Android类加载器ClassLoader

作者: BlueSocks | 来源:发表于2022-11-08 21:01 被阅读0次

一. 概述

Android从5.0开始就采用art虚拟机, 该虚拟机有些类似Java虚拟机, 程序运行过程也需要通过ClassLoader 将目标类加载到内存.

传统Jvm主要是通过读取class字节码来加载, 而art则是从dex字节码来读取. 这是一种更为优化的方案, 可以将多个.class文件合并成一个classes.dex文件. 下面直接来看看ClassLoader的关系

1.1 关系类图

二. 五种类构造器

接下来，依次看看PathClassLoader，DexClassLoader，BaseDexClassLoader，BootClassLoader，ClassLoader这5个类加载器

2.1 PathClassLoader

public class PathClassLoader extends BaseDexClassLoader {

    public PathClassLoader(String dexPath, ClassLoader parent) {
        super(dexPath, null, null, parent);
    }

    public PathClassLoader(String dexPath, String libraryPath,
            ClassLoader parent) {
        super(dexPath, null, libraryPath, parent);
    }
}

PathClassLoader比较简单, 继承于BaseDexClassLoader. 封装了一下构造函数, 默认 optimizedDirectory=null.

2.2 DexClassLoader

public class DexClassLoader extends BaseDexClassLoader {

    public DexClassLoader(String dexPath, String optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        super(dexPath, new File(optimizedDirectory), libraryPath, parent);
    }
}

DexClassLoader也同样,只是简单地封装了BaseDexClassLoader对象,并没有覆写父类的任何方法.

2.3 BaseDexClassLoader

public class BaseDexClassLoader extends ClassLoader {
    private final DexPathList pathList;  //记录dex文件路径信息

    public BaseDexClassLoader(String dexPath, File optimizedDirectory, String libraryPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
    }
}

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

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，这里的getSystemClassLoader()返回的是PathClassLoader类。

public abstract class ClassLoader {

    static private class SystemClassLoader {
        public static ClassLoader loader = ClassLoader.createSystemClassLoader();
    }

    public static ClassLoader getSystemClassLoader() {
        return SystemClassLoader.loader;
    }

    private static ClassLoader createSystemClassLoader() {
        //此处classPath默认值为"."
        String classPath = System.getProperty("java.class.path", ".");
        // BootClassLoader见小节2.5
        return new PathClassLoader(classPath, 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);
    } }

以上所有的ClassLoader都直接或间接着继承于抽象类ClassLoader.

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。例如：

new PathClassLoader("/system/framework/tcmclient.jar", ClassLoader.getSystemClassLoader());

3.1 初始化

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

[-> DexPathList.java]

final class DexPathList {
    private Element[] dexElements;
    private final List<File> nativeLibraryDirectories;
    private final List<File> systemNativeLibraryDirectories;

    final class DexPathList {
    public DexPathList(ClassLoader definingContext, String dexPath,
            String libraryPath, File optimizedDirectory) {
        ...
        this.definingContext = definingContext;
        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();

        //记录所有的dexFile文件
        this.dexElements = makePathElements(splitDexPath(dexPath), optimizedDirectory, suppressedExceptions);

        //app目录的native库
        this.nativeLibraryDirectories = splitPaths(libraryPath, false);
        //系统目录的native库
        this.systemNativeLibraryDirectories = splitPaths(System.getProperty("java.library.path"), true);
        List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
        allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);
        //记录所有的Native动态库
        this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories, null, suppressedExceptions);
        ...
    }
}

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 makePathElements

[-> DexPathList.java]

private static Element[] makePathElements(List<File> files, File optimizedDirectory,
        List<IOException> suppressedExceptions) {
 
    return makeDexElements(files, optimizedDirectory, suppressedExceptions, null);
}

3.2.2 makeDexElements

private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
        List<IOException> suppressedExceptions, ClassLoader loader) {
    return makeDexElements(files, optimizedDirectory, suppressedExceptions, loader, false);
}

private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
        List<IOException> suppressedExceptions, ClassLoader loader, boolean isTrusted) {
  Element[] elements = new Element[files.size()];  //获取文件个数
  int elementsPos = 0;
  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)) {
             
              dex = loadDexFile(file, optimizedDirectory, loader, elements);
              if (dex != null) {
                  elements[elementsPos++] = new Element(dex, null);
              }
          } else {
              dex = loadDexFile(file, optimizedDirectory, loader, elements);              
              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;
}

