先说结论
1、DexClassLoader可以加载jar/apk/dex，可以从SD卡中加载未安装的apk
2、PathClassLoader只能加载系统中已经安装过的apk
PathClassLoader 源码
以下源码全部来自Android6.0.1
package dalvik.system;
public class PathClassLoader extends BaseDexClassLoader {
/** 有兴趣的可以看看注释，故意没删
* 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:
JAR/ZIP/APK files, possibly containing a "classes.dex" file as
* well as arbitrary resources.
*
Raw ".dex" files (not inside a zip file).
*
*
* 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 libraryPath 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 libraryPath,
ClassLoader parent) {
super(dexPath, null, libraryPath, parent);
}
}
DexClassLoader 源码
package dalvik.system;
import java.io.File;
/**
* A class loader that loads classes from {@code .jar} and {@code .apk} files
* containing a {@code classes.dex} entry. This can be used to execute code not
* installed as part of an application.
* * <p>This class loader requires an application-private, writable directory to
* cache optimized classes. Use {@code Context.getCodeCacheDir()} to create
* such a directory: <pre> {@code
* File dexOutputDir = context.getCodeCacheDir();
* }</pre>
* * <p><strong>Do not cache optimized classes on external storage.</strong>
* External storage does not provide access controls necessary to protect your
* application from code injection attacks.
*/
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 directory where optimized dex files
* should be written; must not 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 DexClassLoader(String dexPath, String optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(dexPath, new File(optimizedDirectory), libraryPath, parent);
}
}
父类BaseDexClassLoader源码
/**
* Base class for common functionality between various dex-based
* {@link ClassLoader} implementations.
*/
public class BaseDexClassLoader extends ClassLoader {
private final DexPathList pathList;
/**
* 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);
}
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
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;
}
@Override
protected URL findResource(String name) {
return pathList.findResource(name);
}
@Override
protected Enumeration<URL> findResources(String name) {
return pathList.findResources(name);
}
@Override
public String findLibrary(String name) {
return pathList.findLibrary(name);
}
/**
* Returns package information for the given package.
* Unfortunately, instances of this class don't really have this
* information, and as a non-secure {@code ClassLoader}, it isn't
* even required to, according to the spec. Yet, we want to
* provide it, in order to make all those hopeful callers of
* {@code myClass.getPackage().getName()} happy. Thus we construct
* a {@code Package} object the first time it is being requested
* and fill most of the fields with dummy values. The {@code
* Package} object is then put into the {@code ClassLoader}'s
* package cache, so we see the same one next time. We don't
* create {@code Package} objects for {@code null} arguments or
* for the default package.
*
* <p>There is a limited chance that we end up with multiple
* {@code Package} objects representing the same package: It can
* happen when when a package is scattered across different JAR
* files which were loaded by different {@code ClassLoader}
* instances. This is rather unlikely, and given that this whole
* thing is more or less a workaround, probably not worth the
* effort to address.
*
* @param name the name of the class
* @return the package information for the class, or {@code null}
* if there is no package information available for it
*/
@Override
protected synchronized Package getPackage(String name) {
if (name != null && !name.isEmpty()) {
Package pack = super.getPackage(name);
if (pack == null) {
pack = definePackage(name, "Unknown", "0.0", "Unknown",
"Unknown", "0.0", "Unknown", null);
}
return pack;
}
return null;
}
/**
* @hide
*/
public String getLdLibraryPath() {
StringBuilder result = new StringBuilder();
for (File directory : pathList.getNativeLibraryDirectories()) {
if (result.length() > 0) {
result.append(':');
}
result.append(directory);
}
return result.toString();
}
@Override public String toString() {
return getClass().getName() + "[" + pathList + "]";
}
}
原因
DexClassLoader构造函数
//dexPath ：dex路径
//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。
public DexClassLoader(String dexPath, String optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(dexPath, new File(optimizedDirectory), libraryPath, parent);
}
PathClassLoader构造函数
//dexPath ：dex路径
//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。
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 与 PathClassLoader 构造函数区别就是多了个optimizedDirectory参数。
继续查看父类构造函数：
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(parent);
this.pathList = new DexPathList(this, dexPath, libraryPath,optimizedDirectory);
}
和关键方法findClass
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
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;
}
Class c = pathList.findClass(name, suppressedExceptions);中的pathList是在构造函数中new出来的，继续看DexPathList的findClass方法：
public Class findClass(String name, List<Throwable> suppressed) {
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;
}
DexPathList会执行findClass时，会先将所有的dex包进行遍历，从每个包查找相对应name的class，只要找到，马上返回。
这里有关于热修复实现的知识点，就是将补丁 dex 文件放到 dexElements 数组靠前位置，这样在加载 class 时，优先找到补丁包中的 dex 文件，加载到 class 之后就不再寻找，从而原来的 apk 文件中同名的类就不会再使用，从而达到修复的目的
至于这个dexElements怎么创造出来的呢，在DexPathList构造方法里面创造的：
public DexPathList(ClassLoader definingContext, String dexPath,
String libraryPath, File optimizedDirectory) {
//.....
