Service is an important component in Android development

Service Component

「Service」 is a component in Android that is used to perform long-running tasks or handle operations unrelated to the user interface. It can run independently of the user interface during the life cycle of the application and can continue to run after the application is destroyed.

A Service can perform various tasks, such as playing music, downloading files, handling network requests, etc. It can be executed outside the main thread of the application, so it can avoid blocking the user interface.

There are two types of services: foreground services and background services. Foreground services are usually used to perform tasks that can be perceived by the user and display a persistent notification in the notification bar to keep the user visible to the service. Background services perform tasks silently in the background without any visible notification to the user.

Service is one of the most important components in Android development. It can help developers implement various background tasks and functions and improve the user experience of the application.

Service startup method

In Android, there are two ways to start a Service:

• startService(Intent): Start the Service by calling the startService(Intent) method. This method is a startup method, that is, the Service will run in the background even if the component that starts it (such as Activity) has been destroyed. There is no direct communication channel between the component that starts the Service and the Service. The component only starts the Service through Intent and can stop the Service by calling the stopService(Intent) method.
• bindService(Intent, ServiceConnection, int): Start the Service by calling the bindService(Intent, ServiceConnection, int) method. This method is a binding method, that is, a connection is established between the Service and the component that starts it (such as Activity), and the component can communicate with the Service through ServiceConnection. When all components bound to the Service are unbound, the Service will be destroyed.

It should be noted that for a Service started with startService(Intent), the stopSelf() method needs to be called from within the Service or other components need to call the stopService(Intent) method to stop the Service. For a Service started with bindService(Intent, ServiceConnection, int), all bound components need to be unbound before the Service can be destroyed.

Service Lifecycle

The Service life cycle includes the following methods:

• onCreate(): Called when the Service is created, for one-time initialization operations.
• onStartCommand(): Called when the Service is started to handle the request to start the Service. Time-consuming operations can be performed in this method, but the Service needs to be stopped manually.
• onBind(): Called when binding a Service, it returns an IBinder object so that the client can communicate with the Service.
• onUnbind(): Called when unbinding a Service, it is used to clean up the resources created when binding the Service.
• onDestroy(): Called when the Service is destroyed to release all resources.

onStartCommand is used to process requests to start a service. This method is called when the service is started and needs to return an integer value to indicate the startup type of the service.

The meaning of the return value is as follows:

• START_STICKY: If the system restarts the service after it has been destroyed, the service is recreated and the onStartCommand method is called. However, the previous Intent object is not redelivered. This startup type is suitable for services that do not need to pass data, such as background services that play music.
• START_NOT_STICKY: If the system restarts the service after it has been destroyed, it does not recreate the service or call the onStartCommand method. This startup type is suitable for services that do not need to run continuously, such as services that perform one-time tasks.
• START_REDELIVER_INTENT: If the system restarts the service after it is destroyed, the service is recreated and the onStartCommand method is called, and the previous Intent object is redelivered. This startup type is suitable for services that need to transfer data and need to maintain data integrity, such as services that download files.
• START_STICKY_COMPATIBILITY: Same as START_STICKY, but used in versions prior to Android 2.0.
• START_NOT_STICKY_COMPATIBILITY: Same as START_NOT_STICKY, but used in versions prior to Android 2.0.

The choice of return value depends on the needs and behavior of your service. Depending on the return value, the system will have different processing methods to manage the life cycle of the service.

It should be noted that the Service can be started through the startService() method and can also be bound through the bindService() method. When the Service is started and bound at the same time, you need to manually call the stopSelf() or stopService() method to stop the Service.

Service Usage

Create a class MyService that inherits from Service:

 public class MyService extends Service { @Override public int onStartCommand(Intent intent, int flags, int startId) { // 在这里执行你的后台任务return START_STICKY; } @Override public IBinder onBind(Intent intent) { // 如果你的Service不需要绑定，则返回null return null; } @Override public void onDestroy() { super.onDestroy(); // 在Service销毁时执行清理操作} }

Declare the Service in the AndroidManifest.xml file:

 <service android:name=".MyService" />

Start the Service:

 Intent serviceIntent = new Intent(this, MyService.class); startService(serviceIntent);

In the onStartCommand method of MyService, you can perform some background tasks, such as downloading files, playing music, etc. The return value START_STICKY means that if the Service is killed by the system, the system will try to restart the Service.

