Android熱更新實現原理

ClassLoader

我們知道Java在運行時加載對應的類是通過ClassLoader來實現的，ClassLoader本身是一個抽象來，Android中使用PathClassLoader類作爲Android的默認的類加載器， 
PathClassLoader其實實現的就是簡單的從文件系統中加載類文件。PathClassLoader本身繼承自BaseDexClassLoader，BaseDexClassLoader重寫了findClass方法， 
該方法是ClassLoader的核心

@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;
}

看源碼可知，BaseDexClassLoader將findClass方法委託給了pathList對象的findClass方法，pathList對象是在BaseDexClassLoader的構造函數中new出來的， 
它的類型是DexPathList。看下DexPathList.findClass源碼是如何做的：

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;
}

直接就是遍歷dexElements列表，然後通過調用element.dexFile對象上的loadClassBinaryName方法來加載類，如果返回值不是null，就表示加載類成功，會將這個Class對象返回。 
而dexElements對象是在DexPathList類的構造函數中完成初始化的。

this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions);

makeDexElements所做的事情就是遍歷我們傳遞來的dexPath，然後一次加載每個dex文件。

實現

上面分析了Android中的類的加載的流程，可以看出來DexPathList對象中的dexElements列表是類加載的一個核心，一個類如果能被成功加載，那麼它的dex一定 
會出現在dexElements所對應的dex文件中，並且dexElements中出現的順序也很重要，在dexElements前面出現的dex會被優先加載，一旦Class被加載成功， 
就會立即返回，也就是說，我們的如果想做hotpatch，一定要保證我們的hotpacth dex文件出現在dexElements列表的前面。

要實現熱更新，就需要我們在運行時去更改PathClassLoader.pathList.dexElements，由於這些屬性都是private的，因此需要通過反射來修改。另外，構造我們自己的dex文件 
所對應的dexElements數組的時候，我們也可以採取一個比較取巧的方式，就是通過構造一個DexClassLoader對象來加載我們的dex文件，並且調用一次dexClassLoader.loadClass(dummyClassName); 

方法，這樣，dexClassLoader.pathList.dexElements中，就會包含我們的dex，通過把dexClassLoader.pathList.dexElements插入到系統默認的classLoader.pathList.dexElements列表前面，就可以讓系統優先加載我們的dex中的類，從而可以實現熱更新了。

熱更新實戰

項目地址：https://github.com/lantian0314/hotFix

一、需要熱更新的方法

package com.blue.testapk;

import android.util.Log;

/**
 * Created by xingyatong on 2018/4/9.
 */

public class TestClass {
    private static final String TAG = "MainActivity";
    public void test1() {
        Log.i(TAG, "This is bug class");
    }
}

二、熱更新的類

public class DexLoaderUtil {
    private static final String TAG = "DexLoaderUtil";
    public static final String SECONDARY_DEX_NAME = "app-bug.apk";
    public static final String THIRD_DEX_NAME = "app2.apk";
    private static final int BUF_SIZE = 8 * 1024;
    public static final String TEST_CLASS_NAME = "com.blue.testapk.TestClass";

    public static String getDexPath(Context context, String dexName) {
        return new File(context.getDir("dex", Context.MODE_PRIVATE), dexName).getAbsolutePath();
    }

    public static String getOptimizedDexPath(Context context) {
        return context.getDir("outdex", Context.MODE_PRIVATE).getAbsolutePath();
    }

    public static void copyDex(Context context, String dexName) {
        File dexInternalStoragePath = new File(context.getDir("dex", Context.MODE_PRIVATE),
                dexName);
        BufferedInputStream bis = null;
        OutputStream dexWriter = null;
        try {
            bis = new BufferedInputStream(context.getAssets().open(dexName));
            dexWriter = new BufferedOutputStream(
                    new FileOutputStream(dexInternalStoragePath));
            byte[] buf = new byte[BUF_SIZE];
            int len;
            while ((len = bis.read(buf, 0, BUF_SIZE)) > 0) {
                dexWriter.write(buf, 0, len);
            }
            dexWriter.close();
            bis.close();

