Android classLoader 从源码理解类加载
一、概述
我们知道Java中的 ClassLoader
可以加载 jar 文件和Class文件(本质时加载Class文件)。在Android中,它们加载到是dex文件。
Android中的ClassLoader类型分别是系统类加载器和自定义加载器。其中系统类加载器主要包括3种,分别是 BootClassLoader 、PathClassLoader 和 DexClassLoader 。
概览
- BootClassLoader
Android 系统启动时会使用BootClassLoader
来预加载常用类。- DexClassLoader
DexClassLoader 可以加载dex文件以及包含dex的压缩文件(apk和jar文件)- PathClassLoader
Android 系统使用PathClassLoader来加载系统类和应用程序的类。
framework 源码使用 android10 release 分支
platform/libcore/dalvik/src/main/java/dalvik/system
- PathClassLoader.java
- DexClassLoader.java
- BaseDexClassLoader.java
- DexPathList.java
- DexFile.java
platform/art/runtime/native
- dalvik_system_DexFile.cc
platform/ojluni/src/main/java/java/lang/
- ClassLoader.java
二、五种类加载构造函数
2.1 PathClassLoader
提供了一个简单的{@link ClassLoader}实现,它对列表进行操作的文件和目录,但没有尝试从网络加载类。Android将这个类用于它的系统类加载器和它的应用程序类加载器。
public class PathClassLoader extends BaseDexClassLoader {
public PathClassLoader(String dexPath, ClassLoader parent) {
super(dexPath, null, null, parent);
}
public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
super(dexPath, null, librarySearchPath, parent);
}
@libcore.api.CorePlatformApi
public PathClassLoader(
String dexPath, String librarySearchPath, ClassLoader parent,
ClassLoader[] sharedLibraryLoaders) {
super(dexPath, librarySearchPath, parent, sharedLibraryLoaders);
}
}
2.2 DexClassLoader
一个类装入器,从包含{@code classes.dex}条目的{@code .jar}和{@code .apk}文件中装入类。这可用于执行未作为应用程序的一部分安装的代码。 在API级别26之前,这个类装入器需要一个应用专用的可写目录来缓存优化后的类。使用
File dexOutputDir = context.getCodeCacheDir()
创建这样一个目录 。不要在外部存储上缓存优化的类。外部存储不提供必要的访问控制来保护应用程序免受代码注入攻击。
public class DexClassLoader extends BaseDexClassLoader {
public DexClassLoader(String dexPath, String optimizedDirectory,
String librarySearchPath, ClassLoader parent) {
super(dexPath, null, librarySearchPath, parent);
}
}
2.3 BaseDexClassLoader
用于定制如何报告dex文件加载的钩子。
这使框架能够监视dex文件的使用。其目标是简化优化外部dex文件的机制,并允许进一步优化辅助dex文件。只有当BaseDexClassLoader的新实例被构造时,报告才会发生,并且只有在这个字段被设置为{@link BaseDexClassLoader#setReporter}后才会被激活。
public class BaseDexClassLoader extends ClassLoader {
@UnsupportedAppUsage
private final DexPathList pathList;
public BaseDexClassLoader(String dexPath,
String librarySearchPath, ClassLoader parent, ClassLoader[] sharedLibraryLoaders,
boolean isTrusted) {
super(parent);
//在创建路径列表之前设置共享库。ART依赖于类加载器层次结构在加载dex文件之前完成。
this.sharedLibraryLoaders = sharedLibraryLoaders == null
? null
: Arrays.copyOf(sharedLibraryLoaders, sharedLibraryLoaders.length);
this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);
if (reporter != null) {
reportClassLoaderChain();
}
}
//设置dex加载通知的报告器。一旦设置完毕,BaseDexClassLoader的所有新实例都将在构建已加载的dex文件时报告。
@libcore.api.CorePlatformApi
public static void setReporter(Reporter newReporter) {
reporter = newReporter;
}
}
BaseDexClassLoader构造函数, 有一个非常重要的过程, 那就是初始化DexPathList对象.
另外该构造函数的参数说明:
- dexPath: 包含目标类或资源的apk/jar列表;当有多个路径则采用:分割;
- optimizedDirectory: 优化后dex文件存在的目录, 可以为null;
- libraryPath: native库所在路径列表;当有多个路径则采用:分割;
- ClassLoader:父类的类加载器.
