package dalvik.system;
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);
    }
}

• aseDexClassLoader

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

• 第一个参数指的是我们要加载的 dex 文件的路径，它有可能是多个 dex 路径，取决于我们要加载的 dex 文件的个数，多个路径之间用 : 隔开。
• 第二个参数指的是优化后的 dex 存放目录。实际上，dex 其实还并不能被虚拟机直接加载，它需要系统的优化工具优化后才能真正被利用。优化之后的 dex 文件我们把它叫做 odex （optimized dex，说明这是被优化后的 dex）文件。其实从 class 到 dex 也算是经历了一次优化，这种优化的是机器无关的优化，也就是说不管将来运行在什么机器上，这种优化都是遵循固定模式的，因此这种优化发生在 apk 编译。而从 dex 文件到 odex 文件，是机器相关的优化，它使得 odex 适配于特定的硬件环境，不同机器这一步的优化可能有所不同，所以这一步需要在应用安装等运行时期由机器来完成。需要注意的是，在较早版本的系统中，这个目录可以指定为外部存储中的目录，较新版本的系统为了安全只允许其为应用程序私有存储空间（/data/data/apk-package-name/）下的目录，一般我们可以通过 Context#getDir(String dirName) 得到这个目录。
• 第三个参数的意义是库文件的的搜索路径，一般来说是 .so 库文件的路径，也可以指明多个路径。
• 第四个参数就是要传入的父加载器，一般情况我们可以通过 Context#getClassLoader() 得到应用程序的类加载器然后把它传进去。

public DexPathList(ClassLoader definingContext, String dexPath,
            String librarySearchPath, File optimizedDirectory) {

        if (definingContext == null) {
            throw new NullPointerException("definingContext == null");
        }

        if (dexPath == null) {
            throw new NullPointerException("dexPath == null");
       }

       if (optimizedDirectory != null) {
           if (!optimizedDirectory.exists())  {
               throw new IllegalArgumentException(
                       "optimizedDirectory doesn't exist: "
                       + optimizedDirectory);
           }

           if (!(optimizedDirectory.canRead()
                           && optimizedDirectory.canWrite())) {
               throw new IllegalArgumentException(
                       "optimizedDirectory not readable/writable: "
                       + optimizedDirectory);
           }
       }

       this.definingContext = definingContext;

       ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
       // save dexPath for BaseDexClassLoader
       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.
       this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
       this.systemNativeLibraryDirectories =
               splitPaths(System.getProperty("java.library.path"), true);
       List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
       allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);

       this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories,
                                                         suppressedExceptions,
                                                         definingContext);

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

his.definingContext = definingContext;

       ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
       // save dexPath for BaseDexClassLoader
       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.
       this.nativeLibraryDirectories = splitPaths(librarySearchPath, false);
       this.systemNativeLibraryDirectories =
               splitPaths(System.getProperty("java.library.path"), true);
       List<File> allNativeLibraryDirectories = new ArrayList<>(nativeLibraryDirectories);
       allNativeLibraryDirectories.addAll(systemNativeLibraryDirectories);

       this.nativeLibraryPathElements = makePathElements(allNativeLibraryDirectories,
                                                         suppressedExceptions,
                                                         definingContext);

private static List<File> splitPaths(String searchPath, boolean directoriesOnly) {
       List<File> result = new ArrayList<>();

       if (searchPath != null) {
           for (String path : searchPath.split(File.pathSeparator)) {
               if (directoriesOnly) {
                   try {
                       StructStat sb = Libcore.os.stat(path);
                       if (!S_ISDIR(sb.st_mode)) {
                           continue;
                       }
                   } catch (ErrnoException ignored) {
                       continue;
                   }
               }
               result.add(new File(path));
           }
       }

       return result;
}

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

private static Element[] makeElements(List<File> files, File optimizedDirectory,
                                          List<IOException> suppressedExceptions,
                                          boolean ignoreDexFiles,
                                         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) {
           File zip = null;
           File dir = new File("");
           DexFile dex = null;
           String path = file.getPath();
           String name = file.getName();

           if (path.contains(zipSeparator)) {
               String split[] = path.split(zipSeparator, 2);
               zip = new File(split[0]);
               dir = new File(split[1]);
           } else if (file.isDirectory()) {
               // We support directories for looking up resources and native libraries.
               // Looking up resources in directories is useful for running libcore tests.
               elements[elementsPos++] = new Element(file, true, null, null);
           } else if (file.isFile()) {
               if (!ignoreDexFiles && name.endsWith(DEX_SUFFIX)) {
                   // Raw dex file (not inside a zip/jar).
                   try {
                       dex = loadDexFile(file, optimizedDirectory, loader, elements);
                   } catch (IOException suppressed) {
                       System.logE("Unable to load dex file: " + file, suppressed);
                       suppressedExceptions.add(suppressed);
                   }
               } else {
                   zip = file;

                   if (!ignoreDexFiles) {
                       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);
                       }
                   }
               }
           } else {
               System.logW("ClassLoader referenced unknown path: " + file);
           }

           if ((zip != null) || (dex != null)) {
               elements[elementsPos++] = new Element(dir, false, zip, dex);
           }
        }
       if (elementsPos != elements.length) {
           elements = Arrays.copyOf(elements, elementsPos);
       }
       return elements;
}

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);
       }
}