android8-0-0源码分析-classloader1

前言

本篇开始分析classloader。

参考：
https://blog.csdn.net/itachi85/article/details/78088701
https://blog.csdn.net/itachi85/article/details/78276837?utm_source=blogxgwz0
http://gityuan.com/2017/03/19/android-classloader/
https://www.jianshu.com/p/2f9b1d8be88d
https://www.jianshu.com/p/3912c05d257e
https://www.jianshu.com/p/20dcfcf27004
https://www.jianshu.com/p/b89d0b03e82c
http://liwenkun.me/2016/11/11/android-load-class-dynamically/

jvm

先从java虚拟机的类加载器讲起。以后再补java虚拟机的规范。

java虚拟机的系统加载器有：

Bootstrap ClassLoader（引导类加载器）
用C/C++代码实现的加载器，用于加载指定的JDK的核心类库，比如java.lang.、java.uti.等这些系统类。它用来加载以下目录中的类库：
$JAVA_HOME/jre/lib目录
-Xbootclasspath参数指定的目录
Java虚拟机的启动就是通过引导类加载器创建一个初始类来完成的。由于类加载器是使用平台相关的底层C/C++语言实现的， 所以该加载器不能被Java代码访问到。但是我们可以查询某个类是否被引导类加载器加载过。

Extensions ClassLoader（拓展类加载器）
用于加载 Java 的拓展类 ，用来提供除了系统类之外的额外功能。它用来加载以下目录中的类库：
加载$JAVA_HOME/jre/lib/ext目录
系统属性java.ext.dir所指定的目录。

Application ClassLoader（应用程序类加载器）
又称作System ClassLoader(系统类加载器)，这是因为这个类加载器可以通过ClassLoader的getSystemClassLoader方法获取到。它用来加载以下目录中的类库：
当前应用程序Classpath目录
系统属性java.class.path指定的目录

用户自定义加载器，则是通过继承 java.lang.ClassLoader类的方式来实现自己的类加载器。
除了 Bootstrap ClassLoader，Extensions ClassLoader和App ClassLoader也继承了java.lang.ClassLoader类。

类加载器查找Class所采用的是双亲委托模式，所谓双亲委托模式就是首先判断该Class是否已经加载，如果没有则不是自身去查找而是委托给父加载器进行查找，这样依次的进行递归，直到委托到最顶层的Bootstrap ClassLoader，如果Bootstrap ClassLoader找到了该Class，就会直接返回，如果没找到，则继续依次向下查找，如果还没找到则最后会交由自身去查找。
其父都是创建时存储的。

采取双亲委托模式主要有两点好处：

1. 避免重复加载，如果已经加载过一次Class，就不需要再次加载，而是先从缓存中直接读取。 因为所有加载器都被连接在了一起，加载类时一定先向上委托，再向下加载，因此保证了只加载一次
2. 更加安全，如果不使用双亲委托模式，就可以自定义一个String类来替代系统的String类，这显然会造成安全隐患，采用双亲委托模式会使得系统的String类在Java虚拟机启动时就被加载，也就无法自定义String类来替代系统的String类，除非我们修改 类加载器搜索类的默认算法。还有一点，只有两个类名一致并且被同一个类加载器加载的类，Java虚拟机才会认为它们是同一个类，想要骗过Java虚拟机显然不会那么容易。

android中的classloader

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.

还有几个提供的相关类：

1. ClassLoader是一个抽象类，其中定义了ClassLoader的主要功能。BootClassLoader是它的内部类。
2. SecureClassLoader类和JDK8中的SecureClassLoader类的代码是一样的，它继承了抽象类ClassLoader。SecureClassLoader并不是ClassLoader的实现类，而是拓展了ClassLoader类加入了权限方面的功能，加强了ClassLoader的安全性。
3. URLClassLoader类和JDK8中的URLClassLoader类的代码是一样的，它继承自SecureClassLoader，用来通过URl路径从jar文件和文件夹中加载类和资源。
4. InMemoryDexClassLoader是Android8.0新增的类加载器，继承自BaseDexClassLoader，用于加载内存中的dex文件。
5. BaseDexClassLoader继承自ClassLoader，是抽象类ClassLoader的具体实现类，PathClassLoader和DexClassLoader都继承它。

ClassLoader

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

public abstract class ClassLoader {
	
    protected ClassLoader(ClassLoader parent) {
        this(checkCreateClassLoader(), parent);
    }
	private ClassLoader(Void unused, ClassLoader parent) {
        this.parent = parent;
    }
	protected ClassLoader() {
        this(checkCreateClassLoader(), getSystemClassLoader());
    }
	@CallerSensitive
    public static ClassLoader getSystemClassLoader() {
        return SystemClassLoader.loader;
    }
	
	


    static private class SystemClassLoader {
        public static ClassLoader loader = ClassLoader.createSystemClassLoader();
    }
	
	
	private static ClassLoader createSystemClassLoader() {
		//获取默认的path
        String classPath = System.getProperty("java.class.path", ".");
        String librarySearchPath = System.getProperty("java.library.path", "");
		//构造了一个PathClassLoader，父用BootClassLoader
        return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());
    }
}

BootClassLoader

Android系统启动时会使用BootClassLoader来预加载常用类，与Java中的BootClassLoader不同，它并不是由C/C++代码实现，而是由Java实现的
libcore\ojluni\src\main\java\java\lang\ClassLoader.java

class BootClassLoader extends ClassLoader {

    private static BootClassLoader instance;

    @FindBugsSuppressWarnings("DP_CREATE_CLASSLOADER_INSIDE_DO_PRIVILEGED")
    public static synchronized BootClassLoader getInstance() {
        if (instance == null) {
            instance = new BootClassLoader();
        }

        return instance;
    }

    public BootClassLoader() {
        super(null);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        return Class.classForName(name, false, null);
    }

