docs/html/guide/webapps/debugging.jd

page.title=Debugging Web Apps
@jd:body

<div id="qv-wrapper">
<div id="qv">
<h2>Quickview</h2>
<ul>
  <li>You can debug your web app using console methods in JavaScript</li>
  <li>If debugging in a custom WebView, you need to implement a callback method to handle debug
messages</li>
</ul>

<h2>In this document</h2>
<ol>
  <li><a href="#Browser">Using Console APIs in the Android Browser</a></li>
  <li><a href="#WebView">Using Console APIs in WebView</a></li>
</ol>

<h2>See also</h2>
<ol>
  <li><a class="external-link"
href="https://developers.google.com/chrome-developer-tools/docs/remote-debugging">Remote
Debugging on Android</a></li>
  <li><a href="{@docRoot}tools/debugging/index.html">Debugging</a></li>
</ol>

</div>
</div>

<p>If you are testing your web app with a device running Android 4.4 or higher,
you can remotely debug your web pages in {@link android.webkit.WebView} with
Chrome Developer Tools, while continuing to support older versions of Android.
For more information, see <a class="external-link"
href="https://developers.google.com/chrome-developer-tools/docs/remote-debugging">Remote
Debugging on Android</a>.</p>

<p>If you don't have a device running Android 4.4 or higher, you can debug your JavaScript using the
{@code console} JavaScript APIs and view the output messages to logcat. If you're familiar with
debugging web pages with Firebug or Web Inspector, then you're probably familiar
with using {@code console} (such as {@code console.log()}). Android's WebKit framework supports most
of the same APIs, so you can receive logs from your web page when debugging in Android's Browser
or in your own {@link android.webkit.WebView}. This document describes how to use the
console APIs for debugging.</p>


<h2 id="Browser">Using Console APIs in the Android Browser</h2>

<div class="sidebox-wrapper">
<div class="sidebox">
  <h2>Logcat</h2>
  <p>Logcat is a tool that dumps a log of system messages. The messages include a stack trace when
the device throws an error, as well as log messages written from your application and
those written using JavaScript {@code console} APIs.</p>
  <p>To run logcat and view messages, execute
{@code adb logcat} from your Android SDK {@code tools/} directory, or, from DDMS, select
<strong>Device > Run logcat</strong>. When using the <a href="{@docRoot}tools/sdk/eclipse-adt.html">ADT
plugin for Eclipse</a>, you can also view logcat messages by opening the Logcat view, available from
<strong>Window > Show View > Other > Android > Logcat</strong>.</p>
  <p>See <a href="{@docRoot}tools/debugging/debugging-log.html">Debugging</a>
  for more information about <codelogcat</code>.</p>
</div>
</div>

<p>When you call a {@code console} function (in the DOM's {@code window.console} object),
the output appears in logcat. For example, if your web page executes the following
JavaScript:</p>
<pre>
console.log("Hello World");
</pre>
<p>Then the logcat message looks something like this:</p>
<pre class="no-pretty-print">
Console: Hello World http://www.example.com/hello.html :82
</pre>

<p>The format of the message might appear different depending on which version of Android you're
using. On Android 2.1 and higher, console messages from the Android Browser
are tagged with the name "browser". On Android 1.6 and lower, Android Browser
messages are tagged with the name "WebCore".</p>

<p>Android's WebKit does not implement all of the console APIs available in other desktop browsers.
You can, however, use the basic text logging functions:</p>
<ul>
  <li>{@code console.log(String)}</li>
  <li>{@code console.info(String)}</li>
  <li>{@code console.warn(String)}</li>
  <li>{@code console.error(String)}</li>
</ul>

<p>Other console functions don't raise errors, but might not behave the same as what you
expect from other web browsers.</p>



<h2 id="WebView">Using Console APIs in WebView</h2>

<p>All the console APIs shown above are also
supported when debugging in {@link android.webkit.WebView}. 
If you're targeting Android 2.1 (API level 7) and higher, you must
provide a {@link android.webkit.WebChromeClient}
that implements the {@link android.webkit.WebChromeClient#onConsoleMessage(String,int,String)
onConsoleMessage()} method in order for console messages to appear in logcat.
Then, apply the {@link android.webkit.WebChromeClient} to your {@link
android.webkit.WebView} with {@link android.webkit.WebView#setWebChromeClient(WebChromeClient)
setWebChromeClient()}.

<p>For example, to support API level 7, this is how your code for {@link
android.webkit.WebChromeClient#onConsoleMessage(String,int,String)} might look:</p>

<pre>
WebView myWebView = (WebView) findViewById(R.id.webview);
myWebView.setWebChromeClient(new WebChromeClient() {
  public void onConsoleMessage(String message, int lineNumber, String sourceID) {
    Log.d("MyApplication", message + " -- From line "
                         + lineNumber + " of "
                         + sourceID);
  }
});
</pre>

<p>However, if your lowest supported version is API level 8 or higher, you should instead
implement {@link android.webkit.WebChromeClient#onConsoleMessage(ConsoleMessage)}. For example:</p>

<pre>
WebView myWebView = (WebView) findViewById(R.id.webview);
myWebView.setWebChromeClient(new WebChromeClient() {
  public boolean onConsoleMessage(ConsoleMessage cm) {
    Log.d("MyApplication", cm.{@link android.webkit.ConsoleMessage#message()} + " -- From line "
                         + cm.{@link android.webkit.ConsoleMessage#lineNumber()} + " of "
                         + cm.{@link android.webkit.ConsoleMessage#sourceId()} );
    return true;
  }
});
</pre>

<p>The {@link android.webkit.ConsoleMessage} also includes a {@link
android.webkit.ConsoleMessage.MessageLevel MessageLevel} object to indicate the type of console
message being delivered. You can query the message level with {@link
android.webkit.ConsoleMessage#messageLevel()} to determine the severity of the message, then
use the appropriate {@link android.util.Log} method or take other appropriate actions.</p>

<p>Whether you're using {@link
android.webkit.WebChromeClient#onConsoleMessage(String,int,String)} or {@link
android.webkit.WebChromeClient#onConsoleMessage(ConsoleMessage)}, when you execute a console method
in your web page, Android calls the appropriate {@link
android.webkit.WebChromeClient#onConsoleMessage(String,int,String)
onConsoleMessage()} method so you can report the error. For example, with the example code above,
a logcat message is printed that looks like this:</p>

<pre class="no-pretty-print">
Hello World -- From line 82 of http://www.example.com/hello.html
</pre>