Android ClassLoader类加载机制

Android ClassLoader类加载机制

概述

我们知道Java中的 ClassLoader可以加载 jar 文件和Class文件。在Android中，它们加载的是dex文件。

Android中的ClassLoader类型分别是系统类加载器和自定义加载器。其中系统类加载器主要包括3种，分别是 BootClassLoader 、PathClassLoader 和 DexClassLoader 。

• BootClassLoader
  Android 系统启动时会使用 BootClassLoader来预加载常用类。
• DexClassLoader
  DexClassLoader 可以加载dex文件以及包含dex的压缩文件（apk和jar文件）
• PathClassLoader
  Android 系统使用PathClassLoader来加载系统类和应用程序的类

platform/libcore/dalvik/src/main/java/dalvik/system
    - PathClassLoader.java
    - DexClassLoader.java
    - BaseDexClassLoader.java
    - DexPathList.java
    - DexFile.java
    
platform/art/runtime/native
    - dalvik_system_DexFile.cc

platform/ojluni/src/main/java/java/lang/
    - ClassLoader.java

五种类加载构造函数

1. PathClassLoader

它提供了一个简单的ClassLoader实现，它对列表进行操作的文件和目录，但没有尝试从网络加载类.

public class PathClassLoader extends BaseDexClassLoader {
 
    public PathClassLoader(String dexPath, ClassLoader parent) {
        super(dexPath, null, null, parent);
    }
 
    public PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent{
        super(dexPath, null, librarySearchPath, parent);
    }
    
    @libcore.api.CorePlatformApi
    public PathClassLoader(
            String dexPath, String librarySearchPath, ClassLoader parent,
            ClassLoader[] sharedLibraryLoaders) {
        super(dexPath, librarySearchPath, parent, sharedLibraryLoaders);
    }
}

2. PathClassLoader

一个类装入器，从包含classes.dex条目的.jar和code .apk文件中装入类。这可用于执行未作为应用程序的一部分安装的代码.

public class DexClassLoader extends BaseDexClassLoader {
    public DexClassLoader(String dexPath, String optimizedDirectory,
            String librarySearchPath, ClassLoader parent) {
        super(dexPath, null, librarySearchPath, parent);
    }
}

3. BaseDexClassLoader

用于定制如何报告dex文件加载的钩子。同时负责初始化DexPathList

public class BaseDexClassLoader extends ClassLoader {
    @UnsupportedAppUsage
    private final DexPathList pathList;

    public BaseDexClassLoader(String dexPath,
                              String librarySearchPath, ClassLoader parent, ClassLoader[] sharedLibraryLoaders,
                              boolean isTrusted) {
        super(parent);
        //在创建路径列表之前设置共享库。ART依赖于类加载器层次结构在加载dex文件之前完成。
        this.sharedLibraryLoaders = sharedLibraryLoaders == null
            ? null
            : Arrays.copyOf(sharedLibraryLoaders, sharedLibraryLoaders.length);
        this.pathList = new DexPathList(this, dexPath, librarySearchPath, null, isTrusted);
        if (reporter != null) {
            reportClassLoaderChain();
        }
    }

    //设置dex加载通知的报告器。一旦设置完毕，BaseDexClassLoader的所有新实例都将在构建已加载的dex文件时报告。
    @libcore.api.CorePlatformApi
        public static void setReporter(Reporter newReporter) {
        reporter = newReporter;
    }
}

4. ClassLoader

类加载器是负责装入类的对象

public abstract class ClassLoader {
    private ClassLoader parent;  //记录父类加载器

    protected ClassLoader() {
        this(getSystemClassLoader(), false);
    }

    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;
    }
}

//----------------------------------------
static private class SystemClassLoader {
    public static ClassLoader loader = ClassLoader.createSystemClassLoader();
}
private static ClassLoader createSystemClassLoader() {
    String classPath = System.getProperty("java.class.path", ".");
    String librarySearchPath = System.getProperty("java.library.path", "");
    return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());
}

