Android ClassLoader

一个运行的Android应用至少有2个ClassLoader，一个是BootClassLoader(系统启动时创建的)，一个是PathClassLoader(应用启动时创建的，用于加载/data/data/packagename/apkname.apk)

android中应用的类加载器主要有两种，分别是PathClassLoader和DexClassLoader，PathClassLoader只能用来加载已安装应用的dex文件，而DexClassLoader可以用来加载未安装的apk\jar等文件；

PathClassLoder

/**
 * Provides a simple {@link ClassLoader} implementation that operates on a list
 * of files and directories in the local file system, but does not attempt to
 * load classes from the network. Android uses this class for its system class
 * loader and for its application class loader(s).
 */
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 libraryPath 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 libraryPath,
            ClassLoader parent) {
        super(dexPath, null, libraryPath, parent);
    }
}

PathClassLoader是ClassLoader的简单实现且只能加载本地的列表文件或目录，在Android中也就是已安装好的APK，它不能加载来自网络的类。Android中的系统类加载器与应用类加载器都是PathClassLoader。

PathClassLoader是ClassLoader的简单实现且只能加载本地的列表文件或目录，在Android中也就是已安装好的APK，它不能加载来自网络的类。Android中的系统类加载器与应用类加载器都是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，不管是加载那种文件，最终都是要加载dex文件，在这里为了方便理解，我们将dex文件以及包含dex的apk文件或jar文件统称为dex相关文件。PathClassLoader不建议开发直接使用。PathClassLoader继承自BaseDexClassLoader，很明显PathClassLoader的方法实现都在BaseDexClassLoader中。从PathClassLoader的构造方法也可以看出它遵循了双亲委托模式。

dexPath：包含dex文件的JAR/ZIP/APK文件的路径

librarySearchPath：native library文件的路径

parent：父类加载器

DexClassLoader

/**
 * A class loader that loads classes from {@code .jar} and {@code .apk} files
 * containing a {@code classes.dex} entry. This can be used to execute code not
 * installed as part of an application.
 *
 * <p>This class loader requires an application-private, writable directory to
 * cache optimized classes. Use {@code Context.getDir(String, int)} to create
 * such a directory: <pre>   {@code
 *   File dexOutputDir = context.getDir("dex", 0);
 * }</pre>
 *
 * <p><strong>Do not cache optimized classes on external storage.</strong>
 * External storage does not provide access controls necessary to protect your
 * application from code injection attacks.
 */
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 libraryPath 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 libraryPath, ClassLoader parent) {
        super(dexPath, new File(optimizedDirectory), libraryPath, parent);
    }
}

DexClassLoader可以从包含dex文件的JAR或APK中来加载类，而这些代码源允许不必是安装应用的一部分，因此可用于动态加载。

构造方法中有一个新的参数为optimizedDirectory，它表示优化后的dex文件要写入的路径。通常建议使用如下路径：

context.getCodeCacheDir();

总结

• 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.