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Debugging Android Applications : Eclipse Java Editor (part 3) - Logcat

1/1/2013 5:40:42 PM
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3. Logcat

Granted, the errors we debugged in the last section were pretty straightforward—no different from debugging in any other environment. But most applications are not as simple as DebugTest, and many problems are much harder to isolate and solve. Android provides a general-purpose logging facility that can help with many of those more difficult problems.

As mentioned before, there’s a logcat pane on the Debug perspective (it’s also in the DDMS perspective). The log for DebugTest isn’t very interesting, so instead start MJAndroid in Debug mode and we’ll take a look at its log.

After the application comes up in the emulator, Eclipse switches to the Debug Perspective and shows the logcat pane on the lower right, as it looks in Figure 10.

Figure 10. Logcat pane, minimized

To make the pane large enough to be useful for reading the log, click on the “full screen” symbol at the upper right of the pane, and it will expand to fill the Eclipse window. You will then see that there are hundreds of log messages in the file, going back to when Eclipse first started the instantiation of the emulator that you are using, continuing through the boot process for Android, loading all the applications, and finally loading and executing MJAndroid. How are you supposed to find anything useful in all of that?

Luckily, Android provides you with some handy filters to apply to the logfile. See the V, D, I, W, and E symbols in the toolbar? These filters successively narrow the scope of messages displayed, as follows:

V (Verbose)

Show everything

D (Debug)

Show Debug, Information, Warning, and Error messages (equivalent to V for now)

I (Information)

Show Information, Warning, and Error messages

W (Warning)

Show Warning and Error messages

E (Error)

Show only Error messages

The columns displayed for the log are:


The time the log entry was made

Priority (the column is not actually labeled)

One of the log entry types from the previous list (D, I, W, or E)


The Linux process ID of the process making the entry


A short tag describing the source of the entry


The log entry itself

About two-thirds of the way through the log (if you started a new emulator when you brought up MJAndroid), you’ll see a message entry something like:

11-28 12:10:31.475: INFO/ActivityManager(52): Start proc com.microjobsinc.mjandroid 
   for activity com.microjobsinc.mjandroid/.MicroJobs:
                    pid=163 uid=10017 gids={3003}


which actually appears all on one line; we’ve broken it here so it will fit on a printed page.

This is a log message from the Activity Manager telling us that it started MicroJobs with process ID 163 (it will probably be different as you run it). If you click on the green cross at the top of the logcat pane, it will let you define a custom filter. Fill in a random name and the pid number that you saw in the log. Now the log is filtered to show only the messages that apply to this instance of MicroJobs. There are likely still a lot of messages, which you can filter further (using the D, I, W, and E buttons) or just scan.

If you ask other people for help debugging an error in your own program, one of the first things you’ll likely be asked for is a copy of the logcat output. You can easily extract the content of the logfile to a text file by selecting what you’d like to preserve and clicking on the little down arrow at the upper right of the logcat pane, which brings down a pull-down menu. One of the selections on the menu is “Export Selection as Text...”, which takes you to a dialog where you can name an output file for the log text.

3.1. Looking at logcat to solve runtime errors

Logcat gives you a lot of information about what happened as Android tried to run your program. It is very useful when you get a generic error message from Android that doesn’t tell you much. Let’s demonstrate one of my (least) favorites.

In Eclipse, go to main.xml for MJAndroid and remove the apiKey line under the MapView declaration (save it in a text file or somewhere, so you can restore it; we’re doing this just to generate an error). The apiKey is needed to access mapping information, so removing it brings the program to a screeching halt. When you run the program, the emulator screen looks like Figure 11.

Figure 11. “Stopped unexpectedly” message

Although it’s good to know that the application stopped, the message tells us very little about why. If you now look at the logcat output in the Debug perspective (or the DDMS perspective), you’ll find something like this after MicroJobs starts up, all in red type (we’ve left off the first few columns so it will fit):

java.lang.RuntimeException: Unable to start activity 
                     android.view.InflateException: Binary XML file line #8: Error 
                       inflating class java.lang.reflect.Constructor
    at android.os.Handler.dispatchMessage(
    at android.os.Looper.loop(
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(
    at dalvik.system.NativeStart.main(Native Method)
Caused by: android.view.InflateException: Binary XML file line #8: Error 
 inflating class 
    at android.view.LayoutInflater.createView(
    at android.view.LayoutInflater.createViewFromTag(
    at android.view.LayoutInflater.rInflate(
    at android.view.LayoutInflater.inflate(
    at android.view.LayoutInflater.inflate(
    at android.view.LayoutInflater.inflate(
    at com.microjobsinc.mjandroid.MicroJobs.onCreate(
... 11 more
Caused by: java.lang.reflect.InvocationTargetException at<init>( at java.lang.reflect.Constructor.constructNative(Native Method) at java.lang.reflect.Constructor.newInstance( at android.view.LayoutInflater.createView(
... 21 more
Caused by: java.lang.IllegalArgumentException: You need to specify an API Key for each MapView. See the MapView documentation for details. at<init>( at<init>(
... 25 more

The first three errors basically tell us that Android could not start our application because it could not inflate the Views it found in our layout file. The last error block we showed in the output clearly tells us we need an API Key for each MapView. Logcat is often the best way to get insight into errors where there isn’t specific information otherwise.

3.2. Writing your own logcat entries

To write your own entries from your application into logcat, Android provides methods corresponding to the different entry priorities. The methods are all of the form:

Log.x(String tag, String message, [Throwable exception])

where x can be v, d, i, w, or e, and the optional exception makes it easy to report exceptions that you didn’t anticipate in your code but encounter within a try/catch block. For example, look at the onItemSelected method for the Spinner in

try {
catch (Exception e) {
    Log.i("MicroJobs", "Unable to animate map", e);
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