android SharedPreferences null keys, values and sets<?> - corner cases

问题

Been extensively testing the SharedPreferences framework. While most works as one would expect I run across some cases where I wonder what's the reasoning behind them. I give some tests all of which pass - their common set up is :

Context ctx;
SharedPreferences prefs;
Editor ed;
protected void setUp() throws Exception {
    super.setUp();
    ctx = getContext();
    prefs = PreferenceManager.getDefaultSharedPreferences(ctx);
    ed = prefs.edit();
}
protected void tearDown() throws Exception {
    if (ed != null) ed.clear().commit();
    super.tearDown();
}

Now the weird points :

1. When I put a null value in, I have to ask for it with a null default to get it back - if the default is non null I get this default back. Even if the default is of a different type from the one I put in. Applies to String and Set<String> (but I can get back a boolean for instance) :

   public void testNullString() {
       ed.putString("string_key", null); // putString() and putStringSet() only
       ed.commit();
       assertTrue(prefs.contains("string_key"));
       assertEquals(null, prefs.getString("string_key", null));
       // if I specify a non null default I get this default back, not null
       assertEquals("a string", prefs.getString("string_key", "a string"));
       // *even if I ask for a boolean*
       assertEquals(true, prefs.getBoolean("string_key", true));
   }
   

2. I can easily put a Set<Integer> inside the prefs :

   @SuppressWarnings("unchecked")
   @TargetApi(Build.VERSION_CODES.HONEYCOMB)
   public void testNullStringSetsRaw() {
       if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
           @SuppressWarnings("rawtypes")
           final Set integerHashSet = new HashSet();
           integerHashSet.add(1);
           ed.putStringSet("set_key", integerHashSet);
           ed.commit();
           final Set<String> defValues = new HashSet<String>();
           defValues.add("a string");
           Set<String> s = prefs.getStringSet("set_key", defValues);
           final String msg = "The set I put in: " + integerHashSet
               + " and what I got out :" + s;
           Log.e(TAG, msg); // the same - [1]
           assertTrue(msg, integerHashSet.equals(s));
           assertTrue(s.contains(1)); // !
       }
   }
   

3. What's the deal with null keys ? They seem to be perfectly legal keys:

   public void testNullKeyNonNullString() {
       final String NULL_KEY = null;
       ed.putString(NULL_KEY, "a string");
       ed.commit();
       assertTrue("Contains null key", prefs.contains(NULL_KEY));
       assertEquals("Retrieve the value giving null default", "a string",
           prefs.getString(NULL_KEY, null));
       assertEquals("Retrieve the value giving default", "a string",
           prefs.getString(NULL_KEY, "a string" + "blah"));
       try {
           // no deal !
           prefs.getBoolean(NULL_KEY, true);
           fail("Expected CCE");
       } catch (ClassCastException e) {}
   }
   

   but I have seen in my logs things like : org.xmlpull.v1.XmlPullParserException: Map value without name attribute: boolean - see android getSharedPreferences error: Map value without name attribute: boolean for a discussion. I wonder if this is related to null keys. EDIT : it is related- 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)
   

   and apparently the preferences are cleared when the exception is thrown (bad bad bad)
   

So my questions are : is indeed the behavior as I state it (or have I missed something silly) ? What is the motivation behind it (especially the null values behavior) ? Is it documented and guaranteed to stay so - or may change ? Is point 2 an oversight ?

Test project.

回答1:

1. It's in the code

2. It's me not understanding generics:

   You can easily put a String into a HashSet<Integer> by using the raw type HashSet

3. Posted an issue here: http://code.google.com/p/android/issues/detail?id=63463 (after starting a discussion in google groups with the usual noop results). Actually as of 2014.01.07 the issue is marked Future Release :)

Test project.

来源：https://stackoverflow.com/questions/19610569/android-sharedpreferences-null-keys-values-and-sets-corner-cases