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上一篇文章 自定义ClassLoader和双亲委派机制 讲述了JVM中的类的加载机制，Android也是类JVM虚拟机那么它的类加载机制是什么呢，我们来探究一下(PS：文章源码为Android5.1)。
前言
Android的Dalvik虚拟机和Java虚拟机的运行原理相同都是将对应的java类加载在内存中运行。而Java虚拟机是加载class文件，也可以将一段二进制流通过defineClass方法生产Class进行加载（PS: 自定义ClassLoader和双亲委派机制 文章后面的自定义类加载器就是通过这种方式实现的）。Dalvik虚拟机加载的dex文件。dex文件是Android对与Class文件做的优化，以便于提高手机的性能。可以想象dex为class文件的一个压缩文件。dex在Android中的加载和class在jvm中的相同都是基于双亲委派模型，都是调用ClassLoader的loadClass方法加载类。
Android系统中类加载的双亲委派机制
- Android5.1源码中ClassLoader的loadClass方法
protected Class loadClass(String className, boolean resolve) throws ClassNotFoundException {
Class clazz = findLoadedClass(className);
if (clazz == null) {
ClassNotFoundException suppressed = null;
try {
//先让父类加载器加载
clazz = parent.loadClass(className, false);
} catch (ClassNotFoundException e) {
suppressed = e;
}
//当所有父类节点的类加载器都没有找到该类时，当前加载器调用findClass方法加载。
if (clazz == null) {
try {
clazz = findClass(className);
} catch (ClassNotFoundException e) {
e.addSuppressed(suppressed);
throw e;
}
}
}
- 想要动态加载类，可以用 自定义ClassLoader和双亲委派机制 中自定义ClassLoader的方法加载自己定义的class文件么？看看Android源码中的ClassLoader的findClass方法：
protected Class findClass(String className) throws ClassNotFoundException {
throw new ClassNotFoundException(className);
}
这个方法直接抛出了“ClassNotFoundException”异常，所以在Android中想通过这种方式实现类的加载时不行的。
Android系统中的类加载器
- Android系统屏蔽了ClassLoader的findClass加载方法，那么它自己的类加载时通过什么样的方式实现的呢？
- Android系统中有两个类加载器分别为PathClassLoader和DexclassLoader。
- PathClassLoader和DexClassLoader都是继承与BaseDexClassLoader，BaseDexClassLoader继承与ClassLoader。
提出问题
在这里我们先提一个问题Android为什么会将自己的类加载器派生出两个不同的子类，它们各自有什么用？
BaseDexClassLoader类加载
- 作为ClassLoader的子类，复写了父类的findClass方法。
@Override
protected Class findClass(String name) throws ClassNotFoundException {
List suppressedExceptions = new ArrayList();
//在自己的成员变量DexPathList中寻找，找不到抛异常
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;
}
- DexPathList的findClass方法
public Class findClass(String name, List suppressed) {
//循环便利成员变量dexElements，调用DexFile.loadClassBinaryName加载class
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;
}
通过以上两段代码我们可以看出，虽然Android中的ClassLoader的findClass方法的实现被取消了，但是ClassLoader的基类BaseDexClassLoader实现了findClass方法取加载指定的Class。
PathClassLoader和DexClassLoader比较
- PathClassLoader
public class PathClassLoader extends BaseDexClassLoader {
public PathClassLoader(String dexPath, ClassLoader parent) {
super(dexPath, null, null, parent);
}
public PathClassLoader(String dexPath, String libraryPath,
ClassLoader parent) {
super(dexPath, null, libraryPath, parent);
}
}
- DexClassLoader
public class DexClassLoader extends BaseDexClassLoader {
public DexClassLoader(String dexPath, String optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(dexPath, new File(optimizedDirectory), libraryPath, parent);
}
}
- BaseDexClassLoader的构造函数
/**
* Constructs an instance.
