runtime/class_loader_context.h - LeafOS-Project/android_art - Gitiles

 /*
  * Copyright (C) 2017 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #ifndef ART_RUNTIME_CLASS_LOADER_CONTEXT_H_
 #define ART_RUNTIME_CLASS_LOADER_CONTEXT_H_

 #include <string>
 #include <vector>
 #include <set>

 #include "arch/instruction_set.h"
 #include "base/dchecked_vector.h"
 #include "base/macros.h"
 #include "dex/dex_file.h"
 #include "handle_scope.h"
 #include "mirror/class_loader.h"
 #include "oat/oat_file.h"
 #include "scoped_thread_state_change.h"

 namespace art HIDDEN {

 class DexFile;
 class OatFile;

 // Utility class which holds the class loader context used during compilation/verification.
 class EXPORT ClassLoaderContext {
  public:
   enum class VerificationResult {
     kVerifies,
     kMismatch,
   };

   enum ClassLoaderType {
     kInvalidClassLoader = 0,
     kPathClassLoader = 1,
     kDelegateLastClassLoader = 2,
     kInMemoryDexClassLoader = 3
   };

   // Special encoding used to denote a foreign ClassLoader was found when trying to encode class
   // loader contexts for each classpath element in a ClassLoader. See
   // EncodeClassPathContextsForClassLoader. Keep in sync with PackageDexUsage in the framework
   // (frameworks/base/services/core/java/com/android/server/pm/dex/PackageDexUsage.java) and
   // DexUseManager in ART Services
   // (art/libartservice/service/java/com/android/server/art/DexUseManager.java).
   static constexpr const char* kUnsupportedClassLoaderContextEncoding =
       "=UnsupportedClassLoaderContext=";

   ~ClassLoaderContext();

   // Opens requested class path files and appends them to ClassLoaderInfo::opened_dex_files.
   // If the dex files have been stripped, the method opens them from their oat files which are added
   // to ClassLoaderInfo::opened_oat_files. The 'classpath_dir' argument specifies the directory to
   // use for the relative class paths.
   // Returns true if all dex files where successfully opened.
   // It may be called only once per ClassLoaderContext. Subsequent calls will return the same
   // result without doing anything.
   // If `context_fds` is an empty vector, files will be opened using the class path locations as
   // filenames. Otherwise `context_fds` is expected to contain file descriptors to class path dex
   // files, following the order of dex file locations in a flattened class loader context. If their
   // number (size of `context_fds`) does not match the number of dex files, OpenDexFiles will fail.
   //
   // This will replace the class path locations with the locations of the opened dex files.
   // (Note that one dex file can contain multidexes. Each multidex will be added to the classpath
   // separately.)
   //
   // only_read_checksums controls whether or not we only read the dex locations and the checksums
   // from the apk instead of fully opening the dex files.
   //
   // This method is not thread safe.
   //
   // Note that a "false" return could mean that either an apk/jar contained no dex files or
   // that we hit a I/O or checksum mismatch error.
   // TODO(calin): Currently there's no easy way to tell the difference.
   //
   // TODO(calin): we're forced to complicate the flow in this class with a different
   // OpenDexFiles step because the current dex2oat flow requires the dex files be opened before
   // the class loader is created. Consider reworking the dex2oat part.
   bool OpenDexFiles(const std::string& classpath_dir = "",
                     const std::vector<int>& context_fds = std::vector<int>(),
                     bool only_read_checksums = false);

   // Remove the specified compilation sources from all classpaths present in this context.
   // Should only be called before the first call to OpenDexFiles().
   bool RemoveLocationsFromClassPaths(const dchecked_vector<std::string>& compilation_sources);

   // Creates the entire class loader hierarchy according to the current context.
   // Returns the first class loader from the chain.
   //
   // For example: if the context was built from the spec
   // "ClassLoaderType1[ClasspathElem1:ClasspathElem2...];ClassLoaderType2[...]..."
   // the method returns the class loader correponding to ClassLoader1. The parent chain will be
   // ClassLoader1 --> ClassLoader2 --> ... --> BootClassLoader.
   //
   // The compilation sources are appended to the classpath of the first class loader (in the above
   // example ClassLoader1).
   //
   // If the context is empty, this method only creates a single PathClassLoader with the
   // given compilation_sources.
   //
   // Shared libraries found in the chain will be canonicalized based on the dex files they
   // contain.
   //
   // Implementation notes:
   //   1) the objects are not completely set up. Do not use this outside of tests and the compiler.
   //   2) should only be called before the first call to OpenDexFiles().
   jobject CreateClassLoader(const std::vector<const DexFile*>& compilation_sources) const;