PathClassLoader加载类的过程

在\libcore\dalvik\src\main\java\dalvik\system\DexClassLoader.java文件下：

public class PathDexClassLoader extends BaseDexClassLoader {
    public PathDexClassLoader (String dexPath, String optimizedDirectory,String libraryPath, ClassLoader parent) {
        super(dexPath, new File(optimizedDirectory), libraryPath, parent);
    }
}

在此处调用了父类BaseDexClassLoader的构造函数，在同一目录下的BaseDexClassLoader.java中：

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

此处调用了父类ClassLoader的构造函数，然后生成一个DexPathList对象，在同一目录下的DexPathList.java文件中：

public DexPathList(ClassLoader definingContext, String dexPath,
            String libraryPath, File optimizedDirectory) {
    //省略参数校验以及异常处理的代码
        this.definingContext = definingContext;
        ……
        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions);
        ……
        this.nativeLibraryDirectories = splitLibraryPath(libraryPath);
}

在DexPathList.java文件中有一段关键的代码makeDexElements ：

private static Element[] makeDexElements(ArrayList<File> files, File optimizedDirectory,
                                         ArrayList<IOException> suppressedExceptions) {
    // ……
    for (File file : files) {
        File zip = null;
        DexFile dex = null;
        String name = file.getName();
        if (name.endsWith(DEX_SUFFIX)) {//.dex文件
            // Raw dex file (not inside a zip/jar).
            try {
                dex = loadDexFile(file, optimizedDirectory);
            } catch (IOException ex) {
                System.logE("Unable to load dex file: " + file, ex);
            }
        } else if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX)
                   || name.endsWith(ZIP_SUFFIX)) {
            //.apk  .jar  .zip文件
            zip = file;
            try {
                dex = loadDexFile(file, optimizedDirectory);
            } catch (IOException suppressed) {
                suppressedExceptions.add(suppressed);
            }
        } else if (file.isDirectory()) {
            // We support directories for looking up resources.
            // This is only useful for running libcore tests.
            elements.add(new Element(file, true, null, null));
        } else {
            System.logW("Unknown file type for: " + file);
        }
    }
    //……
    return elements.toArray(new Element[elements.size()]);
}

我们再点进

loadDexFile(file, optimizedDirectory)方法看一下：

private static DexFile loadDexFile(File file, File optimizedDirectory)
    throws IOException {
    if (optimizedDirectory == null) {
        return new DexFile(file);
    } else {
        String optimizedPath = optimizedPathFor(file, optimizedDirectory);
        return DexFile.loadDex(file.getPath(), optimizedPath, 0);
    }
}

//生成odex的目录
private static String optimizedPathFor(File path,
                                       File optimizedDirectory) {
    String fileName = path.getName();
    if (!fileName.endsWith(DEX_SUFFIX)) {
        int lastDot = fileName.lastIndexOf(".");
        if (lastDot < 0) {
            fileName += DEX_SUFFIX;
        } else {
            StringBuilder sb = new StringBuilder(lastDot + 4);
            sb.append(fileName, 0, lastDot);
            sb.append(DEX_SUFFIX);
            fileName = sb.toString();
        }
    }
    File result = new File(optimizedDirectory, fileName);
    return result.getPath();
}

optimizedPathFor主要是对文件的后缀进行修正，如果没有后缀名，就在末尾加上.dex，如果文件结尾不是.dex，就将后缀替换为.dex，然后创建我们的.dex文件，然后返回我们创建的.dex文件的路径，继续执行DexFile.loadDex() 函数：

static public DexFile loadDex(String sourcePathName, String outputPathName,
        int flags) throws IOException {
        return new DexFile(sourcePathName, outputPathName, flags);
}

