最近在研究插件动态加载，记录一下其中ClassLoader部分的内容.
Android中的ClassLoader与Java有些不同，Android中ClassLoader加载的是dex文件，而Java中加载的是jar文件.相同的是两者都采用了双亲委派模型.ClassLoader中loadClass函数体现了这个模型：
protected Class loadClass(String className, boolean resolve)
throws ClassNotFoundException {
Class clazz = findLoadedClass(className);// 首先检查Class是否已经加载过
if (clazz == null) {
ClassNotFoundException suppressed = null;
try {
clazz = parent.loadClass(className, false);//尝试使用parent ClassLoader去加载Class
} catch (ClassNotFoundException e) {
suppressed = e;
}
if (clazz == null) {
try {
clazz = findClass(className);// 如果加载不成功，调用findClass函数来获取class对象.
} catch (ClassNotFoundException e) {
e.addSuppressed(suppressed);
throw e;
}
}
}
return clazz;
}
其中findClass()为protected，直接抛出异常，需要子类来实现具体的find过程.Android中ClassLoader的子类是BaseDexClassLoder,同时有PathClassLoader和DexClassLoder两个类直接继承自BaseDexClassLoder.
public class DexClassLoader extends BaseDexClassLoader {
public DexClassLoader(String dexPath, String optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(dexPath, new File(optimizedDirectory), libraryPath, parent);
}
}
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);
}
}
public class BaseDexClassLoader extends ClassLoader｛
/**
* 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.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;
}
//more code
｝
DexClassLoader和PathClassLoader都直接调用BaseDexClassLoader的构造函数，区别是传入的参数不一样，PathClassLoader的super调用中，传入的optimizedDirectory一直为null.BaseDexClassLoder的构造函数使用入参的三个路径构造了一个DexPathList对象.同时findClass()直接使用DexPathList的findClass()函数.看一下DexPathList的源码：
/*package*/ final class DexPathList {
/** list of dex/resource (class path) elements */
private final Element[] dexElements;
public DexPathList(ClassLoader definingContext, String dexPath,
String libraryPath, File optimizedDirectory) {
// some error checking
this.definingContext = definingContext;
this.dexElements =
makeDexElements(splitDexPath(dexPath), optimizedDirectory);
this.nativeLibraryDirectories = splitLibraryPath(libraryPath);
}
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;
}
/*package*/ static class Element {
public final File file;
public final ZipFile zipFile;
public final DexFile dexFile;
}
}
DexPathList的构造函数将传入的路径转化为Element数组，findClass()中遍历Element中的DexFile来加载Class，看一下Element数组的生成过程：
/**
* Makes an array of dex/resource path elements, one per element of
* the given array.
*/
private static Element[] makeDexElements(ArrayList files,
File optimizedDirectory) {
ArrayList elements = new ArrayList();
for (File file : files) {
ZipFile zip = null;
DexFile dex = null;
String name = file.getName();
if (name.endsWith(DEX_SUFFIX)) { // 文件后缀为.dex
try {
dex = loadDexFile(file, optimizedDirectory);//1.加载dex文件
} 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)) {// 文件为压缩包
try {
zip = new ZipFile(file);//构造zip对象
} catch (IOException ex) {
System.logE("Unable to open zip file: " + file, ex);
}
try {
dex = loadDexFile(file, optimizedDirectory);//2.加载dex文件
} catch (IOException ignored) {
// 如果压缩文件中没有dex文件，抛出这个异常，可以直接无视
}
} else {
System.logW("Unknown file type for: " + file);
}
if ((zip != null) || (dex != null)) {//如果同时有zip和dex文件，就构造对应的Element
elements.add(new Element(file, zip, dex));
}
}
return elements.toArray(new Element[elements.size()]);
}
调用了两次loadDexFile()来生成dex对象，第一次的file为dex文件，第二次的file是压缩文件(zip,jar,apk).看一下对应的代码：
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);
}
}
根据optimizedDirectory是否为null调用不同的方法来构造DexFile.这两种构造方法最后都会调用openDexFile()这个native函数.其中outputName就是optimizedDirectory.回到前面看，PathClassLoader和DexClassLoader的区别是optimizedDirectory是否为null,对应openDexFile()中outputName是否为null.
// java code
native private static int openDexFile(String sourceName, String outputName,
int flags) throws IOException;
// native code
static void Dalvik_dalvik_system_DexFile_openDexFile(const u4* args,
JValue* pResult)
{
// ....
if (hasDexExtension(sourceName)
&& dvmRawDexFileOpen(sourceName, outputName, &pRawDexFile, false) == 0) {
ALOGV("Opening DEX file '%s' (DEX)", sourceName);
pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
pDexOrJar->isDex = true;
pDexOrJar->pRawDexFile = pRawDexFile;
pDexOrJar->pDexMemory = NULL;
} else if (dvmJarFileOpen(sourceName, outputName, &pJarFile, false) == 0) {
ALOGV("Opening DEX file '%s' (Jar)", sourceName);
pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
pDexOrJar->isDex = false;
pDexOrJar->pJarFile = pJarFile;
pDexOrJar->pDexMemory = NULL;
} else {
ALOGV("Unable to open DEX file '%s'", sourceName);
dvmThrowIOException("unable to open DEX file");
}
//....
}
加载dex文件的过程在dvmRawDexFileOpen()和dvmJarFileOpen()方法中完成.看一下dvmJarFileOpen()的代码：
int dvmJarFileOpen(const char* fileName, const char* odexOutputName,
JarFile** ppJarFile, bool isBootstrap)
{
tryArchive:
entry = dexZipFindEntry(&archive, kDexInJarName);
if (entry != NULL) {
if (odexOutputName == NULL) {
cachedName = dexOptGenerateCacheFileName(fileName,
kDexInJarName);
if (cachedName == NULL)
goto bail;
} else {
cachedName = strdup(odexOutputName);
}
}
bail:
/* clean up, closing the open file */
if (archiveOpen && result != 0)
dexZipCloseArchive(&archive);
free(cachedName);
if (fd >= 0) {
if (locked)
(void) dvmUnlockCachedDexFile(fd);
close(fd);
}
return result;
}
在odexOutputName为null的情况下，会直接返回一个ERROR，表示加载失败。
所以PathClassLoader只能加载dex格式的文件，而DexClassLoader可以加载dex文件，对于apk等压缩文件，加载过程会将压缩文件中的class.dex解压到optimizedDirectory目录中进行加载。