   // Encodes the context as a string suitable to be added in oat files.
   // (so that it can be read and verified at runtime against the actual class
   // loader hierarchy).
   // Should only be called if OpenDexFiles() returned true.
   // If stored context is non-null, the stored names are overwritten by the class path from the
   // stored context.
   // E.g. if the context is PCL[a.dex:b.dex] this will return
   // "PCL[a.dex*a_checksum*b.dex*a_checksum]".
   std::string EncodeContextForOatFile(const std::string& base_dir,
                                       ClassLoaderContext* stored_context = nullptr) const;

   // Encodes the context as a string suitable to be passed to dex2oat.
   // This is the same as EncodeContextForOatFile but without adding the checksums
   // and only adding each dex files once (no multidex).
   // Should only be called if OpenDexFiles() returned true.
   std::string EncodeContextForDex2oat(const std::string& base_dir) const;

   // Encodes the contexts for each of the classpath elements in the child-most
   // classloader. Under the hood EncodeContextForDex2oat is used, so no checksums
   // will be encoded.
   // Should only be called if the dex files are opened (either via OpenDexFiles() or by creating the
   // context from a live class loader).
   // Notably, for each classpath element the encoded classloader context will contain only the
   // elements that appear before it in the containing classloader. E.g. if `this` contains
   // (from child to parent):
   //
   // PathClassLoader { multidex.apk!classes.dex, multidex.apk!classes2.dex, foo.dex, bar.dex } ->
   //    PathClassLoader { baz.dex } -> BootClassLoader
   //
   // then the return value will look like:
   //
   // `{ "multidex.apk": "PCL[];PCL[baz.dex]",
   //    "foo.dex"     : "PCL[multidex.apk];PCL[baz.dex]",
   //    "bar.dex"     : "PCL[multidex.apk:foo.dex];PCL[baz.dex]" }`
   std::map<std::string, std::string> EncodeClassPathContexts(const std::string& base_dir) const;

   // Flattens the opened dex files into the given vector.
   // Should only be called if OpenDexFiles() returned true.
   std::vector<const DexFile*> FlattenOpenedDexFiles() const;

   // Return a list of dex file locations from this class loader context after flattening.
   std::vector<std::string> FlattenDexPaths() const;

   // Verifies that the current context is identical to the context encoded as `context_spec`.
   // Identical means:
   //    - the number and type of the class loaders from the chain matches
   //    - the class loader from the same position have the same classpath
   //      (the order and checksum of the dex files matches)
   // This should be called after OpenDexFiles() with only_read_checksums=true. There's no
   // need to fully open the dex files if the only thing that needs to be done is to verify
   // the context.
   //
   // Names are only verified if verify_names is true.
   // Checksums are only verified if verify_checksums is true.
   VerificationResult VerifyClassLoaderContextMatch(const std::string& context_spec,
                                                    bool verify_names = true,
                                                    bool verify_checksums = true) const;

   // Checks if any of the given dex files is already loaded in the current class loader context.
   // It only checks the first class loader.
   // Returns the list of duplicate dex files (empty if there are no duplicates).
   std::set<const DexFile*> CheckForDuplicateDexFiles(
       const std::vector<const DexFile*>& dex_files);

   // Creates the class loader context from the given string.
   // The format: ClassLoaderType1[ClasspathElem1:ClasspathElem2...];ClassLoaderType2[...]...
   // ClassLoaderType is either "PCL" (PathClassLoader) or "DLC" (DelegateLastClassLoader).
   // ClasspathElem is the path of dex/jar/apk file.
   //
   // The spec represents a class loader chain with the natural interpretation:
   // ClassLoader1 has ClassLoader2 as parent which has ClassLoader3 as a parent and so on.
   // The last class loader is assumed to have the BootClassLoader as a parent.
   //
   // Note that we allowed class loaders with an empty class path in order to support a custom
   // class loader for the source dex files.
   static std::unique_ptr<ClassLoaderContext> Create(const std::string& spec);

