Android – 活动构造函数

我注意到使用快捷键Alt + Insert并选择构建器，它尝试使用每个私有属性（例如cManager和mTextView ）创建构造函数。 我记得我已经创建了没有它的项目并且工作正常。

在运行应用程序之后，在完整的logcat和Activity构造函数之后发生错误：

具有私有财产的构造函数：

 private ConnectivityManager cManager; private TextView mTextView; public SplashScreenActivity() { this.cManager = (ConnectivityManager) this.getSystemService(Context.CONNECTIVITY_SERVICE); this.mTextView = (TextView)this.findViewById(R.id.mProgressText); } 

logcat的：

 02-20 18:20:56.155 1591-1591/com.universo91.towersrock I/art﹕ Not late-enabling -Xcheck:jni (already on) 02-20 18:20:58.355 1591-1591/com.universo91.towersrock D/AndroidRuntime﹕ Shutting down VM --------- beginning of crash 02-20 18:20:58.361 1591-1591/com.universo91.towersrock E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.universo91.towersrock, PID: 1591 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.universo91.towersrock/com.universo91.towersrock.Pages.SplashScreenActivity}: java.lang.InstantiationException: class com.universo91.towersrock.Pages.SplashScreenActivity has no zero argument constructor at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) Caused by: java.lang.InstantiationException: class com.universo91.towersrock.Pages.SplashScreenActivity has no zero argument constructor at java.lang.Class.newInstance(Class.java:1563) at android.app.Instrumentation.newActivity(Instrumentation.java:1065) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) Caused by: java.lang.NoSuchMethodException:  [] at java.lang.Class.getConstructor(Class.java:531) at java.lang.Class.getDeclaredConstructor(Class.java:510) at java.lang.Class.newInstance(Class.java:1561) at android.app.Instrumentation.newActivity(Instrumentation.java:1065) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 02-20 18:20:58.772 1591-1603/com.universo91.towersrock I/art﹕ Background sticky concurrent mark sweep GC freed 1747(114KB) AllocSpace objects, 0(0B) LOS objects, 37% free, 391KB/623KB, paused 825us total 382.655ms 02-20 18:21:29.824 1690-1690/com.universo91.towersrock D/AndroidRuntime﹕ Shutting down VM 02-20 18:21:29.829 1690-1690/com.universo91.towersrock E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.universo91.towersrock, PID: 1690 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.universo91.towersrock/com.universo91.towersrock.Pages.SplashScreenActivity}: java.lang.InstantiationException: class com.universo91.towersrock.Pages.SplashScreenActivity has no zero argument constructor at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) Caused by: java.lang.InstantiationException: class com.universo91.towersrock.Pages.SplashScreenActivity has no zero argument constructor at java.lang.Class.newInstance(Class.java:1563) at android.app.Instrumentation.newActivity(Instrumentation.java:1065) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) Caused by: java.lang.NoSuchMethodException:  [] at java.lang.Class.getConstructor(Class.java:531) at java.lang.Class.getDeclaredConstructor(Class.java:510) at java.lang.Class.newInstance(Class.java:1561) at android.app.Instrumentation.newActivity(Instrumentation.java:1065) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 02-20 18:21:30.168 1690-1702/com.universo91.towersrock I/art﹕ Background sticky concurrent mark sweep GC freed 1749(114KB) AllocSpace objects, 0(0B) LOS objects, 37% free, 391KB/623KB, paused 856us total 166.355ms 02-20 18:21:53.341 1690-1690/com.universo91.towersrock I/Process﹕ Sending signal. PID: 1690 SIG: 9 02-20 18:26:00.878 1754-1754/com.universo91.towersrock D/AndroidRuntime﹕ Shutting down VM 02-20 18:26:00.903 1754-1754/com.universo91.towersrock E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.universo91.towersrock, PID: 1754 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.universo91.towersrock/com.universo91.towersrock.Pages.SplashScreenActivity}: java.lang.IllegalStateException: System services not available to Activities before onCreate() at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) Caused by: java.lang.IllegalStateException: System services not available to Activities before onCreate() at android.app.Activity.getSystemService(Activity.java:4989) at com.universo91.towersrock.Pages.SplashScreenActivity.(SplashScreenActivity.java:24) at java.lang.reflect.Constructor.newInstance(Native Method) at java.lang.Class.newInstance(Class.java:1572) at android.app.Instrumentation.newActivity(Instrumentation.java:1065) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) 02-20 18:26:01.144 1754-1766/com.universo91.towersrock I/art﹕ Background sticky concurrent mark sweep GC freed 1745(118KB) AllocSpace objects, 0(0B) LOS objects, 38% free, 382KB/623KB, paused 950us total 131.218ms 02-20 18:26:04.429 1754-1754/com.universo91.towersrock I/Process﹕ Sending signal. PID: 1754 SIG: 9 

Answer 1

所有初始化都应该在Activity的onCreate()方法中执行：

 @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); this.cManager = (ConnectivityManager) this.getSystemService(Context.CONNECTIVITY_SERVICE); this.mTextView = (TextView)this.findViewById(R.id.mProgressText); } 

覆盖Activity的构造函数涉及相当多的繁重工作，并且真的不是在公园散步。 虽然你[当然]可以为Activity创建一个空构造函数，但在Android框架的上下文中它确实是多余的。

相关答案：

1. 为什么我无法将参数传递给Android Activity Constructor 。

2. 使用参数启动活动 。

Answer 2

你的代码应该在onCreate（）中

不要打扰活动的构造函数。

Answer 3

只要采用零参数，Android活动就可以拥有构造函数。 实际上，只要在声明它时初始化任何字段，就会有这样的构造函数。

但正如您的日志所述， System services not available to Activities before onCreate() ， System services not available to Activities before onCreate() ，因此您无法从构造函数中调用getSystemService 。

所以https://stackoverflow.com/a/28636652/53974中的代码是正确的，但原因不是。