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 href="{@docRoot}tools/debugging/index.html">Debugging</a></li>
</ol>

</div>
</div>

<p>If you're developing a web application for Android, you can debug your JavaScript
using the {@code console} JavaScript APIs, which 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}.</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>If you've implemented a custom {@link android.webkit.WebView} in your application, all the
same console APIs are supported when debugging your web page in WebView. On Android
1.6 and lower, console messages are automatically sent to logcat with the
"WebCore" logging tag. If you're targeting Android 2.1 (API Level 7) or higher, then you must
provide a {@link android.webkit.WebChromeClient}
that implements the {@link android.webkit.WebChromeClient#onConsoleMessage(String,int,String)
onConsoleMessage()} callback method, in order for console messages to appear in logcat.</p>

<p>Additionally, the {@link
android.webkit.WebChromeClient#onConsoleMessage(String,int,String)} method introduced in API
Level 7 has been deprecated in favor of {@link
android.webkit.WebChromeClient#onConsoleMessage(ConsoleMessage)} in API Level 8.</p>

<p>Whether you're developing for Android 2.1 (API Level 7) or Android 2.2 (API Level 8 or
greater), you must implement {@link android.webkit.WebChromeClient} and override the appropriate
{@link
android.webkit.WebChromeClient#onConsoleMessage(String,int,String) onConsoleMessage()} callback
method. Then, apply the {@link android.webkit.WebChromeClient} to your {@link
android.webkit.WebView} with {@link android.webkit.WebView#setWebChromeClient(WebChromeClient)
setWebChromeClient()}.

<p>Using 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>With API Level 8 or greater, your code for {@link
android.webkit.WebChromeClient#onConsoleMessage(ConsoleMessage)} might look like this:</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} 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>