   // Creates a context for the given class_loader and dex_elements.
   // The method will walk the parent chain starting from `class_loader` and add their dex files
   // to the current class loaders chain. The `dex_elements` will be added at the end of the
   // classpath belonging to the `class_loader` argument.
   // The ownership of the opened dex files will be retained by the given `class_loader`.
   // If there are errors in processing the class loader chain (e.g. unsupported elements) the
   // method returns null.
   static std::unique_ptr<ClassLoaderContext> CreateContextForClassLoader(jobject class_loader,
                                                                          jobjectArray dex_elements);

   // Returns the default class loader context to be used when none is specified.
   // This will return a context with a single and empty PathClassLoader.
   static std::unique_ptr<ClassLoaderContext> Default();

   // Encodes the contexts for each of the classpath elements in `class_loader`. See
   // ClassLoaderContext::EncodeClassPathContexts for more information about the return value.
   //
   // If `class_loader` does not derive from BaseDexClassLoader then an empty map is returned.
   // Otherwise if a foreign ClassLoader is found in the class loader chain then the results values
   // will all be ClassLoaderContext::kUnsupportedClassLoaderContextEncoding.
   static std::map<std::string, std::string> EncodeClassPathContextsForClassLoader(
       jobject class_loader);

   // Returns whether `encoded_class_loader_context` is a valid encoded ClassLoaderContext or
   // EncodedUnsupportedClassLoaderContext.
   static bool IsValidEncoding(const std::string& possible_encoded_class_loader_context);

   struct ClassLoaderInfo {
     // The type of this class loader.
     ClassLoaderType type;
     // Shared libraries this context has.
     std::vector<std::unique_ptr<ClassLoaderInfo>> shared_libraries;
     // Shared libraries that will be loaded after apks code that this context has.
     std::vector<std::unique_ptr<ClassLoaderInfo>> shared_libraries_after;
     // The list of class path elements that this loader loads.
     // Note that this list may contain relative paths.
     std::vector<std::string> classpath;
     // Original opened class path (ignoring multidex).
     std::vector<std::string> original_classpath;
     // The list of class path elements checksums.
     // May be empty if the checksums are not given when the context is created.
     std::vector<uint32_t> checksums;
     // After OpenDexFiles is called this holds the opened dex files.
     std::vector<std::unique_ptr<const DexFile>> opened_dex_files;
     // After OpenDexFiles, in case some of the dex files were opened from their oat files
     // this holds the list of opened oat files.
     std::vector<std::unique_ptr<OatFile>> opened_oat_files;
     // The parent class loader.
     std::unique_ptr<ClassLoaderInfo> parent;

     explicit ClassLoaderInfo(ClassLoaderType cl_type) : type(cl_type) {}
   };

  private:
   // Creates an empty context (with no class loaders).
   ClassLoaderContext();

   // Get the parent of the class loader chain at depth `index`.
   ClassLoaderInfo* GetParent(size_t index) const {
     ClassLoaderInfo* result = class_loader_chain_.get();
     while ((result != nullptr) && (index-- != 0)) {
       result = result->parent.get();
     }
     return result;
   }

   size_t GetParentChainSize() const {
     size_t result = 0;
     ClassLoaderInfo* info = class_loader_chain_.get();
     while (info != nullptr) {
       ++result;
       info = info->parent.get();
     }
     return result;
   }

   // Constructs an empty context.
   // `owns_the_dex_files` specifies whether or not the context will own the opened dex files
   // present in the class loader chain. If `owns_the_dex_files` is true then OpenDexFiles cannot
   // be called on this context (dex_files_open_attempted_ and dex_files_open_result_ will be set
   // to true as well)
   explicit ClassLoaderContext(bool owns_the_dex_files);

   // Reads the class loader spec in place and returns true if the spec is valid and the
   // compilation context was constructed.
   bool Parse(const std::string& spec, bool parse_checksums = false);
   ClassLoaderInfo* ParseInternal(const std::string& spec, bool parse_checksums);

   // Attempts to parse a single class loader spec.
   // Returns the ClassLoaderInfo abstraction for this spec, or null if it cannot be parsed.
   std::unique_ptr<ClassLoaderInfo> ParseClassLoaderSpec(
       const std::string& class_loader_spec,
       bool parse_checksums = false);

