文章目录
前言
一、什么是dex文件？
二、加载dex文件
1.DexClassLoader
2.加载流程
总结
前言
最近在看tinker热修复相关的文章，想要完整的了解tinker热修复的原理，还需要先对dex文件的加载有一个基本的了解。
一、什么是dex文件？
在明白什么是 Dex 文件之前，要先了解一下 JVM，Dalvik 和 ART。JVM 是 JAVA 虚拟机，用来运行 JAVA 字节码程序。Dalvik 是 Google 设计的用于 Android平台的运行时环境，适合移动环境下内存和处理器速度有限的系统。ART 即 Android Runtime，是 Google 为了替换 Dalvik 设计的新 Android 运行时环境，在Android 4.4推出。ART 比 Dalvik 的性能更好。Android 程序一般使用 Java 语言开发，但是 Dalvik 虚拟机并不支持直接执行 JAVA 字节码，所以会对编译生成的 .class 文件进行翻译、重构、解释、压缩等处理，这个处理过程是由 dx 进行处理，处理完成后生成的产物会以 .dex 结尾，称为 Dex 文件。Dex 文件格式是专为 Dalvik 设计的一种压缩格式。所以可以简单的理解为：Dex 文件是很多 .class 文件处理后的产物，最终可以在 Android 运行时环境执行。
二、加载dex文件
1.DexClassLoader
就像JVM在使用Class之前，先要通过ClassLoader进行类加载，加载后才能使用某个Class。
dex文件在Android系统中是通过DexClassLoader和PathClassLoader进行Class加载的。
Android SDK中可以看到Class.java文件，类加载的作用就是从文件流中读取类信息，生成某个类对应的Class对象。
代码如下：
DexClassLoader.java
public class DexClassLoader extends BaseDexClassLoader {
/**
* Creates a {@code DexClassLoader} that finds interpreted and native
* code. Interpreted classes are found in a set of DEX files contained
* in Jar or APK files.
*
* <p>The path lists are separated using the character specified by the
* {@code path.separator} system property, which defaults to {@code :}.
*
* @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 this parameter is deprecated and has no effect since API level 26.
* @param librarySearchPath the list of directories containing native
* libraries, delimited by {@code File.pathSeparator}; may be
* {@code null}
* @param parent the parent class loader
*/
public DexClassLoader(String dexPath, String optimizedDirectory,
String librarySearchPath, ClassLoader parent) {
super(dexPath, null, librarySearchPath, parent);
}
}PathClassLoader.java
public class PathClassLoader extends BaseDexClassLoader {
/**
* Creates a {@code PathClassLoader} that operates on a given list of files
* and directories. This method is equivalent to calling
* {@link #PathClassLoader(String, String, ClassLoader)} with a
* {@code null} value for the second argument (see description there).
*
* @param dexPath the list of jar/apk files containing classes and
* resources, delimited by {@code File.pathSeparator}, which
* defaults to {@code ":"} on Android
* @param parent the parent class loader
*/
public PathClassLoader(String dexPath, ClassLoader parent) {
super(dexPath, null, null, parent);
}
/**
* Creates a {@code PathClassLoader} that operates on two given
* lists of files and directories. The entries of the first list
* should be one of the following:
*
* <ul>
* <li>JAR/ZIP/APK files, possibly containing a "classes.dex" file as
* well as arbitrary resources.
* <li>Raw ".dex" files (not inside a zip file).
* </ul>
*
* The entries of the second list should be directories containing
* native library files.
*
* @param dexPath the list of jar/apk files containing classes and
* resources, delimited by {@code File.pathSeparator}, which
* defaults to {@code ":"} on Android
* @param librarySearchPath the list of directories containing native
* libraries, delimited by {@code File.pathSeparator}; may be
* {@code null}
* @param parent the parent class loader
*/
public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
super(dexPath, null, librarySearchPath, parent);
}
/**
* @hide
*/
@libcore.api.CorePlatformApi
public PathClassLoader(
String dexPath, String librarySearchPath, ClassLoader parent,
ClassLoader[] sharedLibraryLoaders) {
super(dexPath, librarySearchPath, parent, sharedLibraryLoaders);
}
}2.加载流程
DexClassLoader调用loadClass时，会执行ClassLoaer的loadClass方法
ClassLoader.java
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);//关注这一行
}
}
return c;
}我们这里分析第一次加载某个自定义类的场景。最终会执行 c = findClass(name);//关注这一行
DexClassLoader中没有重写findClass方法，它的父类BaseDexClassLoader中重写了该方法
BaseDexClassLoader.java
protected Class<?> findClass(String name) throws ClassNotFoundException {
// First, check whether the class is present in our shared libraries.
if (sharedLibraryLoaders != null) {
for (ClassLoader loader : sharedLibraryLoaders) {//1
try {
return loader.loadClass(name);
} catch (ClassNotFoundException ignored) {
}
}
}
// Check whether the class in question is present in the dexPath that
// this classloader operates on.
