Overview

Duplicate code can be hard to find, especially in a large project. But PMD's Copy/Paste Detector (CPD) can find it for you! CPD has been through three major incarnations:

• First we wrote it using a variant of Michael Wise's Greedy String Tiling algorithm (our variant is described here)
• Then it was completely rewritten by Brian Ewins using the Burrows-Wheeler transform
• Finally, it was rewritten by Steve Hawkins to use the Karp-Rabin string matching algorithm.

Each rewrite made it much faster, and now it can process the JDK 1.4 java.* packages in about 4 seconds (on my workstation, at least).

Here's a screenshot of CPD after running on the JDK java.lang package.

Note that CPD works with Java, JSP, C, C++, Fortran and PHP code. Your own language is missing ? See how to add it here

CPD is included with PMD, which you can download here. Or, if you have Java Web Start, you can run CPD by clicking here.

Here are the duplicates CPD found in the JDK 1.4 source code.

Here are the duplicates CPD found in the APACHE_2_0_BRANCH branch of Apache (just the httpd-2.0/server/ directory).

Ant task

Andy Glover wrote an Ant task for CPD; here's how to use it:

<target name="cpd">
    <taskdef name="cpd" classname="net.sourceforge.pmd.cpd.CPDTask" />
    <cpd minimumTokenCount="100" outputFile="/home/tom/cpd.txt">
        <fileset dir="/home/tom/tmp/ant">
            <include name="**/*.java"/>
        </fileset>
    </cpd>
</target>
       

There's a ignoreLiterals="true" option which makes CPD ignore literal value differences when evaluating a duplicate block. This means that foo=42; and foo=43; will be seen as equivalent. You may want to run PMD with this option off to start with and then switch it on to see what it turns up. There's also a ignoreIdentifiers="true" option that does the same thing with identifiers; i.e., variable names, methods names, and so forth. The same guidelines apply. There's also an encoding option which allows the encoding used when parsing files to be specified; the default is based upon the locale. Finally, there's an optional language="cpp|java|php|ruby" flag to select the appropriate language; the default language is "java".

Also, you can get verbose output from this task by running ant with the -v flag; i.e.:

 ant -v -f mybuildfile.xml cpd
  

Also, you can get an HTML report from CPD by using the XSLT script in pmd/etc/xslt/cpdhtml.xslt. Just run the CPD task as usual and right after it invoke the Ant XSLT script like this:

<xslt in="cpd.xml" style="etc/xslt/cpdhtml.xslt" out="carview.php?tsp=cpd.html" />

Command line usage

To run CPD from the command line, just give it the minimum duplicate size and the source directory:

$ java net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /usr/local/java/src/java
           

You can also specify the language:

$ java net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /path/to/c/source --language cpp
           

You may wish to check sources that are stored in differents directories:

$ java net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /path/to/other/source  --files /path/to/other/source --files /path/to/other/source --language fortran
           

There should be no limit to the number of '--files', you may add... But if you stumble one, please tell us !

And if you're checking a C source tree with duplicate files in different architecture directories you can skip those using --skip-duplicate-files:

$ java net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /path/to/c/source --language cpp --skip-duplicate-files
           

You can also the encoding to use when parsing files:

$ java net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /usr/local/java/src/java --encoding utf-16le
           

You can also specify a report format - here we're using the XML report:

$ java net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /usr/local/java/src/java --format net.sourceforge.pmd.cpd.XMLRenderer
           

The default format is a text report, and there's also a net.sourceforge.pmd.cpd.CSVRenderer report.

Note that CPD is pretty memory-hungry; you may need to give Java more memory to run it, like this:

$ java -Xmx512m net.sourceforge.pmd.cpd.CPD --minimum-tokens 100 --files /usr/local/java/src/java