android类加载过程,Android类加载机制

缺乏、安全感 2022-10-17 15:58 321阅读 0赞

ClassLoader

Android 中使用PathClassLoader、DexClassLoader等来实现类的加载，这两个类都继承自BaseDexClassLoader，而BaseDexClassLoader继承自ClassLoader。

从Android官方文档中可知：PathClassLoader用来加载系统类及已安装的应用程序的类；DexClassLoader用来加载jar、apk、dex文件中的类。

ClassLoader使用了一种称为“双亲委托”的机制：

public abstract class ClassLoader \{

private final ClassLoader parent;

protected Class> loadClass(String name, boolean resolve)

throws ClassNotFoundException

Class c = findLoadedClass(name);

if (c == null) \{

try \{

if (parent != null) \{

c = parent.loadClass(name, false);

\} else \{

c = findBootstrapClassOrNull(name);

\} catch (ClassNotFoundException e) \{

if (c == null) \{

c = findClass(name);

return c;

调用loadClass方法来加载类时，首先使用findLoadedClass来获取已经被加载到内存中的类，如果类已经加载过，就会直接返回已加载的类；否则，先去调用parent的loadClass方法来加载类，parent无法加载时才会调用自身的findClass方法来加载。

BaseDexClassLoader

public class BaseDexClassLoader extends ClassLoader \{

private final DexPathList pathList;

public BaseDexClassLoader(String dexPath, File optimizedDirectory,

String libraryPath, ClassLoader parent) \{

super(parent);

this.originalPath = dexPath;

this.pathList =

new DexPathList(this, dexPath, libraryPath, optimizedDirectory);

@Override

protected Class> findClass(String name) throws ClassNotFoundException \{

Class clazz = pathList.findClass(name);

if (clazz == null) \{

throw new ClassNotFoundException(name);

return clazz;

BaseDexClassLoader中findClass方法将类的加载委托给DexPathList类型的pathList处理，下面看下DexPathList

DexPathList

DexPathList类关键方法如下：

final class DexPathList \{

private static final String DEX\_SUFFIX = ".dex";

private static final String JAR\_SUFFIX = ".jar";

private static final String ZIP\_SUFFIX = ".zip";

private static final String APK\_SUFFIX = ".apk";

private final ClassLoader definingContext;

private final Element\[\] dexElements;

public DexPathList(ClassLoader definingContext, String dexPath,

String libraryPath, File optimizedDirectory) \{

this.definingContext = definingContext;

this.dexElements =

makeDexElements(splitDexPath(dexPath), optimizedDirectory);

private static Element\[\] makeDexElements(ArrayList files,

File optimizedDirectory) \{

ArrayList elements = new ArrayList();

for (File file : files) \{

ZipFile zip = null;

DexFile dex = null;

String name = file.getName();

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 if (name.endsWith(APK\_SUFFIX) || name.endsWith(JAR\_SUFFIX)

|| name.endsWith(ZIP\_SUFFIX)) \{

try \{

zip = new ZipFile(file);

\} catch (IOException ex) \{

System.logE("Unable to open zip file: " + file, ex);

try \{

dex = loadDexFile(file, optimizedDirectory);

\} catch (IOException ignored) \{

\} else \{

System.logW("Unknown file type for: " + file);

if ((zip != null) || (dex != null)) \{

elements.add(new Element(file, zip, dex));

return elements.toArray(new Element\[elements.size()\]);

DexPathList构造函数参数dexPath包含了dex文件的列表，这些dex文件可以是jar、zip、apk格式的压缩包，也可以直接是dex文件。DexPathList使用makeDexElements方法遍历这些dex文件，使用loadDexFile方法加载dex文件：

final class DexPathList \{

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);

loadDexFile方法则使用DexFile的loadDex方法打开dex文件，同时会将dex优化为odex文件，返回的是DexFile对象，对dex文件的打开文件、加载类等操作都通过DexFile来实现。

DexPathList.makeDexElements将每个dex文件对应的File、ZipFile、DexFile包装成Element变量，这些Element变量存在DexPathList的dexElements数组里。

/\*package\*/ static class Element \{

public final File file;

public final ZipFile zipFile;

public final DexFile dexFile;

public Element(File file, ZipFile zipFile, DexFile dexFile) \{

this.file = file;

this.zipFile = zipFile;

this.dexFile = dexFile;

public URL findResource(String name) \{

if ((zipFile == null) || (zipFile.getEntry(name) == null)) \{

return null;

try \{

return new URL("jar:" + file.toURL() + "!/" + name);

\} catch (MalformedURLException ex) \{

throw new RuntimeException(ex);

DexPathList类里加载class如下：

final class DexPathList \{

public Class findClass(String name) \{

for (Element element : dexElements) \{

DexFile dex = element.dexFile;

if (dex != null) \{

Class clazz = dex.loadClassBinaryName(name, definingContext);

if (clazz != null) \{

return clazz;

return null;

DexPathList加载类时，遍历Element数组，使用element.dexFile的loadClassBinaryName方法来实际加载类：

public final class DexFile \{

public Class loadClassBinaryName(String name, ClassLoader loader) \{

return defineClass(name, loader, mCookie);

private native static Class defineClass(String name, ClassLoader loader, int cookie);

而loadClassBinaryName方法最终调用的是 native 方法defineClass。

总结

DexClassLoader或PathClassLoader加载类的过程：

DexClassLoader.loadClass ->

DexClassLoader.findClass ->

BaseDexClassLoader.findClass ->

DexPathList.findClass ->

DexFile.loadClassBinaryName ->

DexFile.defineClass

参考

android类加载过程,Android类加载机制

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