    @Override
    protected URL findResource(String name) {
        return VMClassLoader.getResource(name);
    }

    @SuppressWarnings("unused")
    @Override
    protected Enumeration<URL> findResources(String resName) throws IOException {
        return Collections.enumeration(VMClassLoader.getResources(resName));
    }


    @Override
    protected Package getPackage(String name) {
        if (name != null && !name.isEmpty()) {
            synchronized (this) {
                Package pack = super.getPackage(name);

                if (pack == null) {
                    pack = definePackage(name, "Unknown", "0.0", "Unknown", "Unknown", "0.0",
                            "Unknown", null);
                }

                return pack;
            }
        }

        return null;
    }

    @Override
    public URL getResource(String resName) {
        return findResource(resName);
    }

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

        if (clazz == null) {
            clazz = findClass(className);
        }

        return clazz;
    }

    @Override
    public Enumeration<URL> getResources(String resName) throws IOException {
        return findResources(resName);
    }
}

BootClassLoader是ClassLoader的内部类，并继承自ClassLoader。BootClassLoader是一个单例类，需要注意的是BootClassLoader的访问修饰符是默认的，只有在同一个包中才可以访问，因此我们在应用程序中是无法直接调用的。

PathClassLoader

Android系统使用PathClassLoader来加载系统类和应用程序的类，如果是加载非系统应用程序类，则会加载data/app/目录下的dex文件以及包含dex的apk文件或jar文件.
libcore\dalvik\src\main\java\dalvik\system\PathClassLoader.java

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);
    }
}

dexPath：dex文件以及包含dex的apk文件或jar文件的路径集合，多个路径用文件分隔符分隔，默认文件分隔符为‘：’。
librarySearchPath：包含 C/C++ 库的路径集合，多个路径用文件分隔符分隔分割，可以为null。
parent：ClassLoader的parent。

DexClassLoader

DexClassLoader可以加载dex文件以及包含dex的apk文件或jar文件，也支持从SD卡进行加载，这也就意味着DexClassLoader可以在应用未安装的情况下加载dex相关文件。
libcore\dalvik\src\main\java\dalvik\system\DexClassLoader.java

public class DexClassLoader extends BaseDexClassLoader {
    /**
     * Creates a {@code DexClassLoader} that finds interpreted and native
     * code.  Interpreted classes are found in a set of DEX files contained
     * in Jar or APK files.
     *
     * <p>The path lists are separated using the character specified by the
     * {@code path.separator} system property, which defaults to {@code :}.
     *
     * @param dexPath the list of jar/apk files containing classes and
     *     resources, delimited by {@code File.pathSeparator}, which
     *     defaults to {@code ":"} on Android
     * @param optimizedDirectory directory where optimized dex files
     *     should be written; must not be {@code null}
     * @param librarySearchPath the list of directories containing native
     *     libraries, delimited by {@code File.pathSeparator}; may be
     *     {@code null}
     * @param parent the parent class loader
     */
    public DexClassLoader(String dexPath, String optimizedDirectory,
            String librarySearchPath, ClassLoader parent) {
        super(dexPath, new File(optimizedDirectory), librarySearchPath, parent);
    }
}

DexClassLoader构造方法的参数要比PathClassLoader多一个optimizedDirectory参数,该参数表示dex的优化存储odex的路径，PathClassLoader默认路径为/data/dalvik-cache

BaseDexClassLoader

BaseDexClassLoader
libcore\dalvik\src\main\java\dalvik\system\BaseDexClassLoader.java

/**
 * Base class for common functionality between various dex-based
 * {@link ClassLoader} implementations.
 */
public class BaseDexClassLoader extends ClassLoader {

    /**
     * Hook for customizing how dex files loads are reported.
     *
     * This enables the framework to monitor the use of dex files. The
     * goal is to simplify the mechanism for optimizing foreign dex files and
     * enable further optimizations of secondary dex files.
     *
     * The reporting happens only when new instances of BaseDexClassLoader
     * are constructed and will be active only after this field is set with
     * {@link BaseDexClassLoader#setReporter}.
     */
    /* @NonNull */ private static volatile Reporter reporter = null;

    private final DexPathList pathList;

	//optimizedDirectory已经弃用
    public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String librarySearchPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, librarySearchPath, null);

        if (reporter != null) {
            reporter.report(this.pathList.getDexPaths());
        }
    }


    public BaseDexClassLoader(ByteBuffer[] dexFiles, ClassLoader parent) {
        // TODO We should support giving this a library search path maybe.
        super(parent);
        this.pathList = new DexPathList(this, dexFiles);
    }



}

path生成pathList，pathList是每次BaseDexClassLoader创建都会创建。代表加载的所有文件与目录

源码分析

2-DexPathList

DexPathList
libcore\dalvik\src\main\java\dalvik\system\DexPathList.java

    /**
     * List of dex/resource (class path) elements.
     * Should be called pathElements, but the Facebook app uses reflection
     * to modify 'dexElements' (http://b/7726934).
     */
    private Element[] dexElements;
    /** class definition context */
    private final ClassLoader definingContext;
    /** List of native library path elements. */
    private final NativeLibraryElement[] nativeLibraryPathElements;
    /**
     * Constructs an instance.
     *
     * @param definingContext the context in which any as-yet unresolved
     * classes should be defined
     * @param dexPath list of dex/resource path elements, separated by
     * {@code File.pathSeparator}
     * @param librarySearchPath list of native library directory path elements,
     * separated by {@code File.pathSeparator}
     * @param optimizedDirectory directory where optimized {@code .dex} files
     * should be found and written to, or {@code null} to use the default
     * system directory for same
     */
	 //optimizedDirectory为null
    public DexPathList(ClassLoader definingContext, String dexPath,
            String librarySearchPath, File optimizedDirectory) {


......

        this.definingContext = definingContext;

