除了Activity的生命周期外，Activity的启动模式也是一个难点，有时候为了满足项目的特殊需求，就必须使用Activity的启动模式。
在默认情况下，当我们多次启动同一个Activity的时候，系统会创建多个实例并把它们放入任务栈中，但是有些场景重复创建多个实例，是没有必要且浪费资源的，这就需要启动模式来修改系统的默认行为。
下面，我将以理论+实践的形式为大家介绍Activity的启动模式。
这是系统的默认启动模式，采用这种模式的Activity无论是否已经存在实例，都会重新创建一个实例，在这种模式下谁启动了这个Activity，那么这个Activity就运行在启动它的那个Activity所在的栈中。
实践：MainActivity 采用 standard 模式
// 核心代码展示
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "MainActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, getTaskId() + "onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button:
Intent intent = new Intent(this,MainActivity.class);
intent.putExtra("id",1);
startActivity(intent);
}
}
}
// 日志结果展示
// 启动MainActivity
2022-04-21 18:37:59.622 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onCreate: 0
2022-04-21 18:37:59.625 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onStart: 0
2022-04-21 18:37:59.626 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onResume: 0
// 点击MainActivity的按钮启动MainActivity
2022-04-21 18:38:03.336 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onPause: 0
2022-04-21 18:38:03.373 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onCreate: 1
2022-04-21 18:38:03.378 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onStart: 1
2022-04-21 18:38:03.379 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onResume: 1
2022-04-21 18:38:03.939 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onStop: 0
2022-04-21 18:38:03.942 4072-4072/com.hnucm.myapplication I/MainActivityLog: 12onSaveInstanceState: 0
在这种模式下，如果新的Activity已经位于任务栈的栈顶，那么此Activity不会被重新创建，同时它的NewIntent方法将会被回调。如果新Activity的实例已存在但不是位于栈顶，那么新Activity依然会被创建。
实践：MainActivity 采用 singleTop 模式
// 核心代码展示
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "MainActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, getTaskId() + "onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button:
Intent intent = new Intent(this,MainActivity.class);
intent.putExtra("id",1);
startActivity(intent);
}
}
}
// 日志结果展示
// 启动MainActivity
2022-04-21 18:52:30.963 4205-4205/com.hnucm.myapplication I/MainActivityLog: 13onCreate: 0
2022-04-21 18:52:30.968 4205-4205/com.hnucm.myapplication I/MainActivityLog: 13onStart: 0
2022-04-21 18:52:30.970 4205-4205/com.hnucm.myapplication I/MainActivityLog: 13onResume: 0
// 点击MainActivity的按钮启动MainActivity
2022-04-21 18:52:35.601 4205-4205/com.hnucm.myapplication I/MainActivityLog: 13onPause: 0
2022-04-21 18:52:35.603 4205-4205/com.hnucm.myapplication I/MainActivityLog: 13onNewIntent: 0
2022-04-21 18:52:35.604 4205-4205/com.hnucm.myapplication I/MainActivityLog: 13onResume: 0
MainActivity 采用 singleTop 模式，SecondActivity采用 standard 模式
// 核心代码展示
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "MainActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, getTaskId() + "onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button:
Intent intent = new Intent(this,SecondActivity.class);
startActivity(intent);
}
}
}
public class SecondActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "SecondActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
Log.i(TAG, getTaskId() +"onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button2);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button2:
Intent intent = new Intent(this,MainActivity.class);
intent.putExtra("id",1);
startActivity(intent);
}
}
}
// 日志结果展示
// 启动MainActivity
2022-04-21 18:58:54.961 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onCreate: 0
2022-04-21 18:58:54.964 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onStart: 0
2022-04-21 18:58:54.966 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onResume: 0
// 点击MainActivity的按钮启动SecondActivity
2022-04-21 19:00:13.845 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onPause: 0
2022-04-21 19:00:13.895 4815-4815/com.hnucm.myapplication I/SecondActivityLog: 20onCreate: 0
2022-04-21 19:00:13.898 4815-4815/com.hnucm.myapplication I/SecondActivityLog: 20onStart: 0