   // CHECKs that the dex files were opened (OpenDexFiles was called and set dex_files_open_result_
   // to true). Aborts if not. The `calling_method` is used in the log message to identify the source
   // of the call.
   void CheckDexFilesOpened(const std::string& calling_method) const;

   // Creates the `ClassLoaderInfo` representing`class_loader` and attach it to `this`.
   // The dex file present in `dex_elements` array (if not null) will be added at the end of
   // the classpath.
   bool CreateInfoFromClassLoader(ScopedObjectAccessAlreadyRunnable& soa,
                                  Handle<mirror::ClassLoader> class_loader,
                                  Handle<mirror::ObjectArray<mirror::Object>> dex_elements,
                                  ClassLoaderInfo* child_info,
                                  bool is_shared_library,
                                  bool is_after)
     REQUIRES_SHARED(Locks::mutator_lock_);

   // Encodes the context as a string suitable to be passed to dex2oat or to be added to the
   // oat file as the class path key.
   // If for_dex2oat is true, the encoding adds each file once (i.e. it does not add multidex
   // location). Otherwise, for oat files, the encoding adds all the dex files (including multidex)
   // together with their checksums.
   // Should only be called if OpenDexFiles() returned true.
   std::string EncodeContext(const std::string& base_dir,
                             bool for_dex2oat,
                             ClassLoaderContext* stored_context) const;

   // Internal version of `EncodeContext`, which will be called recursively
   // on the parent and shared libraries.
   void EncodeContextInternal(const ClassLoaderInfo& info,
                              const std::string& base_dir,
                              bool for_dex2oat,
                              ClassLoaderInfo* stored_info,
                              std::ostringstream& out) const;

   // Encodes e.g. PCL[foo.dex:bar.dex]
   void EncodeClassPath(const std::string& base_dir,
                        const std::vector<std::string>& dex_locations,
                        const std::vector<uint32_t>& checksums,
                        ClassLoaderType type,
                        std::ostringstream& out) const;

   // Encodes the shared libraries classloaders and the parent classloader if
   // either are present in info, e.g. {PCL[foo.dex]#PCL[bar.dex]};PCL[baz.dex]
   void EncodeSharedLibAndParent(const ClassLoaderInfo& info,
                                 const std::string& base_dir,
                                 bool for_dex2oat,
                                 ClassLoaderInfo* stored_info,
                                 std::ostringstream& out) const;

   bool ClassLoaderInfoMatch(const ClassLoaderInfo& info,
                             const ClassLoaderInfo& expected_info,
                             const std::string& context_spec,
                             bool verify_names,
                             bool verify_checksums) const;

   // Extracts the class loader type from the given spec.
   // Return ClassLoaderContext::kInvalidClassLoader if the class loader type is not
   // recognized.
   static ClassLoaderType ExtractClassLoaderType(const std::string& class_loader_spec);

   // Returns the string representation of the class loader type.
   // The returned format can be used when parsing a context spec.
   static const char* GetClassLoaderTypeName(ClassLoaderType type);

   // Encodes the state of processing the dex files associated with the context.
   enum ContextDexFilesState {
     // The dex files are not opened.
     kDexFilesNotOpened = 1,
     // The dex checksums/locations were read from the apk/dex but the dex files were not opened.
     kDexFilesChecksumsRead = 2,
     // The dex files are opened (either because we called OpenDexFiles, or we used a class loader
     // to create the context). This implies kDexFilesChecksumsRead.
     kDexFilesOpened = 3,
     // We failed to open the dex files or read the checksums.
     kDexFilesOpenFailed = 4
   };

   // The class loader chain.
   std::unique_ptr<ClassLoaderInfo> class_loader_chain_;

   // The opening state of the dex files.
   ContextDexFilesState dex_files_state_;

   // Whether or not the context owns the opened dex and oat files.
   // If true, the opened dex files will be de-allocated when the context is destructed.
   // If false, the objects will continue to be alive.
   // Note that for convenience the the opened dex/oat files are stored as unique pointers
   // which will release their ownership in the destructor based on this flag.
   const bool owns_the_dex_files_;

   friend class ClassLoaderContextTest;

   DISALLOW_COPY_AND_ASSIGN(ClassLoaderContext);
 };