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void loadAndCall(Context context, String dexName) {
        final File dexInternalStoragePath = new File(context.getDir("dex", Context.MODE_PRIVATE), dexName);
        final File optimizedDexOutputPath = context.getDir("outdex", Context.MODE_PRIVATE);

        DexClassLoader cl = new DexClassLoader(dexInternalStoragePath.getAbsolutePath(),
                optimizedDexOutputPath.getAbsolutePath(),
                null,
                context.getClassLoader());
        call(cl);
    }

    public static void call(ClassLoader cl) {
        Class myClasz = null;
        try {
            myClasz =
                    cl.loadClass(TEST_CLASS_NAME);
            Object instance = myClasz.getConstructor().newInstance();
            myClasz.getDeclaredMethod("test1").invoke(instance);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        }
    }

    public static synchronized Boolean injectAboveEqualApiLevel14(
            String dexPath, String defaultDexOptPath, String nativeLibPath, String dummyClassName) {
        Log.i(TAG, "--> injectAboveEqualApiLevel14");
        PathClassLoader pathClassLoader = (PathClassLoader) DexLoaderUtil.class.getClassLoader();
        DexClassLoader dexClassLoader = new DexClassLoader(dexPath, defaultDexOptPath, nativeLibPath, pathClassLoader);
        try {
            dexClassLoader.loadClass(dummyClassName);
            Object dexElements = combineArray(
                    getDexElements(getPathList(pathClassLoader)),
                    getDexElements(getPathList(dexClassLoader)));

            Object pathList = getPathList(pathClassLoader);
            setField(pathList, pathList.getClass(), "dexElements", dexElements);
        } catch (Throwable e) {
            e.printStackTrace();
            return false;
        }
        Log.i(TAG, "<-- injectAboveEqualApiLevel14 End.");
        return true;
    }

    private static Object getPathList(Object baseDexClassLoader)
            throws IllegalArgumentException, NoSuchFieldException, IllegalAccessException, ClassNotFoundException {
        return getField(baseDexClassLoader, Class.forName("dalvik.system.BaseDexClassLoader"), "pathList");
    }


    private static Object getDexElements(Object paramObject)
            throws IllegalArgumentException, NoSuchFieldException, IllegalAccessException {
        return getField(paramObject, paramObject.getClass(), "dexElements");
    }


    private static Object getField(Object obj, Class<?> cl, String field)
            throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
        Field localField = cl.getDeclaredField(field);
        localField.setAccessible(true);
        return localField.get(obj);
    }


    private static void setField(Object obj, Class<?> cl, String field, Object value)
            throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
        Field localField = cl.getDeclaredField(field);
        localField.setAccessible(true);
        localField.set(obj, value);
    }

    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;
    }
}

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        new Thread(new Runnable() {
            @Override
            public void run() {
                Log.d(TAG, "------------start");
                DexLoaderUtil.copyDex(MainActivity.this, DexLoaderUtil.SECONDARY_DEX_NAME);
                DexLoaderUtil.copyDex(MainActivity.this, DexLoaderUtil.THIRD_DEX_NAME);

                String secondDexPath = DexLoaderUtil.getDexPath(MainActivity.this, DexLoaderUtil.SECONDARY_DEX_NAME);
                String thirdDexPath = DexLoaderUtil.getDexPath(MainActivity.this, DexLoaderUtil.THIRD_DEX_NAME);

                final String optimizedDexOutputPath = DexLoaderUtil.getOptimizedDexPath(MainActivity.this);
                DexLoaderUtil.injectAboveEqualApiLevel14(thirdDexPath, optimizedDexOutputPath, null, DexLoaderUtil.TEST_CLASS_NAME);
                DexLoaderUtil.injectAboveEqualApiLevel14(secondDexPath, optimizedDexOutputPath, null, DexLoaderUtil.TEST_CLASS_NAME);
                DexLoaderUtil.call(getClassLoader());
                Log.d(TAG, "------------end");
            }
        }).start();
    }
}