Tag: code coverage

In July 2012 a until now rarely documented feature was added to GWT (related Git-Commit). It’s code coverage on the client side.

How it works

You can activate it via the System-Property “gwt.coverage”. The property-value must contain either all named Java source files separated by comma, or it must point to a text file containing all Java source files; one per line. Now the GWT compiler will instrument the corresponding JavaScript which is based on the named Java source files. Additionally the compiler registers an onUnLoad-Event, which stores the code coverage into LocalStorage of the browser.

Maven-Example

I have a simple GWT-Project which is build via Maven. In the main source (src/main/java/org/jajax/gwt/test/client/MyEntryPoint.java) I have the following file:

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package org.jajax.gwt.test.client;
 
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.Window;
 
public class MyEntryPoint implements EntryPoint {
 
   @Override
   public void onModuleLoad() {
       Window.alert("Hello world");
   }
}

Additionally I created a Textfile (target/gwt-coverage-sources.txt) with the following content:

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org/jajax/gwt/test/client/MyEntryPoint.java

Now only the configuration of GWT is missing. You have to add the Parameter gwt.coverage=target/gwt-coverage-sources.txt.

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<plugin>
    <groupid>org.codehaus.mojo</groupid>
    <artifactid>gwt-maven-plugin</artifactid>
    <version>2.7.0</version>
    <configuration>
        <extrajvmargs>-Dgwt.coverage=target/gwt-coverage-sources.txt</extrajvmargs>
    </configuration>
</plugin>

Now if you run you application, you can access the code coverage via JavaScript

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localStorge.getItem("gwt_coverage");

You will get a JSON with the following content:

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{"org/jajax/gwt/test/client/MyEntryPoint.java":{"6":1,"10":1}}

The inner object has for every line as key a value of 0 for a missing coverage or 1 for covered code.

Conclusion

This feature is very interesting, because the alternatives are very rare. The old option to use Emma / Jacoco is not functional anymore with the current versions of Java (1.8), GWT (2.7.0 / 2.8.0-SNAPSHOT), etc. So this feature seems to be a good alternative. Unfortunately there is no tool to extract the code coverage of Unit-Tests or Integration-Tests, but I will release a Tool, which connects the GWT code coverage with Jacoco Reports. So stay tuned!