevelAnnotationDemo

This example demonstrates how tools can harvest source file annotations to produce other files. The generated files don’t have to be source files. Annotation processors may choose to generate XML descriptors, property files, shell scripts, HTML documentation, and so on.

  • + Share This
  • 🔖 Save To Your Account

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