ClassLoader
Android 中使用PathClassLoader、DexClassLoader等来实现类的加载，这两个类都继承自BaseDexClassLoader，而BaseDexClassLoader继承自ClassLoader。
从Android官方文档中可知：PathClassLoader用来加载系统类及已安装的应用程序的类；DexClassLoader用来加载jar、apk、dex文件中的类。
ClassLoader使用了一种称为“双亲委托”的机制：
public abstract class ClassLoader {
private final ClassLoader parent;
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
Class c = findLoadedClass(name);
if (c == null) {
try {
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
}
if (c == null) {
c = findClass(name);
}
}
return c;
}
}
调用loadClass方法来加载类时，首先使用findLoadedClass来获取已经被加载到内存中的类，如果类已经加载过，就会直接返回已加载的类；否则，先去调用parent的loadClass方法来加载类，parent无法加载时才会调用自身的findClass方法来加载。
BaseDexClassLoader
public class BaseDexClassLoader extends ClassLoader {
private final DexPathList pathList;
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(parent);
this.originalPath = dexPath;
this.pathList =
new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
Class clazz = pathList.findClass(name);
if (clazz == null) {
throw new ClassNotFoundException(name);
}
return clazz;
}
}
BaseDexClassLoader中findClass方法将类的加载委托给DexPathList类型的pathList处理，下面看下DexPathList
DexPathList
DexPathList类关键方法如下：
final class DexPathList {
private static final String DEX_SUFFIX = ".dex";
private static final String JAR_SUFFIX = ".jar";
private static final String ZIP_SUFFIX = ".zip";
private static final String APK_SUFFIX = ".apk";
private final ClassLoader definingContext;
private final Element[] dexElements;
public DexPathList(ClassLoader definingContext, String dexPath,
String libraryPath, File optimizedDirectory) {
this.definingContext = definingContext;
this.dexElements =
makeDexElements(splitDexPath(dexPath), optimizedDirectory);
}
private static Element[] makeDexElements(ArrayList<File> files,
File optimizedDirectory) {
ArrayList<Element> elements = new ArrayList<Element>();
for (File file : files) {
ZipFile zip = null;
DexFile dex = null;
String name = file.getName();
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 if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX)
|| name.endsWith(ZIP_SUFFIX)) {
try {
zip = new ZipFile(file);
} catch (IOException ex) {
System.logE("Unable to open zip file: " + file, ex);
}
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException ignored) {
}
} else {
System.logW("Unknown file type for: " + file);
}
if ((zip != null) || (dex != null)) {
elements.add(new Element(file, zip, dex));
}
}
return elements.toArray(new Element[elements.size()]);
}
}
DexPathList构造函数参数dexPath包含了dex文件的列表，这些dex文件可以是jar、zip、apk格式的压缩包，也可以直接是dex文件。DexPathList使用makeDexElements方法遍历这些dex文件，使用loadDexFile方法加载dex文件：
final class DexPathList {
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);
}
}
}
loadDexFile方法则使用DexFile的loadDex方法打开dex文件，同时会将dex优化为odex文件，返回的是DexFile对象，对dex文件的打开文件、加载类等操作都通过DexFile来实现。
DexPathList.makeDexElements将每个dex文件对应的File、ZipFile、DexFile包装成Element变量，这些Element变量存在DexPathList的dexElements数组里。
/*package*/ static class Element {
public final File file;
public final ZipFile zipFile;
public final DexFile dexFile;
public Element(File file, ZipFile zipFile, DexFile dexFile) {
this.file = file;
this.zipFile = zipFile;
this.dexFile = dexFile;
}
public URL findResource(String name) {
if ((zipFile == null) || (zipFile.getEntry(name) == null)) {
return null;
}
try {
return new URL("jar:" + file.toURL() + "!/" + name);
} catch (MalformedURLException ex) {
throw new RuntimeException(ex);
}
}
}
DexPathList类里加载class如下：
final class DexPathList {
public Class findClass(String name) {
for (Element element : dexElements) {
DexFile dex = element.dexFile;
if (dex != null) {
Class clazz = dex.loadClassBinaryName(name, definingContext);
if (clazz != null) {
return clazz;
}
}
}
return null;
}
}
DexPathList加载类时，遍历Element数组，使用element.dexFile的loadClassBinaryName方法来实际加载类：
public final class DexFile {
public Class loadClassBinaryName(String name, ClassLoader loader) {
return defineClass(name, loader, mCookie);
}
private native static Class defineClass(String name, ClassLoader loader, int cookie);
}
而loadClassBinaryName方法最终调用的是 native 方法defineClass。
总结
DexClassLoader或PathClassLoader加载类的过程：
DexClassLoader.loadClass ->
DexClassLoader.findClass ->
BaseDexClassLoader.findClass ->
DexPathList.findClass ->
DexFile.loadClassBinaryName ->
DexFile.defineClass