上一篇文章自定义ClassLoader和双亲委派机制讲述了JVM中的类的加载机制，Android也是类JVM虚拟机那么它的类加载机制是什么呢，我们来探究一下(PS：文章源码为Android5.1)。

前言

Android的Dalvik虚拟机和Java虚拟机的运行原理相同都是将对应的java类加载在内存中运行。而Java虚拟机是加载class文件，也可以将一段二进制流通过defineClass方法生产Class进行加载（PS: 自定义ClassLoader和双亲委派机制文章后面的自定义类加载器就是通过这种方式实现的）。Dalvik虚拟机加载的dex文件。dex文件是Android对与Class文件做的优化，以便于提高手机的性能。可以想象dex为class文件的一个压缩文件。dex在Android中的加载和class在jvm中的相同都是基于双亲委派模型，都是调用ClassLoader的loadClass方法加载类。

Android系统中类加载的双亲委派机制

Android5.1源码中ClassLoader的loadClass方法

protected Class<?> loadClass(String className, boolean resolve) throws ClassNotFoundException {        Class<?> clazz = findLoadedClass(className);        if (clazz == null) {            ClassNotFoundException suppressed = null;            try {                //先让父类加载器加载                clazz = parent.loadClass(className, false);            } catch (ClassNotFoundException e) {                suppressed = e;            }            //当所有父类节点的类加载器都没有找到该类时，当前加载器调用findClass方法加载。            if (clazz == null) {                try {                    clazz = findClass(className);                } catch (ClassNotFoundException e) {                    e.addSuppressed(suppressed);                    throw e;                }            }        }

想要动态加载类，可以用自定义ClassLoader和双亲委派机制中自定义ClassLoader的方法加载自己定义的class文件么？看看Android源码中的ClassLoader的findClass方法：

protected Class<?> findClass(String className) throws ClassNotFoundException {        throw new ClassNotFoundException(className);    }

这个方法直接抛出了“ClassNotFoundException”异常，所以在Android中想通过这种方式实现类的加载时不行的。

Android系统中的类加载器

Android系统屏蔽了ClassLoader的findClass加载方法，那么它自己的类加载时通过什么样的方式实现的呢？
- Android系统中有两个类加载器分别为PathClassLoader和DexclassLoader。
- PathClassLoader和DexClassLoader都是继承与BaseDexClassLoader，BaseDexClassLoader继承与ClassLoader。

提出问题

在这里我们先提一个问题Android为什么会将自己的类加载器派生出两个不同的子类，它们各自有什么用？

BaseDexClassLoader类加载

作为ClassLoader的子类，复写了父类的findClass方法。

@Override    protected Class<?> findClass(String name) throws ClassNotFoundException {        List<Throwable> suppressedExceptions = new ArrayList<Throwable>();        //在自己的成员变量DexPathList中寻找，找不到抛异常        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;    }

DexPathList的findClass方法

public Class findClass(String name, List<Throwable> suppressed) {        //循环便利成员变量dexElements，调用DexFile.loadClassBinaryName加载class        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;    }

通过以上两段代码我们可以看出，虽然Android中的ClassLoader的findClass方法的实现被取消了，但是ClassLoader的基类BaseDexClassLoader实现了findClass方法取加载指定的Class。

PathClassLoader和DexClassLoader比较

PathClassLoader

public class PathClassLoader extends BaseDexClassLoader {    public PathClassLoader(String dexPath, ClassLoader parent) {        super(dexPath, null, null, parent);    }    public PathClassLoader(String dexPath, String libraryPath,            ClassLoader parent) {        super(dexPath, null, libraryPath, parent);    }}

DexClassLoader

public class DexClassLoader extends BaseDexClassLoader {    public DexClassLoader(String dexPath, String optimizedDirectory,            String libraryPath, ClassLoader parent) {        super(dexPath, new File(optimizedDirectory), libraryPath, parent);    }}

