[TOC]

Android ClassLoader浅析
Android中的dex、apk、ClassLoader详解

程序在运行时把对应的类加载到内存中，在Android上来说就是把Dex文件中的类加载到内存。
双亲委派机制

public abstract class ClassLoader {    private ClassLoader(Void unused, ClassLoader parent) {        this.parent = parent;    }        protected ClassLoader(ClassLoader parent) {        this(checkCreateClassLoader(), parent);    }        protected ClassLoader() {        this(checkCreateClassLoader(), getSystemClassLoader());    }        public static ClassLoader getSystemClassLoader() {        return SystemClassLoader.loader;    }        static private class SystemClassLoader {        public static ClassLoader loader = ClassLoader.createSystemClassLoader();    }        private static ClassLoader createSystemClassLoader() {        String classPath = System.getProperty("java.class.path", ".");        String librarySearchPath = System.getProperty("java.library.path", "");        return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());    }        public Class<?> loadClass(String name) throws ClassNotFoundException {        return loadClass(name, false);    }        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) {                try {                    if (parent != null) {                        c = parent.loadClass(name, false);                    } else {                        c = findBootstrapClassOrNull(name);                    }                } catch (ClassNotFoundException e) {                    // ClassNotFoundException thrown if class not found                    // from the non-null parent class loader                }                if (c == null) {                    // If still not found, then invoke findClass in order                    // to find the class.                    c = findClass(name);                }            }            return c;    }        protected Class<?> findClass(String name) throws ClassNotFoundException {        throw new ClassNotFoundException(name);    }}

1. ClassLoader 继承关系

继承关系
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PathClassLoader只能加载已经安装到Android系统中的apk文件（/data/app目录），是Android默认使用的类加载器。
DexClassLoader可以加载任意目录下的dex/jar/apk/zip文件，比PathClassLoader更灵活，是实现热修复的重点。

源码查看（基于API28 9.0）
在8.0以上，optimizedDirectory参数传的都是null，也就是说PathClassLoader和DexClassLoader没差别，DexClassLoader可以加载未安装的apk，PathClassLoader也可以。

/** * Provides a simple {@link ClassLoader} implementation that operates on a list * of files and directories in the local file system, but does not attempt to * load classes from the network. Android uses this class for its system class * loader and for its application class loader(s). *///public class PathClassLoader extends BaseDexClassLoader {        public PathClassLoader(String dexPath, ClassLoader parent) {        super(dexPath, null, null, parent);    }    public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent){        super(dexPath, null, librarySearchPath, parent);   }}

/** * A class loader that loads classes from {@code .jar} and {@code .apk} files * containing a {@code classes.dex} entry. This can be used to execute code not * installed as part of an application.*///public class DexClassLoader extends BaseDexClassLoader {    public DexClassLoader(String dexPath, String optimizedDirectory,                          String librarySearchPath, ClassLoader parent) {        super(dexPath, null, librarySearchPath, parent);    }}

具体实现还得看BaseDexClassLoader的构造方法。

public class BaseDexClassLoader extends ClassLoader {    private final DexPathList pathList;   public BaseDexClassLoader(String dexPath, File optimizedDirectory,                          String librarySearchPath, ClassLoader parent) {        this(dexPath, optimizedDirectory, librarySearchPath, parent, false);    }    // 参数dexPath：待加载的apk/dex/jar文件路径；    // 参数optimizedDirectory：dex的输出路径，将apk/dex/jar解压出dex文件，复制到指定路径，用于dalvik运行    // 参数librarySearchPath：加载时候需要用到的lib库，这个一般不用，可以传入Null    // 参数parent：指定父加载器    public BaseDexClassLoader(String dexPath, File optimizedDirectory,                              String librarySearchPath, ClassLoader parent, boolean isTrusted) {        super(parent);        this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);        ...    }    @Override    protected Class<?> findClass(String name) throws ClassNotFoundException {        ...        Class c = pathList.findClass(name, suppressedExceptions);        ...        return c;    }    @Override    protected URL findResource(String name) {        return pathList.findResource(name);    }        @Override    public String findLibrary(String name) {        return pathList.findLibrary(name);    }}

构造方法中创建了一个DexPathList对象，而BaseDexClassLoader中的各个findxxx()调用的是DexPathList对象.findxxx()

DexPathList的构造方法和findxxx()方法。

final class DexPathList {        private Element[] dexElements;        DexPathList(ClassLoader definingContext, String dexPath,            String librarySearchPath, File optimizedDirectory, boolean isTrusted) {        ...        // 加载dexPath路径下的dex和resource        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory, suppressedExceptions, definingContext, isTrusted);        ...    }        private static Element[] makeDexElements(List files, File optimizedDirectory,        List suppressedExceptions, ClassLoader loader, boolean isTrusted) {           Element[] elements = new Element[files.size()];      int elementsPos = 0;      for (File file : files) {          if (file.isDirectory()) {              elements[elementsPos++] = new Element(file);          } else if (file.isFile()) {              String name = file.getName();              DexFile dex = null;              if (name.endsWith(DEX_SUFFIX)) {                  // Raw dex file (not inside a zip/jar).                  try {                      dex = loadDexFile(file, optimizedDirectory, loader, elements);                      if (dex != null) {                          elements[elementsPos++] = new Element(dex, null);                      }                  } catch (IOException suppressed) {                      System.logE("Unable to load dex file: " + file, suppressed);                      suppressedExceptions.add(suppressed);                  }              } else {                  try {                      dex = loadDexFile(file, optimizedDirectory, loader, elements);                  } catch (IOException suppressed) {                      suppressedExceptions.add(suppressed);                  }                  if (dex == null) {                      elements[elementsPos++] = new Element(file);                  } else {                      elements[elementsPos++] = new Element(dex, file);                  }              }          } else {              System.logW("ClassLoader referenced unknown path: " + file);          }      }      return elements;    }}

