Android: how to resolve Application’s parameter NullPointerException
我也试试老外写博客＝ ＝
怎么防止程序后台运行长时间后切换回来导致application里的value被清空而crash。
1. Scenario
You have a customize Application (named MyApplication). and you initialize some parameters in MyApplication. When you press 'Home' , your app will be suspended by system . If your memory is not enough, the system will kill your app (include MyApplication). In this case , When you switch back to your app . if you access the parameter in MyApplication , the app crashed .
2. How to reproduce this bug manually.
You may not realize this before until the crash log from Umeng or HockeyApp , but you don't know when does this happen , only thing you can do is that wish user don't suspend your app or exit app when they want to brower other apps .
but you can reproduce it .
2.a) run your app through Eclipse , go to the Activity which may access the parameter in MyApplication.
2.b) click 'Home' , suspend your app.
2.c) go to DDMS , choose your process (com.stay.test) ,and stop it .
2.d) now , long press 'Home' or press 'Menu' to resume your app .
2.e) app crash
3.How to resolve
3.a) if you have a good programming style. and you have customized Activity as a super class . like BaseActivity , BaseFragmentActivity. And also you know Activity's lifecircle very clear , you may have a SingleTask Activity always on the bottom of activity stack. In this case , this bug can easily be fixed , just like how to exit app in any Activity
3.b) Well , if your code is not like that , pls change your frame , if not , you will find it hard to resovle bugs when the app became bigger.
3.c) set a flag in MyApplication , to represent your app's state , (-1 default , 0 logout ,1 login)
3.d) in BaseActivity or BaseFragment , override onCreate(Bundle savedInstanceState).
check the flag in MyApplication ,
if is -1 , you should exit app and enter app just like user first to use your app . and the parameters in MyApplication would be initialized in correct workflow.
how to exit? simple .
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finish
(
)
;
Intent
intent
=
new
Intent
(
getApplicationContext
(
)
,
SingleTaskActivity
.
class
)
;
intent
.
addFlags
(
Intent
.
FLAG_ACTIVITY_CLEAR_TOP
)
;
intent
.
putExtra
(
Constants
.
RESTART_APP
,
true
)
;
startActivity
(
intent
)
;
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this will jump to the Activity which is SingleTask . and with flag 'clear top'. all the activities exclude 'single task' in stack will be removed , and the 'single task' would be brought to top of stack . in this case , if you call finish() in 'single task' activity , you will exit app. cause the 'single task' is already exist , so it would not call onCreate(Bundle savedInstanceState), it will call onNewIntent(Intent intent).
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@
Override
protected
void
onNewIntent
(
Intent
intent
)
{
super
.
onNewIntent
(
intent
)
;
if
(
intent
.
getBooleanExtra
(
Constants
.
RESTART_APP
,
false
)
)
{
finish
(
)
;
//TODO start Activity that you defined as MAIN in your manifest.xml
}
}
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4.Check if this bug is fixed , you should test all activities you have , and make sure it is fixed . it may have different case , and you need to give a special solution .
p.s.
解决方案已经写的很明白，概不提供额外解释与说明，如果不能解决，请支付酬劳让我提供服务。(源码和讲解)