        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
        // save dexPath for BaseDexClassLoader
		////记录所有的dexFile文件
        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions, definingContext);

        // Native libraries may exist in both the system and
        // application library paths, and we use this search order:
        //
        //   1. This class loader's library path for application libraries (librarySearchPath):
        //   1.1. Native library directories
        //   1.2. Path to libraries in apk-files
        //   2. The VM's library path from the system property for system libraries
        //      also known as java.library.path
        //
        // This order was reversed prior to Gingerbread; see http://b/2933456.
		//app目录的native库
        this.nativeLibraryDirectories = splitPaths(librarySearchPath, 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);

        if (suppressedExceptions.size() > 0) {
            this.dexElementsSuppressedExceptions =
                suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
        } else {
            dexElementsSuppressedExceptions = null;
        }
    }

	
	public DexPathList(ClassLoader definingContext, ByteBuffer[] dexFiles) {
        if (definingContext == null) {
            throw new NullPointerException("definingContext == null");
        }
        if (dexFiles == null) {
            throw new NullPointerException("dexFiles == null");
        }
        if (Arrays.stream(dexFiles).anyMatch(v -> v == null)) {
            throw new NullPointerException("dexFiles contains a null Buffer!");
        }

        this.definingContext = definingContext;
        // TODO It might be useful to let in-memory dex-paths have native libraries.
        this.nativeLibraryDirectories = Collections.emptyList();
        this.systemNativeLibraryDirectories =
                splitPaths(System.getProperty("java.library.path"), true);
        this.nativeLibraryPathElements = makePathElements(this.systemNativeLibraryDirectories);

        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
        this.dexElements = makeInMemoryDexElements(dexFiles, suppressedExceptions);
        if (suppressedExceptions.size() > 0) {
            this.dexElementsSuppressedExceptions =
                    suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
        } else {
            dexElementsSuppressedExceptions = null;
        }
    }

收集：
dexElements: 记录所有的dexFile文件/目录
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

有一个问题，什么时候从apk、jar中解压的？DexClassLoader的说明中说了可以是压缩包的，虽然我试的时候失败了

3-makeDexElements

makeDexElements
libcore\dalvik\src\main\java\dalvik\system\DexPathList.java

   /**
    * Makes an array of dex/resource path elements, one per element of
    * the given array.
    */
   private static Element[] makeDexElements(List<File> files, File optimizedDirectory,
           List<IOException> suppressedExceptions, ClassLoader loader) {
     Element[] elements = new Element[files.size()];
     int elementsPos = 0;
     /*
      * Open all files and load the (direct or contained) dex files up front.
      */
     for (File file : files) {
         if (file.isDirectory()) {
             // We support directories for looking up resources. Looking up resources in
             // directories is useful for running libcore tests.
             elements[elementsPos++] = new Element(file);
         } else if (file.isFile()) {
             String name = file.getName();

             if (name.endsWith(DEX_SUFFIX)) {//.dex
                 // Raw dex file (not inside a zip/jar). 
			  //这句注释的意思可能是返回的dex是从zip取到内存中的
                 try {
                     DexFile dex = loadDexFile(file, optimizedDirectory, loader, elements);
                     if (dex != null) {
                         elements[elementsPos++] = new Element(dex, null);
                     }
                 } catch (IOException suppressed) {
                     System.logE("Unable to load dex file: " + file, suppressed);
                     suppressedExceptions.add(suppressed);
                 }
             } else {//不是以.dex结尾
                 DexFile dex = null;
                 try {
                     dex = loadDexFile(file, optimizedDirectory, loader, elements);
                 } catch (IOException suppressed) {
                     /*
                      * IOException might get thrown "legitimately" by the DexFile constructor if
                      * the zip file turns out to be resource-only (that is, no classes.dex file
                      * in it).
                      * Let dex == null and hang on to the exception to add to the tea-leaves for
                      * when findClass returns null.
                      */
                     suppressedExceptions.add(suppressed);
                 }

                 if (dex == null) {
                     elements[elementsPos++] = new Element(file);
                 } else {
                     elements[elementsPos++] = new Element(dex, file);
                 }
             }
         } else {
             System.logW("ClassLoader referenced unknown path: " + file);
         }
     }
     if (elementsPos != elements.length) {
         elements = Arrays.copyOf(elements, elementsPos);
     }
     return elements;
   }


   /**
    * Element of the dex/resource path. Note: should be called DexElement, but apps reflect on
    * this.
    */
   /*package*/ static class Element {
       /**
        * A file denoting a zip file (in case of a resource jar or a dex jar), or a directory
        * (only when dexFile is null).
        */
       private final File path;

       private final DexFile dexFile;

       private ClassPathURLStreamHandler urlHandler;
       private boolean initialized;

       /**
        * Element encapsulates a dex file. This may be a plain dex file (in which case dexZipPath
        * should be null), or a jar (in which case dexZipPath should denote the zip file).
        */
       public Element(DexFile dexFile, File dexZipPath) {
           this.dexFile = dexFile;
           this.path = dexZipPath;
       }

       public Element(DexFile dexFile) {
           this.dexFile = dexFile;
           this.path = null;
       }

       public Element(File path) {
         this.path = path;
         this.dexFile = null;
       }	
.......
}

Element对象表示目录或着DexFile对象。makeDexElements通过解析传过来的目录列表将其下对象挨个纳入Element，对于文件调用loadDexFile加载成内存DexFile对象。
值得注意的是是不是.dex结尾都会调用loadDexFile。