这里直接返回了一个DexFile对象，下面来看看这个类的构造函数：

//sourceName 我们要加载的自己的.jar文件路径
// outputName 在optimizedPathFor()中修正的dex的路径
private DexFile(String sourceName, String outputName, int flags) throws IOException {
    if (outputName != null) {
        try {
            String parent = new File(outputName).getParent();
            if (Libcore.os.getuid() != Libcore.os.stat(parent).st_uid) {
                throw new IllegalArgumentException("Optimized data directory " + parent
                                                   + " is not owned by the current user. Shared storage cannot protect"
                                                   + " your application from code injection attacks.");
            }
        } catch (ErrnoException ignored) {
            // assume we'll fail with a more contextual error later
        }
    }
    //我们的重点就是在openDexFile()函数上
    mCookie = openDexFile(sourceName, outputName, flags);
    mFileName = sourceName;
    guard.open("close");
    //System.out.println("DEX FILE cookie is " + mCookie);
}

openDexFile函数的返回值是一个整型，保存在mCookie中，文件名保存在mFileName中：

private static int openDexFile(String sourceName, String outputName,
                               int flags) throws IOException {
    return openDexFileNative(new File(sourceName).getCanonicalPath(),
                             (outputName == null) ? null : new File(outputName).getCanonicalPath(),
                             flags);
}

在openDexFile()中只是调用了openDexFileNative () ,继续跟入在\dalvik\vm\native\dalvik _system_DexFile.cpp文件中的openDexFileNative() 函数，接下重点就在这个函数：

static void Dalvik_dalvik_system_DexFile_openDexFileNative(const u4* args,
    JValue* pResult)
{
//args[0]: sourceName java层传入的
//args[1]: outputName    
    StringObject* sourceNameObj = (StringObject*) args[0];
    StringObject* outputNameObj = (StringObject*) args[1];
    DexOrJar* pDexOrJar = NULL;
    JarFile* pJarFile;
RawDexFile* pRawDexFile;
//DexOrJar*  JarFile*   RawDexFile* 目录
    char* sourceName;
    char* outputName;
    //……
    sourceName = dvmCreateCstrFromString(sourceNameObj);
    if (outputNameObj != NULL)
        outputName = dvmCreateCstrFromString(outputNameObj);
    else
        outputName = NULL;
/*判断要加载的dex是否为系统中的dex文件
* gDvm ？？？
*/
    if (dvmClassPathContains(gDvm.bootClassPath, sourceName)) {
        ALOGW("Refusing to reopen boot DEX '%s'", sourceName);
        dvmThrowIOException(
            "Re-opening BOOTCLASSPATH DEX files is not allowed");
        free(sourceName);
        free(outputName);
        RETURN_VOID();
    }

    /*
     * Try to open it directly as a DEX if the name ends with ".dex".
     * If that fails (or isn't tried in the first place), try it as a
     * Zip with a "classes.dex" inside.
     */
    //判断sourcename扩展名是否是.dex
    if (hasDexExtension(sourceName)
            && dvmRawDexFileOpen(sourceName, outputName, &pRawDexFile, false) == 0) {
        ALOGV("Opening DEX file '%s' (DEX)", sourceName);
        pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
        pDexOrJar->isDex = true;
        pDexOrJar->pRawDexFile = pRawDexFile;
        pDexOrJar->pDexMemory = NULL;
    //.jar文件
    } else if (dvmJarFileOpen(sourceName, outputName, &pJarFile, false) == 0) {
        ALOGV("Opening DEX file '%s' (Jar)", sourceName);
        pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
        pDexOrJar->isDex = false;
        pDexOrJar->pJarFile = pJarFile;
        pDexOrJar->pDexMemory = NULL;
} else {
//都不满足，抛出异常
        ALOGV("Unable to open DEX file '%s'", sourceName);
        dvmThrowIOException("unable to open DEX file");
    }
if (pDexOrJar != NULL) {
        pDexOrJar->fileName = sourceName;
    //把pDexOr这个结构体中的内容加到gDvm中的userDexFile结构的hash表中，便于Dalvik以后的查找
        addToDexFileTable(pDexOrJar);
    } else {
        free(sourceName);
    }
    free(outputName);
    RETURN_PTR(pDexOrJar);
}

