Every Activity in Android is a Process,or One Application is one process

In this page, http://androidapps.org.ua/androidintro_ipc.html , intent switching between activities is described as Inter Process Communication. Now I am confused whether every activity is a separate process in android or All activities inside an application is one process. I have checked the process id of all activities and service in my Application using:

 int id = android.os.Process.myPid();
 System.out.println("Process id of Activity1 :"+id);

But it is showing same process id. Please reply back.

Solution

All activities inside an application run in one process?

It depends on value of android:process attribute in application manifest.

if attribute android:process is not defined for Application/Activity tags in the manifest, by default all the activities will run in single process (process name will be name of the package defined in manifest)

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.so.test" android:versionCode="1" android:versionName="1.0">
    <uses-sdk android:minSdkVersion="8" />

    <application android:icon="@drawable/icon" android:label="@string/app_name">
        <activity android:name=".Activity1">
        </activity>
        <activity android:name=".Activity2">
        </activity>
        <activity android:name=".Activity3">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

In the above manifest all activities run in process com.so.test,ps command output in adb shell:

# ps
app_39    668   33    84492  20672 ffffffff afd0c51c S com.so.test

If android:process is specified for Activity new process will be created with the same userid and the activity runs in that process.

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.so.test" android:versionCode="1" android:versionName="1.0">
    <uses-sdk android:minSdkVersion="8" />

    <application android:icon="@drawable/icon" android:label="@string/app_name" 
     android:process="com.so.p1">
        <activity android:name=".Activity1">
        </activity>
        <activity android:name=".Activity2" android:process="com.so.p2">
        </activity>
        <activity android:name=".Activity3" android:process="com.so.p3">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

If the manifest is defined like above

Activity1 runs in com.so.p1 process
Activity2 runs in com.so.p2 process
Activity3 runs in com.so.p3 process

ps output in adb shell

# ps
app_39    650   33    83192  20900 ffffffff afd0c51c S com.so.p1
app_39    659   33    83188  20864 ffffffff afd0c51c S com.so.p2
app_39    668   33    84492  20672 ffffffff afd0c51c S com.so.p3

If an Activity needs to be run in another process not defined in this manifest, both APKs should be signed with the same certificate.