Android类加载流程

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
{
        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;
}

/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;
        }

libcoredalviksrcmainjavadalviksystemDexFile.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);
    }

libcoredalviksrcmainjavadalviksystemDexFile.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;
}

artruntimenativedalvik_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ClassDef部分，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) {
        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);

      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;
}

artruntimeclass_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);

  ......

  DexFile const* new_dex_file = nullptr;
  DexFile::ClassDef const* new_class_def = nullptr;
  Runtime::Current()->GetRuntimeCallbacks()->ClassPreDefine(descriptor,
                                                            klass,
                                                            class_loader,
                                                            dex_file,
                                                            dex_class_def,
                                                            &new_dex_file,
                                                            &new_class_def);
  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());

  if (UNLIKELY(!init_done_)) {
    if (strcmp(descriptor, "Ljava/lang/String;") == 0) {
      klass->SetStringClass();
    }
  }

  ObjectLock<mirror::Class> lock(self, klass);
  klass->SetClinitThreadId(self->GetTid());

  klass->SetIfTable(GetClassRoot(kJavaLangObject)->GetIfTable());

  ObjPtr<mirror::Class> existing = InsertClass(descriptor, klass.Get(), hash);
  if (existing != nullptr) {

    return EnsureResolved(self, descriptor, existing);
  }

  LoadClass(self, *new_dex_file, *new_class_def, klass);
  if (self->IsExceptionPending()) {
    VLOG(class_linker) << self->GetException()->Dump();

    if (!klass->IsErroneous()) {
      mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
    }
    return nullptr;
  }

  CHECK(!klass->IsLoaded());
  if (!LoadSuperAndInterfaces(klass, *new_dex_file)) {

    if (!klass->IsErroneous()) {
      mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
    }
    return nullptr;
  }
  CHECK(klass->IsLoaded());

  Runtime::Current()->GetRuntimeCallbacks()->ClassLoad(klass);

  CHECK(!klass->IsResolved());
  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)) {

    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;

  if (Runtime::Current()->GetInstrumentation()->AreExitStubsInstalled()) {

    DCHECK_EQ(self->GetState(), kRunnable);
    Runtime::Current()->GetInstrumentation()->InstallStubsForClass(h_new_class.Get());
  }

  Runtime::Current()->GetRuntimeCallbacks()->ClassPrepare(klass, h_new_class);

  jit::Jit::NewTypeLoadedIfUsingJit(h_new_class.Get());

  return h_new_class.Get();
}

在DefineClass里面也能看到许多mirror::Class::SetStatus()函数的调用，用来设置Class的加载状态。它可以有以下几种状态：

enum Status {
  kStatusRetired = -3,  // Retired, should not be used. Use the newly cloned one instead.
  kStatusErrorResolved = -2,
  kStatusErrorUnresolved = -1,
  kStatusNotReady = 0,
  kStatusIdx = 1,  // Loaded, DEX idx in super_class_type_idx_ and interfaces_type_idx_.
  kStatusLoaded = 2,  // DEX idx values resolved.
  kStatusResolving = 3,  // Just cloned from temporary class object.
  kStatusResolved = 4,  // Part of linking.
  kStatusVerifying = 5,  // In the process of being verified.
  kStatusRetryVerificationAtRuntime = 6,  // Compile time verification failed, retry at runtime.
  kStatusVerifyingAtRuntime = 7,  // Retrying verification at runtime.
  kStatusVerified = 8,  // Logically part of linking; done pre-init.
  kStatusSuperclassValidated = 9,  // Superclass validation part of init done.
  kStatusInitializing = 10,  // Class init in progress.
  kStatusInitialized = 11,  // Ready to go.
  kStatusMax = 12,
};

延申：mirror命名空间下的类是底层对Java层类的映射，比如上面的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。

artruntimeclass_linker.cc::SetupClass

SetupClass设置类的一些基本字段信息。并把Class加载状态设置为kStatusIdx

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_);
}

artruntimeclass_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) {
      Runtime::Current()->GetHeap()->WriteBarrierEveryFieldOf(class_loader);
    }
    if (log_new_roots_) {
      new_class_roots_.push_back(GcRoot<mirror::Class>(klass));
    }
  }
  if (kIsDebugBuild) {
    for (ArtMethod& method : klass->GetCopiedMethods(image_pointer_size_)) {
      CHECK_EQ(GetHoldingClassOfCopiedMethod(&method), klass);
    }
  }
  return nullptr;
}

对于InsertClass结果不为空的情况，表示类已经在加载表中了。

这种情况下，调用EnsureResolved来等待类过期并且重新加载完成

mirror::Class* ClassLinker::EnsureResolved(Thread* self,
                                           const char* descriptor,
                                           ObjPtr<mirror::Class> klass) {

  ......