4-loadDexFile

loadDexFile
libcore\dalvik\src\main\java\dalvik\system\DexPathList.java

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

构建DexFile返回

5-DexFile

DexFile
libcore\dalvik\src\main\java\dalvik\system\DexFile.java

 /**
  * If close is called, mCookie becomes null but the internal cookie is preserved if the close
  * failed so that we can free resources in the finalizer.
  */
   private Object mCookie;
   private Object mInternalCookie;
   private final String mFileName;
   /*
    * Private version with class loader argument.
    *
    * @param file
    *            the File object referencing the actual DEX file
    * @param loader
    *            the class loader object creating the DEX file object
    * @param elements
    *            the temporary dex path list elements from DexPathList.makeElements
    */
   DexFile(File file, ClassLoader loader, DexPathList.Element[] elements)
           throws IOException {
       this(file.getPath(), loader, elements);
   }
/*
    * Private version with class loader argument.
    *
    * @param fileName
    *            the filename of the DEX file
    * @param loader
    *            the class loader creating the DEX file object
    * @param elements
    *            the temporary dex path list elements from DexPathList.makeElements
    */
   DexFile(String fileName, ClassLoader loader, DexPathList.Element[] elements) throws IOException {
       mCookie = openDexFile(fileName, null, 0, loader, elements);
       mInternalCookie = mCookie;
       mFileName = fileName;
       //System.out.println("DEX FILE cookie is " + mCookie + " fileName=" + fileName);
   }

/*
    * Open a DEX file.  The value returned is a magic VM cookie.  On
    * failure, an IOException is thrown.
    */
   private static Object openDexFile(String sourceName, String outputName, int flags,
           ClassLoader loader, DexPathList.Element[] elements) throws IOException {
       // Use absolute paths to enable the use of relative paths when testing on host.
       return openDexFileNative(new File(sourceName).getAbsolutePath(),
                                (outputName == null)
                                    ? null
                                    : new File(outputName).getAbsolutePath(),
                                flags,
                                loader,
                                elements);
   }

可见DexFile代表的就是一个dex文件加载到内存中。参数：

1. dex文件的绝对路径
2. outputName：null
3. flags：0
4. ClassLoader就是加载器this
5. DexPathList.Element[]要创建的文件/目录节点数组

7-openDexFileNative

我们还是跟进native接着看
openDexFileNative
D:\androidwsp\android8.0.0_r1\android-8.0.0_r1\art\runtime\native\dalvik_system_DexFile.cc

// TODO(calin): clean up the unused parameters (here and in libcore).
//ATTRIBUTE_UNUSED：属性不使用
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);//名字转化类
  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);

	//这里不空，所以执行
  if (!dex_files.empty()) {
	  //将dex_files/oat_file转成jlongArray返回
    jlongArray array = ConvertDexFilesToJavaArray(env, oat_file, dex_files); 
    if (array == nullptr) {
      ScopedObjectAccess soa(env);
      for (auto& dex_file : dex_files) {
        if (linker->IsDexFileRegistered(soa.Self(), *dex_file)) {
          dex_file.release();
        }
      }
    }
    return array;
  } else {
    ScopedObjectAccess soa(env);
    CHECK(!error_msgs.empty());
    // The most important message is at the end. So set up nesting by going forward, which will
    // wrap the existing exception as a cause for the following one.
    auto it = error_msgs.begin();
    auto itEnd = error_msgs.end();
    for ( ; it != itEnd; ++it) {
      ThrowWrappedIOException("%s", it->c_str());
    }

    return nullptr;
  }
}

参：

1. 文件名的c字串模式
2. jobject的class_loader
3. jobjectArray的dex_elements
4. 出：OatFile指针
5. 字串向量error_msgs

10-ConvertDexFilesToJavaArray

ConvertDexFilesToJavaArray
返回给mCookie，oat与dex结构如何转化的：
art\runtime\native\dalvik_system_DexFile.cc

static jlongArray ConvertDexFilesToJavaArray(JNIEnv* env,
                                             const OatFile* oat_file,
                                             std::vector<std::unique_ptr<const DexFile>>& vec) {
  // Add one for the oat file.
  jlongArray long_array = env->NewLongArray(static_cast<jsize>(kDexFileIndexStart + vec.size()));
  if (env->ExceptionCheck() == JNI_TRUE) {
    return nullptr;
  }

  jboolean is_long_data_copied;
  jlong* long_data = env->GetLongArrayElements(long_array, &is_long_data_copied);
  if (env->ExceptionCheck() == JNI_TRUE) {
    return nullptr;
  }

  long_data[kOatFileIndex] = reinterpret_cast<uintptr_t>(oat_file);
  for (size_t i = 0; i < vec.size(); ++i) {
    long_data[kDexFileIndexStart + i] = reinterpret_cast<uintptr_t>(vec[i].get());
  }

  env->ReleaseLongArrayElements(long_array, long_data, 0);
  if (env->ExceptionCheck() == JNI_TRUE) {
    return nullptr;
  }

  // Now release all the unique_ptrs.
  for (auto& dex_file : vec) {
    dex_file.release();
  }

  return long_array;
}

8-OpenDexFilesFromOat

OpenDexFilesFromOat
art\runtime\oat_file_manager.cc

std::vector<std::unique_ptr<const DexFile>> OatFileManager::OpenDexFilesFromOat(
    const char* dex_location,
    jobject class_loader,
    jobjectArray dex_elements,
    const OatFile** out_oat_file,
    std::vector<std::string>* error_msgs) {
  ScopedTrace trace(__FUNCTION__);
  CHECK(dex_location != nullptr);
  CHECK(error_msgs != nullptr);

  // Verify we aren't holding the mutator lock, which could starve GC if we
  // have to generate or relocate an oat file.
  Thread* const self = Thread::Current();
  Locks::mutator_lock_->AssertNotHeld(self);
  Runtime* const runtime = Runtime::Current();
 //初始化oat_file_assistant
  OatFileAssistant oat_file_assistant(dex_location,
                                      kRuntimeISA,
                                      !runtime->IsAotCompiler());