 }  // namespace art
 #endif  // ART_RUNTIME_CLASS_LOADER_CONTEXT_H_
	/*
	* Copyright (C) 2017 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#ifndef ART_RUNTIME_CLASS_LOADER_CONTEXT_H_
	#define ART_RUNTIME_CLASS_LOADER_CONTEXT_H_

	#include <string>
	#include <vector>
	#include <set>

	#include "arch/instruction_set.h"
	#include "base/dchecked_vector.h"
	#include "base/macros.h"
	#include "dex/dex_file.h"
	#include "handle_scope.h"
	#include "mirror/class_loader.h"
	#include "oat/oat_file.h"
	#include "scoped_thread_state_change.h"

	namespace art HIDDEN {

	class DexFile;
	class OatFile;

	// Utility class which holds the class loader context used during compilation/verification.
	class EXPORT ClassLoaderContext {
	public:
	enum class VerificationResult {
	kVerifies,
	kMismatch,
	};

	enum ClassLoaderType {
	kInvalidClassLoader = 0,
	kPathClassLoader = 1,
	kDelegateLastClassLoader = 2,
	kInMemoryDexClassLoader = 3
	};

	// Special encoding used to denote a foreign ClassLoader was found when trying to encode class
	// loader contexts for each classpath element in a ClassLoader. See
	// EncodeClassPathContextsForClassLoader. Keep in sync with PackageDexUsage in the framework
	// (frameworks/base/services/core/java/com/android/server/pm/dex/PackageDexUsage.java) and
	// DexUseManager in ART Services
	// (art/libartservice/service/java/com/android/server/art/DexUseManager.java).
	static constexpr const char* kUnsupportedClassLoaderContextEncoding =
	"=UnsupportedClassLoaderContext=";

	~ClassLoaderContext();

	// Opens requested class path files and appends them to ClassLoaderInfo::opened_dex_files.
	// If the dex files have been stripped, the method opens them from their oat files which are added
	// to ClassLoaderInfo::opened_oat_files. The 'classpath_dir' argument specifies the directory to
	// use for the relative class paths.
	// Returns true if all dex files where successfully opened.
	// It may be called only once per ClassLoaderContext. Subsequent calls will return the same
	// result without doing anything.
	// If `context_fds` is an empty vector, files will be opened using the class path locations as
	// filenames. Otherwise `context_fds` is expected to contain file descriptors to class path dex
	// files, following the order of dex file locations in a flattened class loader context. If their
	// number (size of `context_fds`) does not match the number of dex files, OpenDexFiles will fail.
	//
	// This will replace the class path locations with the locations of the opened dex files.
	// (Note that one dex file can contain multidexes. Each multidex will be added to the classpath
	// separately.)
	//
	// only_read_checksums controls whether or not we only read the dex locations and the checksums
	// from the apk instead of fully opening the dex files.
	//
	// This method is not thread safe.
	//
	// Note that a "false" return could mean that either an apk/jar contained no dex files or
	// that we hit a I/O or checksum mismatch error.
	// TODO(calin): Currently there's no easy way to tell the difference.
	//
	// TODO(calin): we're forced to complicate the flow in this class with a different
	// OpenDexFiles step because the current dex2oat flow requires the dex files be opened before
	// the class loader is created. Consider reworking the dex2oat part.
	bool OpenDexFiles(const std::string& classpath_dir = "",
	const std::vector<int>& context_fds = std::vector<int>(),
	bool only_read_checksums = false);

	// Remove the specified compilation sources from all classpaths present in this context.
	// Should only be called before the first call to OpenDexFiles().
	bool RemoveLocationsFromClassPaths(const dchecked_vector<std::string>& compilation_sources);

	// Creates the entire class loader hierarchy according to the current context.
	// Returns the first class loader from the chain.
	//
	// For example: if the context was built from the spec
	// "ClassLoaderType1[ClasspathElem1:ClasspathElem2...];ClassLoaderType2[...]..."
	// the method returns the class loader correponding to ClassLoader1. The parent chain will be
	// ClassLoader1 --> ClassLoader2 --> ... --> BootClassLoader.
	//
	// The compilation sources are appended to the classpath of the first class loader (in the above
	// example ClassLoader1).
	//
	// If the context is empty, this method only creates a single PathClassLoader with the
	// given compilation_sources.
	//
	// Shared libraries found in the chain will be canonicalized based on the dex files they
	// contain.
	//
	// Implementation notes:
	// 1) the objects are not completely set up. Do not use this outside of tests and the compiler.
	// 2) should only be called before the first call to OpenDexFiles().
	jobject CreateClassLoader(const std::vector<const DexFile*>& compilation_sources) const;

