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JDoctest is an implementation of Python's doctest for Java, based on the ideas in doctestj and Rhino's doctests.

Unlike doctestj, JDoctest is a javadoc Taglet, rather than a Doclet. It doesn't replace the standard javadoc, it just adds a new @doc.test tag. We also use a multiline format similar to Python's doctest (based on Rhino's implementation) rather than insist on a single evaluated expression, as doctestj does.

Doctest blocks are written in Javascript. Calling from Javascript into Java is straightforward. An example from the top of the net.cscott.jdoctest.JDoctest class:

 * A "@doc.test" tag specifies an interactive javascript
 * session, in an environment where the class' package has been imported.
 * The output of the javascript session should match the output
 * provided.
 * @author C. Scott Ananian
 * @doc.test
 *   This is an example of a test which passes:
 *   js> "a".equals("a")
 *   true
 * @doc.test
 *   This is an example of a test which fails:
 *   js> 1+2
 *   5
 * @doc.test
 *   We can write tests which expect exceptions to be thrown, although
 *   it's a little clunky (implementing doctest.ELLIPSIS or
 *   doctest.IGNORE_EXCEPTION_DETAIL would make this nicer):
 *   js> try {
 *     >   java.lang.String("hi").charAt(3);
 *     > } catch (e) {
 *     >   print(e.javaException.getMessage())
 *     > }
 *   String index out of range: 3
 * @doc.test
 *   This demonstrates that the current package has been imported:
 *   js> JDoctest
 *   [JavaClass net.cscott.jdoctest.JDoctest]
 *   js> Version.PACKAGE_NAME
 *   jdoctest
 * @doc.test
 *   Note that results referencing object hashes are properly handled in
 *   the output comparison function, even when Rhino gives a different hash
 *   at runtime:
 *   js> o = new java.lang.Object()
 *   java.lang.Object@1d2068d
 *   js> o
 *   java.lang.Object@1d2068d
 *   js> new java.lang.Object()
 *   java.lang.Object@1ac2f9c
public class JDoctest implements Taglet { ... }

Build and install

To build:

  • You must have a JDK 1.5 or greater. In JDoctest version 1.5 I removed support for pre-1.5 JDKs.
    • In older JDKs, you might need to copy to and edit it to properly reflect that path to your JDK, so that we can find the JDK's tools.jar file.
  • Run ant javadoc, which will build the source code and then run its doctests.
  • One (ignored) failure is to be expected as the tests are run; it is included to show the results of a failing test in the output javadoc.

To install:

  • Move the generated jdoctest-*.jar as well as lib/rhino*/js.jar to a directory in your project. Below, we will assume that you've placed both these files in lib/jdoctest in your project.
  • If you intend to use the JUnit test support, you wil also need lib/junit-4.6.jar and the tools.jar from your JDK release in your classpath (see junit rules for ant below).

Command-line usage

To invoke from the command line:

 javadoc -taglet net.cscott.jdoctest.JDoctest \
         -tagletpath lib/jdoctest/jdoctest.jar:lib/jdoctest/js.jar:bin \
	 -J-ea \
         <your other javadoc options go here>

The above command-line assumes that your source code has been compiled into bin; change that part of the -tagletpath if your classes are elsewhere. Your compiled .class files must be included on the tagletpath so that we can run the doctests.

Note that you need to use the -J-ea option if you want assertions to be enabled during the evaluation of the doctests. You can use -J-ea:<your package>... to only enable assertions in your code (not in the entire javadoc tool).

Add -J-Dnet.cscott.jdoctest.output=<dir> to emit the javascript doctests into files in <dir> where they can be re-run standalone. This is helpful for debugging failing tests.

Ant integration

Ant rule:

  <javadoc failonerror="true" ...>
    <taglet name="net.cscott.jdoctest.JDoctest"
            path="lib/jdoctest/jdoctest.jar:lib/jdoctest/js.jar:bin" />
    <arg value="-J-ea" />
    <classpath> ...your classpath here... </classpath>

As before, replace bin with the appropriate path to your compiled project classes. The -J-ea option ensures assertions are enabled in your code when doctests are evaluated. Add:

    <arg value="-J-Dnet.cscott.jdoctest.output=your/dir/here" />

to dump the discovered doctests into your/dir/here for re-running standalone.

Pretty output

You may want to add css rules to make the output prettier. JDoctest has hooks to allow the use of google-code-prettify, which will do syntax highlighting on the client side. The build.xml file for JDoctest shows how this might be hooked up. The files in src/doc-files should be copied to a doc-files subdirectory of the top-level source directory of your project.

JUnit test support

There are two ways to hook up doctests to JUnit.

The simplest uses the net.cscott.jdoctest.output option described above: running net.cscott.jdoctest.RerunJDoctests as a JUnit test will rerun all emitted standalone tests. It assumes these tests were emitted into api/tests; if you prefer them someplace else, subclass RerunJDoctests and reimplement the listTests() method to call listTests(String dirName) with the appropriate alternate directory name.

The following ant rule implements this option:

    <target name="retest" ...>
	<formatter type="plain"/>
	<test name="net.cscott.jdoctest.RerunJDoctests" />
	<classpath refid="classpath.path" />

A more elegant (but slower) mechanism invokes Javadoc on each of your source files to extract and execute the doctests. Simply annotate each of your doctest-containing classes with the JUnit annotation:


The src/net/cscott/jdoctest/ file demonstrates how this is done. JUnit can then directly execute the doctests for your class. JDoctestRunner assumes that your sources are found below a directory named src; if your sources are elsewhere you can use the annotation:


If you need to use the @SrcRoot annotation extensively, you might find it easier to subclass JDoctestRunner and override its defaultSrcRoot() method.

The following ant rule implements this option:

    <target name="test" ...>
      <junit printsummary="yes" fork="yes" forkmode="once" haltonfailure="yes">
	<formatter type="plain"/>
	<test name="...your source files here..." />
	<classpath refid="classpath.path" />
	  <enable />

The fork="yes" option is necessary here, or else the embedded javadoc doesn't get the correct classpath: ant seems to use a classloader which knows the correct classpath but leaves the system property java.class.path set arbitrarily (ie, wrong), and the embedded javadoc looks at java.class.path, not its classloader, when resolving source references.

See also


JDoctest, a doctest module for javadoc.
Copyright (C) 2009,2014 C. Scott Ananian <>

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA


JDoctest is an implementation of Python's doctest for Java.




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