2.4 ClassLoader
类加载器是负责装入类的对象。类是一个抽象类。给定类的属性,类加载器应该尝试定位或生成构成类定义的数据。应用程序实现的子类是为了扩展Java虚拟机动态加载类的方式。安全管理器通常可以使用类装入器来指示安全域。
public abstract class ClassLoader {
private ClassLoader parent; //记录父类加载器
protected ClassLoader() {
this(getSystemClassLoader(), false); //见下文
}
protected ClassLoader(ClassLoader parentLoader) {
this(parentLoader, false);
}
ClassLoader(ClassLoader parentLoader, boolean nullAllowed) {
if (parentLoader == null && !nullAllowed) {
//父类的类加载器为空,则抛出异常
throw new NullPointerException("parentLoader == null && !nullAllowed");
}
parent = parentLoader;
}
}
再来看看 SystemClassLoader
static private class SystemClassLoader {
public static ClassLoader loader = ClassLoader.createSystemClassLoader();
}
private static ClassLoader createSystemClassLoader() {
String classPath = System.getProperty("java.class.path", ".");
String librarySearchPath = System.getProperty("java.library.path", "");
return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());
}
2.5 BootClassLoader
class BootClassLoader extends ClassLoader {
private static BootClassLoader instance;
public static synchronized BootClassLoader getInstance() {
if (instance == null) {
instance = new BootClassLoader();
}
return instance;
}
public BootClassLoader() {
super(null, true);
} }
三、PathClassLoader加载类的过程
此处以PathClassLoader为例来说明类的加载过程,先初始化,然后执行loadClass()方法来加载相应的类。
3.1 PathClassLoader构造方法
public class PathClassLoader extends BaseDexClassLoader {
public PathClassLoader(String dexPath, ClassLoader parent) {
super(dexPath, null, null, parent); //见下文
}
}
public class BaseDexClassLoader extends ClassLoader {
private final DexPathList pathList;
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(parent); //见下文
//收集dex文件和Native动态库
this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}
}
public abstract class ClassLoader {
private ClassLoader parent; //父类加载器
protected ClassLoader(ClassLoader parentLoader) {
this(parentLoader, false);
}
ClassLoader(ClassLoader parentLoader, boolean nullAllowed) {
parent = parentLoader;
}
}
3.2 DexPathList
类路径条目可以是任何一个:包含可选的*{@code classes.dex}文件和任意资源的{@code .dex}文件,或者是普通的{@code .dex}文件(不可能有关联的资源)。这个类还包含使用这些列表查找类和资源的方法
public final class DexPathList {
private static final String DEX_SUFFIX = ".dex";
private static final String zipSeparator = "!/";
@UnsupportedAppUsage
private final ClassLoader definingContext;
@UnsupportedAppUsage
private Element[] dexElements;//dex/资源(类路径)元素的列表。应该被称为pathElements,但是Facebook应用程序使用反射来修改“dexelement”
@UnsupportedAppUsage
NativeLibraryElement[] nativeLibraryPathElements; //本地库路径元素列表
@UnsupportedAppUsage
private final List<File> nativeLibraryDirectories;//应用程序本机库目录的列表。
@UnsupportedAppUsage
private final List<File> systemNativeLibraryDirectories;
@UnsupportedAppUsage
private IOException[] dexElementsSuppressedExceptions;//创建dexElements列表时抛出的异常。
@UnsupportedAppUsage
public DexPathList(ClassLoader definingContext, String dexPath,
String librarySearchPath, File optimizedDirectory) {
this(definingContext, dexPath, librarySearchPath, optimizedDirectory, false);
}
DexPathList(ClassLoader definingContext, String dexPath,
String librarySearchPath, File optimizedDirectory, boolean isTrusted) {
if (definingContext == null) {
throw new NullPointerException("definingContext == null");
}
if (dexPath == null) {
throw new NullPointerException("dexPath == null");
}
if (optimizedDirectory != null) {
if (!optimizedDirectory.exists()) {
throw new IllegalArgumentException(
"optimizedDirectory doesn't exist: "
+ optimizedDirectory);
}
if (!(optimizedDirectory.canRead()
&& optimizedDirectory.canWrite())) {
throw new IllegalArgumentException(
"optimizedDirectory not readable/writable: "
+ optimizedDirectory);
}
}
this.definingContext = definingContext;
ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
//记录所有的dexFile文件
this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
suppressedExceptions, definingContext, isTrusted);
//app目录的native库
this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
this.systemNativeLibraryDirectories =
