背景
由于前前前阵子写了个壳，得去了解类的加载流程，当时记了一些潦草的笔记。这几天把这些东西简单梳理了一下，本文分析的代码基于Android8.1.0源码。
流程分析
从loadClass开始，我们来看下Android中类加载的流程
/libcore/ojluni/src/main/java/java/lang/ClassLoader.java::loadClass
loadClass流程如下：
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;
}
/libcore/ojluni/src/main/java/java/lang/ClassLoader.java::findClass
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
ClassLoader类的findClass是没有实际查找代码的，所以调用findClass其实是调用其实现类的findClass函数，例如：BaseDexClassLoader
/libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java::findClass
每个BaseDexClassLoader都持有一个DexPathList，BaseDexClassLoader的findClass类调用了DexPathList的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;
}
/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java::findClass
遍历所有dexElements，并调用Element类的findClass。
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;
}
题外话，dexElements对象其实是DexPathList$Element类的数组，用于存储已加载的dex或者jar的信息。
/libcore/dalvik/src/main/java/dalvik/system/DexPathList$Element::findClass
Element的findClass，又去调用DexFile类的loadClassBinaryName，可以理解为在单独的dex或者jar对象中加载类
public Class<?> findClass(String name, ClassLoader definingContext,
List<Throwable> suppressed) {
return dexFile != null ? dexFile.loadClassBinaryName(name, definingContext, suppressed)
: null;
}
libcore\dalvik\src\main\java\dalvik\system\DexFile.java::loadClassBinaryName
去调用defineClass函数
public Class loadClassBinaryName(String name, ClassLoader loader, List<Throwable> suppressed) {
return defineClass(name, loader, mCookie, this, suppressed);
}
libcore\dalvik\src\main\java\dalvik\system\DexFile.java::defineClass
调用defineClassNative，准备进入Native层
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;
}
art\runtime\native\dalvik_system_DexFile.cc::DexFile_defineClassNative
检查dex是否加载，类名是否合理，并遍历DexFile对象，查找Dex文件中的类的定义，找到就去调用ClassLinker::DefineClass函数。
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;
}
ScopedUtfChars class_name(env, javaName);
if (class_name.c_str() == nullptr) {
VLOG(class_linker) << "Failed to find class_name";
return nullptr;
}
const std::string descriptor(DotToDescriptor(class_name.c_str()));
const size_t hash(ComputeModifiedUtf8Hash(descriptor.c_str()));
for (auto& dex_file : dex_files) {
const DexFile::ClassDef* dex_class_def =
OatDexFile::FindClassDef(*dex_file, descriptor.c_str(), hash);
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);
// Add the used dex file. This only required for the DexFile.loadClass API since normal
// class loaders already keep their dex files live.
class_linker->InsertDexFileInToClassLoader(soa.Decode<mirror::Object>(dexFile),
class_loader.Get());
if (result != nullptr) {
VLOG(class_linker) << "DexFile_defineClassNative returning " << result
<< " for " << class_name.c_str();
return soa.AddLocalReference<jclass>(result);
}
}
}
VLOG(class_linker) << "Failed to find dex_class_def " << class_name.c_str();
return nullptr;
}
art\runtime\class_linker.cc::DefineClass
DefineClass这个函数做了许多工作，相当于底层类加载逻辑的分发器，整体逻辑如下图：
mirror::Class* ClassLinker::DefineClass(Thread* self,
const char* descriptor,
size_t hash,
Handle<mirror::ClassLoader> class_loader,
const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def) {
StackHandleScope<3> hs(self);
auto klass = hs.NewHandle<mirror::Class>(nullptr);
......
// Get the real dex file. This will return the input if there aren't any callbacks or they do
// nothing.
DexFile const* new_dex_file = nullptr;
DexFile::ClassDef const* new_class_def = nullptr;
// TODO We should ideally figure out some way to move this after we get a lock on the klass so it
// will only be called once.
Runtime::Current()->GetRuntimeCallbacks()->ClassPreDefine(descriptor,
klass,
class_loader,
dex_file,
dex_class_def,
&new_dex_file,
&new_class_def);
// Check to see if an exception happened during runtime callbacks. Return if so.
if (self->IsExceptionPending()) {
return nullptr;
}
ObjPtr<mirror::DexCache> dex_cache = RegisterDexFile(*new_dex_file, class_loader.Get());
if (dex_cache == nullptr) {
self->AssertPendingException();
return nullptr;
}
klass->SetDexCache(dex_cache);
SetupClass(*new_dex_file, *new_class_def, klass, class_loader.Get());
// Mark the string class by setting its access flag.
if (UNLIKELY(!init_done_)) {
if (strcmp(descriptor, "Ljava/lang/String;") == 0) {
klass->SetStringClass();
}
}
ObjectLock<mirror::Class> lock(self, klass);
klass->SetClinitThreadId(self->GetTid());
// Make sure we have a valid empty iftable even if there are errors.
klass->SetIfTable(GetClassRoot(kJavaLangObject)->GetIfTable());
// Add the newly loaded class to the loaded classes table.
