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2、系統(tǒng)數(shù)據(jù)檢查
2.1 dex更新
我們開發(fā)的java代碼通過(guò)編譯生成.class文件，然后通過(guò)dx工具生成機(jī)器可以識(shí)別的dex文件。
Android中采用ClassLoader加載dex文件，加載完成之后可以通過(guò)反射調(diào)用其中的方法，適合那些不依賴文件等資源的業(yè)務(wù)，而打點(diǎn)恰好比較適合使用dex加載的方式。
Android中有PathClassLoader和DexClassLoader ，他們都繼承自ClassLoader。他們的繼承關(guān)系如下：
- DexClassLoader：可以加載jar/apk/dex，可以從SD卡中加載未安裝的apk；
- PathClassLoader：要傳入系統(tǒng)中apk的存放Path，所以只能加載已經(jīng)安裝的apk文件。
- BaseDexClassLoader是DexClassLoader和PathClassLoader的父類，針對(duì)傳入不同的參數(shù)做差異化處理。
看看三個(gè)類的源碼：
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中通過(guò)DexPathList類具體的處理Dex，他的構(gòu)造函數(shù)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);
}
}
最終通過(guò)DexFile加載loadDex方法在native層實(shí)現(xiàn)對(duì)dex的加載和處理。
而分析PathClassLoader和DexClassLoader的構(gòu)造函數(shù)可以看到Android 9.0中的DexClassLoader構(gòu)造函數(shù)的optimizedDirectory參數(shù)默認(rèn)是null。所以這里要針對(duì)版本的不同做差異化處理。
我們一般加載Dex的方式是：
classLoader = context.getClassLoader();
classLoader.loadClass("you class path");
這里做一下差異化：
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
當(dāng)sdk_int大于等于25時(shí)，通過(guò)反射BaseDexClassLoader的addDexPath方法直接添加dex文件到DexPathList的Element[]數(shù)組中，而后續(xù)findClass方法的邏輯就是遍歷這個(gè)數(shù)據(jù)找到對(duì)應(yīng)的dex文件。
- SDK_INI小于25
這種情況下，先反射獲取ClassLoader的pathList對(duì)象，這里的ClassLoader實(shí)際是PathClassLoader，但是最終都會(huì)到BaseDexClassLoader的pathList。
獲取到這個(gè)變量之后，先調(diào)用makeDexElements方法將生成的dex對(duì)象放到一個(gè)數(shù)組中，接著在expandFieldArray方法中將就的dex數(shù)組和新的dex數(shù)組合并：
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);
}
這里的makeDexElements方法對(duì)應(yīng)的是DexPathList的makeDexElements方法，最終目的是將dex對(duì)象添加到Element[]數(shù)組中，作為新的數(shù)組返回。
expandFieldArray對(duì)應(yīng)的是DexPathList的addDexPath方法，將新舊Element[]數(shù)組合到一個(gè)數(shù)組中，舊的數(shù)組在前面。這樣就導(dǎo)致了相同文件名的dex文件，最新修復(fù)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");
}
對(duì)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);
}
}
可見我們自己的操作對(duì)應(yīng)著DexPathList的addDexPath方法。
在我們自己的expandFieldArray方法最后通過(guò)執(zhí)行jlrField.set(instance, combined);，將合并后的Element[]數(shù)組賦值給DexPathList的Element[] dexElements。
當(dāng)上述操作完成之后，就要調(diào)用loadClass方法加載dex文件中的類了，這個(gè)方法在ClassLoader類中定義：
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所屬的PathClassLoader傳遞，一直從BaseDexClassLoader到ClassLoader。
到這里要熟悉應(yīng)用被創(chuàng)建初始化的流程了，這里先不引申過(guò)去，只需要知道這個(gè)parent是BootClassLoader類型。
@Override
protected Class<?> loadClass(String className, boolean resolve)
throws ClassNotFoundException {
Class<?> clazz = findLoadedClass(className);
if (clazz == null) {
clazz = findClass(className);
}
return clazz;
}
如果還找不到，就調(diào)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;
}
到這里如果還找不到就拋異常出來(lái)了，ClassNotFoundException。
上邊的流程就是各大博客上面說(shuō)的雙親委派機(jī)制，父類先從已經(jīng)加載的類里面找，找不到的話，再?gòu)淖约?code>BaseDexClassLoader的findClass方法里面去找。
2.1.5 問(wèn)題復(fù)盤
到這里，基本就將Dex加載的流程搞清楚了，但是這樣的加載會(huì)導(dǎo)致新加載的Dex不能立即生效，必須重新啟動(dòng)應(yīng)用之后才能生效。針對(duì)這種問(wèn)題，可以將Dex熱更新模塊放到一個(gè)單獨(dú)的進(jìn)程中，當(dāng)Dex加載完畢之后，調(diào)用killProcess方法自殺，然后由另一個(gè)進(jìn)程拉活重啟。
