Figure 1. The Choose Device window enables you to select a physical Android device or a virtual device to debug your app.
Android Studio enables you to debug apps running on the emulator or on an Android device. With Android Studio, you can:
To debug your app, Android Studio builds a debuggable version of your app, connects to a device or to the emulator, installs the app and runs it. The IDE shows the system log while your app is running and provides debugging tools to filter log messages, work with breakpoints, and control the execution flow.
Figure 1. The Choose Device window enables you to select a physical Android device or a virtual device to debug your app.
To run your app in debug mode, you build an APK signed with a debug key and install it on a physical Android device or on the Android emulator. To set up an Android device for development, see Using Hardware Devices. For more information about the emulator provided by the Android SDK, see Using the Emulator.
To debug your app in Android Studio:
in the toolbar.Figure 1 shows the Choose Device window. The list shows all the Android devices
connected to your computer. Select Launch Emulator to use an Android virtual device
instead. Click the ellipsis 
to open the
Android Virtual Device Manager.
Android Studio opens the Debug tool window when you debug your app. To open the
Debug window manually, click Debug
.
This window shows threads and variables in the Debugger tab, the device status in the
Console tab, and the system log in the Logcat tab. The Debug tool
window also provides other debugging tools covered in the following sections.
Figure 2. The Debug tool window in Android Studio showing the current thread and the object tree for a variable.
You don't always have to restart your app to debug it. To debug an app that you're already running:
..The system log shows system messages while you debug your app. These messages include information from apps running on the device. If you want to use the system log to debug your app, make sure your code writes log messages and prints the stack trace for exceptions while your app is in the development phase.
To write log messages in your code, use the {@link android.util.Log} class. Log messages help you understand the execution flow by collecting the system debug output while you interact with your app. Log messages can tell you what part of your application failed. For more information about logging, see Reading and Writing Logs.
The following example shows how you might add log messages to determine if previous state information is available when your activity starts:
import android.util.Log;
...
public class MyActivity extends Activity {
private static final String TAG = MyActivity.class.getSimpleName();
...
@Override
public void onCreate(Bundle savedInstanceState) {
if (savedInstanceState != null) {
Log.d(TAG, "onCreate() Restoring previous state");
/* restore state */
} else {
Log.d(TAG, "onCreate() No saved state available");
/* initialize app */
}
}
}
During development, your code can also catch exceptions and write the stack trace to the system log:
void someOtherMethod() {
try {
...
} catch (SomeException e) {
Log.d(TAG, "someOtherMethod()", e);
}
}
Note: Remove debug log messages and stack trace print calls from
your code when you are ready to publish your app. You could do this by setting a DEBUG
flag and placing debug log messages inside conditional statements.
Both the Android DDMS (Dalvik Debug Monitor Server) and the Debug tool windows show the system log; however, the Android DDMS tool window lets you view only log messages for a particular process. To view the system log on the Android DDMS tool window:
to open the Android DDMS
tool window.
.
Figure 4. The system log in the Android DDMS tool window.
The Android DDMS tool window gives you access to some DDMS features from Android Studio. For more information about DDMS, see Using DDMS.
The system log shows messages from Android services and other Android apps. To filter the log messages to view only the ones you are interested in, use the tools in the Android DDMS window:
. If the Devices view
is not available, click Restore Devices View
on the right of the Android
DDMS tool window. This button is only visible when you hide the Devices
window.Breakpoints enable you to pause the execution of your app at a particular line of code, examine variables, evaluate expressions, and continue the execution line by line. Use breakpoints to determine the causes of run-time errors that you can't fix by looking at your code only. To debug your app using breakpoints:
Android Studio pauses the execution of your app when it reaches the breakpoint. You can then use the tools in the Debug tool window to identify the cause of the error.
Figure 5. A red dot appears next to the line when you set a breakpoint.
To view all the breakpoints and configure breakpoint settings, click View
Breakpoints 
on the left side of the Debug tool
window. The Breakpoints window appears, as shown in figure 6.
Figure 6. The Breakpoints window lists all the current breakpoints and includes behavior settings for each.
The Breakpoints window lets you enable or disable each breakpoint from the list on the left. If a breakpoint is disabled, Android Studio does not pause your app when it hits that breakpoint. Select a breakpoint from the list to configure its settings. You can configure a breakpoint to be disabled at first and have the system enable it after a different breakpoint is hit. You can also configure whether a breakpoint should be disabled after it is hit. To set a breakpoint for any exception, select Exception Breakpoints in the list of breakpoints.
After you set breakpoints in your code, click Rerun
to start the app again. When a breakpoint is
hit, Android Studio pauses the app and highlights the breakpoint in the source code. The
Debug tool window lets you examine variables and control the execution step by
step:
To examine the object tree for a variable, expand it in the Variables view. If
the Variables view is not visible, click Restore Variables View
.
To evaluate an expression at the current execution point, click Evaluate
Expression 
.
To advance to the next line in the code (without entering a method), click Step
Over 
.
To advance to the first line inside a method call, click Step
Into 
.
To advance to the next line outside the current method, click Step
Out 
.
To continue running the app normally, click Resume Program
.
Figure 7. The Variables view in the Debug tool window.
Android Studio lets you track objects that are being allocated on the Java heap and see which classes and threads are allocating these objects. This allows you to see the list of objects allocated during a period of interest. This information is valuable for assessing memory usage that can affect application performance.
To track memory allocation of objects:
to open the Android DDMS
tool window.
Android Studio shows the objects that the system allocated with the following information:
Figure 8. Object allocation tracking in Android Studio.
Even if your application does not generate runtime errors, this does not mean it is free of problems. You should also consider the following issues:
The Android Device Monitor is a stand-alone tool with a graphical user interface for serveral Android application debugging and analysis tools, including the Dalvik Debug Monitor Server (DDMS). You can use the Android Device Monitor to analyze memory usage, profile methods, monitor network traffic and simulate incoming calls and messages.
To open the Android Device Monitor from Android Studio, click
Monitor 
on the toolbar. The Android Device Monitor
opens in a new window.
For more information about the Android Device Monitor and DDMS, see Device Monitor and Using DDMS.
Android Studio enables you to capture a screenshot or a short video of the device screen while your app is running. Screenshots and videos are useful as promotional materials for your app, and you can also attach them to bug reports that you send to your development team.
To take a screenshot of your app:
to open the Android DDMS
tool window.
on the left side of the
Android DDMS tool window.To take a video recording of your app:
to open the Android DDMS
tool window.
on the left side of the
Android DDMS tool window.