  if (init_done_ && klass->IsTemp()) {
    CHECK(!klass->IsResolved());
    if (klass->IsErroneousUnresolved()) {
      ThrowEarlierClassFailure(klass);
      return nullptr;
    }
    StackHandleScope<1> hs(self);
    Handle<mirror::Class> h_class(hs.NewHandle(klass));
    ObjectLock<mirror::Class> lock(self, h_class);
    while (!h_class->IsRetired() && !h_class->IsErroneousUnresolved()) {
      lock.WaitIgnoringInterrupts();
    }
    if (h_class->IsErroneousUnresolved()) {
      ThrowEarlierClassFailure(h_class.Get());
      return nullptr;
    }
    CHECK(h_class->IsRetired());
    klass = LookupClass(self, descriptor, h_class.Get()->GetClassLoader());
  }

  size_t index = 0;

  static const size_t kNumYieldIterations = 1000;

  static const size_t kSleepDurationUS = 1000;  // 1 ms.
  while (!klass->IsResolved() && !klass->IsErroneousUnresolved()) {
    StackHandleScope<1> hs(self);
    HandleWrapperObjPtr<mirror::Class> h_class(hs.NewHandleWrapper(&klass));
    {
      ObjectTryLock<mirror::Class> lock(self, h_class);

      if (lock.Acquired()) {
        if (!h_class->IsResolved() && h_class->GetClinitThreadId() == self->GetTid()) {
          ThrowClassCircularityError(h_class.Get());
          mirror::Class::SetStatus(h_class, mirror::Class::kStatusErrorUnresolved, self);
          return nullptr;
        }
      }
    }
    {
      ScopedThreadSuspension sts(self, kSuspended);
      if (index < kNumYieldIterations) {
        sched_yield();
      } else {
        usleep(kSleepDurationUS);
      }
    }
    ++index;
  }

  if (klass->IsErroneousUnresolved()) {
    ThrowEarlierClassFailure(klass);
    return nullptr;
  }
  CHECK(klass->IsResolved()) << klass->PrettyClass();
  self->AssertNoPendingException();
  return klass.Ptr();
}

然后将EnsureResolved函数的返回的Class作为DefineClass函数的返回值

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) {
  ......

  ObjPtr<mirror::Class> existing = InsertClass(descriptor, klass.Get(), hash);
  if (existing != nullptr) {

    return EnsureResolved(self, descriptor, existing);
  }
  
  ......
}

artruntimeclass_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;
  }
  LoadClassMembers(self, dex_file, class_data, klass);
}

artruntimeclass_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);
      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;
      }
    }

    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); 
      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) {
        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++;
    }
    ......
}

artruntimeclass_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());
}

artruntimeclass_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。

artruntimeclass_linker.cc::LinkCode

LinkCode函数主要功能是判断代码是否编译，以及函数的类型来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()) {
    return;
  }
  DCHECK(method->GetEntryPointFromQuickCompiledCode() == nullptr);
  if (oat_class != nullptr) {
    const OatFile::OatMethod oat_method = oat_class->GetOatMethod(class_def_method_index);
    oat_method.LinkMethod(method);
  }

  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()) {
    method->SetEntryPointFromQuickCompiledCode(GetQuickResolutionStub());
  } else if (quick_code == nullptr && method->IsNative()) {
    method->SetEntryPointFromQuickCompiledCode(GetQuickGenericJniStub());
  } else if (enter_interpreter) {
    method->SetEntryPointFromQuickCompiledCode(GetQuickToInterpreterBridge());
  }

  if (method->IsNative()) {
    method->UnregisterNative();

    if (enter_interpreter || quick_code == nullptr) {
      const void* entry_point = method->GetEntryPointFromQuickCompiledCode();
      DCHECK(class_linker->IsQuickGenericJniStub(entry_point) ||
             class_linker->IsQuickResolutionStub(entry_point));
    }
  }
}

跟到这里，DefineClass里面的流程已经过了一半了，接下来继续来跟LinkClass那一分支。

从LinkClass函数定义来看，LinkClass有一个参数是h_new_class_out，它是一个指针，并且根据变量名猜测，它是会被LinkClass修改和赋值的

bool ClassLinker::LinkClass(Thread* self,
                            const char* descriptor,
                            Handle<mirror::Class> klass,
                            Handle<mirror::ObjectArray<mirror::Class>> interfaces,
                            MutableHandle<mirror::Class>* h_new_class_out);

看DefineClass里调用LinkClass的代码，它传了一个为nullptr的h_new_class进去，也印证了上面的猜想：

......