//省略其他不重要代码
ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
// save dexPath for BaseDexClassLoader
this.dexElements = makePathElements(splitDexPath(dexPath), optimizedDirectory, suppressedExceptions);
//.....
//省略部分代码
}
继续看 makePathElements 方法：
private static Element[] makePathElements(List<File> files, File optimizedDirectory, List<IOException> suppressedExceptions) {
List<Element> elements = new ArrayList<>();
for (File file : files) {
File zip = null;
File dir = new File("");
DexFile dex = null;
String path = file.getPath();
String name = file.getName();
if (path.contains(zipSeparator)) {
String split[] = path.split(zipSeparator, 2);
zip = new File(split[0]);
dir = new File(split[1]);
} else if (file.isDirectory()) {
// We support directories for looking up resources and native libraries.
// Looking up resources in directories is useful for running libcore tests.
elements.add(new Element(file, true, null, null));
} 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(dir, false, zip, dex));
}
}
return elements.toArray(new Element[elements.size()]);
}
通过遍历files（就是传入的dexPath），执行loadDexFile转变为dex，然后添加到elements中。
到这里，依然没有看出有和无optimizedDirectory这个参数的区别，那就继续看loadDexFile这个方法：
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);
}
}
尼玛，终于找到区别的了...
1. optimizedDirectory如果为null，loadDexFile返回的是new DexFile(file)
2. optimizedDirectory 不为null，返回的是DexFile.loadDex(file.getPath(), optimizedPath, 0)
继续查DexFile，看这两个的区别,先是new DexFile（File）：
public DexFile(File file) throws IOException {
this(file.getPath());
}
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);
}
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);
}
最终指向了DexFile(String sourceName, String outputName, int flags)这个方法。
继续看上面的DexFile.loadDex(file.getPath(), optimizedPath, 0)方法：
static public DexFile loadDex(String sourcePathName, String outputPathName,
int flags) throws IOException {
return new DexFile(sourcePathName, outputPathName, flags);
}
同样指向的是new DexFile(sourcePathName, outputPathName, flags);
上面两者区别是，outputPathName参数为不为空。
好了，现在PathClassLoader 和 DexClassLoader的区别就变成了看这个outputPathName参数为不为空的区别了。
而outputPathName为不为空，结果会有什么影响呢？继续看DexFile中构造函数openDexFile的源码：
private static Object openDexFile(String sourceName, String outputName, int flags) throws IOException {
return openDexFileNative(new File(sourceName).getAbsolutePath(), (outputName == null) ? null : new File(outputName).getAbsolutePath(), flags);
}
openDexFileNative是个Native方法，在源码art/runtime/native/dalvik_system_DexFile.cc中找到（不要问我怎么找到的，我也是看别人才找到的）：
static jobject DexFile_openDexFileNative(
JNIEnv* env, jclass, jstring javaSourceName, jstring javaOutputName, jint) {
ScopedUtfChars sourceName(env, javaSourceName);
ScopedUtfChars sourceName(env, javaSourceName);
if (sourceName.c_str() == nullptr) {
return 0;
}
NullableScopedUtfChars outputName(env, javaOutputName);
if (env->ExceptionCheck()) {
return 0;
}
ClassLinker* linker = Runtime::Current()->GetClassLinker();
std::vector<std::unique_ptr<const DexFile>> dex_files;
std::vector<std::string> error_msgs;
dex_files = linker->OpenDexFilesFromOat(sourceName.c_str(), outputName.c_str(), &error_msgs);
//省略部分代码
//......
