The Doclet API (also called the Javadoc API) provides a mechanism for clients to inspect the source-level structure of programs and libraries, including javadoc comments embedded in the source. This is useful for documentation, program checking, automatic code generation and many other tools.

Doclets are invoked by javadoc and use this API to write out program information to files. For example, the standard doclet is called by default and writes out documentation to HTML files.

The invocation is defined by the abstract {@link com.sun.javadoc.Doclet} class -- the entry point is the {@link com.sun.javadoc.Doclet#start(RootDoc) start} method: 

    public static boolean start(RootDoc root)
The {@link com.sun.javadoc.RootDoc} instance holds the root of the program structure information. From this root all other program structure information can be extracted.

Terminology

When calling javadoc, you pass in package names and source file names -- these are called the specified packages and classes. You also pass in Javadoc options; the access control Javadoc options (-public, -protected, -package, and -private) filter program elements, producing a result set, called the included set, or "documented" set. (The unfiltered set is also available through {@link com.sun.javadoc.PackageDoc#allClasses(boolean) allClasses(false)}.)

Throughout this API, the term class is normally a shorthand for "class or interface", as in: {@link com.sun.javadoc.ClassDoc}, {@link com.sun.javadoc.PackageDoc#allClasses() allClasses()}, and {@link com.sun.javadoc.PackageDoc#findClass(String) findClass(String)}. In only a couple of other places, it means "class, as opposed to interface", as in: {@link com.sun.javadoc.Doc#isClass()}. In the second sense, this API calls out four kinds of classes: {@linkplain com.sun.javadoc.Doc#isOrdinaryClass() ordinary classes}, {@linkplain com.sun.javadoc.Doc#isEnum() enums}, {@linkplain com.sun.javadoc.Doc#isError() errors} and {@linkplain com.sun.javadoc.Doc#isException() exceptions}. Throughout the API, the detailed description of each program element describes explicitly which meaning is being used.

A qualified class or interface name is one that has its package name prepended to it, such as java.lang.String. A non-qualified name has no package name, such as String.

Example

The following is an example doclet that displays information in the @param tags of the processed classes: 
import com.sun.javadoc.*;

public class ListParams extends Doclet {

    public static boolean start(RootDoc root) {
        ClassDoc[] classes = root.classes();
        for (int i = 0; i < classes.length; ++i) {
            ClassDoc cd = classes[i];
            printMembers(cd.constructors());
            printMembers(cd.methods());
        }
        return true;
    }

    static void printMembers(ExecutableMemberDoc[] mems) {
        for (int i = 0; i < mems.length; ++i) {
            ParamTag[] params = mems[i].paramTags();
            System.out.println(mems[i].qualifiedName());
            for (int j = 0; j < params.length; ++j) {
                System.out.println("   " + params[j].parameterName()
                    + " - " + params[j].parameterComment());
            }
        }
    }        
}
Interfaces and methods from the Javadoc API are marked in red. {@link com.sun.javadoc.Doclet Doclet} is an abstract class that specifies the invocation interface for doclets, {@link com.sun.javadoc.Doclet Doclet} holds class or interface information, {@link com.sun.javadoc.ExecutableMemberDoc} is a superinterface of {@link com.sun.javadoc.MethodDoc} and {@link com.sun.javadoc.ConstructorDoc}, and {@link com.sun.javadoc.ParamTag} holds information from "@param" tags.

This doclet when invoked with a command line like: 

    javadoc -doclet ListParams -sourcepath <source-location> java.util
producing output like: 
    ...
    java.util.ArrayList.add
       index - index at which the specified element is to be inserted.
       element - element to be inserted.
    java.util.ArrayList.remove
       index - the index of the element to removed.
    ...
@see com.sun.javadoc.Doclet @see com.sun.javadoc.RootDoc