Finally, when you don't need the Service, remember to call the stopService method to stop the Service:

 Intent serviceIntent = new Intent(this, MyService.class); stopService(serviceIntent);

IntentService

IntentService is a class in Android that is used to handle background tasks. It is a subclass of Service and can perform time-consuming operations in the background without blocking the main thread.

The characteristic of IntentService is that it automatically handles all Intent requests and executes them one by one in the background. It creates a worker thread to handle each Intent request, and when all requests are processed, IntentService automatically stops.

IntentService can be used to easily handle some tasks that need to be performed in the background, such as downloading files, uploading data, etc. It can avoid the complexity of manually creating threads and handling inter-thread communication, and provides a simple API for developers to use.

IntentService principle

IntentService is a special service provided by Android to handle asynchronous tasks. Its principle is as follows:

• IntentService inherits from the Service class, so it is also a service component that can run in the background.
• When we start IntentService, it creates a worker thread to handle the task. This worker thread is created inside IntentService.
• IntentService will process each Intent request in the order in which it is started, and each request will be processed in a working thread.
• In the working thread, IntentService will call the onHandleIntent() method to handle specific tasks. This method is an abstract method and needs to be implemented by ourselves.
• After processing an Intent request, IntentService will automatically stop itself without us manually calling the stopService() method.
• If multiple Intent requests arrive at the same time, IntentService will process them in sequence to ensure the orderliness of the tasks.
• IntentService also provides a default implementation that can handle time-consuming tasks, such as network requests, database operations, etc. It will perform these time-consuming tasks in the onHandleIntent() method and automatically stop the service after the task is completed.

The principle of IntentService is to process Intent requests by creating a working thread, and to ensure the order of tasks and the automatic stop of services. This allows us to easily handle asynchronous tasks without worrying about thread management and the life cycle of services.

IntentService usage

Create a class MyIntentService that inherits from IntentService:

 public class MyIntentService extends IntentService { public MyIntentService() { super("MyIntentService"); } @Override protected void onHandleIntent(Intent intent) { // 在这里执行后台任务// 可以通过intent获取传递的参数} @Override public void onDestroy() { super.onDestroy(); // 在服务销毁时执行一些清理操作} }

Declare the Service in the AndroidManifest.xml file:

 <service android:name=".MyIntentService" />

Start the Service:

 Intent serviceIntent = new Intent(this, MyIntentService.class); startService(serviceIntent);

In the above code, a subclass named MyIntentService is created and the onHandleIntent method is overridden to perform background tasks. Some cleanup operations can be performed in the onDestroy method.

The difference between Service and IntentService

Service and IntentService are two common service types in Android. They have the following differences:

• Lifecycle: Service is a general service type, its lifecycle is controlled by the developer, and it can run in the background for a long time. Whereas IntentService is a subclass of Service, it is a special service type, it will automatically stop once all the work is completed.
• Multithreading: By default, the Service runs in the main thread. If you need to perform time-consuming operations in the Service, you need to manually create a new thread. However, the IntentService has implemented a multithreading mechanism, which automatically creates a working thread to handle all Intent requests, avoiding the complexity of manual thread processing.
• Task queue: Service processes a single request, while IntentService can process multiple requests. It will put all Intent requests into the task queue in sequence and then process them one by one.
• Startup method: Service can be started through the startService() method, or bound through the bindService() method. IntentService can only be started through the startService() method.

To sum up, Service is suitable for background tasks that need to run for a long time and require manual processing of multi-threading and task queues; while IntentService is suitable for executing a series of independent background tasks, automatically processing multi-threading and task queues, and is suitable for simple asynchronous operations.