1.概述
今天我们来看一看纯java代码打补丁的方式会是怎样，纯Java代码是什么意思？因为上一期讲到阿里开源的热补丁里面涉及到NDK，会是会用但要自己去写NDK很多人估计不考谱，今天我们就用一种最简单的方式去实现，灵感来自腾讯提供的解决方案Tinker，但是我们自己的实现方式与它又不相同。上一周要大家去看类的加载机制也不知道大家看得怎么样了，某些估计连BaseDexClassLoader的源码都找不到，这里提供一个在线阅读网站http://androidxref.com。
视频讲解：http://pan.baidu.com/s/1dE4UsbZ
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2017Android进阶之路与你同行
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2.源码阅读
2.1 Activity启动流程
为很么要读Activity的流程呢？因为到后面我们要讲插件开发那也是个蒙B的坎，了解了解也好，但这里我就不介绍那么详细，后面插件开发再说，我只想知道Activity是怎么创建的呢？我贴点源码出来：
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// ........省略代码
Activity activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
// ........省略代码
}
/**
* Perform instantiation of the process's {@link Activity} object. The
* default implementation provides the normal system behavior.
*
* @param cl The ClassLoader with which to instantiate the object.
* @param className The name of the class implementing the Activity
* object.
* @param intent The Intent object that specified the activity class being
* instantiated.
*
* @return The newly instantiated Activity object.
*/
public Activity newActivity(ClassLoader cl, String className,
Intent intent)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
// 利用ClassLoader通过名字加载类然后通过反射创建对象
return (Activity)cl.loadClass(className).newInstance();
}
2.2 ClassLoader源码解析
Activity的ClassLoader我们仔细看源码是 PathClassLoader extends BaseDexClassLoader extends ClassLoader 而loadClass这个方法在ClassLoader 中：
20160314140715580.png
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
// First, check if the class has already been loaded
Class c = findLoadedClass(name);
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
c = findClass(name);
// this is the defining class loader; record the stats
}
return c;
}
BaseDexClassLoader部分源码：
public class BaseDexClassLoader extends ClassLoader {
private final DexPathList pathList;
/**
* Constructs an instance.
*
* @param dexPath the list of jar/apk files containing classes and
* resources, delimited by {@code File.pathSeparator}, which
* defaults to {@code ":"} on Android
* @param optimizedDirectory directory where optimized dex files
* should be written; may be {@code null}
* @param libraryPath the list of directories containing native
* libraries, delimited by {@code File.pathSeparator}; may be
* {@code null}
* @param parent the parent class loader
*/
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(parent);
this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
Class c = pathList.findClass(name, suppressedExceptions);
if (c == null) {
ClassNotFoundException cnfe = new ClassNotFoundException("Didn't find class \"" + name + "\" on path: " + pathList);
for (Throwable t : suppressedExceptions) {
cnfe.addSuppressed(t);
}
throw cnfe;
}
return c;
}
}
这部分代码请务必看懂很简单，如果看不懂周末可以看看视频，从源码得知，当我们需要加载一个class时，实际是从pathList中去找的，而pathList则是DexPathList的一个实体。
DexPathList部分源码：
/*package*/ final class DexPathList {
private static final String DEX_SUFFIX = ".dex";
private static final String JAR_SUFFIX = ".jar";
private static final String ZIP_SUFFIX = ".zip";
private static final String APK_SUFFIX = ".apk";
/** class definition context */
private final ClassLoader definingContext;
/**
* List of dex/resource (class path) elements.
* Should be called pathElements, but the Facebook app uses reflection
* to modify 'dexElements' (http://b/7726934).
*/
private final Element[] dexElements;
/**
* Finds the named class in one of the dex files pointed at by
* this instance. This will find the one in the earliest listed
* path element. If the class is found but has not yet been
* defined, then this method will define it in the defining
* context that this instance was constructed with.
