Configuring GeckoView for Automation

How to set environment variables, Gecko arguments, and Gecko preferences for automation and debugging.

Configuring GeckoView

GeckoView and the underlying Gecko engine have many, many options, switches, and toggles “under the hood”. Automation (and to a lesser extent, debugging) can require configuring the Gecko engine to allow (or disallow) specific actions or features.

Some such actions and features are controlled by the GeckoRuntimeSettings instance you configure in your consuming project. For example, remote debugging web content via the Firefox Developer Tools is configured by GeckoRuntimeSettings.Builder#remoteDebuggingEnabled

Not all actions and features have GeckoView API interfaces. Generally, actions and features that do not have GeckoView API interfaces are not intended for broad usage. Configuration for these types of things is controlled by:

  • environment variables in GeckoView’s runtime environment

  • command line arguments to the Gecko process

  • internal Gecko preferences

Automation-specific configuration is generally in this category.

Running GeckoView with environment variables

After a successful ./mach build, ./mach run --setenv can be used to run GeckoView with the given environment variables.

For example, to enable extended logging for JSComponentLoader, run ./mach run --setenv MOZ_LOG=JSComponentLoader:5.

Reading configuration from a file

When GeckoView is embedded into a debugabble application (i.e., when your manifest includes android:debuggable="true"), by default GeckoView reads configuration from a file named /data/local/tmp/$PACKAGE-geckoview-config.yaml. For example, if your Android package name is com.yourcompany.yourapp, GeckoView will read configuration from:


Configuration file format

The configuration file format is YAML. The following keys are recognized:

  • env is a map from string environment variable name to string value to set in GeckoView’s runtime environment

  • args is a list of string command line arguments to pass to the Gecko process

  • prefs is a map from string Gecko preference name to boolean, string, or integer value to set in the Gecko profile

# Contents of /data/local/tmp/com.yourcompany.yourapp-geckoview-config.yaml

  MOZ_LOG: nsHttp:5

  - --marionette
  - --profile
  - "/path/to/gecko-profile"

prefs: true "string" 500

Verifying configuration from a file

When configuration from a file is read, GeckoView logs to adb logcat, like:

    GeckoRuntime  I  Adding debug configuration from: /data/local/tmp/org.mozilla.geckoview_example-geckoview-config.yaml
GeckoDebugConfig  D  Adding environment variables from debug config: {MOZ_LOG=nsHttp:5}
GeckoDebugConfig  D  Adding arguments from debug config: [--marionette]
GeckoDebugConfig  D  Adding prefs from debug config: {}

When a configuration file is found but cannot be parsed, an error is logged and the file is ignored entirely. When a configuration file is not found, nothing is logged.

Controlling configuration from a file

By default, GeckoView provides a secure web rendering engine. Custom configuration can compromise security in many ways: by storing sensitive data in insecure locations on the device, by trusting websites with incorrect security configurations, by not validating HTTP Public Key Pinning configurations; the list goes on.

You should only allow such configuration if your end-user opts-in to the configuration!

GeckoView will always read configuration from a file if the consuming Android package is set as the current Android “debug app” (see set-debug-app and clear-debug-app in the adb documentation). An Android package can be set as the “debug app” without regard to the android:debuggable flag. There can only be one “debug app” set at a time. To disable the “debug app” check, disable reading configuration from a file entirely. Setting an Android package as the “debug app” requires privileged shell access to the device (generally via adb shell am ..., which is only possible on devices which have ADB debugging enabled) and therefore it is safe to act on the “debug app” flag.

To enable reading configuration from a file:

adb shell am set-debug-app --persistent com.yourcompany.yourapp

To disable reading configuration from a file:

adb shell am clear-debug-app

Enabling reading configuration from a file unconditionally

Some applications (for example, web browsers) may want to allow configuration for automation unconditionally, i.e., even when the application is not debuggable, like release builds that have android:debuggable="false". In such cases, you can use GeckoRuntimeSettings.Builder#configFilePath to force GeckoView to read configuration from the given file path, like:

new GeckoRuntimeSettings.Builder()

Disabling reading configuration from a file entirely

To force GeckoView to never read configuration from a file, even when the embedding application is debuggable, invoke GeckoRuntimeSettings.Builder#configFilePath with an empty path, like:

new GeckoRuntimeSettings.Builder()

The empty path is recognized and no file I/O is performed.