  // Lock the target oat location to avoid races generating and loading the
  // oat file.
  std::string error_msg;
  if (!oat_file_assistant.Lock(/*out*/&error_msg)) {
    // Don't worry too much if this fails. If it does fail, it's unlikely we
    // can generate an oat file anyway.
    VLOG(class_linker) << "OatFileAssistant::Lock: " << error_msg;
  }

  const OatFile* source_oat_file = nullptr;

  
  //dex文件是否已经oat化
  if (!oat_file_assistant.IsUpToDate()) {
    // Update the oat file on disk if we can, based on the --compiler-filter
    // option derived from the current runtime options.
    // This may fail, but that's okay. Best effort is all that matters here.
	//MakeUpToDate执行了dex文件的优化工作，即利用dex文件生成oat文件
    switch (oat_file_assistant.MakeUpToDate(/*profile_changed*/false, /*out*/ &error_msg)) {
      case OatFileAssistant::kUpdateFailed:
        LOG(WARNING) << error_msg;
        break;

      case OatFileAssistant::kUpdateNotAttempted:
        // Avoid spamming the logs if we decided not to attempt making the oat
        // file up to date.
        VLOG(oat) << error_msg;
        break;

      case OatFileAssistant::kUpdateSucceeded:
        // Nothing to do.
        break;
    }
  }

  // Get the oat file on disk.
  //从磁盘获取oat文件，返回
  //oat_file_assistant.GetBestOatFile().release() 获取oat对象
  std::unique_ptr<const OatFile> oat_file(oat_file_assistant.GetBestOatFile().release());

  if (oat_file != nullptr) {
    // Take the file only if it has no collisions, or we must take it because of preopting.
    bool accept_oat_file =
        !HasCollisions(oat_file.get(), class_loader, dex_elements, /*out*/ &error_msg);
    if (!accept_oat_file) {
      // Failed the collision check. Print warning.
      if (Runtime::Current()->IsDexFileFallbackEnabled()) {
        if (!oat_file_assistant.HasOriginalDexFiles()) {
          // We need to fallback but don't have original dex files. We have to
          // fallback to opening the existing oat file. This is potentially
          // unsafe so we warn about it.
          accept_oat_file = true;

          LOG(WARNING) << "Dex location " << dex_location << " does not seem to include dex file. "
                       << "Allow oat file use. This is potentially dangerous.";
        } else {
          // We have to fallback and found original dex files - extract them from an APK.
          // Also warn about this operation because it's potentially wasteful.
          LOG(WARNING) << "Found duplicate classes, falling back to extracting from APK : "
                       << dex_location;
          LOG(WARNING) << "NOTE: This wastes RAM and hurts startup performance.";
        }
      } else {
        // TODO: We should remove this. The fact that we're here implies -Xno-dex-file-fallback
        // was set, which means that we should never fallback. If we don't have original dex
        // files, we should just fail resolution as the flag intended.
        if (!oat_file_assistant.HasOriginalDexFiles()) {
          accept_oat_file = true;
        }

        LOG(WARNING) << "Found duplicate classes, dex-file-fallback disabled, will be failing to "
                        " load classes for " << dex_location;
      }

      LOG(WARNING) << error_msg;
    }

    if (accept_oat_file) {
      VLOG(class_linker) << "Registering " << oat_file->GetLocation();
      source_oat_file = RegisterOatFile(std::move(oat_file));
	  //出oatfile
      *out_oat_file = source_oat_file;
    }
  }

  std::vector<std::unique_ptr<const DexFile>> dex_files;

  // Load the dex files from the oat file.
  //从oat文件中加载dex文件，返回
  if (source_oat_file != nullptr) {
    bool added_image_space = false;
    if (source_oat_file->IsExecutable()) {
      std::unique_ptr<gc::space::ImageSpace> image_space =
          kEnableAppImage ? oat_file_assistant.OpenImageSpace(source_oat_file) : nullptr;
      if (image_space != nullptr) {
        ScopedObjectAccess soa(self);
        StackHandleScope<1> hs(self);
        Handle<mirror::ClassLoader> h_loader(
            hs.NewHandle(soa.Decode<mirror::ClassLoader>(class_loader)));
        // Can not load app image without class loader.
        if (h_loader != nullptr) {
          std::string temp_error_msg;
          // Add image space has a race condition since other threads could be reading from the
          // spaces array.
          {
            ScopedThreadSuspension sts(self, kSuspended);
            gc::ScopedGCCriticalSection gcs(self,
                                            gc::kGcCauseAddRemoveAppImageSpace,
                                            gc::kCollectorTypeAddRemoveAppImageSpace);
            ScopedSuspendAll ssa("Add image space");
            runtime->GetHeap()->AddSpace(image_space.get());
          }
          {
            ScopedTrace trace2(StringPrintf("Adding image space for location %s", dex_location));
            added_image_space = runtime->GetClassLinker()->AddImageSpace(image_space.get(),
                                                                         h_loader,
                                                                         dex_elements,
                                                                         dex_location,
                                                                         /*out*/&dex_files,
                                                                         /*out*/&temp_error_msg);
          }
          if (added_image_space) {
            // Successfully added image space to heap, release the map so that it does not get
            // freed.
            image_space.release();
          } else {
            LOG(INFO) << "Failed to add image file " << temp_error_msg;
            dex_files.clear();
            {
              ScopedThreadSuspension sts(self, kSuspended);
              gc::ScopedGCCriticalSection gcs(self,
                                              gc::kGcCauseAddRemoveAppImageSpace,
                                              gc::kCollectorTypeAddRemoveAppImageSpace);
              ScopedSuspendAll ssa("Remove image space");
              runtime->GetHeap()->RemoveSpace(image_space.get());
            }
            // Non-fatal, don't update error_msg.
          }
        }
      }
    }
    if (!added_image_space) {
      DCHECK(dex_files.empty());
      dex_files = oat_file_assistant.LoadDexFiles(*source_oat_file, dex_location);
    }
    if (dex_files.empty()) {
      error_msgs->push_back("Failed to open dex files from " + source_oat_file->GetLocation());
    }
  }

