对于SingleTop模式，即使栈中存在活动的实例，但是如果不在栈顶的位置，系统还是会创建该活动的情况下。如果将启动模式设置为SingleTask模式，那么在启动该活动时，系统检测到栈中存在该活动的实例，就不会再创建该活动了，而只是会将处于该活动上方的所有活动都出栈，使得该活动处于栈顶位置。
同样的，贴上代码来检测一下。创建项目LearnLaunchMode_SingleTask，其各部分代码如下：
FirstActivity.java：
1 public class FirstActivity extends ActionBarActivity { 2 private Button launch_second, launch_third; 3 @Override 4 protected void onCreate(Bundle savedInstanceState) { 5 super.onCreate(savedInstanceState); 6 setContentView(R.layout.activity_first); 7 8 launch_second = (Button) findViewById(R.id.launch_second); 9 launch_third = (Button) findViewById(R.id.launch_third); 10 launch_second.setOnClickListener(new View.OnClickListener() { 11 @Override 12 public void onClick(View v) { 13 Intent intent = new Intent().setClass(FirstActivity.this, SecondActivity.class); 14 startActivity(intent); 15 } 16 }); 17 launch_third.setOnClickListener(new View.OnClickListener() { 18 @Override 19 public void onClick(View v) { 20 Intent intent = new Intent().setClass(FirstActivity.this, ThirdActivity.class); 21 startActivity(intent); 22 } 23 }); 24 Log.i("TAG", this + " is launch"); 25 } 26 27 @Override 28 protected void onDestroy() { 29 super.onDestroy(); 30 Log.i("TAG", this + " is destroy！"); 31 } 32 }
SecondActivity.java：
1 public class SecondActivity extends Activity { 2 private Button launch_first, launch_third; 3 @Override 4 protected void onCreate(Bundle savedInstanceState) { 5 super.onCreate(savedInstanceState); 6 requestWindowFeature(Window.FEATURE_NO_TITLE); 7 setContentView(R.layout.activity_second); 8 9 launch_first = (Button) findViewById(R.id.launch_first); 10 launch_third = (Button) findViewById(R.id.launch_third); 11 launch_first.setOnClickListener(new View.OnClickListener() { 12 @Override 13 public void onClick(View v) { 14 Intent intent = new Intent().setClass(SecondActivity.this, FirstActivity.class); 15 startActivity(intent); 16 } 17 }); 18 launch_third.setOnClickListener(new View.OnClickListener() { 19 @Override 20 public void onClick(View v) { 21 Intent intent = new Intent().setClass(SecondActivity.this, ThirdActivity.class); 22 startActivity(intent); 23 } 24 }); 25 Log.i("TAG", this + " is launch"); 26 } 27 28 @Override 29 protected void onDestroy() { 30 super.onDestroy(); 31 Log.i("TAG", this + " is destroy！"); 32 } 33 }
ThirdActivity.java：
1 public class ThirdActivity extends Activity { 2 private Button launch_first, launch_second; 3 @Override 4 protected void onCreate(Bundle savedInstanceState) { 5 super.onCreate(savedInstanceState); 6 requestWindowFeature(Window.FEATURE_NO_TITLE); 7 setContentView(R.layout.activity_third); 8 9 launch_first = (Button) findViewById(R.id.launch_first); 10 launch_second = (Button) findViewById(R.id.launch_second); 11 launch_first.setOnClickListener(new View.OnClickListener() { 12 @Override 13 public void onClick(View v) { 14 Intent intent = new Intent().setClass(ThirdActivity.this, FirstActivity.class); 15 startActivity(intent); 16 } 17 }); 18 launch_second.setOnClickListener(new View.OnClickListener() { 19 @Override 20 public void onClick(View v) { 21 Intent intent = new Intent().setClass(ThirdActivity.this, SecondActivity.class); 22 startActivity(intent); 23 } 24 }); 25 Log.i("TAG", this + " is launch"); 26 } 27 28 @Override 29 protected void onDestroy() { 30 super.onDestroy(); 31 Log.i("TAG", this + " is destroy！"); 32 } 33 }
activity_first.xml：
1 <?xml version="1.0" encoding="utf-8"?> 2 <LinearLayout 3 xmlns:android="http://schemas.android.com/apk/res/android" 4 android:layout_width="match_parent" 5 android:layout_height="match_parent" 6 android:orientation="vertical"> 7 <Button 8 android:id="@+id/launch_second" 9 android:layout_width="match_parent" 10 android:layout_height="wrap_content" 11 android:text="启动第二个Activity！" /> 12 <Button 13 android:id="@+id/launch_third" 14 android:layout_width="match_parent" 15 android:layout_height="wrap_content" 16 android:text="启动第三个Activity！" /> 17 </LinearLayout>
activity_second.xml：
1 <?xml version="1.0" encoding="utf-8"?> 2 <LinearLayout 3 xmlns:android="http://schemas.android.com/apk/res/android" 4 android:layout_width="match_parent" 5 android:layout_height="match_parent" 6 android:orientation="vertical"> 7 <Button 8 android:id="@+id/launch_first" 9 android:layout_width="match_parent" 10 android:layout_height="wrap_content" 11 android:text="启动第一个Activity！" /> 12 <Button 13 android:id="@+id/launch_third" 14 android:layout_width="match_parent" 15 android:layout_height="wrap_content" 16 android:text="启动第三个Activity！" /> 17 </LinearLayout>
activity_third.xml：
1 <?xml version="1.0" encoding="utf-8"?> 2 <LinearLayout 3 xmlns:android="http://schemas.android.com/apk/res/android" 4 android:layout_width="match_parent" 5 android:layout_height="match_parent" 6 android:orientation="vertical"> 7 <Button 8 android:id="@+id/launch_first" 9 android:layout_width="match_parent" 10 android:layout_height="wrap_content" 11 android:text="启动第一个Activity！" /> 12 <Button 13 android:id="@+id/launch_second" 14 android:layout_width="match_parent" 15 android:layout_height="wrap_content" 16 android:text="启动第二个Activity！" /> 17 </LinearLayout>
同样，还需要在AndroidManifest.java中对SecondActivity以及ThirdActivity两个活动进行注册，并设置上述三个活动的启动模式为SingleTask：
1 <activity 2 android:name=".FirstActivity" 3 android:launchMode="singleTask" 4 android:label="@string/app_name"> 5 <intent-filter> 6 <action android:name="android.intent.action.MAIN" /> 7 <category android:name="android.intent.category.LAUNCHER" /> 8 </intent-filter> 9 </activity> 10 <activity android:name=".SecondActivity" android:launchMode="singleTask" /> 11 <activity android:name=".ThirdActivity" android:launchMode="singleTask" />
在该程序中，系统先启动FirstActivity，此时打印信息显示该活动启动了；然后我们点击"启动第二个Activity!"按钮，可以看到显示信息中SecondActivity is launch；再点击"启动第三个Activity!"，也可以看到第三个活动启动了。最后再点击"启动第一个Activity!"。从时观察打印信息可以看到；SecondActivity is destroy；以及ThirdActivity is destroy。表示此时SecondActivity与ThirdActivity都已出栈。现在只需要按一次back键即可退出程序。其他的各种创建活动的顺序就不再做演示了。
综上所述，可以画出该启动模式的原理示意图如下：
