为什么使用Fragment时必须提供一个无参的构造函数？

问题

最近在线上bugly看到一个近两个版本的出现的一个bug：

#5712 java.lang.NoSuchMethodException
<init> []
com.jess.arms.a.c.onCreate(BaseActivity.java:86)
java.lang.RuntimeException:Unable to start activity 
ComponentInfo{xxxx.VehicleDetailActivity}: 
android.support.v4.app.Fragment$InstantiationException: 
Unable to instantiate fragment xxxx.mvp.ui.fragment.c: 
could not find Fragment constructor

这个bug是方法找不到异常，因为项目中用了mvparms框架，刚开始看到异常指向他的BaseActivity类，还以为是框架出了问题。但是后面注意到是项目的VehicleDetailActivity报的could not find Fragment constructor，即fragment构造器找不到的异常。于是马上将问题定位到了该类新添加的一个DialogFragment上，先看一下这个类的构造方法：

public MorePlanFragment(String modelId, HashMap<String, Integer> map) {
    this.modelId = modelId;
    this.posMap = map;
    this.tabPos = map.get(POS_MAP_TAB);
}

这个MorePlanFragment使用了有参构造函数，而问题就出现在这里。

分析

既然报的找不到构造方法的错误，我们先来看一下Fragment的构造函数：

/**
 * Default constructor.  <strong>Every</strong> fragment must have an
 * empty constructor, so it can be instantiated when restoring its
 * activity's state.  It is strongly recommended that subclasses do not
 * have other constructors with parameters, since these constructors
 * will not be called when the fragment is re-instantiated; instead,
 * arguments can be supplied by the caller with {@link #setArguments}
 * and later retrieved by the Fragment with {@link #getArguments}.
 *
 * <p>Applications should generally not implement a constructor. Prefer
 * {@link #onAttach(Context)} instead. It is the first place application code can run where
 * the fragment is ready to be used - the point where the fragment is actually associated with
 * its context. Some applications may also want to implement {@link #onInflate} to retrieve
 * attributes from a layout resource, although note this happens when the fragment is attached.
 */
public Fragment() {
}

构造函数上有一段注释：

默认构造器。
每一个Fragment必须有一个无参的构造函数，以便当Activity恢复状态时fragment可以实例化。
强烈建议fragment的子类不要有其他的有参构造函数，因为当fragment重新实例化时不会调用这些有参构造函数；
如果要传值应该使用setArguments方法，在需要获取这些值时调用getArguments方法。

这一段注释明确的告诉我们使用有参构造函数会出问题，建议使用无参构造函数，但是并没有告诉我们具体是哪里的问题。我们在Fragment中搜索could not find Fragment constructor这个异常，发现是在instantiate方法中抛出的。

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);
            sClassMap.put(fname, clazz);
        }
        Fragment f = (Fragment) clazz.getConstructor().newInstance();
        if (args != null) {
            args.setClassLoader(f.getClass().getClassLoader());
            f.setArguments(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);
    } catch (NoSuchMethodException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": could not find Fragment constructor", e);
    } catch (InvocationTargetException e) {
        throw new InstantiationException("Unable to instantiate fragment " + fname
                + ": calling Fragment constructor caused an exception", e);
    }
}

看上面的代码我们可以知道，Fragment的实例化是通过调用类对象的getConstructor()方法获取构造器对象并调用其newInstance()方法创建对象的。此时还会将args参数设置给Fragment。现在找到了具体报错的地方，但是这个方法是在哪里调用触发的呢？在Fragment没有找到调用的地方，由于Fragment是由FragmentManager管理的，在该类发现是在restoreAllState方法中调用的。

void restoreAllState(Parcelable state, FragmentManagerNonConfig nonConfig) {
        // Build the full list of active fragments, instantiating them from
        // their saved state.
        mActive = new SparseArray<>(fms.mActive.length);
        for (int i=0; i<fms.mActive.length; i++) {
            FragmentState fs = fms.mActive[i];
            if (fs != null) {
                FragmentManagerNonConfig childNonConfig = null;
                if (childNonConfigs != null && i < childNonConfigs.size()) {
                    childNonConfig = childNonConfigs.get(i);
                }
                ViewModelStore viewModelStore = null;
                if (viewModelStores != null && i < viewModelStores.size()) {
                    viewModelStore = viewModelStores.get(i);
                }
                Fragment f = fs.instantiate(mHost, mContainer, mParent, childNonConfig,
                        viewModelStore);
                if (DEBUG) Log.v(TAG, "restoreAllState: active #" + i + ": " + f);
                mActive.put(f.mIndex, 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;
            }
        }
    ...   
    }

这方法名意为恢复所有的状态，而其中注释为创建激活Fragment的列表，并将他们从保存的状态中实例化。这个方法应该是Fragment重新实例化时调用的方法。该方法在Fragment的restoreChildFragmentState被调用。

void restoreChildFragmentState(@Nullable Bundle savedInstanceState) {
    if (savedInstanceState != null) {
        Parcelable p = savedInstanceState.getParcelable(
                FragmentActivity.FRAGMENTS_TAG);
        if (p != null) {
            if (mChildFragmentManager == null) {
                instantiateChildFragmentManager();
            }
            mChildFragmentManager.restoreAllState(p, mChildNonConfig);
            mChildNonConfig = null;
            mChildFragmentManager.dispatchCreate();
        }
    }
}

restoreChildFragmentState方法又在Fragment的onCreate方法中调用，这里将保存的savedInstanceState状态又传递给了restoreChildFragmentState以完成Fragment的重新实例化。

@CallSuper
public void onCreate(@Nullable Bundle savedInstanceState) {
    mCalled = true;
    restoreChildFragmentState(savedInstanceState);
    if (mChildFragmentManager != null
            && !mChildFragmentManager.isStateAtLeast(Fragment.CREATED)) {
        mChildFragmentManager.dispatchCreate();
    }
}

结论

经过以上的分析，我们就知道了为什么这个错误出在了Fragment的有参构造函数上。因为当Fragment因为某种原因重新创建时，会调用到onCreate方法传入之前保存的状态，在instantiate方法中通过反射无参构造函数创建一个Fragment，并且为Arguments初始化为原来保存的值，而此时如果没有无参构造函数就会抛出异常，造成程序崩溃。
所以Fragment的构造函数以及参数传递正确使用方式为如下：

public static MorePlanFragment newInstance(String modelId,HashMap<String, Integer> map) {
    Bundle args = new Bundle();
    args.putString("modelId",modelId);
    args.putSerializable("map",map);
    MorePlanFragment fragment = new MorePlanFragment();
    fragment.setArguments(args);
    return fragment;
}