[android-developers] Re: why does SharedPreferences framework allow you to insert a preference with a null key - given it can't reload the preferences afterwards?

I quickly peeked into the source code and well, this is the way how it is programmed. When an exception occurs while the preferences data file gets read, SharedPreferences sets internally an empty map so you start from scratch. I even dug a bit deeper. The XML serializer just ignores NULL keys and creates XML output that cannot be properly read anymore through the map deserialization method which seems to expect an existing key value.

As for why it has been programmed like that... I think the reasoning may be that preferences are not deemed to be of so much importance that it should make the app crash in case of failure. This error state is silently discarded and you start over with the defaults. I think that's a reasonable approach since any app should be able to start over with empty preferences.

In this particular case you may have discovered a tiny bug you may want to report. But to be honest, using null keys is a pretty unusual thing to do.

On Wednesday, October 30, 2013 5:48:12 AM UTC-5, Palmer Eldritch wrote:

The preferences are apparently cleared when one tries to load them when there is a null key which is bad ! Reproducer :

    public class XmlExceptionTest extends AndroidTestCase {
        /** Run it twice - on the second run the exception is thrown */
        public void testXmlException() {
            Context ctx = getContext();
            SharedPreferences prefs = PreferenceManager
                .getDefaultSharedPreferences(ctx); // exception thrown here (line 18)
            // and apparently it clears the prefs as the condition below is false
            if (prefs.contains("run_once")) { // false
                Log.w("XmlExceptionTest",
                    "contains null key :" + prefs.contains(null));
            }
            Editor e = prefs.edit();
            e.putBoolean("run_once", true).commit();
            e.putString(null, "I put a sting with null key").commit();
            assertTrue("Contains null", prefs.contains(null));
            PreferenceManager.getDefaultSharedPreferences(ctx); // exception
            // NOT thrown here  - why ? - apparently there is a static factory
            // returning the instance it already constructed
            // e.clear().commit(); // this eliminates the exception
        }
    }
   
exception :

    W/ApplicationContext(): getSharedPreferences
    W/ApplicationContext(): org.xmlpull.v1.XmlPullParserException: Map value without name attribute: string
    W/ApplicationContext():     at com.android.internal.util.XmlUtils.readThisMapXml(XmlUtils.java:521)
    W/ApplicationContext():     at com.android.internal.util.XmlUtils.readThisValueXml(XmlUtils.java:733)
    W/ApplicationContext():     at com.android.internal.util.XmlUtils.readValueXml(XmlUtils.java:667)
    W/ApplicationContext():     at com.android.internal.util.XmlUtils.readMapXml(XmlUtils.java:470)
    W/ApplicationContext():     at android.app.ContextImpl.getSharedPreferences(ContextImpl.java:361)
    W/ApplicationContext():     at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:348)
    W/ApplicationContext():     at gr.uoa.di.android.helpers.test.XmlExceptionTest.testXmlException(XmlExceptionTest.java:18)
    W/ApplicationContext():     at java.lang.reflect.Method.invokeNative(Native Method)
    W/ApplicationContext():     at java.lang.reflect.Method.invoke(Method.java:521)
    W/ApplicationContext():     at junit.framework.TestCase.runTest(TestCase.java:154)
    W/ApplicationContext():     at junit.framework.TestCase.runBare(TestCase.java:127)
    W/ApplicationContext():     at junit.framework.TestResult$1.protect(TestResult.java:106)
    W/ApplicationContext():     at junit.framework.TestResult.runProtected(TestResult.java:124)
    W/ApplicationContext():     at junit.framework.TestResult.run(TestResult.java:109)
    W/ApplicationContext():     at junit.framework.TestCase.run(TestCase.java:118)
    W/ApplicationContext():     at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:169)
    W/ApplicationContext():     at android.test.AndroidTestRunner.runTest(AndroidTestRunner.java:154)
    W/ApplicationContext():     at android.test.InstrumentationTestRunner.onStart(InstrumentationTestRunner.java:520)
    W/ApplicationContext():     at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1447)

Posted in SO here and in the relevant thread here - but still no answers

Any ideas ?

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