  // Fall back to running out of the original dex file if we couldn't load any
  // dex_files from the oat file.
  //如果oat文件生成失败会返回原始的dex文件
  if (dex_files.empty()) {
    if (oat_file_assistant.HasOriginalDexFiles()) {
      if (Runtime::Current()->IsDexFileFallbackEnabled()) {
        static constexpr bool kVerifyChecksum = true;
        if (!DexFile::Open(
            dex_location, dex_location, kVerifyChecksum, /*out*/ &error_msg, &dex_files)) {
          LOG(WARNING) << error_msg;
          error_msgs->push_back("Failed to open dex files from " + std::string(dex_location)
                                + " because: " + error_msg);
        }
      } else {
        error_msgs->push_back("Fallback mode disabled, skipping dex files.");
      }
    } else {
      error_msgs->push_back("No original dex files found for dex location "
          + std::string(dex_location));
    }
  }

  return dex_files;
}

将dex_location生成OatFileAssistant(助手)，查看是否为oat文件，决定dex到oat文件的优化，之后从磁盘中读取oat到内存，从oat中读取dex，并返回oat与dex。
值的注意的是如果hook了execv函数阻止oat文件的生成，最终会调用DexFile::Open方法来直接加载原始的DEX文件。

9-OatFileAssistant

OatFileAssistant
art\runtime\oat_file_assistant.cc

OatFileAssistant::OatFileAssistant(const char* dex_location,
                                   const InstructionSet isa,
                                   bool load_executable)
    : isa_(isa),
      load_executable_(load_executable),
      odex_(this, /*is_oat_location*/ false),
      oat_(this, /*is_oat_location*/ true) {
  CHECK(dex_location != nullptr) << "OatFileAssistant: null dex location";

  // Try to get the realpath for the dex location.
  //
  // This is OK with respect to dalvik cache naming scheme because we never
  // generate oat files starting from symlinks which go into dalvik cache.
  // (recall that the oat files in dalvik cache are encoded by replacing '/'
  // with '@' in the path).
  // The boot image oat files (which are symlinked in dalvik-cache) are not
  // loaded via the oat file assistant.
  //
  // The only case when the dex location may resolve to a different path
  // is for secondary dex files (e.g. /data/user/0 symlinks to /data/data and
  // the app is free to create its own internal layout). Related to this it is
  // worthwhile to mention that installd resolves the secondary dex location
  // before calling dex2oat.
  UniqueCPtr<const char[]> dex_location_real(realpath(dex_location, nullptr));
  if (dex_location_real != nullptr) {
	  //获取dex的位置
    dex_location_.assign(dex_location_real.get());
  } else {
    // If we can't get the realpath of the location there's not much point in trying to move on.
    PLOG(ERROR) << "Could not get the realpath of dex_location " << dex_location;
    return;
  }

  if (load_executable_ && isa != kRuntimeISA) {
    LOG(WARNING) << "OatFileAssistant: Load executable specified, "
      << "but isa is not kRuntimeISA. Will not attempt to load executable.";
    load_executable_ = false;
  }

  // Get the odex filename.
  //   location = /foo/bar/baz.jar
  //   odex_location = /foo/bar/oat/<isa>/baz.odex
  std::string error_msg;
  std::string odex_file_name;
  if (DexLocationToOdexFilename(dex_location_, isa_, &odex_file_name, &error_msg)) {
    odex_.Reset(odex_file_name);
  } else {
    LOG(WARNING) << "Failed to determine odex file name: " << error_msg;
  }

  // Get the oat filename.
  std::string oat_file_name;
  if (DexLocationToOatFilename(dex_location_, isa_, &oat_file_name, &error_msg)) {
    oat_.Reset(oat_file_name);
  } else {
    LOG(WARNING) << "Failed to determine oat file name for dex location "
        << dex_location_ << ": " << error_msg;
  }

  // Check if the dex directory is writable.
  // This will be needed in most uses of OatFileAssistant and so it's OK to
  // compute it eagerly. (the only use which will not make use of it is
  // OatFileAssistant::GetStatusDump())
  size_t pos = dex_location_.rfind('/');
  if (pos == std::string::npos) {
    LOG(WARNING) << "Failed to determine dex file parent directory: " << dex_location_;
  } else {
    std::string parent = dex_location_.substr(0, pos);
    if (access(parent.c_str(), W_OK) == 0) {
      dex_parent_writable_ = true;
    } else {
      VLOG(oat) << "Dex parent of " << dex_location_ << " is not writable: " << strerror(errno);
    }
  }
}

代表文件对象，在构造中通过dex路径获取了odex和oat的路径

11-IsUpToDate

IsUpToDate
art\runtime\oat_file_assistant.cc

bool OatFileAssistant::IsUpToDate() {
	//OatFileInfo的Status
  return GetBestInfo().Status() == kOatUpToDate;
}

OatFileAssistant::OatStatus OatFileAssistant::OatFileInfo::Status() {
  if (!status_attempted_) {
    status_attempted_ = true;
	
    const OatFile* file = GetFile();//??????
	//file==null才执行，猜测为不是OatFile返回null
    if (file == nullptr) {
      // Check to see if there is a vdex file we can make use of.
      std::string error_msg;
      std::string vdex_filename = GetVdexFilename(filename_);
      std::unique_ptr<VdexFile> vdex = VdexFile::Open(vdex_filename,
                                                      /*writeable*/false,
                                                      /*low_4gb*/false,
                                                      /*unquicken*/false,
                                                      &error_msg);
		//是vdex
      if (vdex == nullptr) {
        status_ = kOatCannotOpen;
        VLOG(oat) << "unable to open vdex file " << vdex_filename << ": " << error_msg;
      } else {
        if (oat_file_assistant_->DexChecksumUpToDate(*vdex, &error_msg)) {
          // The vdex file does not contain enough information to determine
          // whether it is up to date with respect to the boot image, so we
          // assume it is out of date.
          VLOG(oat) << error_msg;
          status_ = kOatBootImageOutOfDate;
        } else {
          status_ = kOatDexOutOfDate;
        }
      }
    } else {
		//继续检查是不是其它的file
      status_ = oat_file_assistant_->GivenOatFileStatus(*file);
      VLOG(oat) << file->GetLocation() << " is " << status_
          << " with filter " << file->GetCompilerFilter();
    }
  }
  return status_;
}