该方法的主要功能是创建Element数组

3.2.3 loadDexFile

[-> DexPathList.java]

private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader,
                                   Element[] elements)
        throws IOException {
    if (optimizedDirectory == null) {
        return new DexFile(file, loader, elements);  //创建DexFile对象
    } else {
        String optimizedPath = optimizedPathFor(file, optimizedDirectory);
        return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements);
    }
}

3.2.4 创建对象DexFile

[-> DexFile]

DexFile(File file, ClassLoader loader, DexPathList.Element[] elements)
        throws IOException {
    this(file.getPath(), loader, elements);
}

DexFile(String fileName, ClassLoader loader, DexPathList.Element[] elements) throws IOException {
   
    mCookie = openDexFile(fileName, null, 0, loader, elements);
    mInternalCookie = mCookie;
    mFileName = fileName;
}

3.2.5 openDexFile

[-> DexFile]

private static Object openDexFile(String sourceName, String outputName, int flags,
        ClassLoader loader, DexPathList.Element[] elements) throws IOException {
   
    return openDexFileNative(new File(sourceName).getAbsolutePath(),
                             (outputName == null) ? null : new File(outputName).getAbsolutePath(),
                             flags,
                             loader,
                             elements);
}

此时参数取值说明：

sourceName为PathClassLoader构造函数传递的dexPath中以分隔符划分之后的文件名；
outputName为null；
flags=0
loader为null；
elements为makeDexElements()过程生成的Element数组；

3.2.6 DexFile_openDexFileNative

[-> dalvik_system_DexFile.cc]

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 0;
  }
  Runtime* const runtime = Runtime::Current();
  ClassLinker* linker = runtime->GetClassLinker();
  std::vector<std::unique_ptr<const DexFile>> dex_files;
  std::vector<std::string> error_msgs;
  const OatFile* oat_file = nullptr;

  dex_files = runtime->GetOatFileManager().OpenDexFilesFromOat(sourceName.c_str(),
                                                               class_loader,
                                                               dex_elements,
                                                               /*out*/ &oat_file,
                                                               /*out*/ &error_msgs);

  if (!dex_files.empty()) {
    jlongArray array = ConvertDexFilesToJavaArray(env, oat_file, dex_files);
    ...
    return array;
  } else {
    ...
    return nullptr;
  }
}

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 loadClass

[-> ClassLoader.java]

public abstract class ClassLoader {

    public Class<?> loadClass(String className) throws ClassNotFoundException {
        return loadClass(className, false);
    }

    protected Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException {
        //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
        Class<?> clazz = findLoadedClass(className);

        if (clazz == null) { 
            //如果没有加载过，则调用parent的类加载递归加载该类，若已加载则直接返回
            clazz = parent.loadClass(className, false);

            if (clazz == null) {
                //还没加载，则调用当前类加载器来加载
                clazz = findClass(className);
            }
        }
        return clazz;
    }
}

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。

3.3.1 findLoadedClass

[-> ClassLoader.java]

protected final Class<?> findLoadedClass(String name) {
    ClassLoader loader;
    if (this == BootClassLoader.getInstance())
        loader = null;
    else
        loader = this;
    return VMClassLoader.findLoadedClass(loader, name);
}

3.4 findClass

[-> BaseDexClassLoader.java]

public class BaseDexClassLoader extends ClassLoader {
    protected Class<?> findClass(String name) throws ClassNotFoundException {
      
        Class c = pathList.findClass(name, suppressedExceptions);
        ...
        return c;
    }
}

3.5 DexPathList.findClass

[-> DexPathList.java]

public Class findClass(String name, List<Throwable> suppressed) {
    for (Element element : dexElements) {
        DexFile dex = element.dexFile;
        if (dex != null) {
            //找到目标类，则直接返回[见小节3.6]
            Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);
            if (clazz != null) {
                return clazz;
            }
        }
    }
    return null;
}

功能说明：

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精髓，将需要修复的类所打包的dex文件插入到dexElements前面。

3.6 DexFile.loadClassBinaryName

[-> DexFile.java]

public final class DexFile {

    public Class loadClassBinaryName(String name, ClassLoader loader, List<Throwable> suppressed) {
        return defineClass(name, loader, mCookie, suppressed);  
    }

    private static Class defineClass(String name, ClassLoader loader, Object cookie, List<Throwable> suppressed) {
        Class result = null;
        try {
            result = defineClassNative(name, loader, cookie); 
        } catch (NoClassDefFoundError e) {
            if (suppressed != null) {
                suppressed.add(e);
            }
        } catch (ClassNotFoundException e) {
            if (suppressed != null) {
                suppressed.add(e);
            }
        }
        return result;
    }
}

defineClassNative()这是native方法, 进入如下方法.