	// Encodes the context as a string suitable to be added in oat files.
	// (so that it can be read and verified at runtime against the actual class
	// loader hierarchy).
	// Should only be called if OpenDexFiles() returned true.
	// If stored context is non-null, the stored names are overwritten by the class path from the
	// stored context.
	// E.g. if the context is PCL[a.dex:b.dex] this will return
	// "PCL[a.dexa_checksumb.dex*a_checksum]".
	std::string EncodeContextForOatFile(const std::string& base_dir,
	ClassLoaderContext* stored_context = nullptr) const;

	// Encodes the context as a string suitable to be passed to dex2oat.
	// This is the same as EncodeContextForOatFile but without adding the checksums
	// and only adding each dex files once (no multidex).
	// Should only be called if OpenDexFiles() returned true.
	std::string EncodeContextForDex2oat(const std::string& base_dir) const;

	// Encodes the contexts for each of the classpath elements in the child-most
	// classloader. Under the hood EncodeContextForDex2oat is used, so no checksums
	// will be encoded.
	// Should only be called if the dex files are opened (either via OpenDexFiles() or by creating the
	// context from a live class loader).
	// Notably, for each classpath element the encoded classloader context will contain only the
	// elements that appear before it in the containing classloader. E.g. if `this` contains
	// (from child to parent):
	//
	// PathClassLoader { multidex.apk!classes.dex, multidex.apk!classes2.dex, foo.dex, bar.dex } ->
	// PathClassLoader { baz.dex } -> BootClassLoader
	//
	// then the return value will look like:
	//
	// `{ "multidex.apk": "PCL[];PCL[baz.dex]",
	// "foo.dex" : "PCL[multidex.apk];PCL[baz.dex]",
	// "bar.dex" : "PCL[multidex.apk:foo.dex];PCL[baz.dex]" }`
	std::map<std::string, std::string> EncodeClassPathContexts(const std::string& base_dir) const;

	// Flattens the opened dex files into the given vector.
	// Should only be called if OpenDexFiles() returned true.
	std::vector<const DexFile*> FlattenOpenedDexFiles() const;

	// Return a list of dex file locations from this class loader context after flattening.
	std::vector<std::string> FlattenDexPaths() const;

	// Verifies that the current context is identical to the context encoded as `context_spec`.
	// Identical means:
	// - the number and type of the class loaders from the chain matches
	// - the class loader from the same position have the same classpath
	// (the order and checksum of the dex files matches)
	// This should be called after OpenDexFiles() with only_read_checksums=true. There's no
	// need to fully open the dex files if the only thing that needs to be done is to verify
	// the context.
	//
	// Names are only verified if verify_names is true.
	// Checksums are only verified if verify_checksums is true.
	VerificationResult VerifyClassLoaderContextMatch(const std::string& context_spec,
	bool verify_names = true,
	bool verify_checksums = true) const;

	// Checks if any of the given dex files is already loaded in the current class loader context.
	// It only checks the first class loader.
	// Returns the list of duplicate dex files (empty if there are no duplicates).
	std::set<const DexFile*> CheckForDuplicateDexFiles(
	const std::vector<const DexFile*>& dex_files);

	// Creates the class loader context from the given string.
	// The format: ClassLoaderType1[ClasspathElem1:ClasspathElem2...];ClassLoaderType2[...]...
	// ClassLoaderType is either "PCL" (PathClassLoader) or "DLC" (DelegateLastClassLoader).
	// ClasspathElem is the path of dex/jar/apk file.
	//
	// The spec represents a class loader chain with the natural interpretation:
	// ClassLoader1 has ClassLoader2 as parent which has ClassLoader3 as a parent and so on.
	// The last class loader is assumed to have the BootClassLoader as a parent.
	//
	// Note that we allowed class loaders with an empty class path in order to support a custom
	// class loader for the source dex files.
	static std::unique_ptr<ClassLoaderContext> Create(const std::string& spec);

