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;
}
