Do you use thread or service?

In the last article we said Threa and Service It doesn't matter . It basically explains their differences , But the depth is not enough , In this article, we continue to discuss thread and Service！

Let's talk about it first. Thread and Service Some of the differences ：

Thread：Thread ( Threads ) Is the smallest unit of program execution , It can be used Thread To perform some asynchronous operations .

Thread The operation of is independent . When one Activity By finish after , If there is no active stop Thread perhaps Thread Inside run If the method is not finished ,Thread And it's going to be carried out all the time . So there's a problem here ： When Activity By finish after , No longer hold the Thread References to , And you can no longer control the Thread. On the other hand , There is no way in different Activity Center to same Thread Control .

Service：Service yes android A mechanism of , When it's running, if it's Local Service, So the corresponding Service Is running on the main thread of the main process . If it is Remote Service, So the corresponding Service It runs on the main thread of an independent process .（ We talked about that in the last post RemoteService Of ）

for example ： If One Thread You need to connect to the server at regular intervals to verify the data , The Thread It needs to be running all the time in the background . At this time, if you create this Thread Of Activity It's over and it's time Thread Didn't stop , Then there will be no way to control the Thread, Unless you kill the process of the program . At this point, if you create and start a Service , stay Service It creates 、 Run and control the Thread, This solves the problem ( Because of anything Activity Can be bound to the same Service, And the system will only create a corresponding Service Example ).

So we can Service Think of it as a messaging service , Can be anywhere there is Context Local call to startService or bindService To control it , It can also be in Service Registered in the BroadcastReceiver, By sending broadcast To achieve the purpose of control , These are all Thread Impossible .

Look again. Service Several characteristics of ：

（1） By default ,Service Is running in the main thread , If you need to perform complex and time-consuming operations , Must be in Service Create another Thread To perform the task , To avoid ANR.

（2） Service Priority of is higher than that of Activity. Of course , Also higher than in Activity Created in Thread. therefore , The system may give priority to killing the background when the memory is insufficient Activity perhaps Thread, And will not easily kill Service Components , Even if forced to kill Service, It will also restart the killed when resources are available Service.

Actually ,Service and Thread It is not a level thing at all ,Service Is one of the four components of the system . and Thread It is just a tool class used to perform background tasks , It can be Activity Is created in , It can also be in Service Is created in .

therefore , The root of this problem should be , We should not discuss the use of Service still Thread, Instead, we should discuss Where to create Thread？

Typical applications , It can be created in three places , Different locations , Its life cycle is different , therefore , We should be According to the Thread The target life cycle is determined in Service Created in Thread Still Activity Create it in .

（1） stay Activity Is created in

Generally in onCreate Created on , stay onDestroy() Medium destruction , otherwise ,Activity After destruction ,Thread Will still be running in the background .

In this case ,Thread The life cycle of is the whole Activity Life cycle of . therefore , stay Activity Created in the Thread Only suitable for completing some dependencies Activity Tasks related to itself , For example, update regularly Activity Control status, etc .

Core features ： The Thread That's why Activity Service , Complete this particular Activity The task assigned , Take the initiative to inform the Activity Some news and events ,Activity After destruction , The Thread There's no point in living .

（2） stay Application Is created in

General customization Application class , heavy load onCreate Method , And create Thread, Of course , Will also be in onTerminate() Method of destruction Thread, otherwise , If Thread If you don't quit , Even if the whole Application Out of the ,Thread Still running in the background .

In this case ,Thread The life cycle of is the whole Application Life cycle of . therefore , stay Application Created in the Thread, It can perform some tasks at the whole application level , For example, regularly check the network connection status and so on .

Core features ： The Thread The ultimate goal is for this APP Each of them Activity Service , Including completing a Activity The task assigned , Take the initiative to inform someone Activity Some news and events, etc ,APP After exiting, you should Thread There's no point in living .

In both cases ,Thread The life cycle of should not exceed the entire application life cycle , That is to say , Whole APP After you quit ,Thread Should quit completely , So there won't be memory leaks or zombie threads . that , If you want the whole APP Can still run this after exiting Thread, Then we should put Thread Put it in Service To create and launch .

（3） stay Service Is created in

This is to guarantee the longest life cycle Thread The only way , As long as the whole Service Do not exit ,Thread It can be executed in the background all the time , Generally in Service Of onCreate() Created in , stay onDestroy() Medium destruction .

therefore , stay Service Created in the Thread, Suitable for long-term execution some independent of APP Background tasks for , The common one is ： stay Service Keep a long connection with the server side .

Core features ： The Thread It can be for APP Provide some “ service ” perhaps “ State query ”, But it's time to Thread There is no need to take the initiative to inform APP Any incident , You don't even need to know APP Who is it? .

All in all , We're not thinking about using Thread Or should I use it Service, It should be for Thread Choose the right life cycle , This is my right to Service and Thread Thinking and understanding .