2022-04-21 19:00:13.900 4815-4815/com.hnucm.myapplication I/SecondActivityLog: 20onResume: 0
2022-04-21 19:00:14.531 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onStop: 0
2022-04-21 19:00:14.534 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onSaveInstanceState: 0
// 点击SecondActivity的按钮启动MainActivity
2022-04-21 19:00:21.287 4815-4815/com.hnucm.myapplication I/SecondActivityLog: 20onPause: 0
2022-04-21 19:00:21.321 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onCreate: 1
2022-04-21 19:00:21.329 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onStart: 1
2022-04-21 19:00:21.331 4815-4815/com.hnucm.myapplication I/MainActivityLog: 20onResume: 1
2022-04-21 19:00:21.865 4815-4815/com.hnucm.myapplication I/SecondActivityLog: 20onStop: 0
2022-04-21 19:00:21.866 4815-4815/com.hnucm.myapplication I/SecondActivityLog: 20onSaveInstanceState: 0
这是一种单实例模式，在这种模式下，只要Activity在一个栈中存在，那么多次启动此Activity都不会重新创建实例，而是回调onNewIntent()。
实践：MainActivity 采用 singleTask 模式
// 核心代码展示
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "MainActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, getTaskId() + "onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button:
Intent intent = new Intent(this,MainActivity.class);
intent.putExtra("id",1);
startActivity(intent);
}
}
}
// 日志结果展示
// 启动MainActivity
2022-04-21 19:05:26.977 4945-4945/com.hnucm.myapplication I/MainActivityLog: 21onCreate: 0
2022-04-21 19:05:26.981 4945-4945/com.hnucm.myapplication I/MainActivityLog: 21onStart: 0
2022-04-21 19:05:26.982 4945-4945/com.hnucm.myapplication I/MainActivityLog: 21onResume: 0
// 点击MainActivity的按钮启动MainActivity
2022-04-21 19:05:29.539 4945-4945/com.hnucm.myapplication I/MainActivityLog: 21onPause: 0
2022-04-21 19:05:29.540 4945-4945/com.hnucm.myapplication I/MainActivityLog: 21onNewIntent: 0
2022-04-21 19:05:29.541 4945-4945/com.hnucm.myapplication I/MainActivityLog: 21onResume: 0
MainActivity 采用 singleTask 模式，SecondActivity采用 standard 模式
// 核心代码展示
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "MainActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, getTaskId() + "onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button:
Intent intent = new Intent(this,SecondActivity.class);
startActivity(intent);
}
}
}
public class SecondActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "SecondActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
Log.i(TAG, getTaskId() +"onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button2);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button2:
Intent intent = new Intent(this,MainActivity.class);
intent.putExtra("id",1);
startActivity(intent);
}
}
}
// 日志结果展示
// 启动MainActivity
2022-04-21 19:11:04.891 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onCreate: 0
2022-04-21 19:11:04.894 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onStart: 0
2022-04-21 19:11:04.897 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onResume: 0
// 点击MainActivity的按钮启动SecondActivity
2022-04-21 19:11:18.653 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onPause: 0
2022-04-21 19:11:18.690 5045-5045/com.hnucm.myapplication I/SecondActivityLog: 22onCreate: 0
2022-04-21 19:11:18.694 5045-5045/com.hnucm.myapplication I/SecondActivityLog: 22onStart: 0
2022-04-21 19:11:18.696 5045-5045/com.hnucm.myapplication I/SecondActivityLog: 22onResume: 0
2022-04-21 19:11:19.232 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onStop: 0
2022-04-21 19:11:19.235 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onSaveInstanceState: 0
// 点击SecondActivity的按钮启动MainActivity
2022-04-21 19:11:29.909 5045-5045/com.hnucm.myapplication I/SecondActivityLog: 22onPause: 0
2022-04-21 19:11:29.928 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onRestart: 0
2022-04-21 19:11:29.930 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onStart: 0
2022-04-21 19:11:29.931 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onNewIntent: 0
