在Activity.onCreate()中,为什么Intent.getExtras()有时会返回null？

Question

这可能是一个误报,请参阅我自己的答案.原始问题如下:

活动有一个按钮,可以将用户带到另一个活动.为了启动新活动,我们使用extras和onCreate()填充我们的Intent,新活动通过Intent.getExtras()从这些额外内容中读取.我们假设返回的包是非空的,但是当发现客户崩溃报告时,getExtras()有时会返回null.

正如这个答案所示,无法保护额外内容是完全正常的,但是如果你填充了intent的附加内容,那么为什么它会在以后返回null呢？是否有更好的地方(如onResume())来阅读额外内容？

编辑:可能是因为我们没有遵循密钥所需的名称约定:

名称必须包含包前缀,例如app com.android.contacts将使用"com.android.contacts.ShowAll"之类的名称.

这是来自Intent.putExtras的javadoc.如果您不遵循此名称惯例会发生什么; 是甚至定义的行为？

这是相关的代码:

class FromActivity extends Activity {

    ImageButton button;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.from_view);

        button = (ImageButton)findViewById(R.id.button);
        button.setVisibility(View.VISIBLE);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(FromActivity.this, ToActivity.class);
                i.putExtra(ToActivity.SERVER_PARAM, ...);
                i.putExtra(ToActivity.UUID_PARAM, ...);
                i.putExtra(ToActivity.TEMPLATE_PARAM, ...);
                startActivityForResult(i, 0); 
                overrideTransition(R.anim.slide_left_in, R.anim.slide_left_out);
            }
        });
    }

}

class ToActivity extends Activity {

    public static final String SERVER_PARAM = "server";
    public static final String UUID_PARAM = "uuid";
    public static final String TEMPLATE_PARAM = "template";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Bundle extras = getIntent().getExtras();
        if (extras == null) {
            finish();
            return;
        }

        // do stuff with extras
    }
}

以下是此问题的示例堆栈跟踪:

java.lang.RuntimeException: Unable to start activity ComponentInfo{ToActivity}: java.lang.NullPointerException
    at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2355)
    at  android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2391)
    at  android.app.ActivityThread.access$600(ActivityThread.java:151)
    at  android.app.ActivityThread$H.handleMessage(ActivityThread.java:1335)
    at  android.os.Handler.dispatchMessage(Handler.java:99)
    at  android.os.Looper.loop(Looper.java:155)
    at  android.app.ActivityThread.main(ActivityThread.java:5493)
    at  java.lang.reflect.Method.invokeNative(Native Method)
    at  java.lang.reflect.Method.invoke(Method.java:511)
    at  com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
    at  com.android.internal.os.ZygoteInit.main(ZygoteInit.java:795)
    at  dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
    at  ToActivity.onCreate(SourceFile:49)
    at  android.app.Activity.performCreate(Activity.java:5066)
    at  android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1101)
    at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2311)
     ... 11 more
     java.lang.NullPointerException
    at  ToActivity.onCreate(SourceFile:49)
    at  android.app.Activity.performCreate(Activity.java:5066)
    at  android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1101)
    at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2311)
    at  android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2391)
    at  android.app.ActivityThread.access$600(ActivityThread.java:151)
    at  android.app.ActivityThread$H.handleMessage(ActivityThread.java:1335)
    at  android.os.Handler.dispatchMessage(Handler.java:99)
    at  android.os.Looper.loop(Looper.java:155)
    at  android.app.ActivityThread.main(ActivityThread.java:5493)
    at  java.lang.reflect.Method.invokeNative(Native Method)
    at  java.lang.reflect.Method.invoke(Method.java:511)
    at  com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1028)
    at  com.android.internal.os.ZygoteInit.main(ZygoteInit.java:795)
    at  dalvik.system.NativeStart.main(Native Method)

Answer 1

你可以这样得到它:

getIntent().getStringExtra(key);

要么:

getIntent().getExtras().getString(key)

并在"FromActivity"中将其设置为:

Bundle extras = new Bundle();
extras.putString(key, value);
intent.putExtras(extras);
//And start your activity...

要么:

intent.putExtra(key, string);
//And start your activity...

无论哪种方式都应该工作,希望这有助于......

问候!

Answer 2

这可能是一场虚惊。这更有可能是由于我们自己的实例变量为空：

class ToActivity extends Activity {

    public static final String SERVER_PARAM = "server";
    public static final String UUID_PARAM = "uuid";
    public static final String TEMPLATE_PARAM = "template";

    private State state;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        state = initializeState();
        // MISSING NULL CHECK HERE!

        Bundle extras = getIntent().getExtras();
        if (extras == null) {
            finish();
            return;
        }

        // do stuff with state and extras
    }
}