Net debugging in WebView

Net log

WebView supports the kLogNetLog flag to log debugging network info to a JSON file on disk.

Please do not request netlogs from reporters

Important: at the moment, WebView netlog requires applying commandline flags. It's not typically possible for external reporters to apply commandline flags, so please do not ask them to follow this guide.

This guide is only for chromium developers who are set up for WebView development. Specifically, this guide requires the reader to use a userdebug or eng Android image, see device setup and commandline flags for more information.

Python script

If you have a chromium checkout, the preferred way to set the netlog flag is to use the script like so:

# Optional: set any flags of your choosing before running the script. Don't set
# --log-net-log though; this is set by
$ build/android/adb_system_webview_command_line --enable-features=MyFeature,MyOtherFeature
Wrote command line file. Current flags (in webview-command-line):
  005d1ac915b0c7d6 (bullhead-userdebug 6.0 MDB08M 2353240 dev-keys): --enable-features=MyFeature,MyOtherFeature

# Replace "<app package name>" with your app's package name (ex. the
# WebView Shell is "org.chromium.webview_shell"). This script will set an
# appropriate value for --log-net-log and handle setup/cleanup.
$ android_webview/tools/ --package="<app package name>"
Netlog will start recording as soon as app starts up. Press ctrl-C to stop recording.
Pulling netlog to "netlog.json"

Then import the JSON file (netlog.json in the working directory) into the NetLog viewer.

Manual steps

  1. Figure out the app's data directory
    # appPackageName is the package name of whatever app you're interested (ex.
    # WebView shell is "org.chromium.webview_shell").
    appDataDir="$(adb shell dumpsys package ${appPackageName} | grep 'dataDir=' | sed 's/^ *dataDir=//')" && \
  2. Pick a name for the JSON file. This must be under the WebView folder in the app‘s data directory (ex. jsonFile="${appDataDir}/app_webview/foo.json"). Note: it’s important this is inside the data directory, otherwise multiple WebView apps might try (and succeed) to write to the file simultaneously.
  3. Kill the app, if running
  4. Set the netlog flag:
    adb shell "echo '_ --log-net-log=${jsonFile}' > ${FLAG_FILE}"
  5. Restart the app. Reproduce whatever is of interest, and then kill the app when finished
  6. Get the netlog off the device:
    adb pull "${appDataDir}/app_webview/${jsonFile}"
    adb shell "rm '${appDataDir}/app_webview/${jsonFile}'"
  7. Optional: view the data in the NetLog viewer
  8. Optional: clear the commandline flags:
    adb shell "rm ${FLAG_FILE}"