接下来再看对.dex文件的处理函数dvmRawDexFileOpen

int dvmRawDexFileOpen(const char* fileName, const char* odexOutputName,
    RawDexFile** ppRawDexFile, bool isBootstrap)
{
    DvmDex* pDvmDex = NULL;
    char* cachedName = NULL;
    int result = -1;
    int dexFd = -1;
    int optFd = -1;
    u4 modTime = 0;
    u4 adler32 = 0;
    size_t fileSize = 0;
    bool newFile = false;
    bool locked = false;
    dexFd = open(fileName, O_RDONLY);  //打开dex文件
    if (dexFd < 0) goto bail;
    /* If we fork/exec into dexopt, don't let it inherit the open fd. */
dvmSetCloseOnExec(dexFd);//dexfd不继承
//校验dex文件的标志，将第8字节开始的4个字节赋值给adler32。
    if (verifyMagicAndGetAdler32(dexFd, &adler32) < 0) {
        ALOGE("Error with header for %s", fileName);
        goto bail;
    }
    //得到dex文件的大小和修改时间，保存在modTime和filesize中
    if (getModTimeAndSize(dexFd, &modTime, &fileSize) < 0) {
        ALOGE("Error with stat for %s", fileName);
        goto bail;
    }

    //odexOutputName就是odex文件名，如果odexOutputName为空，则自动生成一个。
    if (odexOutputName == NULL) {
        cachedName = dexOptGenerateCacheFileName(fileName, NULL);
        if (cachedName == NULL)
            goto bail;
    } else {
        cachedName = strdup(odexOutputName);   
    }
    //主要是验证缓存文件名的正确性，之后将dexOptHeader结构写入fd中
    optFd = dvmOpenCachedDexFile(fileName, cachedName, modTime,
        adler32, isBootstrap, &newFile, /*createIfMissing=*/true);
    locked = true;
  
    if (newFile) {
        u8 startWhen, copyWhen, endWhen;
        bool result;
        off_t dexOffset;
        dexOffset = lseek(optFd, 0, SEEK_CUR);  //文件指针的位置
        result = (dexOffset > 0);
        if (result) {
            startWhen = dvmGetRelativeTimeUsec();
    //将dex文件中的内容拷贝到当前odex文件，也就是dexOffset开始
            result = copyFileToFile(optFd, dexFd, fileSize) == 0;
            copyWhen = dvmGetRelativeTimeUsec();
        }
        if (result) {
    //优化odex文件
            result = dvmOptimizeDexFile(optFd, dexOffset, fileSize,
                fileName, modTime, adler32, isBootstrap);
        }
    }
    /*
     * Map the cached version.  This immediately rewinds the fd, so it
     * doesn't have to be seeked anywhere in particular.
     */
//将odex文件映射到内存空间(mmap)，并用mprotect将属性置为只读属性，并将映射的dex结构放在pDvmDex数据结构中，具体代码在下面。
    if (dvmDexFileOpenFromFd(optFd, &pDvmDex) != 0) {
        ALOGI("Unable to map cached %s", fileName);
        goto bail;
    }
……
}

int dvmDexFileOpenFromFd(int fd, DvmDex** ppDvmDex)
{
    DvmDex* pDvmDex;
    DexFile* pDexFile;
    MemMapping memMap;
    int parseFlags = kDexParseDefault;
    int result = -1;

