android.app.Fragment$InstantiationException的原因分析

1. Fragment$InstantiationException的原因分析

在编写Fragment类的代码时候，Android Lint有时会提示如下error：

如果的Fragment没有无参构造方法，app在恢复Activity时（例如旋转设备），会出现crash。

Q: 为什么必须要有一个无参构造方法，而且含参构造方法在Fragment重新实例化时不会调用？

接下来分析一下Activity恢复状态的过程。

1. Activity的onCreate(Bundle savedInstanceState)的方法

package android.app;

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback {

    final FragmentManagerImpl mFragments = new FragmentManagerImpl();

    protected void onCreate(@Nullable Bundle savedInstanceState) {
        ...
        if (savedInstanceState != null) {
            Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
            mFragments.restoreAllState(p, mLastNonConfigurationInstances != null
                    ? mLastNonConfigurationInstances.fragments : null);
        }
        mFragments.dispatchCreate();
        ...
    }
    ...
}

2. FragmentManagerImpl的restoreAllState(Parcelable state, ArrayList<Fragment> nonConfig)方法

该方法会调用FragmentState的instantiate(Activity activity, Fragment parent)方法。

package android.app;

final class FragmentManagerImpl extends FragmentManager implements LayoutInflater.Factory2 {
    ...
    void restoreAllState(Parcelable state, ArrayList<Fragment> nonConfig) {
        // If there is no saved state at all, then there can not be
        // any nonConfig fragments either, so that is that.
        if (state == null) return;
        FragmentManagerState fms = (FragmentManagerState)state;
        if (fms.mActive == null) return;
        
        ...
        for (int i=0; i<fms.mActive.length; i++) {
            FragmentState fs = fms.mActive[i];
            if (fs != null) {
                Fragment f = fs.instantiate(mActivity, mParent);
                if (DEBUG) Log.v(TAG, "restoreAllState: active #" + i + ": " + f);
                mActive.add(f);
                // Now that the fragment is instantiated (or came from being
                // retained above), clear mInstance in case we end up re-restoring
                // from this FragmentState again.
                fs.mInstance = null;
            } else {
                mActive.add(null);
                if (mAvailIndices == null) {
                    mAvailIndices = new ArrayList<Integer>();
                }
                if (DEBUG) Log.v(TAG, "restoreAllState: avail #" + i);
                mAvailIndices.add(i);
            }
        }
        ...
    }
}

最终会调用Fragment的静态工厂方法instantiate(Context context, String fname, @Nullable Bundle args)。

package android.app;

final class FragmentState implements Parcelable { 
    ...
    public Fragment instantiate(Activity activity, Fragment parent) {
        if (mInstance != null) {
            return mInstance;
        }

        if (mArguments != null) {
            mArguments.setClassLoader(activity.getClassLoader());
        }

        mInstance = Fragment.instantiate(activity, mClassName, mArguments);

        if (mSavedFragmentState != null) {
            mSavedFragmentState.setClassLoader(activity.getClassLoader());
            mInstance.mSavedFragmentState = mSavedFragmentState;
        }
        mInstance.setIndex(mIndex, parent);
        mInstance.mFromLayout = mFromLayout;
        mInstance.mRestored = true;
        mInstance.mFragmentId = mFragmentId;
        mInstance.mContainerId = mContainerId;
        mInstance.mTag = mTag;
        mInstance.mRetainInstance = mRetainInstance;
        mInstance.mDetached = mDetached;
        mInstance.mFragmentManager = activity.mFragments;
        if (FragmentManagerImpl.DEBUG) Log.v(FragmentManagerImpl.TAG,
                "Instantiated fragment " + mInstance);

        return mInstance;
    }
}

Fragment的重新实例化是利用Java反射机制，并且调用的是Fragment的无参构造方法，所以这一步是不会调用其他有参构造方法的。若Fragment没有无参构造方法，则clazz.newInstance()会抛出InstantiationExecption，然后就会打印出常见的一个Exception，"Unable to instantiate fragment " + fname + ": make sure class name exists, is public, and has an" + " empty constructor that is public"。

</pre><p><pre name="code" class="java">package android.app;

public class Fragment implements ComponentCallbacks2, OnCreateContextMenuListener { 
    private static final ArrayMap<String, Class<?>> sClassMap = new ArrayMap<String, Class<?>>(); 

    public static Fragment instantiate(Context context, String fname, @Nullable Bundle args) {
        try {
            Class<?> clazz = sClassMap.get(fname);
            if (clazz == null) {
                // Class not found in the cache, see if it's real, and try to add it
                clazz = context.getClassLoader().loadClass(fname);
                if (!Fragment.class.isAssignableFrom(clazz)) {
                    throw new InstantiationException("Trying to instantiate a class " + fname
                            + " that is not a Fragment", new ClassCastException());
                }
                sClassMap.put(fname, clazz);
            }
            Fragment f = (Fragment)clazz.newInstance();
            if (args != null) {
                args.setClassLoader(f.getClass().getClassLoader());
                f.mArguments = args;
            }
            return f;
        } catch (ClassNotFoundException e) {
            throw new InstantiationException("Unable to instantiate fragment " + fname
                    + ": make sure class name exists, is public, and has an"
                    + " empty constructor that is public", e);
        } catch (java.lang.InstantiationException e) {
            throw new InstantiationException("Unable to instantiate fragment " + fname
                    + ": make sure class name exists, is public, and has an"
                    + " empty constructor that is public", e);
        } catch (IllegalAccessException e) {
            throw new InstantiationException("Unable to instantiate fragment " + fname
                    + ": make sure class name exists, is public, and has an"
                    + " empty constructor that is public", e);
        }
    }
}

android.app.Fragment$InstantiationException的原因分析

原文：http://blog.csdn.net/sun927/article/details/46629919