2022-04-21 19:11:29.932 5045-5045/com.hnucm.myapplication I/MainActivityLog: 22onResume: 0
2022-04-21 19:11:30.498 5045-5045/com.hnucm.myapplication I/SecondActivityLog: 22onStop: 0
2022-04-21 19:11:30.499 5045-5045/com.hnucm.myapplication I/SecondActivityLog: 22onDestroy: 0
这是一种加强的singleTask模式，它除了具有singleTask模式的所有特性外，还加强了一点，那就是具有此模式的Activity只能单独的位于一个任务栈中。
实践：MainActivity 采用 singleInstance 模式
// 核心代码展示
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "MainActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, getTaskId() + "onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button:
Intent intent = new Intent(this,MainActivity.class);
intent.putExtra("id",1);
startActivity(intent);
}
}
}
// 日志结果展示
// 启动MainActivity
2022-04-21 19:18:03.183 5193-5193/com.hnucm.myapplication I/MainActivityLog: 23onCreate: 0
2022-04-21 19:18:03.186 5193-5193/com.hnucm.myapplication I/MainActivityLog: 23onStart: 0
2022-04-21 19:18:03.188 5193-5193/com.hnucm.myapplication I/MainActivityLog: 23onResume: 0
// 点击MainActivity的按钮启动MainActivity
2022-04-21 19:18:11.416 5193-5193/com.hnucm.myapplication I/MainActivityLog: 23onPause: 0
2022-04-21 19:18:11.419 5193-5193/com.hnucm.myapplication I/MainActivityLog: 23onNewIntent: 0
2022-04-21 19:18:11.419 5193-5193/com.hnucm.myapplication I/MainActivityLog: 23onResume: 0
MainActivity 采用 singleInstance 模式，SecondActivity采用 standard 模式
// 核心代码展示
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "MainActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, getTaskId() + "onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button:
Intent intent = new Intent(this,SecondActivity.class);
startActivity(intent);
}
}
}
public class SecondActivity extends AppCompatActivity implements View.OnClickListener {
final static String TAG = "SecondActivityLog";
Button button;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
Log.i(TAG, getTaskId() +"onCreate: " + getIntent().getIntExtra("id",0));
button = findViewById(R.id.button2);
button.setOnClickListener(this);
}
@Override
protected void onStart() {
super.onStart();
Log.i(TAG, getTaskId() +"onStart: " + getIntent().getIntExtra("id",0));
}
//此处省略重复代码
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.button2:
Intent intent = new Intent(this,MainActivity.class);
intent.putExtra("id",1);
startActivity(intent);
}
}
}
// 日志结果展示
// 启动MainActivity
2022-04-21 19:19:25.783 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onCreate: 0
2022-04-21 19:19:25.786 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onStart: 0
2022-04-21 19:19:25.787 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onResume: 0
// 点击MainActivity的按钮启动SecondActivity
2022-04-21 19:19:34.745 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onPause: 0
2022-04-21 19:19:34.825 5261-5261/com.hnucm.myapplication I/SecondActivityLog: 25onCreate: 0
2022-04-21 19:19:34.868 5261-5261/com.hnucm.myapplication I/SecondActivityLog: 25onStart: 0
2022-04-21 19:19:34.871 5261-5261/com.hnucm.myapplication I/SecondActivityLog: 25onResume: 0
2022-04-21 19:19:35.464 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onStop: 0
2022-04-21 19:19:35.466 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onSaveInstanceState: 0
// 点击SecondActivity的按钮启动MainActivity
2022-04-21 19:19:45.068 5261-5261/com.hnucm.myapplication I/SecondActivityLog: 25onPause: 0
2022-04-21 19:19:45.078 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onRestart: 0
2022-04-21 19:19:45.079 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onStart: 0
2022-04-21 19:19:45.080 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onNewIntent: 0
2022-04-21 19:19:45.081 5261-5261/com.hnucm.myapplication I/MainActivityLog: 24onResume: 0
2022-04-21 19:19:45.802 5261-5261/com.hnucm.myapplication I/SecondActivityLog: 25onStop: 0
2022-04-21 19:19:45.803 5261-5261/com.hnucm.myapplication I/SecondActivityLog: 25onSaveInstanceState: 0
以上就是Activity启动模式的介绍。
欢迎留言指出错误。
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