一、类型转换问题
插件化过程中经常遇到这么一个问题：
java.lang.ClassCastException: com.trampcr.CustomView cannot be cast to com.trampcr.CustomView
明明相同的两个类，为什么会出现类型转换错误呢？
我们知道判断两个类是否相同的依据有两点：
- 全路径是否相同
- 加载这两个类的 ClassLoader 是否相同
这里明显全路径相同，那就是加载这两个类的 ClassLoader 不同，验证一下：
// 打印当前类（CustomView.java）和获取到的 view 的 Classloader
View rootView = LayoutInflater.from(context).inflate(R.layout.custom_view_layout, null);
View view = null;
try {
view = (View) rootView.findViewById(R.id.custom_view);
} catch (Exception e) {
e.printStackTrace();
}
ClassLoader curClassloader = this.class.getClassLoader();
Log.e("trampcr", "current ClassLoader = " + curClassloader);
if (null != view) {
ClassLoader classloader = view.getClass().getClassLoader();
while (null != classloader) {
Log.e("trampcr", "inflate ClassLoader = " + classloader);
classloader = classloader.getParent();
}
}
//打印结果
current ClassLoader = BundleClassLoader[Bundlecom.trampcr.plugin.alpha]
inflate ClassLoader = BundleClassLoader[Bundlecom.trampcr.plugin.beta]
inflate ClassLoader = java.lang.BootClassLoader@9e408d8
验证发现：当前类和获取到的 view 的 Classloader 确实不同。
这里先说明原因及解决方案，后续再通过源码分析进行说明。
原因： LayoutInflater.from(context).inflate() 底层是通过反射创建 View 对象，使用的 ClassLoader 和当前的 ClassLoader 不相同。
解决方案：：重写 LayoutInflater、LayoutInflater.Factory、LayoutInflater.Factory2，将 xml 转换成 View 对象时传入 ClassLoader，保证创建 View 的 ClassLoader 和当前 ClassLoader 相同。
这里说的可能有点抽象，看完源码分析和解决方案分析，再返回来看这句话。
二、LayoutInflater 源码解析
从 LayoutInflater 的使用入手看起：
LayoutInflater.from(context).inflate(R.layout.custom_view_layout, null);
这里可以拆为两步：
- LayoutInflater.from()：创建 LayoutInflater 对象
- layoutInflater.inflate()：创建 View 对象
1、LayoutInflater.from()：创建 LayoutInflater 对象
// LayoutInflater.java from() 方法作用：创建 LayoutInflater 对象
public static LayoutInflater from(Context context) {
LayoutInflater LayoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (LayoutInflater == null) {
throw new AssertionError("LayoutInflater not found.");
}
return LayoutInflater;
}
这里调用 context.getSystemService(Context.LAYOUT_INFLATER_SERVICE) 创建 LayoutInflater 对象，Context 是一个抽象类，它的实现是 ContextImpl。
// ContextImpl.java
public Object getSystemService(String name) {
return SystemServiceRegistry.getSystemService(this, name);
}
// SystemServiceRegistry.java
// private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new HashMap<String, ServiceFetcher<?>>();
public static Object getSystemService(ContextImpl ctx, String name) {
ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
return fetcher != null ? fetcher.getService(ctx) : null;
}
ContextImpl.getSystemService() 调用了 SystemServiceRegistry.getSystemService()，然后从 SYSTEM_SERVICE_FETCHERS 这个 HashMap 中获取 ServiceFetcher 对象，通过 ServiceFetcher.getService() 获取我们想要的对象。
有 get() 肯定有 put()，找到 put() 是在 registerService() 方法中。
// SystemServiceRegistry.java
// 静态注册系统服务，这个方法在初始化时必须调用
private static <T> void registerService(String serviceName, Class<T> serviceClass, ServiceFetcher<T> serviceFetcher) {
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}
接着往回看，哪里调用了 registerService()，最后发现在 SystemServiceRegistry 类的开头，有一个 static 代码块，其中包含了很多 registerService() 方法，我们常用的 ActivityManager、WindowManger 等服务都在这里注册的。
SystemServiceRegistry.java
final class SystemServiceRegistry {
...
static {
...