}
}
代码太多我只列出了追踪javaOutputName这个参数的关键代码，javaOutputName被转换成了NullableScopedUtfChars类型的 outputName
NullableScopedUtfChars outputName(env, javaOutputName);
然后这个outputName被linker的对象执行了OpenDexFilesFromOat方法使用：
ClassLinker* linker = Runtime::Current()->GetClassLinker();
//省略部分代码
//.....
dex_files = linker->OpenDexFilesFromOat(sourceName.c_str(), outputName.c_str(), &error_msgs);
有兴趣的可以在art\runtime\class_linker.cc文件中查找OpenDexFilesFromOat方法：
std::vector<std::unique_ptr<const DexFile>> ClassLinker::OpenDexFilesFromOat(
const char* dex_location, const char* oat_location,
std::vector<std::string>* error_msgs) {
//代码太多，只列出部分....
//注意这里传入了oat_location，生成了OatFileAssistant 对象
OatFileAssistant oat_file_assistant(dex_location, oat_location, kRuntimeISA,
!Runtime::Current()->IsAotCompiler());
//省略部分代码...
// If we didn't have an up-to-date oat file open, try to load one from disk.
//source_oat_file就是从已经加载过的ota文件中查找到的目标dex文件
if (source_oat_file == nullptr) {
// Update the oat file on disk if we can. This may fail, but that's okay.
// Best effort is all that matters here.
//source_oat_file为空的话，会通过oat_file_assistant.MakeUpToDate(&error_msg)方法在其他路径尝试加载
if (!oat_file_assistant.MakeUpToDate(&error_msg)) {
LOG(WARNING) << error_msg;
}
// Get the oat file on disk.
std::unique_ptr<OatFile> oat_file = oat_file_assistant.GetBestOatFile();
if (oat_file.get() != nullptr) {
// Take the file only if it has no collisions, or we must take it because of preopting.
bool accept_oat_file = !HasCollisions(oat_file.get(), &error_msg);
//省略部分代码
// Fall back to running out of the original dex file if we couldn't load any
// dex_files from the oat file.
if (dex_files.empty()) {
if (oat_file_assistant.HasOriginalDexFiles()) {
if (Runtime::Current()->IsDexFileFallbackEnabled()) {
if (!DexFile::Open(dex_location, dex_location, &error_msg, &dex_files)) {
LOG(WARNING) << error_msg;
error_msgs->push_back("Failed to open dex files from " + std::string(dex_location));
}
} else {
error_msgs->push_back("Fallback mode disabled, skipping dex files.");
}
} else {
error_msgs->push_back("No original dex files found for dex location "
+ std::string(dex_location));
}
}
return dex_files;
}
主要看核心代码oat_file_assistant.MakeUpToDate，在oat_file_assistant.cc这个文件里：
bool OatFileAssistant::MakeUpToDate(std::string* error_msg) {
switch (GetDexOptNeeded()) {
case kNoDexOptNeeded: return true;
case kDex2OatNeeded: return GenerateOatFile(error_msg);
case kPatchOatNeeded: return RelocateOatFile(OdexFileName(), error_msg);
case kSelfPatchOatNeeded: return RelocateOatFile(OatFileName(), error_msg);
}
UNREACHABLE();
}
网上很多分析都不是基于Android6.0，外加因为c不太熟，这个方法分析理解如下（说的不对请指出啊）：
case kNoDexOptNeeded: return true;是指已经在中加载过的dex文件，只要apk安装过，运行过，都可以直接可以找到，不需要再加载了；
case kDex2OatNeeded: return GenerateOatFile(error_msg);内存中没有的，就进行第一次加载dex，转换成oat文件。这个时候，如果之前传入的outputName为空的话，就不能加载了。
下面两个case就看不太懂了。不过文章标题的问题似乎已经找到答案了。
再次总结
1、DexClassLoader可以加载jar/apk/dex，可以从SD卡中加载未安装的apk
2、PathClassLoader只能加载系统中已经安装过的apk