splitPaths(System.getProperty("java.library.path"), true);
//记录所有的Native动态库
this.nativeLibraryPathElements = makePathElements(getAllNativeLibraryDirectories());
if (suppressedExceptions.size() > 0) {
this.dexElementsSuppressedExceptions =
suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
} else {
dexElementsSuppressedExceptions = null;
}
}
}
DexPathList初始化过程,主要功能是收集以下两个变量信息:
- dexElements: 根据多路径的分隔符“;”将dexPath转换成File列表,记录所有的dexFile
- nativeLibraryPathElements: 记录所有的Native动态库, 包括app目录的native库和系统目录的native库。
3.2.1 DexPathList::makePathElements
private static Element[] makePathElements(List<File> files, File optimizedDirectory,
List<IOException> suppressedExceptions) {
return makeDexElements(files, optimizedDirectory, suppressedExceptions, null);
}
该方法的主要功能是创建Element数组
3.2.2 DexPathList::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;
//打开所有文件并预先加载(直接或包含的)dex文件。
for (File file : files) {
if (file.isDirectory()) {
elements[elementsPos++] = new Element(file);
} else if (file.isFile()) {
String name = file.getName();
DexFile dex = null;
//匹配以.dex为后缀的文件
if (name.endsWith(DEX_SUFFIX)) {
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) {
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;
}
3.2.3 DexPathList::loadDexFile
@UnsupportedAppUsage
private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader,
Element[] elements)
throws IOException {
if (optimizedDirectory == null) {
return new DexFile(file, loader, elements);
} else {
String optimizedPath = optimizedPathFor(file, optimizedDirectory);
return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements);
}
}
3.2.4 创建DexFile对象
应用程序不应该直接使用这个类。在大多数情况下,这会损害性能,在最坏的情况下会导致不正确的字节码执行。应用程序应该使用一个标准的类加载器,比如{@link dalvik.system。PathClassLoader}。这个API将在未来的Android版本中被删除。
DexFile(File file, ClassLoader loader, DexPathList.Element[] elements)
throws IOException {
this(file.getPath(), loader, elements);
}
private DexFile(String sourceName, String outputName, int flags, ClassLoader loader,
DexPathList.Element[] elements) 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, loader, elements);
mInternalCookie = mCookie;
mFileName = sourceName;
}
参数介绍
- sourceName : 带有class .dex的Jar或APK文件
- outputName : 保存DEX数据的优化格式的文件。
- elements : makeDexElements()过程生成的临时Element数组;
3.2.5 DexFile::openDexFile
private static Object openDexFile(String sourceName, String outputName, int flags,
ClassLoader loader, DexPathList.Element[] elements) throws IOException {
// Use absolute paths to enable the use of relative paths when testing on host.
return openDexFileNative(new File(sourceName).getAbsolutePath(),
(outputName == null)
? null
: new File(outputName).getAbsolutePath(),
flags,
loader,
elements);
}
3.2.6 dalvik_system_DexFile.cc::openDexFileNative
static jobject DexFile_openDexFileNative(JNIEnv* env,
jclass,
jstring javaSourceName,
jstring javaOutputName ATTRIBUTE_UNUSED,
jint flags ATTRIBUTE_UNUSED,
jobject class_loader,
jobjectArray dex_elements) {
ScopedUtfChars sourceName(env, javaSourceName);
if (sourceName.c_str() == nullptr) {
return nullptr;
}
std::vector<std::string> error_msgs;
const OatFile* oat_file = nullptr;
std::vector<std::unique_ptr<const DexFile>> dex_files =
Runtime::Current()->GetOatFileManager().OpenDexFilesFromOat(sourceName.c_str(),
class_loader,
dex_elements,
/*out*/ &oat_file,
/*out*/ &error_msgs);
return CreateCookieFromOatFileManagerResult(env, dex_files, oat_file, error_msgs);
}
PathClassLoader创建完成后,就已经拥有了目标程序的文件路径,native lib路径,以及parent类加载器对象。接下来开始执行loadClass()来加载相应的类。
3.3 ClassLoader::loadClass
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);
}
}
return c;
}
该方法的加载流程如下:
- 判断当前类加载器是否已经加载过指定类,若已加载则直接返回,否则继续执行
- 调用parent的类加载递归加载该类,检测是否加载,若已加载则直接返回,否则继续执行;
- 调用当前类加载器,通过findClass加载
3.3 ClassLoader::findLoadedClass
protected final Class<?> findLoadedClass(String name) {
ClassLoader loader;
if (this == BootClassLoader.getInstance())
loader = null;
else
loader = this;
return VMClassLoader.findLoadedClass(loader, name);
}
3.4 BaseDexClassLoader:: findClass
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
...
