Android类加载之PathClassLoader和DexClassLoader

北京的初雪.jpg

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

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    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:

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    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。

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    @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

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    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

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    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

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    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

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        /**
    * 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:
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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:

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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。

dalvik-cache.jpg

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。

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文章目录
  1. 1. 前言
  2. 2. Android系统中类加载的双亲委派机制
  3. 3. Android系统中的类加载器
  • 提出问题
    1. 1. BaseDexClassLoader类加载
    2. 2. PathClassLoader和DexClassLoader比较
  • 解决疑问
    1. 1. 总结
  • ,