ObjPtr<mirror::Class> existing = InsertClass(descriptor, klass.Get(), hash);
if (existing != nullptr) {
// We failed to insert because we raced with another thread. Calling EnsureResolved may cause
// this thread to block.
return EnsureResolved(self, descriptor, existing);
}
// Load the fields and other things after we are inserted in the table. This is so that we don't
// end up allocating unfree-able linear alloc resources and then lose the race condition. The
// other reason is that the field roots are only visited from the class table. So we need to be
// inserted before we allocate / fill in these fields.
LoadClass(self, *new_dex_file, *new_class_def, klass);
if (self->IsExceptionPending()) {
VLOG(class_linker) << self->GetException()->Dump();
// An exception occured during load, set status to erroneous while holding klass' lock in case
// notification is necessary.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
}
return nullptr;
}
// Finish loading (if necessary) by finding parents
CHECK(!klass->IsLoaded());
if (!LoadSuperAndInterfaces(klass, *new_dex_file)) {
// Loading failed.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
}
return nullptr;
}
CHECK(klass->IsLoaded());
// At this point the class is loaded. Publish a ClassLoad event.
// Note: this may be a temporary class. It is a listener's responsibility to handle this.
Runtime::Current()->GetRuntimeCallbacks()->ClassLoad(klass);
// Link the class (if necessary)
CHECK(!klass->IsResolved());
// TODO: Use fast jobjects?
auto interfaces = hs.NewHandle<mirror::ObjectArray<mirror::Class>>(nullptr);
MutableHandle<mirror::Class> h_new_class = hs.NewHandle<mirror::Class>(nullptr);
if (!LinkClass(self, descriptor, klass, interfaces, &h_new_class)) {
// Linking failed.
if (!klass->IsErroneous()) {
mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
}
return nullptr;
}
self->AssertNoPendingException();
CHECK(h_new_class != nullptr) << descriptor;
CHECK(h_new_class->IsResolved() && !h_new_class->IsErroneousResolved()) << descriptor;
// Instrumentation may have updated entrypoints for all methods of all
// classes. However it could not update methods of this class while we
// were loading it. Now the class is resolved, we can update entrypoints
// as required by instrumentation.
if (Runtime::Current()->GetInstrumentation()->AreExitStubsInstalled()) {
// We must be in the kRunnable state to prevent instrumentation from
// suspending all threads to update entrypoints while we are doing it
// for this class.
DCHECK_EQ(self->GetState(), kRunnable);
Runtime::Current()->GetInstrumentation()->InstallStubsForClass(h_new_class.Get());
}
/*
* We send CLASS_PREPARE events to the debugger from here. The
* definition of "preparation" is creating the static fields for a
* class and initializing them to the standard default values, but not
* executing any code (that comes later, during "initialization").
*
* We did the static preparation in LinkClass.
*
* The class has been prepared and resolved but possibly not yet verified
* at this point.
*/
Runtime::Current()->GetRuntimeCallbacks()->ClassPrepare(klass, h_new_class);
// Notify native debugger of the new class and its layout.
jit::Jit::NewTypeLoadedIfUsingJit(h_new_class.Get());
return h_new_class.Get();
}
art\runtime\class_linker.cc::SetupClass
SetupClass设置类的一些基本字段信息。
void ClassLinker::SetupClass(const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def,
Handle<mirror::Class> klass,
ObjPtr<mirror::ClassLoader> class_loader) {
CHECK(klass != nullptr);
CHECK(klass->GetDexCache() != nullptr);
CHECK_EQ(mirror::Class::kStatusNotReady, klass->GetStatus());
const char* descriptor = dex_file.GetClassDescriptor(dex_class_def);
CHECK(descriptor != nullptr);
klass->SetClass(GetClassRoot(kJavaLangClass));
uint32_t access_flags = dex_class_def.GetJavaAccessFlags();
CHECK_EQ(access_flags & ~kAccJavaFlagsMask, 0U);
klass->SetAccessFlags(access_flags);
klass->SetClassLoader(class_loader);
DCHECK_EQ(klass->GetPrimitiveType(), Primitive::kPrimNot);
mirror::Class::SetStatus(klass, mirror::Class::kStatusIdx, nullptr);
klass->SetDexClassDefIndex(dex_file.GetIndexForClassDef(dex_class_def));
klass->SetDexTypeIndex(dex_class_def.class_idx_);
}
延申：mirror命名空间下的类是底层对Java层类的映射，比如：mirror::Class类就是对java.lang.Class类的映射，SetAccessFlags就是对Class类的accessFlags字段赋值。
art\runtime\class_linker.cc::InsertClass
InsertClass函数判断类是否在列表中：
- 如果在列表中，则直接返回；
- 如果没有，则添加到列表。
mirror::Class* ClassLinker::InsertClass(const char* descriptor, ObjPtr<mirror::Class> klass, size_t hash) {
if (VLOG_IS_ON(class_linker)) {
ObjPtr<mirror::DexCache> dex_cache = klass->GetDexCache();
std::string source;
if (dex_cache != nullptr) {
source += " from ";
source += dex_cache->GetLocation()->ToModifiedUtf8();
}
LOG(INFO) << "Loaded class " << descriptor << source;
}
{
WriterMutexLock mu(Thread::Current(), *Locks::classlinker_classes_lock_);
ObjPtr<mirror::ClassLoader> const class_loader = klass->GetClassLoader();
ClassTable* const class_table = InsertClassTableForClassLoader(class_loader);
ObjPtr<mirror::Class> existing = class_table->Lookup(descriptor, hash);
if (existing != nullptr) {
return existing.Ptr();
}
VerifyObject(klass);
class_table->InsertWithHash(klass, hash);
if (class_loader != nullptr) {
// This is necessary because we need to have the card dirtied for remembered sets.
