ClassLoader源码阅读

安卓10

java层

libcore\ojluni\src\main\java\java\lang\ClassLoader.java

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;
}

    /**
     * Returns the class with the given binary name if this
     * loader has been recorded by the Java virtual machine as an initiating
     * loader of a class with that binary name.  Otherwise
     * null is returned.
     *
     * @param  name
     *         The binary name of the class
     *
     * @return  The Class object, or null if the class has
     *          not been loaded
     *
     * @since  1.1
     */
    protected final Class<?> findLoadedClass(String name) {
        ClassLoader loader;
        if (this == BootClassLoader.getInstance())
            loader = null;
        else
            loader = this;
        return VMClassLoader.findLoadedClass(loader, name);
    }

libcore\libart\src\main\java\java\lang\VMClassLoader.java

    @FastNative
    native static Class findLoadedClass(ClassLoader cl, String name);

native层

art\runtime\native\java_lang_VMClassLoader.cc

static jclass VMClassLoader_findLoadedClass(JNIEnv* env, jclass, jobject javaLoader,
                                            jstring javaName) {

  // Hard-coded performance optimization: We know that all failed libcore calls to findLoadedClass
  //                                      are followed by a call to the the classloader to actually
  //                                      load the class.
  if (loader != nullptr) {
    // Try the common case.
    StackHandleScope<1> hs(soa.Self());
    c = VMClassLoader::FindClassInPathClassLoader(cl,
                                                  soa,
                                                  soa.Self(),
                                                  descriptor.c_str(),
                                                  descriptor_hash,
                                                  hs.NewHandle(loader));
    if (c != nullptr) {
      return soa.AddLocalReference<jclass>(c);
    }
  }

  // The class wasn't loaded, yet, and our fast-path did not apply (e.g., we didn't understand the
  // classloader chain).
  return nullptr;
}


  static ObjPtr<mirror::Class> FindClassInPathClassLoader(ClassLinker* cl,
                                                          ScopedObjectAccessAlreadyRunnable& soa,
                                                          Thread* self,
                                                          const char* descriptor,
                                                          size_t hash,
                                                          Handle<mirror::ClassLoader> class_loader)
      REQUIRES_SHARED(Locks::mutator_lock_) {
    ObjPtr<mirror::Class> result;
    if (cl->FindClassInBaseDexClassLoader(soa, self, descriptor, hash, class_loader, &result)) {
      DCHECK(!self->IsExceptionPending());
      return result;
    }
    if (self->IsExceptionPending()) {
      self->ClearException();
    }
    return nullptr;
  }

art\runtime\class_linker.cc

bool ClassLinker::FindClassInBaseDexClassLoader(ScopedObjectAccessAlreadyRunnable& soa,
                                                Thread* self,
                                                const char* descriptor,
                                                size_t hash,
                                                Handle<mirror::ClassLoader> class_loader,
                                                /*out*/ ObjPtr<mirror::Class>* result) {
  // Termination case: boot class loader.
  if (IsBootClassLoader(soa, class_loader.Get())) {
    *result = FindClassInBootClassLoaderClassPath(self, descriptor, hash);
    return true;
  }

  if (IsPathOrDexClassLoader(soa, class_loader) || IsInMemoryDexClassLoader(soa, class_loader)) {
    // For regular path or dex class loader the search order is:
    //    - parent
    //    - shared libraries
    //    - class loader dex files

    // Handles as RegisterDexFile may allocate dex caches (and cause thread suspension).
    StackHandleScope<1> hs(self);
    Handle<mirror::ClassLoader> h_parent(hs.NewHandle(class_loader->GetParent()));
    if (!FindClassInBaseDexClassLoader(soa, self, descriptor, hash, h_parent, result)) {
      return false;  // One of the parents is not supported.
    }
    if (*result != nullptr) {
      return true;  // Found the class up the chain.
    }

    if (!FindClassInSharedLibraries(soa, self, descriptor, hash, class_loader, result)) {
      return false;  // One of the shared library loader is not supported.
    }
    if (*result != nullptr) {
      return true;  // Found the class in a shared library.
    }

    // Search the current class loader classpath.
    *result = FindClassInBaseDexClassLoaderClassPath(soa, descriptor, hash, class_loader);
    return !soa.Self()->IsExceptionPending();
  }

  if (IsDelegateLastClassLoader(soa, class_loader)) {
    // For delegate last, the search order is:
    //    - boot class path
    //    - shared libraries
    //    - class loader dex files
    //    - parent
    *result = FindClassInBootClassLoaderClassPath(self, descriptor, hash);
    if (*result != nullptr) {
      return true;  // The class is part of the boot class path.
    }
    if (self->IsExceptionPending()) {
      // Pending exception means there was an error other than ClassNotFound that must be returned
      // to the caller.
      return false;
    }

    if (!FindClassInSharedLibraries(soa, self, descriptor, hash, class_loader, result)) {
      return false;  // One of the shared library loader is not supported.
    }
    if (*result != nullptr) {
      return true;  // Found the class in a shared library.
    }

    *result = FindClassInBaseDexClassLoaderClassPath(soa, descriptor, hash, class_loader);
    if (*result != nullptr) {
      return true;  // Found the class in the current class loader
    }
    if (self->IsExceptionPending()) {
      // Pending exception means there was an error other than ClassNotFound that must be returned
      // to the caller.
      return false;
    }

    // Handles as RegisterDexFile may allocate dex caches (and cause thread suspension).
    StackHandleScope<1> hs(self);
    Handle<mirror::ClassLoader> h_parent(hs.NewHandle(class_loader->GetParent()));
    return FindClassInBaseDexClassLoader(soa, self, descriptor, hash, h_parent, result);
  }

  // Unsupported class loader.
  *result = nullptr;
  return false;
}