List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
Class c = pathList.findClass(name, suppressedExceptions);
...
return c;
}
3.5 DexPathList.findClass
在此实例指向的dex文件中查找已命名类。这将在最早列出的path元素中找到一个。如果找到了类,但还没有定义,则此方法将在构造此实例时使用的定义上下文中定义它。
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;
}
这里是核心逻辑,一个ClassLoader可以包含多个dex文件,每个dex文件被封装到一个Element对象,这些Element对象排列成有序的数组 dexElements 。当查找某个类时,会遍历所有的dex文件,如果找到则直接返回,不再继续遍历dexElements。也就是说当两个类不同的dex中出现,会优先处理排在前面的dex文件,这便是热修复的核心,将需要修复的类所打包的dex文件插入到dexElements前面。
DexFile::loadClassBinaryName
element.findClass
最后走到 dexFile.loadClassBinaryName
@UnsupportedAppUsage
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;
}
dalvik_system_DexFile.cc:: defineClassNative
static jclass DexFile_defineClassNative(JNIEnv* env,
jclass,
jstring javaName,
jobject javaLoader,
jobject cookie,
jobject dexFile) {
std::vector<const DexFile*> dex_files;
const OatFile* oat_file;
if (!ConvertJavaArrayToDexFiles(env, cookie, /*out*/ dex_files, /*out*/ oat_file)) {
VLOG(class_linker) << "Failed to find dex_file";
DCHECK(env->ExceptionCheck());
return nullptr; //dex文件为空, 则直接返回
}
ScopedUtfChars class_name(env, javaName);
if (class_name.c_str() == nullptr) {
VLOG(class_linker) << "Failed to find class_name";
return nullptr; //类名为空, 则直接返回
}
const std::string descriptor(DotToDescriptor(class_name.c_str()));
const size_t hash(ComputeModifiedUtf8Hash(descriptor.c_str()));
for (auto& dex_file : dex_files) {
const dex::ClassDef* dex_class_def =
OatDexFile::FindClassDef(*dex_file, descriptor.c_str(), hash); //将类名转换为hash码
if (dex_class_def != nullptr) {
ScopedObjectAccess soa(env);
ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
StackHandleScope<1> hs(soa.Self());
Handle<mirror::ClassLoader> class_loader(
hs.NewHandle(soa.Decode<mirror::ClassLoader>(javaLoader)));
ObjPtr<mirror::DexCache> dex_cache =
class_linker->RegisterDexFile(*dex_file, class_loader.Get());
if (dex_cache == nullptr) {
// OOME or InternalError (dexFile already registered with a different class loader).
soa.Self()->AssertPendingException();
return nullptr;
}
ObjPtr<mirror::Class> result = class_linker->DefineClass(soa.Self(),
descriptor.c_str(),
hash,
class_loader,
*dex_file,
*dex_class_def);
//添加使用过的dex文件。这只对DexFile是必需的。因为通常的类加载器已经保持它们的dex文件的活动。>InsertDexFileInToClassLoader(soa.Decode<mirror::Object>(dexFile),
class_loader.Get());
if (result != nullptr) {
// 找到目标对象
return soa.AddLocalReference<jclass>(result);
}
}
}
return nullptr;
}
在native层创建目标类的对象并添加到虚拟机列表。
总结
这篇文章我们要理解Android常用的classLoader,以及通过将补丁dex文件插入到替换dex文件的Elements的热修复原理。
可以尝试着回答以下问题来巩固
- 1.虚拟机如何加载这些class文件?
- 2.Class文件中的信息进入到虚拟机后会发生什么变化?
- 3.如何打破双亲委派模型
参考 :
http://gityuan.com/2017/03/19/android-classloader/
本文地址:https://blog.csdn.net/qq_38366777/article/details/110178787
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