Fragment强烈不推荐使用自定义带参的构造函数

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我是靠谱客的博主无语悟空，最近开发中收集的这篇文章主要介绍Fragment强烈不推荐使用自定义带参的构造函数，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

项目在运行monkey63小时左右，出现9次CRASH：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.aliyun.easylauncher/com.aliyun.easylauncher.dialpad.PhoneRecordActivity}: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.aliyun.easylauncher.dialpad.DialFragment: make sure class name exists, is public, and has an empty constructor that is public

正常的使用一切都是正常，Google后发现，Fragment有下面的注意事项：

public Fragment ()

Default constructor. Every 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 setArguments(Bundle) and later retrieved by the Fragment withgetArguments().

Applications should generally not implement a constructor. The first place application code an run where the fragment is ready to be used is in onAttach(Activity), the point where the fragment is actually associated with its activity. Some applications may also want to implementonInflate(Activity, AttributeSet, Bundle) to retrieve attributes from a layout resource, though should take care here because this happens for the fragment is attached to its activity.

看了一下frameworks/support/v4/java/android/support/v4/app/Fragment.java（因为import android.support.v4.app.Fragment）

394 public static Fragment instantiate(Context context, String fname, Bundle args) {
395 try {
396 Class<?> clazz = sClassMap.get(fname);
397 if (clazz == null) {
398 // Class not found in the cache, see if it's real, and try to add it
399 clazz = context.getClassLoader().loadClass(fname);
400 sClassMap.put(fname, clazz);
401 }
402 Fragment f = (Fragment)clazz.newInstance(); //问题出在这里
403 if (args != null) {
404 args.setClassLoader(f.getClass().getClassLoader());
405 f.mArguments = args;
406 }
407 return f;
408 } catch (ClassNotFoundException e) {
409 throw new InstantiationException("Unable to instantiate fragment " + fname
410 + ": make sure class name exists, is public, and has an"
411 + " empty constructor that is public", e);
412 } catch (java.lang.InstantiationException e) {
413 throw new InstantiationException("Unable to instantiate fragment " + fname
414 + ": make sure class name exists, is public, and has an"
415 + " empty constructor that is public", e);
416 } catch (IllegalAccessException e) {
417 throw new InstantiationException("Unable to instantiate fragment " + fname
418 + ": make sure class name exists, is public, and has an"
419 + " empty constructor that is public", e);
420 }
421

解决方案（来自stackoverflow），原文链接： http://stackoverflow.com/questions/10450348/do-fragments-really-need-an-empty-constructor

You shouldn't really be overriding the constructor anyway. You should have a newInstance() static method defined and pass any parameters via arguments (bundle)

For example:

public static final AlertFragment newInstance(int title, String message)
{
    AlertFragment f = new AlertFragment();
    Bundle bdl = new Bundle(2);
    bdl.putInt(EXTRA_TITLE, title);
    bdl.putString(EXTRA_MESSAGE, message);
    f.setArguments(bdl);
    return f;
}

And of course grabbing the args this way:

@Override
public void onCreate(Bundle savedInstanceState)
{
    title = getArguments().getInt(EXTRA_TITLE);
    message = getArguments().getString(EXTRA_MESSAGE);

    //...
    //etc
    //...
}

Then you would instantiate from your fragment manager like so:

public onCreate(Bundle savedInstanceState) {
    if(savedInstanceState == null){
        getSupportFragmentManager()
            .beginTransaction()
            .replace(R.id.content,AlertFragment.newInstance(
                R.string.alert_title,
                "Oh noes an error occured!")
            )
            .commit();
    }
}