  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;
  }

......

artruntimeclass_linker.cc::LinkClass

LinkClass中调用LinkSuperClass、LinkMethods、LinkInstanceFields和LinkStaticFields这些函数。接着给h_new_class_out分配一个值。这个值返回到DexFile_defineClassNative转成jclass，jclass到Java层就变成了java.lang.Class

bool ClassLinker::LinkClass(Thread* self,
                            const char* descriptor,
                            Handle<mirror::Class> klass,
                            Handle<mirror::ObjectArray<mirror::Class>> interfaces,
                            MutableHandle<mirror::Class>* h_new_class_out) {
  CHECK_EQ(mirror::Class::kStatusLoaded, klass->GetStatus());

  if (!LinkSuperClass(klass)) {
    return false;
  }
  ArtMethod* imt_data[ImTable::kSize];
  bool new_conflict = false;
  std::fill_n(imt_data, arraysize(imt_data), Runtime::Current()->GetImtUnimplementedMethod());
  if (!LinkMethods(self, klass, interfaces, &new_conflict, imt_data)) {
    return false;
  }
  if (!LinkInstanceFields(self, klass)) {
    return false;
  }
  size_t class_size;
  if (!LinkStaticFields(self, klass, &class_size)) {
    return false;
  }
  CreateReferenceInstanceOffsets(klass);
  CHECK_EQ(mirror::Class::kStatusLoaded, klass->GetStatus());

  ImTable* imt = nullptr;
  if (klass->ShouldHaveImt()) {

    if (!new_conflict) {
      ImTable* super_imt = FindSuperImt(klass.Get(), image_pointer_size_);
      if (super_imt != nullptr) {
        bool imt_equals = true;
        for (size_t i = 0; i < ImTable::kSize && imt_equals; ++i) {
          imt_equals = imt_equals && (super_imt->Get(i, image_pointer_size_) == imt_data[i]);
        }
        if (imt_equals) {
          imt = super_imt;
        }
      }
    }
    if (imt == nullptr) {
      LinearAlloc* allocator = GetAllocatorForClassLoader(klass->GetClassLoader());
      imt = reinterpret_cast<ImTable*>(
          allocator->Alloc(self, ImTable::SizeInBytes(image_pointer_size_)));
      if (imt == nullptr) {
        return false;
      }
      imt->Populate(imt_data, image_pointer_size_);
    }
  }

  if (!klass->IsTemp() || (!init_done_ && klass->GetClassSize() == class_size)) {
    CHECK_EQ(klass->GetClassSize(), class_size) << klass->PrettyDescriptor();

    if (klass->ShouldHaveEmbeddedVTable()) {
      klass->PopulateEmbeddedVTable(image_pointer_size_);
    }
    if (klass->ShouldHaveImt()) {
      klass->SetImt(imt, image_pointer_size_);
    }

    if (cha_ != nullptr) {
      cha_->UpdateAfterLoadingOf(klass);
    }

    mirror::Class::SetStatus(klass, mirror::Class::kStatusResolved, self);
    h_new_class_out->Assign(klass.Get());
  } else {
    CHECK(!klass->IsResolved());

    StackHandleScope<1> hs(self);
    auto h_new_class = hs.NewHandle(klass->CopyOf(self, class_size, imt, image_pointer_size_));

    klass->SetMethodsPtrUnchecked(nullptr, 0, 0);
    klass->SetSFieldsPtrUnchecked(nullptr);
    klass->SetIFieldsPtrUnchecked(nullptr);
    if (UNLIKELY(h_new_class == nullptr)) {
      self->AssertPendingOOMException();
      mirror::Class::SetStatus(klass, mirror::Class::kStatusErrorUnresolved, self);
      return false;
    }

    CHECK_EQ(h_new_class->GetClassSize(), class_size);
    ObjectLock<mirror::Class> lock(self, h_new_class);
    FixupTemporaryDeclaringClass(klass.Get(), h_new_class.Get());

    {
      WriterMutexLock mu(self, *Locks::classlinker_classes_lock_);
      ObjPtr<mirror::ClassLoader> const class_loader = h_new_class.Get()->GetClassLoader();
      ClassTable* const table = InsertClassTableForClassLoader(class_loader);
      ObjPtr<mirror::Class> existing = table->UpdateClass(descriptor, h_new_class.Get(),
                                                   ComputeModifiedUtf8Hash(descriptor));
      if (class_loader != nullptr) {
        Runtime::Current()->GetHeap()->WriteBarrierEveryFieldOf(class_loader);
      }
      CHECK_EQ(existing, klass.Get());
      if (log_new_roots_) {
        new_class_roots_.push_back(GcRoot<mirror::Class>(h_new_class.Get()));
      }
    }

    if (cha_ != nullptr) {
      cha_->UpdateAfterLoadingOf(h_new_class);
    }

    mirror::Class::SetStatus(klass, mirror::Class::kStatusRetired, self);

    CHECK_EQ(h_new_class->GetStatus(), mirror::Class::kStatusResolving);

    mirror::Class::SetStatus(h_new_class, mirror::Class::kStatusResolved, self);

    h_new_class_out->Assign(h_new_class.Get());
  }
  return true;
}

后记

本文只是跟踪了loadClass的大体流程，还有许多东西没有展开，比如LinkMethods。如有兴趣，可以自行研究。

原文始发于微信公众号（luoyesiqiu）：Android类加载流程