Runtime::Current()->GetHeap()->WriteBarrierEveryFieldOf(class_loader);
}
if (log_new_roots_) {
new_class_roots_.push_back(GcRoot<mirror::Class>(klass));
}
}
if (kIsDebugBuild) {
// Test that copied methods correctly can find their holder.
for (ArtMethod& method : klass->GetCopiedMethods(image_pointer_size_)) {
CHECK_EQ(GetHoldingClassOfCopiedMethod(&method), klass);
}
}
return nullptr;
}
art\runtime\class_linker.cc::LoadClass
LoadClass函数获取了dex文件中的classData部分，然后去调用LoadClassMembers
void ClassLinker::LoadClass(Thread* self,
const DexFile& dex_file,
const DexFile::ClassDef& dex_class_def,
Handle<mirror::Class> klass) {
const uint8_t* class_data = dex_file.GetClassData(dex_class_def);
if (class_data == nullptr) {
return; // no fields or methods - for example a marker interface
}
LoadClassMembers(self, dex_file, class_data, klass);
}
art\runtime\class_linker.cc::LoadClassMembers
LoadClassMembers函数主要逻辑是遍历类中的所有字段和函数，然后分别调用LoadField，LoadMethod和LinkCode
void ClassLinker::LoadClassMembers(Thread* self,
const DexFile& dex_file,
const uint8_t* class_data,
Handle<mirror::Class> klass){
......
LinearAlloc* const allocator = GetAllocatorForClassLoader(klass->GetClassLoader());
ClassDataItemIterator it(dex_file, class_data);
LengthPrefixedArray<ArtField>* sfields = AllocArtFieldArray(self,
allocator,
it.NumStaticFields());
size_t num_sfields = 0;
uint32_t last_field_idx = 0u;
for (; it.HasNextStaticField(); it.Next()) {
uint32_t field_idx = it.GetMemberIndex();
DCHECK_GE(field_idx, last_field_idx); // Ordering enforced by DexFileVerifier.
if (num_sfields == 0 || LIKELY(field_idx > last_field_idx)) {
DCHECK_LT(num_sfields, it.NumStaticFields());
LoadField(it, klass, &sfields->At(num_sfields));
++num_sfields;
last_field_idx = field_idx;
}
}
// Load instance fields.
LengthPrefixedArray<ArtField>* ifields = AllocArtFieldArray(self,
allocator,
it.NumInstanceFields());
size_t num_ifields = 0u;
last_field_idx = 0u;
for (; it.HasNextInstanceField(); it.Next()) {
uint32_t field_idx = it.GetMemberIndex();
DCHECK_GE(field_idx, last_field_idx); // Ordering enforced by DexFileVerifier.
if (num_ifields == 0 || LIKELY(field_idx > last_field_idx)) {
DCHECK_LT(num_ifields, it.NumInstanceFields());
LoadField(it, klass, &ifields->At(num_ifields));
++num_ifields;
last_field_idx = field_idx;
}
}
......
size_t class_def_method_index = 0;
uint32_t last_dex_method_index = DexFile::kDexNoIndex;
size_t last_class_def_method_index = 0;
for (size_t i = 0; it.HasNextDirectMethod(); i++, it.Next()) {
ArtMethod* method = klass->GetDirectMethodUnchecked(i, image_pointer_size_);
LoadMethod(dex_file, it, klass, method);
LinkCode(this, method, oat_class_ptr, class_def_method_index);
uint32_t it_method_index = it.GetMemberIndex();
if (last_dex_method_index == it_method_index) {
// duplicate case
method->SetMethodIndex(last_class_def_method_index);
} else {
method->SetMethodIndex(class_def_method_index);
last_dex_method_index = it_method_index;
last_class_def_method_index = class_def_method_index;
}
class_def_method_index++;
}
for (size_t i = 0; it.HasNextVirtualMethod(); i++, it.Next()) {
ArtMethod* method = klass->GetVirtualMethodUnchecked(i, image_pointer_size_);
LoadMethod(dex_file, it, klass, method);
DCHECK_EQ(class_def_method_index, it.NumDirectMethods() + i);
LinkCode(this, method, oat_class_ptr, class_def_method_index);
class_def_method_index++;
}
......
