基本知识
Java的类加载设计了一套双亲代理的模式，使得用户没法替换系统的核心类，从而让应用更安全。所谓双亲代理就是指，当加载类的时候首先去Bootstrap中加载类，如果没有则去Extension中加载，如果再没有才去AppClassLoader中去加载。从而实现安全和稳定。
Java ClassLoader
BootstrapClassLoader
引导类加载器 ，用来加载Java的核心库。通过底层代码来实现的，基本上只要parent为null，那就表示引导类加载器。
比如：charsets.jar、deploy.jar、javaws.jar、jce.jar、jfr.jar、jfxswt.jar、jsse.jar、management-agent.jar、plugin.jar、resources.jar、rt.jar
ExtClassLoader
拓展类加载器 ，用来加载Java的拓展的类库， ${JAVA_HOME}/jre/lib/ext/ 目录中的所有jar。
比如：cldrdata.jar、dnsns.jar、jfxrt.jar、localedata.jar、nashorn.jar、sunec.jar、sunjce_provider.jar、sunpkcs11.jar、zipfs.jar等等
AppClassLoader
系统类加载器 (不要被名字给迷惑)，用来加载Java应用中的类。一般来说自己写的类都是通过这个加载的。而Java中 ClassLoader.getSystemClassLoader() 返回的就是AppClassLoader。(Android中修改了ClassLoader的逻辑，返回的会是一个PathClassLoader)
自定义ClassLoader
用户如果想自定义ClassLoader的话，只需要继承自 java.lang.ClassLoader 即可。
ClassLoader中与加载类相关的方法：
也许你不太了解上面几个函数的区别，没关系，我们来看下源码是如何实现的。
//ClassLoader.java
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
// First, check if the class has already been loaded
Class c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
}
}
return c;
}
所以优先级大概如下:
loadClass → findLoadedClass → parent.loadClass/findBootstrapClassOrNull → findClass → defineClass
Android ClassLoader
在Android中ClassLoader主要有两个直接子类，叫做 BaseDexClassLoader 和 SecureClassLoader 。而前者有两个直接子类是 PathClassLoader 和 DexClassLoader (Android O添加了 InMemoryDexClassLoader ，略)。
我们只讨论PathClassLoader和DexClassLoader
PathClassLoader
用来加载安装了的应用中的dex文件。它也是Android里面的一个最核心的ClassLoader了。相当于Java中的那个AppClassLoader。
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);
}
}
它的实例化是通过调用 ApplicationLoaders.getClassLoader 来实现的。
它是在ActivityThread启动时发送一个BIND_APPLICATION消息后在handleBindApplication中创建ContextImpl时调用LoadedApk里面的 getResources(ActivityThread mainThread) 最后回到ActivityThread中又调用LoadedApk的 getClassLoader 生成的，具体的在LoadedApk的 createOrUpdateClassLoaderLocked 。
那么问题来了，当Android加载class的时候，LoadedApk中的ClassLoader是怎么被调用到的呢？
其实Class里面，如果你不给ClassLoader的话，它默认会去拿Java虚拟机栈里面的 CallingClassLoader ，而这个就是LoadedApk里面的同一个ClassLoader。
//Class.java
public static Class<?> forName(String className)
throws ClassNotFoundException {
return forName(className, true, VMStack.getCallingClassLoader());
}
查看VMStack的源码发现 getCallingClassLoader 其实是一个native函数，Android通过底层实现了这个。
//dalvik.system.VMStack
/**
* Returns the defining class loader of the caller's caller.
*
* @return the requested class loader, or {@code null} if this is the
* bootstrap class loader.
*/
@FastNative
native public static ClassLoader getCallingClassLoader();
底层想必最终也是拿到LoadedApk里面的ClassLoader。
DexClassLoader
它是一个可以用来加载包含dex文件的jar或者apk文件的，但是它可以用来加载非安装的apk。比如加载sdcard上面的，或者NetWork的。
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);
}
}
比如现在很流行的插件化/热补丁，其实都是通过DexClassLoader来实现的。具体思路是： 创建一个DexClassLoader，通过反射将前者的DexPathList跟系统的PathClassLoader中的DexPathList合并，就可以实现优先加载我们自己的新类，从而替换旧类中的逻辑了。
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。
