ClassLoader分类

在JVM中，有三个ClassLoader，分别是：

BootsTrap ClassLoader,Extension ClassLoader,App ClassLoader.

同样的，在Android DVM(ART类似，具体实现不同)中也有三个ClassLoader：

BootClassLoader:主要用于加载系统类，包括Java和Android的类库
PathClassLoader:主要加载应用(DVM已安装)中的类，它的加载路径固定，无法指定
DexClassLoader :可以加载任意路径的Zip，Jar，Apk，可以实现动态加载（热更新，插件化）

PathClassLoader和DexClassLoader的源码

public class PathClassLoader extends BaseDexClassLoader {
    /**
     * Creates a {@code PathClassLoader} that operates on a given list of files
     * and directories. This method is equivalent to calling
     * {@link #PathClassLoader(String, String, ClassLoader)} with a
     * {@code null} value for the second argument (see description there).
     *
     * @param dexPath the list of jar/apk files containing classes and
     * resources, delimited by {@code File.pathSeparator}, which
     * defaults to {@code ":"} on Android
     * @param parent the parent class loader
     */
    public PathClassLoader(String dexPath, ClassLoader parent) {
        super(dexPath, null, null, parent);
    }

    /**
     * Creates a {@code PathClassLoader} that operates on two given
     * lists of files and directories. The entries of the first list
     * should be one of the following:
     *
     * <ul>
     * <li>JAR/ZIP/APK files, possibly containing a "classes.dex" file as
     * well as arbitrary resources.
     * <li>Raw ".dex" files (not inside a zip file).
     * </ul>
     *
     * The entries of the second list should be directories containing
     * native library files.
     *
     * @param dexPath the list of jar/apk files containing classes and
     * resources, delimited by {@code File.pathSeparator}, which
     * defaults to {@code ":"} on Android
     * @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 PathClassLoader(String dexPath, String librarySearchPath, ClassLoader parent) {
        super(dexPath, null, librarySearchPath, parent);
    }
}

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);
    }
}

可以看到，两者只有构造方法，均无具体实现方法。
ClassLoader加载过程
================
PathClassLoader和DexClassLoader 都继承BaseDexClassLoader,先看BaseDexClassLoader的构造方法：

 /**
     * Constructs an instance.
     *
     * @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; may 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 BaseDexClassLoader(String dexPath, File optimizedDirectory/*odex目录*/,
            String libraryPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
    }

findClass()方法：

#BaseDexClassLoader.java
protected Class<?> findClass(String name) throws ClassNotFoundException {
        List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
       //调用DexPathList.findClass,找到直接返回，找不到直接throw exception
        Class c = pathList.findClass(name, suppressedExceptions);
        if (c == null) {
            ClassNotFoundException cnfe = new ClassNotFoundException("Didn't find class \"" + name + "\" on path: " + pathList);
            for (Throwable t : suppressedExceptions) {
                cnfe.addSuppressed(t);
            }
            throw cnfe;
        }
        return c;
    }

这里的pathList是DexPathList实例：

#DexPathList.java
  public Class findClass(String name, List<Throwable> suppressed) {
        for (Element element : dexElements) {
            DexFile dex = element.dexFile;
            //找到dex file
            if (dex != null) {
                Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);
                if (clazz != null) {
                    return clazz;
                }
            }
        }
        if (dexElementsSuppressedExceptions != null) {
            suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
        }
        return null;
    }

DexFile.loadClassBinaryName():

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

    private static Class defineClass(String name, ClassLoader loader, long 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;
    }
   //调用本地方法defineClassNative
   private static native Class defineClassNative(String name, ClassLoader loader, 
          Object cookie, DexFile dexFile)throws ClassNotFoundException, NoClassDefFoundError;

DexFile.defineClassNative()具体实现请看这里：
art\runtime\native\dalvik_system_DexFile.cc
可以看出，BaseDexClassLoader中有个pathList对象，pathList中包含一个DexFile的数组dexElements，由上面分析知道，dexPath传入的原始dex(.apk,.zip,.jar等)文件在optimizedDirectory文件夹中生成相应的优化后的odex文件，dexElements数组就是这些odex文件的集合，如果不分包一般这个数组只有一个Element元素，也就只有一个DexFile文件，而对于类加载呢，就是遍历这个集合，通过DexFile去寻找。最终调用native方法的defineClass。

参考

Android动态加载之ClassLoader详解
Android中的ClassLoader