	// Creates a context for the given class_loader and dex_elements.
	// The method will walk the parent chain starting from `class_loader` and add their dex files
	// to the current class loaders chain. The `dex_elements` will be added at the end of the
	// classpath belonging to the `class_loader` argument.
	// The ownership of the opened dex files will be retained by the given `class_loader`.
	// If there are errors in processing the class loader chain (e.g. unsupported elements) the
	// method returns null.
	static std::unique_ptr<ClassLoaderContext> CreateContextForClassLoader(jobject class_loader,
	jobjectArray dex_elements);

	// Returns the default class loader context to be used when none is specified.
	// This will return a context with a single and empty PathClassLoader.
	static std::unique_ptr<ClassLoaderContext> Default();

	// Encodes the contexts for each of the classpath elements in `class_loader`. See
	// ClassLoaderContext::EncodeClassPathContexts for more information about the return value.
	//
	// If `class_loader` does not derive from BaseDexClassLoader then an empty map is returned.
	// Otherwise if a foreign ClassLoader is found in the class loader chain then the results values
	// will all be ClassLoaderContext::kUnsupportedClassLoaderContextEncoding.
	static std::map<std::string, std::string> EncodeClassPathContextsForClassLoader(
	jobject class_loader);

	// Returns whether `encoded_class_loader_context` is a valid encoded ClassLoaderContext or
	// EncodedUnsupportedClassLoaderContext.
	static bool IsValidEncoding(const std::string& possible_encoded_class_loader_context);

	struct ClassLoaderInfo {
	// The type of this class loader.
	ClassLoaderType type;
	// Shared libraries this context has.
	std::vector<std::unique_ptr<ClassLoaderInfo>> shared_libraries;
	// Shared libraries that will be loaded after apks code that this context has.
	std::vector<std::unique_ptr<ClassLoaderInfo>> shared_libraries_after;
	// The list of class path elements that this loader loads.
	// Note that this list may contain relative paths.
	std::vector<std::string> classpath;
	// Original opened class path (ignoring multidex).
	std::vector<std::string> original_classpath;
	// The list of class path elements checksums.
	// May be empty if the checksums are not given when the context is created.
	std::vector<uint32_t> checksums;
	// After OpenDexFiles is called this holds the opened dex files.
	std::vector<std::unique_ptr<const DexFile>> opened_dex_files;
	// After OpenDexFiles, in case some of the dex files were opened from their oat files
	// this holds the list of opened oat files.
	std::vector<std::unique_ptr<OatFile>> opened_oat_files;
	// The parent class loader.
	std::unique_ptr<ClassLoaderInfo> parent;

	explicit ClassLoaderInfo(ClassLoaderType cl_type) : type(cl_type) {}
	};

	private:
	// Creates an empty context (with no class loaders).
	ClassLoaderContext();

	// Get the parent of the class loader chain at depth `index`.
	ClassLoaderInfo* GetParent(size_t index) const {
	ClassLoaderInfo* result = class_loader_chain_.get();
	while ((result != nullptr) && (index-- != 0)) {
	result = result->parent.get();
	}
	return result;
	}

	size_t GetParentChainSize() const {
	size_t result = 0;
	ClassLoaderInfo* info = class_loader_chain_.get();
	while (info != nullptr) {
	++result;
	info = info->parent.get();
	}
	return result;
	}

	// Constructs an empty context.
	// `owns_the_dex_files` specifies whether or not the context will own the opened dex files
	// present in the class loader chain. If `owns_the_dex_files` is true then OpenDexFiles cannot
	// be called on this context (dex_files_open_attempted_ and dex_files_open_result_ will be set
	// to true as well)
	explicit ClassLoaderContext(bool owns_the_dex_files);

	// Reads the class loader spec in place and returns true if the spec is valid and the
	// compilation context was constructed.
	bool Parse(const std::string& spec, bool parse_checksums = false);
	ClassLoaderInfo* ParseInternal(const std::string& spec, bool parse_checksums);

	// Attempts to parse a single class loader spec.
	// Returns the ClassLoaderInfo abstraction for this spec, or null if it cannot be parsed.
	std::unique_ptr<ClassLoaderInfo> ParseClassLoaderSpec(
	const std::string& class_loader_spec,
	bool parse_checksums = false);