    if (gDvm.verifyDexChecksum)
        parseFlags |= kDexParseVerifyChecksum;
    if (lseek(fd, 0, SEEK_SET) < 0) {
        ALOGE("lseek rewind failed");
        goto bail;
    }
    //mmap映射fd文件,就是我们之前的odex文件
   if (sysMapFileInShmemWritableReadOnly(fd, &memMap) != 0) {
        ALOGE("Unable to map file");
        goto bail;
    }
    pDexFile = dexFileParse((u1*)memMap.addr, memMap.length, parseFlags);
    if (pDexFile == NULL) {
        ALOGE("DEX parse failed");
        sysReleaseShmem(&memMap);
        goto bail;
    }
    pDvmDex = allocateAuxStructures(pDexFile);
    if (pDvmDex == NULL) {
        dexFileFree(pDexFile);
        sysReleaseShmem(&memMap);
        goto bail;
    }
/* tuck this into the DexFile so it gets released later */
//将映射odex文件的内存拷贝到DvmDex的结构中
    sysCopyMap(&pDvmDex->memMap, &memMap);
    pDvmDex->isMappedReadOnly = true;
    *ppDvmDex = pDvmDex;
    result = 0;

bail:
    return result;
}


/*dalvik\libdex\SysUtil.cpp
*/
int sysMapFileInShmemWritableReadOnly(int fd, MemMapping* pMap)
{
    off_t start;
    size_t length;
    void* memPtr;
assert(pMap != NULL);
//获得文件长度和文件开始地址
    if (getFileStartAndLength(fd, &start, &length) < 0)
        return -1;
//映射文件
    memPtr = mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_FILE | MAP_PRIVATE,
            fd, start);
    //……
//将保护属性置为只读属性
    if (mprotect(memPtr, length, PROT_READ) < 0) {
      //…….
    }
    pMap->baseAddr = pMap->addr = memPtr;
    pMap->baseLength = pMap->length = length;
return 0;
//……
}

简单梳理一下整个的加载流程：

首先是对文件名的修正，后缀名置为”.dex”作为输出文件，然后生个一个DexPathList对象函数直接返回一个DexPathList对象；

在DexPathList的构造函数中调用makeDexElements()函数，在makeDexElement()函数中调用loadDexFile()开始对.dex或者是.jar .zip .apk文件进行处理；

跟入loadDexFile()函数中，调用optimizedPathFor()函数对optimizedDiretcory路径进行修正；

之后才真正通过DexFile.loadDex()开始加载文件中的数据，其中的加载也只是返回一个DexFile对象；

在DexFile类的构造函数中，重点便放在了其调用的openDexFile()函数，在openDexFile()中调用了openDexFileNative()真正进入native层；

DexClassLoader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。

ClassLoader::loadClass

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

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;
}

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

ClassLoader::findLoadedClass

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

BaseDexClassLoader:: findClass

@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
    ...
        List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
    Class c = pathList.findClass(name, suppressedExceptions);
    ...
        return c;
}

DexPathList.findClass

在此实例指向的dex文件中查找已命名类。这将在最早列出的path元素中找到一个。如果找到了类，但还没有定义，则此方法将在构造此实例时使用的定义上下文中定义它。

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;
}

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，将需要修复的类所打包的dex文件插入到dexElements前面。

DexFile::loadClassBinaryName

element.findClass` 最后走到 `dexFile.loadClassBinaryName

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

dalvik_system_DexFile.cc:: defineClassNative

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;  //dex文件为空, 则直接返回
  }
  ScopedUtfChars class_name(env, javaName);
  if (class_name.c_str() == nullptr) {
    VLOG(class_linker) << "Failed to find class_name";
    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()));
  for (auto& dex_file : dex_files) {
    const dex::ClassDef* dex_class_def =
        OatDexFile::FindClassDef(*dex_file, descriptor.c_str(), hash); //将类名转换为hash码
    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);
//添加使用过的dex文件。这只对DexFile是必需的。因为通常的类加载器已经保持它们的dex文件的活动。>InsertDexFileInToClassLoader(soa.Decode(dexFile),
                                                 class_loader.Get());
      if (result != nullptr) {
       // 找到目标对象
        return soa.AddLocalReference<jclass>(result);
      }
    }
  }
  return nullptr;
}