Why doesn't "System.out.println" work in Android?

Question

I want to print something in console, so that I can debug it. But for some reason, nothing prints in my Android application.

How do I debug then?

public class HelloWebview extends Activity {
    WebView webview;    
    private static final String LOG_TAG = "WebViewDemo";
    private class HelloWebViewClient extends WebViewClient {
        @Override
        public boolean shouldOverrideUrlLoading(WebView view, String url) {
            view.loadUrl(url);
            return true;
        }
    }

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        webview = (WebView) findViewById(R.id.webview);
        webview.setWebViewClient(new HelloWebViewClient());
        webview.getSettings().setJavaScriptEnabled(true);
        webview.setWebChromeClient(new MyWebChromeClient());
        webview.loadUrl("http://example.com/");    
        System.out.println("I am here");
    }

Solution 1

Correction:

On the emulator and most devices System.out.println gets redirected to LogCat and printed using Log.i(). This may not be true on very old or custom Android versions.

Original:

There is no console to send the messages to so the System.out.println messages get lost. In the same way this happens when you run a "traditional" Java application with javaw.

Instead, you can use the Android Log class:

Log.d("MyApp","I am here");

You can then view the log either in the Logcat view in Eclipse, or by running the following command:

adb logcat

It's good to get in to the habit of looking at logcat output as that is also where the Stack Traces of any uncaught Exceptions are displayed.

The first Entry to every logging call is the log tag which identifies the source of the log message. This is helpful as you can filter the output of the log to show just your messages. To make sure that you're consistent with your log tag it's probably best to define it once as a static final String somewhere.

Log.d(MyActivity.LOG_TAG,"Application started");

There are five one-letter methods in Log corresponding to the following levels:

• e() - Error
• w() - Warning
• i() - Information
• d() - Debug
• v() - Verbose
• wtf() - What a Terrible Failure

The documentation says the following about the levels:

Verbose should never be compiled into an application except during development. Debug logs are compiled in but stripped at runtime. Error, warning and info logs are always kept.

Solution 2

Use the Log class. Output visible with LogCat

Solution 3

Yes it does. If you're using the emulator, it will show in the Logcat view under the System.out tag. Write something and try it in your emulator.

Solution 4

it is not displayed in your application... it is under your emulator's logcat

Solution 5

if you really need System.out.println to work(eg. it's called from third party library). you can simply use reflection to change out field in System.class:

try{
    Field outField = System.class.getDeclaredField("out");
    Field modifiersField = Field.class.getDeclaredField("accessFlags");
    modifiersField.setAccessible(true);
    modifiersField.set(outField, outField.getModifiers() & ~Modifier.FINAL);
    outField.setAccessible(true);
    outField.set(null, new PrintStream(new RedirectLogOutputStream()); 
}catch(NoSuchFieldException e){
    e.printStackTrace(); 
}catch(IllegalAccessException e){
    e.printStackTrace(); 
}

RedirectLogOutputStream class:

public class RedirectLogOutputStream extends OutputStream{
    private String mCache;

    @Override
    public void write(int b) throws IOException{
        if(mCache == null) mCache = "";

        if(((char) b) == '\n'){
            Log.i("redirect from system.out", mCache);
            mCache = "";
        }else{
            mCache += (char) b;
        }
    }
}

View more solutions

Comments

[Samuh]

Though you have got your answer below,I would like to add that output of SOP statements are directed to LogCat too: only that the tag name would be System.out

[Charlie Collins]

Before 0.9 System.out was lost, I think. After it was passed to the logcat output: code.google.com/p/android/issues/detail?id=92. (If you're using an existing library or such that uses System.out, right or wrong, it will show up in logcat with later versions of Android.)

[rDroid]

we can see the System.out.printlns inside the logcat.

[kaspermoerch]

Actually - System.out prints to LogCat through Log.i().

[David Webb]

@JosephEarl - Feel free to use the Edit button.

[Zorb]

there is also Log.wtf() :-)

[Byron Gavras]

Additional to this answer bear in mind that some times "something happens" and no input is printed with the usage of System.out. If this the case try to close and restart the emulator. This worked for me.

[PVS]

System.out.println does show in Android Studio, under Android Monitor. These are shown as "I/System.out"

[PaulMcG]

The new Console add-in library is also very useful.

[Admin]

When I click on Android Monitor and then logcat, I see many many messages constantly being printed... Is it supposed to be like this? Then how would I see my own debug messages?

[Valentin Filyov]

I just scroll through the other things and find it :D

[Zoe stands with Ukraine]

It's worth noting that Log.wtf can throw an exception when used. It shouldn't be used unless there is a case that never should happen in the first place

[Farid]

This is the very first sentence of the OP "I want to print something in console"

[Michał Jaroń]

There is a race when reading/writing variable LogUtil.log. You should use e.g. condition variable or monitor to synchronize variables and avoid active waiting loop.