	// CHECKs that the dex files were opened (OpenDexFiles was called and set dex_files_open_result_
	// to true). Aborts if not. The `calling_method` is used in the log message to identify the source
	// of the call.
	void CheckDexFilesOpened(const std::string& calling_method) const;

	// Creates the `ClassLoaderInfo` representing`class_loader` and attach it to `this`.
	// The dex file present in `dex_elements` array (if not null) will be added at the end of
	// the classpath.
	bool CreateInfoFromClassLoader(ScopedObjectAccessAlreadyRunnable& soa,
	Handle<mirror::ClassLoader> class_loader,
	Handle<mirror::ObjectArray<mirror::Object>> dex_elements,
	ClassLoaderInfo* child_info,
	bool is_shared_library,
	bool is_after)
	REQUIRES_SHARED(Locks::mutator_lock_);

	// Encodes the context as a string suitable to be passed to dex2oat or to be added to the
	// oat file as the class path key.
	// If for_dex2oat is true, the encoding adds each file once (i.e. it does not add multidex
	// location). Otherwise, for oat files, the encoding adds all the dex files (including multidex)
	// together with their checksums.
	// Should only be called if OpenDexFiles() returned true.
	std::string EncodeContext(const std::string& base_dir,
	bool for_dex2oat,
	ClassLoaderContext* stored_context) const;

	// Internal version of `EncodeContext`, which will be called recursively
	// on the parent and shared libraries.
	void EncodeContextInternal(const ClassLoaderInfo& info,
	const std::string& base_dir,
	bool for_dex2oat,
	ClassLoaderInfo* stored_info,
	std::ostringstream& out) const;

	// Encodes e.g. PCL[foo.dex:bar.dex]
	void EncodeClassPath(const std::string& base_dir,
	const std::vector<std::string>& dex_locations,
	const std::vector<uint32_t>& checksums,
	ClassLoaderType type,
	std::ostringstream& out) const;

	// Encodes the shared libraries classloaders and the parent classloader if
	// either are present in info, e.g. {PCL[foo.dex]#PCL[bar.dex]};PCL[baz.dex]
	void EncodeSharedLibAndParent(const ClassLoaderInfo& info,
	const std::string& base_dir,
	bool for_dex2oat,
	ClassLoaderInfo* stored_info,
	std::ostringstream& out) const;

	bool ClassLoaderInfoMatch(const ClassLoaderInfo& info,
	const ClassLoaderInfo& expected_info,
	const std::string& context_spec,
	bool verify_names,
	bool verify_checksums) const;

	// Extracts the class loader type from the given spec.
	// Return ClassLoaderContext::kInvalidClassLoader if the class loader type is not
	// recognized.
	static ClassLoaderType ExtractClassLoaderType(const std::string& class_loader_spec);

	// Returns the string representation of the class loader type.
	// The returned format can be used when parsing a context spec.
	static const char* GetClassLoaderTypeName(ClassLoaderType type);

	// Encodes the state of processing the dex files associated with the context.
	enum ContextDexFilesState {
	// The dex files are not opened.
	kDexFilesNotOpened = 1,
	// The dex checksums/locations were read from the apk/dex but the dex files were not opened.
	kDexFilesChecksumsRead = 2,
	// The dex files are opened (either because we called OpenDexFiles, or we used a class loader
	// to create the context). This implies kDexFilesChecksumsRead.
	kDexFilesOpened = 3,
	// We failed to open the dex files or read the checksums.
	kDexFilesOpenFailed = 4
	};

	// The class loader chain.
	std::unique_ptr<ClassLoaderInfo> class_loader_chain_;

	// The opening state of the dex files.
	ContextDexFilesState dex_files_state_;

	// Whether or not the context owns the opened dex and oat files.
	// If true, the opened dex files will be de-allocated when the context is destructed.
	// If false, the objects will continue to be alive.
	// Note that for convenience the the opened dex/oat files are stored as unique pointers
	// which will release their ownership in the destructor based on this flag.
	const bool owns_the_dex_files_;

	friend class ClassLoaderContextTest;

	DISALLOW_COPY_AND_ASSIGN(ClassLoaderContext);
	};

	} // namespace art
	#endif // ART_RUNTIME_CLASS_LOADER_CONTEXT_H_