registerService(Context.ACTIVITY_SERVICE, ActivityManager.class,
new CachedServiceFetcher<ActivityManager>() {
@Override
public ActivityManager createService(ContextImpl ctx) {
return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
}});
...
registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
new CachedServiceFetcher<LayoutInflater>() {
@Override
public LayoutInflater createService(ContextImpl ctx) {
return new PhoneLayoutInflater(ctx.getOuterContext());
}});
...
registerService(Context.WINDOW_SERVICE, WindowManager.class,
new CachedServiceFetcher<WindowManager>() {
@Override
public WindowManager createService(ContextImpl ctx) {
return new WindowManagerImpl(ctx.getDisplay());
}});
...
}
}
这里我们只看 LayoutInflater 的创建，因为 LayoutInflater 也是一个抽象类，所以这里创建的是 PhoneLayoutInflater 对象。
2、layoutInflater.inflate()：创建 View 对象
// LayoutInflater.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
// LayoutInflater.java
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
// LayoutInflater.java
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;
try {
...
final String name = parser.getName();
if (TAG_MERGE.equals(name)) {
// merge 标签情况下，root 不能为空
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
+ "ViewGroup root and attachToRoot=true");
}
// 这里其实最终调用的也是 createViewFromTag()
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 通过标签创建 View
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
...
// 加载子 View
rInflateChildren(parser, temp, attrs, true);
...
}
}
...
return result;
}
}
// LayoutInflater.java
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
...
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
...
final String name = parser.getName();
// requestFocus
if (TAG_REQUEST_FOCUS.equals(name)) {
parseRequestFocus(parser, parent);
// tag
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
// include
} else if (TAG_INCLUDE.equals(name)) {
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
// merge
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
// 最终还是调用 createViewFromTag()
final View view = createViewFromTag(parent, name, context, attrs);
...
}
}
...
}
// LayoutInflater.java
private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
return createViewFromTag(parent, name, context, attrs, false);
}
// LayoutInflater.java
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs, boolean ignoreThemeAttr) {
...
try {
View view;
// 尝试用 Factory 和 Factory2 来创建 view，这里也是解决类型转换的关键
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
// 如果 Factory 和 Factory2 没有创建出 View，则最终会调用 createView()
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
// onCreateView() 最终会调用 createView()
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
...
}
}
return view;
}
...
}
// LayoutInflater.java
public final View createView(String name, String prefix, AttributeSet attrs) throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(name);
Class<? extends View> clazz = null;
try {
...
if (constructor == null) {
clazz = mContext.getClassLoader().loadClass(prefix != null ? (prefix + name) : name).asSubclass(View.class);
...
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
sConstructorMap.put(name, constructor);
} else {
...
}
...
// 最终调用 constructor.newInstance() 创建 View
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
return view;
}
...
}
总结一下创建 View 对象的过程：
1、传入 resId，根据 resId 获取到 Xml 文件。
2、解析 Xml 文件，根据标签进行不同的操作（优化等）。
3、通过 Factory 和 Factory2 尝试创建 View。
4、Factory 和 Factory2 如果没有成功创建 View，最终通过 mContext.getClassLoader() 反射创建 View。
由于插件化的原因，这里的 mContext.getClassLoader() 和我们当前的 ClassLoader 不同，所以出现类型转换错误。
这里比较关键的是最后两步，我们可以通过继承 Factory 和 Factory2 自定义 Factory，把 ClassLoader 传进来，然后通过同一个 ClassLoader 来创建 View，具体过程看第三部分。
三、解决类型转换方案
1、封装 LayoutInflater 的 from() 方法，传入 ClassLoader。
2、继承 Factory 和 Factory2 自定义 Factory，实现 onCreateView() 方法，自己实现 ClassLoader 创建 View 的过程。
public class PluginLayoutInflater {
// 1、封装 LayoutInflater 的 from() 方法，传入 ClassLoader
public static LayoutInflater from(Context context, ClassLoader classLoader) {