3.7 defineClassNative

[-> dalvik_system_DexFile.cc]

static jclass DexFile_defineClassNative(JNIEnv* env, jclass, jstring javaName, jobject javaLoader,
                                        jobject cookie) {
  std::unique_ptr<std::vector<const DexFile*>> dex_files = ConvertJavaArrayToNative(env, cookie);
  if (dex_files.get() == nullptr) {
    return nullptr; //dex文件为空, 则直接返回
  }

  ScopedUtfChars class_name(env, javaName);
  if (class_name.c_str() == nullptr) {
    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())); //将类名转换为hash码
  for (auto& dex_file : *dex_files) {
    const DexFile::ClassDef* dex_class_def = dex_file->FindClassDef(descriptor.c_str(), hash);
    if (dex_class_def != nullptr) {
      ScopedObjectAccess soa(env);
      ClassLinker* class_linker = Runtime::Current()->GetClassLinker();
      class_linker->RegisterDexFile(*dex_file);
      StackHandleScope<1> hs(soa.Self());
      Handle<mirror::ClassLoader> class_loader(
          hs.NewHandle(soa.Decode<mirror::ClassLoader*>(javaLoader)));
      //获取目标类
      mirror::Class* result = class_linker->DefineClass(soa.Self(), descriptor.c_str(), hash,
                                                        class_loader, *dex_file, *dex_class_def);
      if (result != nullptr) {
        // 找到目标对象
        return soa.AddLocalReference<jclass>(result);
      }
    }
  }
  return nullptr; //没有找到目标类
}

在native层创建目标类的对象并添加到虚拟机列表。

四. 总结

几种类加载器：

PathClassLoader: 主要用于系统和app的类加载器,其中optimizedDirectory为null, 采用默认目录/data/dalvik-cache/
DexClassLoader: 可以从包含classes.dex的jar或者apk中，加载类的类加载器, 可用于执行动态加载,但必须是app私有可写目录来缓存odex文件. 能够加载系统没有安装的apk或者jar文件，因此很多插件化方案都是采用DexClassLoader;
BaseDexClassLoader: 比较基础的类加载器, PathClassLoader和DexClassLoader都只是在构造函数上对其简单封装而已.
BootClassLoader: 作为父类的类构造器。

热修复核心逻辑：在DexPathList.findClass()过程，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精髓，将需要修复的类所打包的dex文件插入到dexElements前面。

类加载过程常见的ClassNotFound原因：

ABI异常：常见在系统APP，为了减小system分区大小会将apk源文件中的classes.dex文件移除，对于既然可运行在64位又可运行在32位模式的应用，当被强制设置32位时，openDexFileNative在查找不到oat文件时会运行在解释模式，而classes.dex文件不再则出现ClassNotFound异常。
MultiDex处理不当，由于每个Dex文件中方法个数不能超过65536，引入MultiDex机制。dex2oat会自动查找Apk文件中的classes.dex，classes2.dex，…classesN.dex等文件，编译到/data/dalvik-cache下生成oat文件。这里需要文件名跟classesN.dex格式，并且一定要与classes.dex一起放置在第一级目录，有些APP不按照要求来，导致ClassNotFound异常。

Android类加载器ClassLoader

一. 概述

1.1 关系类图

二. 五种类构造器

2.1 PathClassLoader

2.2 DexClassLoader

2.3 BaseDexClassLoader

2.4 ClassLoader

2.5 BootClassLoader

三. PathClassLoader加载类的过程

3.1 初始化

3.2 DexPathList

3.2.1 makePathElements

3.2.2 makeDexElements

3.2.3 loadDexFile

3.2.4 创建对象DexFile

3.2.5 openDexFile

3.2.6 DexFile_openDexFileNative

3.3 loadClass

3.3.1 findLoadedClass

3.4 findClass

3.5 DexPathList.findClass

3.6 DexFile.loadClassBinaryName

3.7 defineClassNative

四. 总结

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