BaseDexClassLoader的构造函数

    /**     * 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 libraryPath 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 libraryPath, ClassLoader parent) {        super(parent);        this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);    }

dexPath:指定的是dex文件地址，多个地址可以用”:”进行分隔

optimizedDirectory:制定输出dex优化后的odex文件，可以为null

libraryPath:动态库路径（将被添加到app动态库搜索路径列表中）

parent:制定父类加载器，以保证双亲委派机制从而实现每个类只加载一次。

可以看出 PathClassLoader和DexClassLoader的区别就在于构造函数中optimizedDirectory这个参数。PathClassLoader中optimizedDirectory为null，DexClassLoader中为new File(optimizedDirectory)。

optimizedDirectory的干活
BaseDexClassLoader的构造函数利用optimizedDirectory创建了一个DexPathList，看看DexPathList中optimizedDirectory:

public DexPathList(ClassLoader definingContext, String dexPath,        String libraryPath, File optimizedDirectory) {    /******部分代码省略******/    this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,                                       suppressedExceptions);    /******部分代码省略******/}private static Element[] makeDexElements(ArrayList<File> files, File optimizedDirectory,                                         ArrayList<IOException> suppressedExceptions) {   /******部分代码省略******/    for (File file : files) {        /******部分代码省略******/        if (file.isDirectory()) {           /******部分代码省略******/        } else if (file.isFile()){            if (name.endsWith(DEX_SUFFIX)) {                // Raw dex file (not inside a zip/jar).                try {                    dex = loadDexFile(file, optimizedDirectory);                } catch (IOException ex) {                    System.logE("Unable to load dex file: " + file, ex);                }            } else {                zip = file;                try {                    dex = loadDexFile(file, optimizedDirectory);                } catch (IOException suppressed) {                    suppressedExceptions.add(suppressed);                }            }        } else {            System.logW("ClassLoader referenced unknown path: " + file);        }        if ((zip != null) || (dex != null)) {            elements.add(new Element(file, false, zip, dex));        }    }    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);    }}

从这里可以看出optimizedDirectory不同生产的DexFile对象不同，我们继续看看optimizedDirectory在DexFile中的作用：

public DexFile(File file) throws IOException {    this(file.getPath());}/** * Opens a DEX file from a given filename. 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 fileName * the filename of the 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(String fileName) throws IOException {    mCookie = openDexFile(fileName, null, 0);    mFileName = fileName;    guard.open("close");    //System.out.println("DEX FILE cookie is " + mCookie + " fileName=" + fileName);}/** * Opens a DEX file from a given filename, using a specified file * to hold the optimized data. * * @param sourceName * Jar or APK file with "classes.dex". * @param outputName * File that will hold the optimized form of the DEX data. * @param flags * Enable optional features. */private DexFile(String sourceName, String outputName, int flags) throws IOException {    if (outputName != null) {        try {            String parent = new File(outputName).getParent();            if (Libcore.os.getuid() != Libcore.os.stat(parent).st_uid) {                throw new IllegalArgumentException("Optimized data directory " + parent                        + " is not owned by the current user. Shared storage cannot protect"                        + " your application from code injection attacks.");            }        } catch (ErrnoException ignored) {            // assume we'll fail with a more contextual error later        }    }    mCookie = openDexFile(sourceName, outputName, flags);    mFileName = sourceName;    guard.open("close");    //System.out.println("DEX FILE cookie is " + mCookie + " sourceName=" + sourceName + " outputName=" + outputName);}static public DexFile loadDex(String sourcePathName, String outputPathName,    int flags) throws IOException {    return new DexFile(sourcePathName, outputPathName, flags);}

从注释当中就可以看到new DexFile(file)的dex输出路径只能为/data/dalvik-cache，而DexFile.loadDex()的dex输出路径为自己输入的optimizedDirectory路径。

解决疑问

我们在文章开始提出的问题就这样一步步得到了答案。

DexClassLoader：能够加载自定义的jar/apk/dex
PathClassLoader：只能加载系统中已经安装过的apk
所以Android系统默认的类加载器为PathClassLoader，而DexClassLoader可以像JVM的ClassLoader一样提供动态加载。

总结

ClassLoader的loadClass方法保证了双亲委派机。
BaseDexClassLoader提供了两种派生类使我们可以加载自定义类。

[置顶] Android类加载之PathClassLoader和DexClassLoader

前言