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2、系统数据检查
2.1 dex更新
我们开发的java代码通过编译生成.class文件,然后通过dx工具生成机器可以识别的dex文件。
Android中采用ClassLoader加载dex文件,加载完成之后可以通过反射调用其中的方法,适合那些不依赖文件等资源的业务,而打点恰好比较适合使用dex加载的方式。
Android中有PathClassLoader和DexClassLoader ,他们都继承自ClassLoader。他们的继承关系如下:
- DexClassLoader:可以加载jar/apk/dex,可以从SD卡中加载未安装的apk;
- PathClassLoader:要传入系统中apk的存放Path,所以只能加载已经安装的apk文件。
- BaseDexClassLoader是DexClassLoader和PathClassLoader的父类,针对传入不同的参数做差异化处理。
看看三个类的源码:
2.1.1 PathClassLoader
dalvik/system/PathClassLoader.java
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);}
}
2.1.2 DexClassLoader
dalvik/system/DexClassLoader.java
public class DexClassLoader extends BaseDexClassLoader {public DexClassLoader(String dexPath, String optimizedDirectory, String librarySearchPath, ClassLoader parent) {super(dexPath, null, librarySearchPath, parent);}
}
2.1.3 BaseDexClassLoader
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);}public BaseDexClassLoader(String dexPath, File optimizedDirectory,String librarySearchPath, ClassLoader parent, boolean isTrusted) {super(parent);this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);if (reporter != null) {reportClassLoaderChain();}}public BaseDexClassLoader(ByteBuffer[] dexFiles, ClassLoader parent) {// TODO We should support giving this a library search path maybe.super(parent);this.pathList = new DexPathList(this, dexFiles);}
在BaseDexClassLoader中通过DexPathList类具体的处理Dex,他的构造函数如下:
2.1.4 DexPathList
private Element[] dexElements;
DexPathList(ClassLoader definingContext, String dexPath, String librarySearchPath, File optimizedDirectory, boolean isTrusted) {this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory, suppressedExceptions, definingContext, isTrusted);
}private static Element[] makeDexElements(List<File> files, File optimizedDirectory, List<IOException> 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;dex = loadDexFile(file, optimizedDirectory, loader, elements);}}retrun elements;
}private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader, Element[] elements) throws IOException {if (optimizedDirectory == null) {return new DexFile(file, loader, elements);} else {String optimizedPath = optimizedPathFor(file, optimizedDirectory);return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements);}
}
最终通过DexFile加载loadDex方法在native层实现对dex的加载和处理。
而分析PathClassLoader
和DexClassLoader
的构造函数可以看到Android 9.0中的DexClassLoader构造函数的optimizedDirectory参数默认是null。所以这里要针对版本的不同做差异化处理。
我们一般加载Dex的方式是:
classLoader = context.getClassLoader();
classLoader.loadClass("you class path");
这里做一下差异化:
public ClassLoader load(Context context,String dexName) {mLoaded = false;ClassLoader classLoader = null;File dexOutputDir = context.getDir("dex", 0);File dexFile = new File(dexOutputDir.getAbsolutePath(), dexName);Log.d(TAG, "load start");if (!dexFile.exists()) {return null;}String dexPath = dexFile.getAbsolutePath();Log.d(TAG, "dPath = " + dexPath);int version = android.os.Build.VERSION.SDK_INT;if (version >= 25) {BaseDexClassLoader parent = (BaseDexClassLoader) context.getClassLoader();Class<BaseDexClassLoader> c = BaseDexClassLoader.class;Method method;try {method = c.getMethod("addDexPath", String.class);method.invoke(parent, dexPath);mLoaded = true;classLoader = parent;return classLoader;} catch (Exception e) {e.printStackTrace();}}Log.d(TAG, "mLoaded1 = " + mLoaded);if (!mLoaded) {ArrayList<File> files = new ArrayList<File>();files.add(dexFile);classLoader = context.getClassLoader();classLoader.loadClass("you class path");try {Field pathListField = findField(classLoader, "pathList");Object dexPathList = pathListField.get(classLoader);ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();if (version < 19) {expandFieldArray(dexPathList, "dexElements",makeDexElements(dexPathList, files, null));} else if (version < 23) {expandFieldArray(dexPathList, "dexElements",makeDexElements(dexPathList, files, null, suppressedExceptions));} else {expandFieldArray(dexPathList, "dexElements",makePathElements(dexPathList, files, null, suppressedExceptions));}} catch (Exception e1) {e1.printStackTrace();}}Log.d(TAG, "mLoaded2 = " + mLoaded);if (mLoaded) {return classLoader;}return null;
