• PatchClassLoader

/*** 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).*/

这个类被用作系统类加载器和应用类(已安装的应用)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.*/

注释可以看出，这个类是可以用来从.jar文件和.apk文件中加载classed.dex，可以用来执行没有安装的程序代码。

通过上面的两个注释可以清楚这两个类的作用了，很显然我们要用的是DexClassLoader，对插件化了解的小伙伴们对这个类肯定不会陌生的，对插件化不了解的也没关系。下面会更详细的介绍。

我们知道了PathClassLoader和DexClassLoader的应用场景，接下来看一下是如何加载类的，看上面的继承关系这里两个类都是继承自BaseDexClassLoader，所以查找类的方法也在BaseDexClassLoader中，下面是部分源码：

/*** Base class for common functionality between various dex-based* {@link ClassLoader} implementations.*/
public class BaseDexClassLoader extends ClassLoader {/** structured lists of path elements */private final DexPathList pathList;//...some code@Overrideprotected Class<?> findClass(String name) throws ClassNotFoundException {Class clazz = pathList.findClass(name);if (clazz == null) {throw new ClassNotFoundException(name);}return clazz;}//...some code
}

可以看到在findClass()方法中用到了pathList.findClass(name)，而pathList的类型是DexPathList，下面看一下DexPathList的findClass()方法源码：

/*** A pair of lists of entries, associated with a {@code ClassLoader}.* One of the lists is a dex/resource path — typically referred* to as a "class path" — list, and the other names directories* containing native code libraries. Class path entries may be any of:* a {@code .jar} or {@code .zip} file containing an optional* top-level {@code classes.dex} file as well as arbitrary resources,* or a plain {@code .dex} file (with no possibility of associated* resources).** <p>This class also contains methods to use these lists to look up* classes and resources.</p>*/
/*package*/ final class DexPathList {/** list of dex/resource (class path) elements */private final Element[] dexElements;/*** Finds the named class in one of the dex files pointed at by* this instance. This will find the one in the earliest listed* path element. If the class is found but has not yet been* defined, then this method will define it in the defining* context that this instance was constructed with.** @return the named class or {@code null} if the class is not* found in any of the dex files*/public Class findClass(String name) {for (Element element : dexElements) {DexFile dex = element.dexFile;if (dex != null) {Class clazz = dex.loadClassBinaryName(name, definingContext);if (clazz != null) {return clazz;}}}return null;}
}

这个方法里面有调用了dex.loadClassBinaryName(name, definingContext)，然后我们来看一下DexFile的这个方法：

/*** Manipulates DEX files. The class is similar in principle to* {@link java.util.zip.ZipFile}. It is used primarily by class loaders.* <p>* Note we don't directly open and read the DEX file here. They're memory-mapped* read-only by the VM.*/
public final class DexFile {/*** See {@link #loadClass(String, ClassLoader)}.** This takes a "binary" class name to better match ClassLoader semantics.** @hide*/public Class loadClassBinaryName(String name, ClassLoader loader){return defineClass(name, loader, mCookie);}private native static Class defineClass(String name, ClassLoader loader, int cookie);
}

好了，关联的代码全部贴上了，理解起来并不难，总结一下流程：BaseDexClassLoader中有一个DexPathList对象pathList，pathList中有个Element数组dexElements（Element是DexPathList的静态内部类，在Element中会保存DexFile的对象），然后遍历Element数组，通过DexFile对象去查找类。

更通俗的说：

一个ClassLoader可以包含多个dex文件，每个dex文件是一个Element，多个dex文件排列成一个有序的数组dexElements，当找类的时候，会按顺序遍历dex文件，然后从当前遍历的dex文件中找类，如果找类则返回，如果找不到从下一个dex文件继续查找。(出自安卓App热补丁动态修复技术介绍)

so，通过上面介绍，我们可以将patch.jar(补丁包)，放在dexElements数组的第一个元素，这样优先找到我们patch.jar中的新类去替换之前存在bug的类。