如果返回值==kOatUpToDate就表示优化过了，kOatUpToDate定义在一个enum，用于表示文件的状态。IsUpToDate中，Status检测(状态)

这里检测过程之后还比较长，先留下等用到再看。

13-MakeUpToDate

接下来看MakeUpToDate
art\runtime\oat_file_assistant.cc

OatFileAssistant::ResultOfAttemptToUpdate
OatFileAssistant::MakeUpToDate(bool profile_changed, std::string* error_msg) {
  CompilerFilter::Filter target;
  if (!GetRuntimeCompilerFilterOption(&target, error_msg)) {
    return kUpdateNotAttempted;
  }

  OatFileInfo& info = GetBestInfo();
  //GetDexOptNeeded()方法再次确认是否需要oat优化
  switch (info.GetDexOptNeeded(target, profile_changed)) {
    case kNoDexOptNeeded:
      return kUpdateSucceeded;

    // TODO: For now, don't bother with all the different ways we can call
    // dex2oat to generate the oat file. Always generate the oat file as if it
    // were kDex2OatFromScratch.
    case kDex2OatFromScratch:
    case kDex2OatForBootImage:
    case kDex2OatForRelocation:
    case kDex2OatForFilter:
      return GenerateOatFileNoChecks(info, target, error_msg);
  }
  UNREACHABLE();
}


OatFileAssistant::ResultOfAttemptToUpdate OatFileAssistant::GenerateOatFileNoChecks(
      OatFileAssistant::OatFileInfo& info, CompilerFilter::Filter filter, std::string* error_msg) {
  CHECK(error_msg != nullptr);
	
  Runtime* runtime = Runtime::Current();
  //检查是否开启了允许优化
  if (!runtime->IsDex2OatEnabled()) {
    *error_msg = "Generation of oat file for dex location " + dex_location_
      + " not attempted because dex2oat is disabled.";
    return kUpdateNotAttempted;
  }

  
  //获取优化后的oat文件路径
  if (info.Filename() == nullptr) {
    *error_msg = "Generation of oat file for dex location " + dex_location_
      + " not attempted because the oat file name could not be determined.";
    return kUpdateNotAttempted;
  }
  const std::string& oat_file_name = *info.Filename();
  const std::string& vdex_file_name = GetVdexFilename(oat_file_name);

  // dex2oat ignores missing dex files and doesn't report an error.
  // Check explicitly here so we can detect the error properly.
  // TODO: Why does dex2oat behave that way?
  struct stat dex_path_stat;
  if (TEMP_FAILURE_RETRY(stat(dex_location_.c_str(), &dex_path_stat)) != 0) {
    *error_msg = "Could not access dex location " + dex_location_ + ":" + strerror(errno);
    return kUpdateNotAttempted;
  }

  // If this is the odex location, we need to create the odex file layout (../oat/isa/..)
  if (!info.IsOatLocation()) {
    if (!PrepareOdexDirectories(dex_location_, oat_file_name, isa_, error_msg)) {
      return kUpdateNotAttempted;
    }
  }

  // Set the permissions for the oat and the vdex files.
  // The user always gets read and write while the group and others propagate
  // the reading access of the original dex file.
  mode_t file_mode = S_IRUSR | S_IWUSR |
      (dex_path_stat.st_mode & S_IRGRP) |
      (dex_path_stat.st_mode & S_IROTH);

  std::unique_ptr<File> vdex_file(OS::CreateEmptyFile(vdex_file_name.c_str()));
  if (vdex_file.get() == nullptr) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the vdex file " + vdex_file_name
      + " could not be opened.";
    return kUpdateNotAttempted;
  }

  if (fchmod(vdex_file->Fd(), file_mode) != 0) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the vdex file " + vdex_file_name
      + " could not be made world readable.";
    return kUpdateNotAttempted;
  }

  std::unique_ptr<File> oat_file(OS::CreateEmptyFile(oat_file_name.c_str()));
  if (oat_file.get() == nullptr) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the oat file could not be created.";
    return kUpdateNotAttempted;
  }

  
  
  //设置存放oat文件地址的可写权限
  if (fchmod(oat_file->Fd(), file_mode) != 0) {
    *error_msg = "Generation of oat file " + oat_file_name
      + " not attempted because the oat file could not be made world readable.";
    oat_file->Erase();
    return kUpdateNotAttempted;
  }

  
  //配置参数  这个vdex待了解
  std::vector<std::string> args;
  args.push_back("--dex-file=" + dex_location_);
  args.push_back("--output-vdex-fd=" + std::to_string(vdex_file->Fd()));
  args.push_back("--oat-fd=" + std::to_string(oat_file->Fd()));
  args.push_back("--oat-location=" + oat_file_name);
  args.push_back("--compiler-filter=" + CompilerFilter::NameOfFilter(filter));