*
* @param name of class to find
* @param suppressed exceptions encountered whilst finding the class
* @return the named class or {@code null} if the class is not
* found in any of the dex files
*/
public Class findClass(String name, List<Throwable> suppressed) {
for (Element element : dexElements) {
DexFile dex = element.dexFile;
if (dex != null) {
Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);
if (clazz != null) {
return clazz;
}
}
}
if (dexElementsSuppressedExceptions != null) {
suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
}
return null;
}
}
从这段源码可以看出，dexElements是用来保存dex的数组，而每个dex文件其实就是DexFile对象。遍历dexElements，然后通过DexFile去加载class文件，加载成功就返回，否则返回null，看到这里应该基本知道我们想干啥了，我打算在dexElements上面做手脚，而且解释也说了Should be called pathElements, but the Facebook app uses reflection 我想问问跟Facebook啥子关系，不管了它说可以用reflection反射。
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我们打算采用dex分包，从服务上获取到fix.dex后，采用反射机制往正在运行的ClassLoader中的PathList中的dexElements中去插入我们fix.dex中的dexElements，并且把它插入到正在运行的ClassLoader最前面，这样我们for循环找class类的时候就会找我们的fix.dex中的class了，原来有Bug的calss就不会被遍历到了。
3.Dex分包问题
如果你是用的Eclipse做开发，不过现在应该很少了吧，Eclipse分包比较蛋疼需要去写脚本不过好就好在这一快的资料比较多。
Android Studio我们可以直接配置，但是网上的资料都是用来解决方法数超过65536，问题是我们现在没有超过按照网上提供的配置根本就不会分包，当然我们还有很多方法如获取到class类自己用命令制作dex包，但是我想问一下就算是知道哪一个class报错了修改混淆后牵连的东西会太多估计也行不同。
我们其实还是可以用Android Studio自带的我们去官网找最新的，百度搜索提供的分包方案比较老了所以行不通，且看我如何配置:
dexOptions {//dex配置
javaMaxHeapSize "4g"
preDexLibraries = false
def listFile = project.rootDir.absolutePath+'/app/maindexlist.txt'
additionalParameters = [//dex参数详见 dx --help
'--multi-dex',//多分包
'--set-max-idx-number=60000',//每个包内方法数上限
'--main-dex-list='+listFile,//打包进主classes.dex的文件列表
'--minimal-main-dex'//使上一句生效
]
}
在build.gradle中加入以上配置，我们再在maindexlist.txt中保存我们主dex的类即可运行，解压apk如果可以看到有两个dex，代表这一步已经成功了，如果想保险一点可以反编译dex看看主classes.dex里面到底是不是只要我们配置的下面这三个类。
com/hc/multidexdemo/MainActivity.class
com/hc/multidexdemo/BuildConfig.class
com/hc/multidexdemo/BaseApplication.class
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4.合并补丁Dex包
假如我们某个类出现了异常闪退的情况，那么我们修改完成重新打包获取classes2.dex作为我们的补丁包fix.dex放在我们的服务器上面，我们客户端访问服务器下载fix.dex进行合并即可修复。
/**
* 合并注入
* @param context
* @throws Exception
*/
private static void injectDexElements(Context context) throws Exception {
ClassLoader pathClassLoader = context.getClassLoader();
File outDexFile = new File(context.getDir("odex", Context.MODE_PRIVATE).getAbsolutePath()
+ File.separator + "out_dex");
if (!outDexFile.exists()) {
outDexFile.mkdirs();
}
// 合并成一个数组
Object applicationDexElement = getDexElementByClassLoader(pathClassLoader);
for (File dexFile : mFixDex) {
ClassLoader classLoader = new DexClassLoader(dexFile.getAbsolutePath(),// dexPath
outDexFile.getAbsolutePath(),// optimizedDirectory
null,
pathClassLoader
);
// 获取这个classLoader中的Element
Object classElement = getDexElementByClassLoader(classLoader);
Log.e("TAG", classElement.toString());
applicationDexElement = combineArray(classElement, applicationDexElement);
}
// 注入到pathClassLoader中
injectDexElements(pathClassLoader, applicationDexElement);
}
/**
* 把dexElement注入到已运行classLoader中
* @param classLoader
* @param dexElement
* @throws Exception
*/
private static void injectDexElements(ClassLoader classLoader, Object dexElement) throws Exception {
Class<?> classLoaderClass = Class.forName("dalvik.system.BaseDexClassLoader");
Field pathListField = classLoaderClass.getDeclaredField("pathList");
pathListField.setAccessible(true);
Object pathList = pathListField.get(classLoader);
Class<?> pathListClass = pathList.getClass();
Field dexElementsField = pathListClass.getDeclaredField("dexElements");
dexElementsField.setAccessible(true);
dexElementsField.set(pathList, dexElement);
}
/**
* 合并两个dexElements数组
*
* @param arrayLhs
* @param arrayRhs
* @return
*/
private static Object combineArray(Object arrayLhs, Object arrayRhs) {
Class<?> localClass = arrayLhs.getClass().getComponentType();
int i = Array.getLength(arrayLhs);
int j = i + Array.getLength(arrayRhs);
Object result = Array.newInstance(localClass, j);
for (int k = 0; k < j; ++k) {
if (k < i) {
Array.set(result, k, Array.get(arrayLhs, k));
} else {
Array.set(result, k, Array.get(arrayRhs, k - i));
}
}
return result;
}
/**
* 获取classLoader中的DexElement
* @param classLoader ClassLoader
*/
public static Object getDexElementByClassLoader(ClassLoader classLoader) throws Exception {
Class<?> classLoaderClass = Class.forName("dalvik.system.BaseDexClassLoader");
Field pathListField = classLoaderClass.getDeclaredField("pathList");
pathListField.setAccessible(true);
Object pathList = pathListField.get(classLoader);
Class<?> pathListClass = pathList.getClass();
Field dexElementsField = pathListClass.getDeclaredField("dexElements");
dexElementsField.setAccessible(true);
Object dexElements = dexElementsField.get(pathList);
return dexElements;
}
就这么几个方法即可完成修复其实挺简单的，大家也可以去看看腾讯提供的修复方案，但是我一看到要防止打标记就开始蒙B了，后面几篇我们又要回归设计模式的讲解了，视频讲解需要等每周六日晚上八点。
视频讲解：http://pan.baidu.com/s/1dE4UsbZ
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