Android中的ClassLoader
1. Android中有哪几种ClassLoader？它们的作用和区别是什么？
有PathClassLoader, DexClassLoader, BootClassLoader
PathClassLoader: 用来加载系统类和应用程序代码(具体见PathClassLoader的构造方法的注释),只能本地文件系统上的文件和目录，而不能是网络上的
DexClassLoader: 用来加载dex,jar,apk,可以加载未安装在应用程序上的代码(DexClassLoader需要提供一个应用私有的可写入的目录，用来存放被优化的classes)(具体可以看DexClassLoader构造方法上英文的注释文档)
PathClassLoader, DexClassLoader 都是继承自 BaseDexClassLoader，两者都是调用的BaseDexClassLoader的方法
/**
* 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 PathClassLoader(String dexPath, String libraryPath,
ClassLoader parent) {
super(dexPath, null, libraryPath, parent);
}
/**
* 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.
*
* This class loader requires an application-private, writable directory to
* cache optimized classes. Use {@code Context.getDir(String, int)} to create
* such a directory:
{@code
* File dexOutputDir = context.getDir("dex", 0);
* }
*
* Do not cache optimized classes on external storage.
* External storage does not provide access controls necessary to protect your
* application from code injection attacks.
*/
public DexClassLoader(String dexPath, String optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(dexPath, new File(optimizedDirectory), libraryPath, parent);
}
区别就在于optimizedDirectory是否为null。具体可以看BaseDexClassLoader的构造方法
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(parent);
this.originalPath = dexPath;
this.pathList =
new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}
从这里我们找到了DexPathList类，那么还需要看看DexPathList是干嘛的
public DexPathList(ClassLoader definingContext, String dexPath,
String libraryPath, File optimizedDirectory) {
...
this.dexElements =
makeDexElements(splitDexPath(dexPath), optimizedDirectory);
}
/**
* Makes an array of dex/resource path elements, one per element of
* the given array.
*/
private static Element[] makeDexElements(ArrayList files,
File optimizedDirectory) {
ArrayList elements = new ArrayList();
for (File file : files) {
...
dex = loadDexFile(file, optimizedDirectory);
}
return elements.toArray(new Element[elements.size()])
}
private static DexFile loadDexFile(File file, File optimizedDirectory)
throws IOException {
if (optimizedDirectory == null) {
return new DexFile(file);
} else {
String optimizedPath = optimizedPathFor(file, optimizedDirectory);
return DexFile.loadDex(file.getPath(), optimizedPath, 0);
}
}
...
/**
* Opens a DEX file from a given File object. This will usually be a ZIP/JAR
* file with a "classes.dex" inside.
*
* The VM will generate the name of the corresponding file in
* /data/dalvik-cache and open it, possibly creating or updating
* it first if system permissions allow. Don't pass in the name of
* a file in /data/dalvik-cache, as the named file is expected to be
* in its original (pre-dexopt) state.
*
* @param file
* the File object referencing the actual DEX file
*
* @throws IOException
* if an I/O error occurs, such as the file not being found or
* access rights missing for opening it
*/
public DexFile(File file) throws IOException {
this(file.getPath());
}
DexPathList主要是加载全部的dex文件，内部通过dexElements属性维护全部的dex
我们还需要看一下DexPathList类的loadDexFile方法，里面有我们关心的一个参数optimizedDirectory，optimizedDirectory为null时，构造一个DexFile对象，不为null的时候,直接通过DexFile.loadDex(sourcePathName, outputPathName, flag)加载一个DexFile文件
先来看看DexFile.loadDex方法
static public DexFile loadDex(String sourcePathName, String outputPathName,
int flags) throws IOException {
return new DexFile(sourcePathName, outputPathName, flags);
}
public DexFile(String fileName) throws IOException {
mCookie = openDexFile(fileName, null, 0);
mFileName = fileName;
guard.open("close");
//System.out.println("DEX FILE cookie is " + mCookie);
}
private DexFile(String sourceName, String outputName, int flags) throws IOException {
mCookie = openDexFile(sourceName, outputName, flags);
mFileName = sourceName;
guard.open("close");
//System.out.println("DEX FILE cookie is " + mCookie);
}
本质上还是调用的DexFile的构造方法，那么不同之处在哪，在于构造方法里面openDexFile传入的参数outputName,对于optimizedDirectory为null，传入的outputName为null，这是为什么呢，因为PathClassLoader加载的是已经安装的应用，这部分classes已经被optimized处理过了，位于/data/dalvik-cache/目录下,不需要在为outputName指定路径了，而对于DexClassLoader而言，加载的是没有安装过的dex文件，那么是需要将优化的classes存储到应用的私有目录。
BootClassLoader: 继承的是ClassLoader类，作为PathClassLoader的父类加载器存在(也是Android中类加载器的根节点)，也是一个package级别的类，对外获取不到
2. 简述ClassLoader的双亲委托模型
双亲委托模型，classLoader用于将一个class文件加载到虚拟机中，供程序运行时使用，动态加载。
在加载class的过程中，首先会先判断父加载器有没有加载这个类，如果加载了，那么由父加载器返回这个class对象，如果没有的话，那么就由当前的类加载器加载。(父加载器也是按照这个过程来)
这样做的好处是避免重复加载class，安全
3. 简述双亲委托模型在热修复领域的应用
首先我们需要看一下，在Android中，如何加载一个类，看看ClassLoader的loadClass方法
protected Class loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
...
c = findClass(name);
...
}
//对于android应用程序中加载class的加载器，我们确定无疑就是PathClassLoader,而PathClassLoader继承自BaseDexClassLoader，BaseDexClassLoader的findClass方法是重写过的
@Override
protected Class findClass(String name) throws ClassNotFoundException {
Class clazz = pathList.findClass(name);
if (clazz == null) {
throw new ClassNotFoundException(name);
}
return clazz;
}
pathList是什么呢，它的类型是DexPathList，DexPathList有一个成员变量叫做dexElements，维护了全部的dex文件，而我们要查找的类都是通过遍历这个dexElements数组，来获取到的。
在热修复中，我们可以将修复后的bugFixdex插入到这个dexElements的最前面，那么在查找某一个类A(存在bug的话)的时候，就优先从bugFixdex中查找到，并返回，而不会使用dexElements里面原来有问题的类A了，这样就达到了修复bug的问题