Android中的ClassLoader _classloader

智者不为愚者谋，勇者不为怯者死。这篇文章主要讲述Android中的ClassLoader相关的知识，希望能为你提供帮助。
android中有哪几种ClassLoader?他们的作用是什么?与java 类似,Android 中也有相应的类加载机制,只是java 加载的是class 字节码文件.而Android中记载的是dex 字节码,继承自ClassLoader 抽象类有以下几种:

BootClassLoader ,是ClassLoader 的内部类,在系统启动时用来加载一些系统相关的类
?
PathClassLoader
官方说明:

提供一个简单的ClassLoader 实现，用来操作文件列表和本地文件系统中的文件和目录，但不会尝试从网络加载类。Android 使用这个类作为它的系统类加载器和其应用程序类加载器。
DexClassLoader
官方说明:

一个类加载器，用于加载包含一个classes.dex 条目的.jar 和.apk 文件中的类,这个可以用来执行不是作为应用程序安装的代码.

PathClassLoader 和DexClassLoader 都继承自BaseDexClassLoader ,接下来看一下源码

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); } }

DexClassLoader 只有一个构造方法,基本实现都在BaseDexClassLoader 中,
参数列表

dexPath包含jar 或apk 文件路径的字符串,多个文件之间用":"隔开
optimizedDirectory : 优化后的dex 存储路径.
librarySearchPath : 本地库文件路径
parent : 关联的父类加载器

接下来我们看一下BaseDexClassLoader ,

public class BaseDexClassLoader extends ClassLoader { private final DexPathList pathList; /** * 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, String librarySearchPath, ClassLoader parent) { super(parent); this.pathList = new DexPathList(this, dexPath, librarySearchPath,optimizedDirectory); }

在BaseDexClassLoader 的构造方法中实例化了一个DexPathList ,接下来看看这个类是干嘛的

public DexPathList(ClassLoader definingContext, String dexPath, String librarySearchPath, File optimizedDirectory) {if (definingContext == null) { throw new NullPointerException("definingContext == null"); }if (dexPath == null) { throw new NullPointerException("dexPath == null"); }if (optimizedDirectory != null) { if (!optimizedDirectory.exists()){ throw new IllegalArgumentException( "optimizedDirectory doesn‘t exist: " + optimizedDirectory); }if (!(optimizedDirectory.canRead() & & optimizedDirectory.canWrite())) { throw new IllegalArgumentException( "optimizedDirectory not readable/writable: " + optimizedDirectory); } }this.definingContext = definingContext; ArrayList< IOException> suppressedExceptions = new ArrayList< IOException> (); // save dexPath for BaseDexClassLoader this.dexElements = makeDexElements(splitDexPath(dexPath),optimizedDirectory, suppressedExceptions,definingContext);

调用makeDexElements() 初始化dexElements

private static Element[] makeElements(List< File> files, File optimizedDirectory, List< IOException> suppressedExceptions, boolean ignoreDexFiles, ClassLoader loader) { Element[] elements = new Element[files.size()]; int elementsPos = 0; /* * Open all files and load the (direct or contained) dex files * up front. */ //遍历文件列表 for (File file : files) { File zip = null; File dir = new File(""); DexFile dex = null; String path = file.getPath(); String name = file.getName(); if (path.contains(zipSeparator)) { String split[] = path.split(zipSeparator, 2); zip = new File(split[0]); dir = new File(split[1]); } else if (file.isDirectory()) { // We support directories for looking up resources and native libraries. // Looking up resources in directories is useful for running libcore tests. elements[elementsPos++] = new Element(file, true, null, null); } else if (file.isFile()) { if (!ignoreDexFiles & & name.endsWith(DEX_SUFFIX)) { // Raw dex file (not inside a zip/jar). try { //生成DexFile dex = loadDexFile(file, optimizedDirectory, loader, elements); } catch (IOException suppressed) { System.logE("Unable to load dex file: " + file, suppressed); suppressedExceptions.add(suppressed); } } else { zip = file; if (!ignoreDexFiles) { try { dex = loadDexFile(file, optimizedDirectory, loader, elements); } catch (IOException suppressed) { /* * IOException might get thrown "legitimately" by the DexFile constructor if * the zip file turns out to be resource-only (that is, no classes.dex file * in it). * Let dex == null and hang on to the exception to add to the tea-leaves for * when findClass returns null. */ suppressedExceptions.add(suppressed); } } } } else { System.logW("ClassLoader referenced unknown path: " + file); }if ((zip != null) || (dex != null)) { //根据zip或dex文件新建Element对象,并添加到数组中 elements[elementsPos++] = new Element(dir, false, zip, dex); } } if (elementsPos != elements.length) { elements = Arrays.copyOf(elements, elementsPos); } return elements; }

loadDexFile 是如何生成DexFile 实例的?

private static DexFile loadDexFile(File file, File optimizedDirectory, ClassLoader loader, Element[] elements) throws IOException { if (optimizedDirectory == null) { //如果优化dex路径为空,直接调用构造方法 return new DexFile(file, loader, elements); } else { //根据优化文件夹路径和文件名生成文件路径 String optimizedPath = optimizedPathFor(file, optimizedDirectory); return DexFile.loadDex(file.getPath(), optimizedPath, 0, loader, elements); } }

static DexFile loadDex(String sourcePathName, String outputPathName, int flags, ClassLoader loader, DexPathList.Element[] elements) throws IOException {/* * TODO: we may want to cache previously-opened DexFile objects. * The cache would be synchronized with close().This would help * us avoid mapping the same DEX more than once when an app * decided to open it multiple times.In practice this may not * be a real issue. */ return new DexFile(sourcePathName, outputPathName, flags, loader, elements); }

这里待优化,所以最后还是调用了构造函数

#BaseDexClassLoader#findClass @Override protected Class< ?> findClass(String name) throws ClassNotFoundException { List< Throwable> suppressedExceptions = new ArrayList< Throwable> (); 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; }

重点关注BaseClassLoader 是如何加载类,通过类加载的双亲委派模型,最终会调用该ClassLoader
【Android中的ClassLoader】的findClass 方法,这里又调用了pathList 的findClass ,

#DexPathList#findClass public Class findClass(String name, List< Throwable> suppressed) { for (Element element : dexElements) { DexFile dex = element.dexFile; 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; }

可以发现是通过遍历dexElements 数组来加载类的.
由于类加载是按需加载的,现在的热修复技术基本上都是基于此,通过加载修复好的dex 文件,并将其放到dexElement 数组前面,下次加载就能够加载到修复好的类了.
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