}
- SDK_INI大于等于25
当sdk_int大于等于25时,通过反射BaseDexClassLoader的addDexPath方法直接添加dex文件到DexPathList的Element[]数组中,而后续findClass方法的逻辑就是遍历这个数据找到对应的dex文件。
- SDK_INI小于25
这种情况下,先反射获取ClassLoader的pathList对象,这里的ClassLoader实际是PathClassLoader,但是最终都会到BaseDexClassLoader的pathList。
获取到这个变量之后,先调用makeDexElements
方法将生成的dex对象放到一个数组中,接着在expandFieldArray
方法中将就的dex数组和新的dex数组合并:
private Object[] makeDexElements(Object dexPathList, ArrayList<File> files, File optimizedDirectory) throws IllegalAccessException, InvocationTargetException, NoSuchMethodException {Method makeDexElements = findMethod(dexPathList, "makeDexElements", ArrayList.class, File.class);return (Object[]) makeDexElements.invoke(dexPathList, files, optimizedDirectory);
}
这里的makeDexElements
方法对应的是DexPathList
的makeDexElements
方法,最终目的是将dex对象添加到Element[]
数组中,作为新的数组返回。
expandFieldArray
对应的是DexPathList
的addDexPath
方法,将新旧Element[]
数组合到一个数组中,旧的数组在前面。这样就导致了相同文件名的dex文件,最新修复了bug的dex不能立即生效。
private void expandFieldArray(Object instance, String fieldName, Object[] extraElements) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {Field jlrField = findField(instance, fieldName);Object[] original = (Object[]) jlrField.get(instance);Object[] combined = (Object[]) Array.newInstance(original.getClass().getComponentType(), original.length + extraElements.length);System.arraycopy(original, 0, combined, 0, original.length);System.arraycopy(extraElements, 0, combined, original.length,extraElements.length);jlrField.set(instance, combined);mLoaded = true;Log.d(TAG, "expandFieldArray");
}
对比看下DexPathList
中的addDexPath
方法:
public void addDexPath(String dexPath, File optimizedDirectory, boolean isTrusted) {final List<IOException> suppressedExceptionList = new ArrayList<IOException>();final Element[] newElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,suppressedExceptionList, definingContext, isTrusted);if (newElements != null && newElements.length > 0) {final Element[] oldElements = dexElements;dexElements = new Element[oldElements.length + newElements.length];System.arraycopy(oldElements, 0, dexElements, 0, oldElements.length);System.arraycopy(newElements, 0, dexElements, oldElements.length, newElements.length);}
}
可见我们自己的操作对应着DexPathList
的addDexPath
方法。
在我们自己的expandFieldArray
方法最后通过执行jlrField.set(instance, combined);
,将合并后的Element[]
数组赋值给DexPathList
的Element[] dexElements
。
当上述操作完成之后,就要调用loadClass方法加载dex文件中的类了,这个方法在ClassLoader类中定义:
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
{Class<?> c = findLoadedClass(name);if (parent != null) {c = parent.loadClass(name, false);} else {c = findBootstrapClassOrNull(name);}if (c == null) {c = findClass(name);}return c;
}
parent
由Context.getClassLoader
所属的PathClassLoader传递,一直从BaseDexClassLoader
到ClassLoader
。
到这里要熟悉应用被创建初始化的流程了,这里先不引申过去,只需要知道这个parent是BootClassLoader
类型。
@Override
protected Class<?> loadClass(String className, boolean resolve)throws ClassNotFoundException {Class<?> clazz = findLoadedClass(className);if (clazz == null) {clazz = findClass(className);}return clazz;
}
如果还找不到,就调用BaseDexClassLoader
的findClass
方法了。
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {List<Throwable> suppressedExceptions = new ArrayList<Throwable>();Class c = pathList.findClass(name, suppressedExceptions);return c;
}
到这里如果还找不到就抛异常出来了,ClassNotFoundException
。
上边的流程就是各大博客上面说的双亲委派机制,父类先从已经加载的类里面找,找不到的话,再从自己BaseDexClassLoader
的findClass
方法里面去找。
2.1.5 问题复盘
到这里,基本就将Dex加载的流程搞清楚了,但是这样的加载会导致新加载的Dex不能立即生效,必须重新启动应用之后才能生效。针对这种问题,可以将Dex热更新模块放到一个单独的进程中,当Dex加载完毕之后,调用killProcess方法自杀,然后由另一个进程拉活重启。
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