*
* @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 directory where optimized dex files
* should be written; may be {@code null}
* @param libraryPath the list of directories containing native
* libraries, delimited by {@code File.pathSeparator}; may be
* {@code null}
* @param parent the parent class loader
*/
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(parent);
this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}
- dexPath:指定的是dex文件地址，多个地址可以用":"进行分隔
- optimizedDirectory:制定输出dex优化后的odex文件，可以为null
- libraryPath:动态库路径（将被添加到app动态库搜索路径列表中）
- parent:制定父类加载器，以保证双亲委派机制从而实现每个类只加载一次。
可以看出 PathClassLoader和DexClassLoader的区别就在于构造函数中optimizedDirectory这个参数。PathClassLoader中optimizedDirectory为null，DexClassLoader中为new File(optimizedDirectory)。
- optimizedDirectory的干活
BaseDexClassLoader的构造函数利用optimizedDirectory创建了一个DexPathList，看看DexPathList中optimizedDirectory:
public DexPathList(ClassLoader definingContext, String dexPath,
String libraryPath, File optimizedDirectory) {
/******部分代码省略******/
this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
suppressedExceptions);
/******部分代码省略******/
}
private static Element[] makeDexElements(ArrayList files, File optimizedDirectory,
ArrayList suppressedExceptions) {
/******部分代码省略******/
for (File file : files) {
/******部分代码省略******/
if (file.isDirectory()) {
/******部分代码省略******/
} else if (file.isFile()){
if (name.endsWith(DEX_SUFFIX)) {
// Raw dex file (not inside a zip/jar).
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException ex) {
System.logE("Unable to load dex file: " + file, ex);
}
} else {
zip = file;
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException suppressed) {
suppressedExceptions.add(suppressed);
}
}
} else {
System.logW("ClassLoader referenced unknown path: " + file);
}
if ((zip != null) || (dex != null)) {
elements.add(new Element(file, false, zip, dex));
}
}
return elements.toArray(new Element[elements.size()]);
}
private static DexFile loadDexFile(File file, File optimizedDirectory)
throws IOException {
if (optimizedDirectory == null) {
return new DexFile(file);
} else {
String optimizedPath = optimizedPathFor(file, optimizedDirectory);
return DexFile.loadDex(file.getPath(), optimizedPath, 0);
}
}
从这里可以看出optimizedDirectory不同生产的DexFile对象不同，我们继续看看optimizedDirectory在DexFile中的作用：
public DexFile(File file) throws IOException {
this(file.getPath());
}
/**
* Opens a DEX file from a given filename. This will usually be a ZIP/JAR
* file with a "classes.dex" inside.
*
* The VM will generate the name of the corresponding file in
* /data/dalvik-cache and open it, possibly creating or updating
* it first if system permissions allow. Don't pass in the name of
* a file in /data/dalvik-cache, as the named file is expected to be
* in its original (pre-dexopt) state.
*
* @param fileName
* the filename of the DEX file
*
* @throws IOException
* if an I/O error occurs, such as the file not being found or
* access rights missing for opening it
*/
public DexFile(String fileName) throws IOException {
mCookie = openDexFile(fileName, null, 0);
mFileName = fileName;
guard.open("close");
//System.out.println("DEX FILE cookie is " + mCookie + " fileName=" + fileName);
}
/**
* Opens a DEX file from a given filename, using a specified file
* to hold the optimized data.
*
* @param sourceName
* Jar or APK file with "classes.dex".
* @param outputName
* File that will hold the optimized form of the DEX data.
* @param flags
* Enable optional features.
*/
private DexFile(String sourceName, String outputName, int flags) throws IOException {
if (outputName != null) {
try {
String parent = new File(outputName).getParent();
if (Libcore.os.getuid() != Libcore.os.stat(parent).st_uid) {
throw new IllegalArgumentException("Optimized data directory " + parent
+ " is not owned by the current user. Shared storage cannot protect"
+ " your application from code injection attacks.");
}
} catch (ErrnoException ignored) {
// assume we'll fail with a more contextual error later
}
}
mCookie = openDexFile(sourceName, outputName, flags);
mFileName = sourceName;
guard.open("close");
//System.out.println("DEX FILE cookie is " + mCookie + " sourceName=" + sourceName + " outputName=" + outputName);
}
static public DexFile loadDex(String sourcePathName, String outputPathName,
int flags) throws IOException {
return new DexFile(sourcePathName, outputPathName, flags);
}
从注释当中就可以看到new DexFile(file)的dex输出路径只能为/data/dalvik-cache，而DexFile.loadDex()的dex输出路径为自己输入的optimizedDirectory路径。
dalvik-cache.jpg
解决疑问
我们在文章开始提出的问题就这样一步步得到了答案。
DexClassLoader：能够加载自定义的jar/apk/dex
PathClassLoader：只能加载系统中已经安装过的apk
所以Android系统默认的类加载器为PathClassLoader，而DexClassLoader可以像JVM的ClassLoader一样提供动态加载。
总结
- ClassLoader的loadClass方法保证了双亲委派机。
- BaseDexClassLoader提供了两种派生类使我们可以加载自定义类。
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