Debugging Frontend Applications

This document contains instructions and tips for debugging oVirt web applications built with Google Web Toolkit, an open source set of tools for building web applications using Java programming language. One GWT tool we’ll use in particular is Development Mode, which allows debugging GWT application without having to manually translate it to JavaScript.

This document assumes that you have Engine development environment and Java IDE configured accordingly.

Tip: Working Java IDE is required in order to debug GWT applications via Development Mode. This is due to JPDA being used as the debugging protocol between IDE and Development Mode.

GWT Development Mode

Launching Development Mode spawns a separate JVM instance (Java application) that executes GWT application code as bytecode, providing a bridge between web browser and Java IDE:

  • Java IDE connects to Development Mode to debug GWT application code, allowing to set breakpoints and debug code as if it was Java
  • browser connects to Development Mode via GWT Developer Plugin, passing instructions to Development Mode which executes them and sends the result back to browser

Following commands should be executed from within the Engine source directory, unless noted otherwise.

First, build Engine from source:

$ make clean install-dev PREFIX="$OVIRT_OUT" DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS="-Dgwt.userAgent=$GWT_USER_AGENT" BUILD_GWT_WEBADMIN="1" BUILD_GWT_USERPORTAL="1" [extra makefile options if necessary]
  • ` OVIRT_OUT` points to Engine build output directory
  • GWT_USER_AGENT specifies web browser(s) for which to build GWT application(s), supported values:
    • ie8 - Microsoft Internet Explorer 8 - UserPortal only, WebAdmin requires IE9+
    • ie9 - Microsoft Internet Explorer 9 and above
    • gecko1_8 - Mozilla Firefox
    • safari - Safari & Google Chrome
    • opera - Opera

Tip: Never use ie6 with gwt.userAgent, Microsoft Internet Explorer 6 and 7 are not supported by oVirt web applications.

Note that you can control GWT compilation using BUILD_GWT_WEBADMIN and BUILD_GWT_USERPORTAL flags. If such flag is set to 0 then the corresponding application won’t be available in Engine build. Since GWT compilation takes some time, make sure to disable GWT applications you’re not debugging.

For example, to build Engine with WebAdmin (excluding UserPortal) for Firefox and Chrome browsers:

$ make clean install-dev PREFIX="$HOME/ovirt-engine" DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS="-Dgwt.userAgent=gecko1_8,safari" BUILD_GWT_WEBADMIN="1" BUILD_GWT_USERPORTAL="0"

Tip: To avoid problems with GWT permutation selector script *.nocache.js being optimized-out, always build Engine for at least two browsers.

To start Development Mode, execute following commands:

  • GWT_APP can be one of following:
    • webadmin for debugging WebAdmin application
    • userportal-gwtp for debugging UserPortal application

For example, to start Development Mode for debugging WebAdmin in Chrome browser:

$ make gwt-debug DEBUG_MODULE="webadmin" DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS="-Dgwt.userAgent=safari"

Development Mode will inform you that it’s awaiting debug connection from your Java IDE:

[INFO] Listening for transport dt_socket at address: 8000

In your Java IDE, create new “Remote Java Application” debug configuration for localhost:8000 with classpath that includes related frontend projects.

IntelliJ Idea: “Run Edit Configurations”
  • Click “Add New Configuration” and choose “Remote”
  • On Configuration tab, set Host=”localhost” and Port=”8000”
Eclipse: “Run Debug Configurations”
  • Select “Remote Java Application” and click “New launch configuration”
  • On Connect tab, set Host=”localhost” and Port=”8000”
  • On Source tab, click “Add Java Project” and choose related frontend projects

After your IDE connects to Development Mode, Development Mode GUI (graphical window) will show up. Development Mode is now ready.

GWT Development Mode

Start a web browser (the one you’ve specified earlier through GWT_USER_AGENT) and navigate to GWT application’s debug URL:

  • WebAdmin:
  • UserPortal:

Tip: GWT application’s debug URL contains the suffix ?gwt.codesvr= which tells GWT Developer Plugin to connect to Development Mode at

If you open the debug URL for the first time, you will be prompted to install GWT Developer Plugin for the given web browser. Just proceed with plugin installation and restart the browser.

Next time you open the debug URL, GWT Developer Plugin will connect to Development Mode and new debugging session will be started for the given browser. This can take some time, please be patient and wait while the application gets loaded.

GWT Development Cycle

Change frontend application’s code while Development Mode is running, your IDE might prompt you to restart debug connection (i.e. hot swap failed) - just reconnect again.

Reload (refresh) GWT application in your browser in order for your changes to take effect.

GWT Draft Compile

Sometimes it’s necessary to profile or analyze GWT applications, e.g. fixing memory leaks or identifying performance bottlenecks in different web browsers. GWT compiler produces optimized and obfuscated JavaScript and CSS by default, which is hard to work with.

To compile oVirt web applications in draft mode, use DEV_BUILD_GWT_DRAFT makefile option:

$ make clean install-dev [usual makefile options] DEV_BUILD_GWT_DRAFT="1"

Tip: Do not use draft mode for regular Engine builds.

Compiling GWT application in draft mode has following effects:

  • prevent code obfuscation (useful when analyzing cause of runtime errors or other JavaScript issues)
  • prevent CSS class name obfuscation (useful when correlating CSS rules with HTML elements)
  • reduce the level of code optimizations (this maps to GWT compiler’s draftCompile option)

Frequently asked questions

Q: My web browser doesn’t prompt me to install GWT Developer Plugin.

A: Make sure your browser is officially supported by GWT Developer Plugin.

Q: The web page is blank after navigating to debug URL.

A: Make sure to build Engine from source prior to debugging for at least two different browsers.

Q: Client-side logs are not persisted on Engine, e.g. $JBOSS_HOME/standalone/log/engine/engine-ui.log.

A: At the moment, client-side logging works only when debugging GWT application via Development Mode. In future, we might consider adding infrastructure to support persisting client-side logs on server.

Q: I’m getting -bindAddress host "" unknown error message when launching Development Mode.

A: Using means that Development Mode will listen for incoming connections on all network interfaces, as opposed to which listens for incoming connections only on loopback interface available from local machine only. Make sure that host name is properly set in /etc/hosts. For Windows machines, check %windir%\system32\drivers\etc\hosts.

Q: I’m getting Exception: java.lang.OutOfMemoryError or similar error during GWT compilation.

A: You can tweak GWT compiler’s JVM arguments using DEV_EXTRA_BUILD_FLAGS makefile option, for example:

$ make clean install-dev [usual makefile options] DEV_EXTRA_BUILD_FLAGS="-Dgwt-plugin.extraJvmArgs="-Xms1024M -Xmx2048M -XX:PermSize=256M -XX:MaxPermSize=512M""