  //执行dex到oat的优化，
  if (!Dex2Oat(args, error_msg)) {
    // Manually delete the oat and vdex files. This ensures there is no garbage
    // left over if the process unexpectedly died.
    vdex_file->Erase();
    unlink(vdex_file_name.c_str());
    oat_file->Erase();
    unlink(oat_file_name.c_str());
    return kUpdateFailed;
  }

  if (vdex_file->FlushCloseOrErase() != 0) {
    *error_msg = "Unable to close vdex file " + vdex_file_name;
    unlink(vdex_file_name.c_str());
    return kUpdateFailed;
  }

  if (oat_file->FlushCloseOrErase() != 0) {
    *error_msg = "Unable to close oat file " + oat_file_name;
    unlink(oat_file_name.c_str());
    return kUpdateFailed;
  }

  // Mark that the odex file has changed and we should try to reload.
  //更新文件信息，此时oat可以获取了
  info.Reset();
  return kUpdateSucceeded;
}

获取、设置路径，执行优化

info.Filename()
art\runtime\oat_file_assistant.cc

//？？？？？？？？？
const std::string* OatFileAssistant::OatFileInfo::Filename() {
  return filename_provided_ ? &filename_ : nullptr;
}

15-Dex2Oat

Dex2Oat
art\runtime\oat_file_assistant.cc

bool OatFileAssistant::Dex2Oat(const std::vector<std::string>& args,
                               std::string* error_msg) {
  Runtime* runtime = Runtime::Current();
  std::string image_location = ImageLocation();
  if (image_location.empty()) {
    *error_msg = "No image location found for Dex2Oat.";
    return false;
  }

  std::vector<std::string> argv;
  argv.push_back(runtime->GetCompilerExecutable());
  argv.push_back("--runtime-arg");
  argv.push_back("-classpath");
  argv.push_back("--runtime-arg");
  std::string class_path = runtime->GetClassPathString();
  if (class_path == "") {
    class_path = OatFile::kSpecialSharedLibrary;
  }
  argv.push_back(class_path);
  if (runtime->IsJavaDebuggable()) {
    argv.push_back("--debuggable");
  }
  runtime->AddCurrentRuntimeFeaturesAsDex2OatArguments(&argv);

  if (!runtime->IsVerificationEnabled()) {
    argv.push_back("--compiler-filter=verify-none");
  }

  if (runtime->MustRelocateIfPossible()) {
    argv.push_back("--runtime-arg");
    argv.push_back("-Xrelocate");
  } else {
    argv.push_back("--runtime-arg");
    argv.push_back("-Xnorelocate");
  }

  if (!kIsTargetBuild) {
    argv.push_back("--host");
  }

  argv.push_back("--boot-image=" + image_location);

  std::vector<std::string> compiler_options = runtime->GetCompilerOptions();
  argv.insert(argv.end(), compiler_options.begin(), compiler_options.end());

  argv.insert(argv.end(), args.begin(), args.end());

  std::string command_line(android::base::Join(argv, ' '));
  return Exec(argv, error_msg);
}




//art\runtime\runtime.cc
std::string Runtime::GetCompilerExecutable() const {
  if (!compiler_executable_.empty()) {
    return compiler_executable_;
  }
  std::string compiler_executable(GetAndroidRoot());
  compiler_executable += (kIsDebugBuild ? "/bin/dex2oatd" : "/bin/dex2oat");
  return compiler_executable;
}

进一步添加参数，调用Exec,其中参数第一项是/bin/dex2oat。

16-Exec

Exec
art\runtime\exec_utils.cc

bool Exec(std::vector<std::string>& arg_vector, std::string* error_msg) {
  int status = ExecAndReturnCode(arg_vector, error_msg);
  if (status != 0) {
    const std::string command_line(android::base::Join(arg_vector, ' '));
    *error_msg = StringPrintf("Failed execv(%s) because non-0 exit status",
                              command_line.c_str());
    return false;
  }
  return true;
}

int ExecAndReturnCode(std::vector<std::string>& arg_vector, std::string* error_msg) {
  const std::string command_line(android::base::Join(arg_vector, ' '));
  CHECK_GE(arg_vector.size(), 1U) << command_line;

  // Convert the args to char pointers.
  const char* program = arg_vector[0].c_str();
  std::vector<char*> args;
  for (size_t i = 0; i < arg_vector.size(); ++i) {
    const std::string& arg = arg_vector[i];
    char* arg_str = const_cast<char*>(arg.c_str());
    CHECK(arg_str != nullptr) << i;
    args.push_back(arg_str);
  }
  args.push_back(nullptr);

  // fork and exec
  pid_t pid = fork();
  if (pid == 0) {
    // no allocation allowed between fork and exec

    // change process groups, so we don't get reaped by ProcessManager
    setpgid(0, 0);

    // (b/30160149): protect subprocesses from modifications to LD_LIBRARY_PATH, etc.
    // Use the snapshot of the environment from the time the runtime was created.
    char** envp = (Runtime::Current() == nullptr) ? nullptr : Runtime::Current()->GetEnvSnapshot();
    if (envp == nullptr) {
      execv(program, &args[0]);
    } else {
		//子进程运行
      execve(program, &args[0], envp);
    }
    PLOG(ERROR) << "Failed to execve(" << command_line << ")";
    // _exit to avoid atexit handlers in child.
    _exit(1);
  } else {
    if (pid == -1) {
      *error_msg = StringPrintf("Failed to execv(%s) because fork failed: %s",
                                command_line.c_str(), strerror(errno));
      return -1;
    }

    // wait for subprocess to finish
	//等待子进程结束
    int status = -1;
    pid_t got_pid = TEMP_FAILURE_RETRY(waitpid(pid, &status, 0));
    if (got_pid != pid) {
      *error_msg = StringPrintf("Failed after fork for execv(%s) because waitpid failed: "
                                "wanted %d, got %d: %s",
                                command_line.c_str(), pid, got_pid, strerror(errno));
      return -1;
    }
    if (WIFEXITED(status)) {
      return WEXITSTATUS(status);
    }
    return -1;
  }
}

fork出子进程，子进程调用dex2oat执行优化，父进程等待子结束。