List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
Class c = pathList.findClass(name, suppressedExceptions);//2
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;
}//1处 sharedLibraryLoaders构造函数中为null，本次流程不关注
//2处 最终调用pathList.findClass，pathList初始化逻辑
public BaseDexClassLoader(String dexPath,
String librarySearchPath, ClassLoader parent, ClassLoader[] sharedLibraryLoaders,
boolean isTrusted) {
super(parent);
// Setup shared libraries before creating the path list. ART relies on the class loader
// hierarchy being finalized before loading dex files.
this.sharedLibraryLoaders = sharedLibraryLoaders == null
? null
: Arrays.copyOf(sharedLibraryLoaders, sharedLibraryLoaders.length);
this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);
if (reporter != null) {
reportClassLoaderChain();
}
}DexPathList.java
findClass方法实现
public Class<?> findClass(String name, List<Throwable> 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;
}可以看到关键代码在 dexElements ，通过遍历该数据去查找class，该数组在构造函数中生成
DexPathList(ClassLoader definingContext, String dexPath,
String librarySearchPath, File optimizedDirectory, boolean isTrusted){
...
this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
suppressedExceptions, definingContext, isTrusted);
...
}先跟踪 splitDexPath方法
private static List<File> splitDexPath(String path) {
return splitPaths(path, false);
}private static List<File> splitPaths(String searchPath, boolean directoriesOnly) {
List<File> result = new ArrayList<>();
if (searchPath != null) {
for (String path : searchPath.split(File.pathSeparator)) {
if (directoriesOnly) {
try {
StructStat sb = Libcore.os.stat(path);
if (!S_ISDIR(sb.st_mode)) {
continue;
}
} catch (ErrnoException ignored) {
continue;
}
}
result.add(new File(path));
}
}
return result;
}主要是判断入参searchPath是否是多个文件/目录，如果是多个文件/目录，则将它们通过系统的分割符分割成File数组
继续追踪方法 makeDexElements
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;
/*
* Open all files and load the (direct or contained) dex files up front.
*/
for (File file : files) {
if (file.isDirectory()) {
// We support directories for looking up resources. Looking up resources in
// directories is useful for running libcore tests.
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) {
/*
* IOException might get thrown "legitimately" by the DexFile constructor if
* the zip file turns out to be resource-only (that is, no classes.dex file
* in it).
* Let dex == null and hang on to the exception to add to the tea-leaves for
* when findClass returns null.
*/
suppressedExceptions.add(suppressed);
}
if (dex == null) {
elements[elementsPos++] = new Element(file);
} else {
elements[elementsPos++] = new Element(dex, file);
}
}
if (dex != null && isTrusted) {
dex.setTrusted();
}
} else {
System.logW("ClassLoader referenced unknown path: " + file);
}
}
if (elementsPos != elements.length) {
elements = Arrays.copyOf(elements, elementsPos);
}
return elements;
}根据File类型的不同，生成对应的Element。
- File是文件夹
- File是文件并且以.dex结尾
- File是文件并且不以.dex结尾
- File是无法解析的类型
继续回到前面定位的查找class逻辑，遍历dexElements实际上调用了Element的findClass方法
DexPathList$Element.java
构造函数
public Element(DexFile dexFile, File dexZipPath) {
if (dexFile == null && dexZipPath == null) {
throw new NullPointerException("Either dexFile or path must be non-null");
}
this.dexFile = dexFile;
this.path = dexZipPath;
// Do any I/O in the constructor so we don't have to do it elsewhere, eg. toString().
this.pathIsDirectory = (path == null) ? null : path.isDirectory();
}查找方法
public Class<?> findClass(String name, ClassLoader definingContext,
List<Throwable> suppressed) {
return dexFile != null ? dexFile.loadClassBinaryName(name, definingContext, suppressed)
: null;
}最终需要查看DexFile的loadClassBinaryName方法
DexFile.java
public Class loadClassBinaryName(String name, ClassLoader loader, List<Throwable> suppressed) {
return defineClass(name, loader, mCookie, this, suppressed);
}private static Class defineClass(String name, ClassLoader loader, Object cookie,
DexFile dexFile, List<Throwable> suppressed) {
Class result = null;
try {
result = defineClassNative(name, loader, cookie, dexFile);
} catch (NoClassDefFoundError e) {
if (suppressed != null) {
suppressed.add(e);
}
} catch (ClassNotFoundException e) {
if (suppressed != null) {
suppressed.add(e);
}
}
return result;
}最终走到了 defineClassNative 方法。（native方法暂不继续往下分析了，这篇文章暂时只了解到这一层。将native当作黑盒，知道它可以根据name查找class即可）
三、部分自测
写代码来测试 jar或者apk路径 算目录还是文件？-- 两个都算文件 最终DexFile的fileName是 jar文件/apk文件 的完整路径
总结
本次粗略的dex加载分析过程可以知道
BaseDexClassLoader->DexPathList pathList
DexPathList->Element[] dexElements
通过遍历dexElements来查找Class，通过修改 dexElements 可以影响最终查找到的Class