How to print to the console in GWT

Question

Quoting the documentation:

Adding GWT logging is really quite simple, as simple as the following code example. However — understanding how logging works, and how to correctly configure it is important, so please do take the time to read the rest of this document.

http://code.google.com/webtoolkit/doc/latest/DevGuideLogging.html

The simplest way to enable logging is:

# In your .gwt.xml file
<inherits name="com.google.gwt.logging.Logging"/>

# In your .java file
Logger logger = java.util.logging.Logger.getLogger("NameOfYourLogger");
logger.log(Level.SEVERE, "this message should get logged");

I needed to do this in the context of a GWT application that was deployed to an Android device/emulator via PhoneGap (and gwt-phonegap). Neither System.out.println() nor GWT logging as above (with module declaration) showed up in Android's logcat, so I resorted to a simple JSNI wrapper to console.log:

  public void onModuleLoad()
  {
    Logger logger = Logger.getLogger("Test1.java");
    logger.log(Level.INFO, "ash: starting onModuleLoad (1)"); // not in logcat
    System.out.println( "ash: starting onModuleLoad (2)" ); // not in logcat
    consoleLog( "ash: starting onModuleLoad (3)" ); // This shows up
    ...
  }

  native void consoleLog( String message) /*-{
      console.log( "me:" + message );
  }-*/;

In GWT version 2.6.0, method GWT.log writes message to browser console, you don't need to write native methods.

To log to browsers console you can do it using native, in a very simple way. Very helpful in debugging.

If you add a native method like in below, you can send a string to it from where you want and it will log it in the browsers console.

public static native void console(String text)
/*-{
    console.log(text);
}-*/;

For more information about using native in GWT: http://www.gwtproject.org/doc/latest/DevGuideCodingBasicsJSNI.html

Just summing up the different possibilities shown in the answer's of mreppy and Strelok in one snippet. I also added one possible workaround for IE exceptions as described here: Why does JavaScript only work after opening developer tools in IE once?

    java.util.logging.Logger logger = Logger.getLogger(this.getClass().getSimpleName());

    native void jsConsoleLog(String message) /*-{
        try {
            console.log(message);
        } catch (e) {
        }
    }-*/;

    private void log(final String message) {
        // Logs to Dev mode console only
        GWT.log(message);
        // Logs to Dev mode and JavaScript console (requires configuration)
        this.logger.log(Level.FINEST, message);
        // Logs to JavaScript console only
        jsConsoleLog(message);

Yet another variation using the native console...

Add this class:

package XXX.XXX.XXX.XXX;

public class Debug {
    private static boolean isEnabled_ = false;
    public static void enable() { isEnabled_ = true; }
    public static void setEnabled( final boolean isEnabled ) 
    { isEnabled_ = isEnabled; }

    public static void log( final String s ) 
    { if( isEnabled_ ) nativeConsoleLog( s ); }

    private static native void nativeConsoleLog( String s ) 
    /*-{ console.log( s ); }-*/;
}

Then, enable debugging with it at some point, like upon starting the app:

public class XXXXXX implements EntryPoint {
    @Override
    public void onModuleLoad() {
        Debug.enable();
        ...
    }
}

Then just use it like so:

Debug.log("Hello World!");