Java 调用方法时的NPE

为了删除应用程序数据，我尝试使用反射，但在调用方法时获得NPE

Class<?> myClass = Class.forName("android.content.pm.IPackageManager");
Method method = myClass.getMethod("clearApplicationUserData", String.class,IPackageDataObserver.class);
method.setAccessible(true);
Log.v("info",method.getName());
Object c = myClass .newInstance();
method.invoke(c,"com.example.android.apis",null);  //NPE

01-31 17:50:26.405:W/System.err（969）：在dalvik.System.NativeStart.main（本机方法）

和完整的堆栈跟踪

 01-31 17:26:38.285: E/AndroidRuntime(920): Uncaught handler: thread main exiting due to uncaught exception
 01-31 17:26:38.295: E/AndroidRuntime(920): java.lang.RuntimeException: Unable to start activity ComponentInfo{f.c.v/f.c.v.ClearUserDataUsingInterfaceActivity}: java.lang.NullPointerException
 01-31 17:26:38.295: E/AndroidRuntime(920):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.access$2200(ActivityThread.java:119)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.os.Looper.loop(Looper.java:123)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.main(ActivityThread.java:4363)
01-31 17:26:38.295: E/AndroidRuntime(920):  at java.lang.reflect.Method.invokeNative(Native Method)
01-31 17:26:38.295: E/AndroidRuntime(920):  at java.lang.reflect.Method.invoke(Method.java:521)
01-31 17:26:38.295: E/AndroidRuntime(920):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
01-31 17:26:38.295: E/AndroidRuntime(920):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
01-31 17:26:38.295: E/AndroidRuntime(920):  at dalvik.system.NativeStart.main(Native Method)
01-31 17:26:38.295: E/AndroidRuntime(920): Caused by: java.lang.NullPointerException
01-31 17:26:38.295: E/AndroidRuntime(920):  at java.lang.reflect.Method.invokeNative(Native Method)
1-31 17:26:38.295: E/AndroidRuntime(920):   at java.lang.reflect.Method.invoke(Method.java:521)
01-31 17:26:38.295: E/AndroidRuntime(920):  at f.c.v.ClearUserDataUsingInterfaceActivity.onCreate(ClearUserDataUsingInterfaceActivity.java:57)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)
01-31 17:26:38.295: E/AndroidRuntime(920):  ... 11 more

似乎应该调用myClass.newInstance（）而不是deleteClass.newInstance（）

确保clearApplicationUserData（）方法知道为IPackageDataObserver处理空值

---

希望有帮助。

什么是

deleteClass

？您是否应该尝试在

myClass

上调用您的方法？

IPackageManager

是一个接口（如“I”前缀所示）。不能通过反射实例化接口

---

我猜你的意思是

android.content.pm.PackageManager

什么是

deleteClass

？给我们看stacktrace.给我们看stacktrace。NPE是否在

clearApplicationUserData

中抛出？很可能它不支持null作为参数。因此根本不支持NPE。实际上，当您尝试实例化和调用接口上的方法时，您会得到InstanceionException。如果我更改为method.invoke（myClass，“com.example.android.api”，new IPackageDataObserver.Stub（）{public void on removeCompleted（字符串packageName，boolean successed）抛出RemoteException{Log.v（“info”，“success”）；}}）；然后我得到一个异常，

对象不是类的实例

您应该更改

类myClass=class.forName（“android.content.pm.IPackageManager”）

因为IPackageManager是一个接口。检查后，您将了解为什么会得到InstanceionException。如何通过实例化实现IPackageManager接口的类来访问IPackageManager.AIDL中定义的这些方法。

 01-31 17:26:38.285: E/AndroidRuntime(920): Uncaught handler: thread main exiting due to uncaught exception
 01-31 17:26:38.295: E/AndroidRuntime(920): java.lang.RuntimeException: Unable to start activity ComponentInfo{f.c.v/f.c.v.ClearUserDataUsingInterfaceActivity}: java.lang.NullPointerException
 01-31 17:26:38.295: E/AndroidRuntime(920):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.access$2200(ActivityThread.java:119)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.os.Handler.dispatchMessage(Handler.java:99)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.os.Looper.loop(Looper.java:123)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.main(ActivityThread.java:4363)
01-31 17:26:38.295: E/AndroidRuntime(920):  at java.lang.reflect.Method.invokeNative(Native Method)
01-31 17:26:38.295: E/AndroidRuntime(920):  at java.lang.reflect.Method.invoke(Method.java:521)
01-31 17:26:38.295: E/AndroidRuntime(920):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
01-31 17:26:38.295: E/AndroidRuntime(920):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
01-31 17:26:38.295: E/AndroidRuntime(920):  at dalvik.system.NativeStart.main(Native Method)
01-31 17:26:38.295: E/AndroidRuntime(920): Caused by: java.lang.NullPointerException
01-31 17:26:38.295: E/AndroidRuntime(920):  at java.lang.reflect.Method.invokeNative(Native Method)
1-31 17:26:38.295: E/AndroidRuntime(920):   at java.lang.reflect.Method.invoke(Method.java:521)
01-31 17:26:38.295: E/AndroidRuntime(920):  at f.c.v.ClearUserDataUsingInterfaceActivity.onCreate(ClearUserDataUsingInterfaceActivity.java:57)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
01-31 17:26:38.295: E/AndroidRuntime(920):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)
01-31 17:26:38.295: E/AndroidRuntime(920):  ... 11 more