LayoutInflater layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
if (layoutInflater == null) {
throw new AssertionError("LayoutInflater not found.");
}
LayoutInflater.Factory2 baseFactory2 = layoutInflater.getFactory2();
LayoutInflater.Factory baseFactory1 = layoutInflater.getFactory();
PluginInflaterFactory factory;
if (checkBaseFactory2(baseFactory2)) {
// 2、将 ClassLoader 进一步传入自定义 Factory 中
factory = new PluginInflaterFactory(baseFactory2, classLoader);
setFactory2(layoutInflater, factory);
} else if (checkBaseFactory1(baseFactory1)) {
// 2、将 ClassLoader 进一步传入自定义 Factory 中
factory = new PluginInflaterFactory(baseFactory1, classLoader);
setFactory(layoutInflater, factory);
}
return layoutInflater;
}
private static boolean checkBaseFactory1(LayoutInflater.Factory baseFactory) {
if (baseFactory == null) {
return true;
}
if (baseFactory instanceof PluginInflaterFactory) {
return false;
}
return true;
}
private static boolean checkBaseFactory2(LayoutInflater.Factory2 baseFactory2) {
if (baseFactory2 == null || baseFactory2 instanceof PluginInflaterFactory) {
return false;
}
return true;
}
private static void setFactory(LayoutInflater layoutInflater, PluginInflaterFactory factory) {
try {
Field mFactory = LayoutInflater.class.getDeclaredField("mFactory");
mFactory.setAccessible(true);
mFactory.set(layoutInflater, factory);
} catch (NoSuchFieldException e1) {
e1.printStackTrace();
} catch (IllegalAccessException e1) {
e1.printStackTrace();
}
}
private static void setFactory2(LayoutInflater layoutInflater, PluginInflaterFactory factory) {
try {
Field mFactory = LayoutInflater.class.getDeclaredField("mFactory2");
mFactory.setAccessible(true);
mFactory.set(layoutInflater, factory);
} catch (NoSuchFieldException e1) {
e1.printStackTrace();
} catch (IllegalAccessException e1) {
e1.printStackTrace();
}
}
}
// 3、继承 Factory 和 Factory2 自定义 Factory
public class PluginInflaterFactory implements LayoutInflater.Factory, LayoutInflater.Factory2 {
private static final String TAG = "PluginInflaterFactory";
private LayoutInflater.Factory mBaseFactory;
private LayoutInflater.Factory2 mBaseFactory2;
private ClassLoader mClassLoader;
public PluginInflaterFactory(LayoutInflater.Factory base, ClassLoader classLoader) {
if (null == classLoader) {
throw new IllegalArgumentException("classLoader is null");
}
mBaseFactory = base;
mClassLoader = classLoader;
}
public PluginInflaterFactory(LayoutInflater.Factory2 base2, ClassLoader classLoader) {
if (null == classLoader) {
throw new IllegalArgumentException("classLoader is null");
}
mBaseFactory2 = base2;
mClassLoader = classLoader;
}
// 4、实现 onCreateView() 方法
@Override
public View onCreateView(String s, Context context, AttributeSet attributeSet) {
if (!s.contains(".")) {
return null;
}
View v = getView(s, context, attributeSet);
if (v != null) {
return v;
}
if (mBaseFactory != null && !(mBaseFactory instanceof PluginInflaterFactory)) {
v = mBaseFactory.onCreateView(s, context, attributeSet);
}
return v;
}
// 4、实现 onCreateView() 方法
@Override
public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
if (!name.contains(".")) {
return null;
}
View v = getView(name, context, attrs);
if (v != null) {
return v;
}
if (mBaseFactory2 != null && !(mBaseFactory2 instanceof PluginInflaterFactory)) {
v = mBaseFactory2.onCreateView(parent, name, context, attrs);
}
return v;
}
// 5、自己实现 ClassLoader 创建 View 的过程
private View getView(String name, Context context, AttributeSet attrs) {
View v = null;
try {
Class<?> clazz = mClassLoader.loadClass(name);
Constructor c = clazz.getConstructor(Context.class, AttributeSet.class);
v = (View) c.newInstance(context, attrs);
} catch (ClassNotFoundException e) {
} catch (NoSuchMethodException e) {
} catch (IllegalAccessException e) {
} catch (InstantiationException e) {
} catch (InvocationTargetException e) {
}
return v;
}
}
具体使用：
LayoutInflater pluginInflater = PluginLayoutInflater.from(this, this.getClass().getClassLoader());