}
art\runtime\class_linker.cc::LoadField
LoadField设置ArtField结构中字段的一些值
void ClassLinker::LoadField(const ClassDataItemIterator& it,
Handle<mirror::Class> klass,
ArtField* dst) {
const uint32_t field_idx = it.GetMemberIndex();
dst->SetDexFieldIndex(field_idx);
dst->SetDeclaringClass(klass.Get());
dst->SetAccessFlags(it.GetFieldAccessFlags());
}
art\runtime\class_linker.cc::LoadMethod
LoadMethod函数主要做设置ArtMethod结构的一些属性，比如函数的MethodIdx，CodeItem在dex文件中的偏移，函数的AccessFlag等。
void ClassLinker::LoadMethod(const DexFile& dex_file,
const ClassDataItemIterator& it,
Handle<mirror::Class> klass,
ArtMethod* dst){
uint32_t dex_method_idx = it.GetMemberIndex();
const DexFile::MethodId& method_id = dex_file.GetMethodId(dex_method_idx);
const char* method_name = dex_file.StringDataByIdx(method_id.name_idx_);
ScopedAssertNoThreadSuspension ants("LoadMethod");
dst->SetDexMethodIndex(dex_method_idx);
dst->SetDeclaringClass(klass.Get());
dst->SetCodeItemOffset(it.GetMethodCodeItemOffset());
dst->SetDexCacheResolvedMethods(klass->GetDexCache()->GetResolvedMethods(), image_pointer_size_);
uint32_t access_flags = it.GetMethodAccessFlags();
......
dst->SetAccessFlags(access_flags);
}
延申：ArtMethod是存储Java函数在虚拟机内相关信息的结构，它不同于mirror命名空间下的Method类，ArtMethod在Java层没有类与之直接映射。
art\runtime\class_linker.cc::LinkCode
LinkCode函数主要功能是判断代码是否编译从而为函数设置入口代码。
static void LinkCode(ClassLinker* class_linker,
ArtMethod* method,
const OatFile::OatClass* oat_class,
uint32_t class_def_method_index){
Runtime* const runtime = Runtime::Current();
if (runtime->IsAotCompiler()) {
// The following code only applies to a non-compiler runtime.
return;
}
// Method shouldn't have already been linked.
DCHECK(method->GetEntryPointFromQuickCompiledCode() == nullptr);
if (oat_class != nullptr) {
// Every kind of method should at least get an invoke stub from the oat_method.
// non-abstract methods also get their code pointers.
const OatFile::OatMethod oat_method = oat_class->GetOatMethod(class_def_method_index);
oat_method.LinkMethod(method);
}
// Install entry point from interpreter.
const void* quick_code = method->GetEntryPointFromQuickCompiledCode();
bool enter_interpreter = class_linker->ShouldUseInterpreterEntrypoint(method, quick_code);
if (!method->IsInvokable()) {
EnsureThrowsInvocationError(class_linker, method);
return;
}
if (method->IsStatic() && !method->IsConstructor()) {
// For static methods excluding the class initializer, install the trampoline.
// It will be replaced by the proper entry point by ClassLinker::FixupStaticTrampolines
// after initializing class (see ClassLinker::InitializeClass method).
method->SetEntryPointFromQuickCompiledCode(GetQuickResolutionStub());
} else if (quick_code == nullptr && method->IsNative()) {
method->SetEntryPointFromQuickCompiledCode(GetQuickGenericJniStub());
} else if (enter_interpreter) {
// Set entry point from compiled code if there's no code or in interpreter only mode.
method->SetEntryPointFromQuickCompiledCode(GetQuickToInterpreterBridge());
}
if (method->IsNative()) {
// Unregistering restores the dlsym lookup stub.
method->UnregisterNative();
if (enter_interpreter || quick_code == nullptr) {
// We have a native method here without code. Then it should have either the generic JNI
// trampoline as entrypoint (non-static), or the resolution trampoline (static).
// TODO: this doesn't handle all the cases where trampolines may be installed.
const void* entry_point = method->GetEntryPointFromQuickCompiledCode();
DCHECK(class_linker->IsQuickGenericJniStub(entry_point) ||
class_linker->IsQuickResolutionStub(entry_point));
}
}
}
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