挨个文件中寻找Class对象

 public Class<?> findClass(String name, List suppressed) {        for (Element element : dexElements) {            Class<?> clazz = element.findClass(name, definingContext, suppressed);            if (clazz != null) {                return clazz;            }        }        if (dexElementsSuppressedExceptions != null) {            suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));        }        return null;    }

2. 热更新实现原理

源码

如果是.dex后缀的文件，会直接放到dexElements数组中，否则就从这个文件中寻找dex文件，找到后放到dexElements数组。
ClassLoader#loadClass就是遍历dexElements数组，从dex文件中找到要找的class文件。

那我们是不是在这个数组前安插一个自己的数组，是不是就可以了？

做法：
拿到我当前应用的dexElements数组,然后拿到已修复的dex或apk文件的dexElements数组，把已修复的数组放到当前应用的数组前面。

public class HotfixApplication extends Application {    @Override    protected void attachBaseContext(Context base) {        super.attachBaseContext(base);        File apk = new File(getCacheDir() + "/hotfix.dex");        if (apk.exists()) {            try {            //1. 获取当前应用的dexElements数组                ClassLoader classLoader = getClassLoader();                Class loaderClass = BaseDexClassLoader.class;                Field pathListField = loaderClass.getDeclaredField("pathList");                pathListField.setAccessible(true);                Object pathListObject = pathListField.get(classLoader);                Class pathListClass = pathListObject.getClass();                Field dexElementsField = pathListClass.getDeclaredField("dexElements");                dexElementsField.setAccessible(true);                Object dexElementsObject = dexElementsField.get(pathListObject);            //2. 获取已修复dex或apk的dexElements数组                PathClassLoader newClassLoader = new PathClassLoader(apk.getPath(), null);                Object newPathListObject = pathListField.get(newClassLoader);                Object newDexElementsObject = dexElementsField.get(newPathListObject);            //3. 数组合并，把修复的数组放在自己的数组前面                int oldLength = Array.getLength(dexElementsObject);                int newLength = Array.getLength(newDexElementsObject);                Object concatDexElementsObject = Array.newInstance(dexElementsObject.getClass().getComponentType(), oldLength + newLength);                for (int i = 0; i < newLength; i++) {                    Array.set(concatDexElementsObject, i, Array.get(newDexElementsObject, i));                }                for (int i = 0; i < oldLength; i++) {                    Array.set(concatDexElementsObject, newLength + i, Array.get(dexElementsObject, i));                }                dexElementsField.set(pathListObject, concatDexElementsObject);            } catch (NoSuchFieldException e) {                e.printStackTrace();            } catch (IllegalAccessException e) {                e.printStackTrace();            }        }    }}

3. 插件化原理

利用ClassLoader在新的apk中loadClass，利用反射获取新的apk中的class

3.1 问题

Q：怎么启动插件apk中的Activity？
A：由于启动Activity系统会校验清单文件，如果没有该Activity会异常。我们可以创建一个代理的Activity，启动这个代理Activity，然后在代理Activity中执行真实对象的方法（这个真实对象需要有和Activity一样的方法）。

Q：怎样获取插件apk中的Resource文件
A：我们要知道，Resource也是利用AssetManager获取的，所以我们要自定义一个AssetManager，安放到Resource对象中。

public class ProxyActivity extends Activity {    Object realActivity;//    realActivity = ???;    @Override    protected void onCreate(@Nullable Bundle savedInstanceState) {        super.onCreate(savedInstanceState);//        realActivity.onCreate(savedInstanceState);    }    @Override    public AssetManager getAssets() {        try {            Class assetManagerClass = AssetManager.class;            AssetManager assetManager = (AssetManager) assetManagerClass.newInstance();            Method addAssetPath = assetManagerClass.getDeclaredMethod("adAssetPath", String.class);            addAssetPath.invoke(assetManager, "apkPath");            return assetManager;        } catch (IllegalAccessException e) {            e.printStackTrace();        } catch (InstantiationException e) {            e.printStackTrace();        } catch (NoSuchMethodException e) {            e.printStackTrace();        } catch (InvocationTargetException e) {            e.printStackTrace();        }        return super.getAssets();    }    @Override    public Resources getResources() {        return new Resources(getAssets(), getResources().getDisplayMetrics(), getResources().getConfiguration());    }}

ClassLoader解析以及应用

1. ClassLoader 继承关系

2. 热更新实现原理

3. 插件化原理

3.1 问题

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