ClassLoader原理剖析

ClassLoader原理剖析

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Android 中 ClassLoader 种类

BootClassLoader

加载 Android FrameWork 层的class字节码文件

PathClassLoader

加载已经安装到系统中的apk 文件中的class 字节码文件

DexClassLoader

加载指定目录中的 class字节码文件

BaseClassLoader

是 PathClassLoader 和 DexClassLoader 的父类

Android 中 ClassLoader 的特点

• 双亲代理模式的特点
• 类加载的共享功能
• 类加载的隔离功能

ClassLoader源码分析

protected Class loadClass(String className, boolean resolve) throws ClassNotFoundException {
        //检测当前类是否被加载
        Class clazz = findLoadedClass(className);

        if (clazz == null) {
            ClassNotFoundException suppressed = null;
            try {
                clazz = parent.loadClass(className, false);
            } catch (ClassNotFoundException e) {
                suppressed = e;
            }

            if (clazz == null) {
                try {
                    clazz = findClass(className);
                } catch (ClassNotFoundException e) {
                    e.addSuppressed(suppressed);
                    throw e;
                }
            }
        }

        return clazz;
    }
//空实现，交由子类去实现
protected Class findClass(String className) throws ClassNotFoundException {
        throw new ClassNotFoundException(className);
    }  
// A class loader that loads classes from {@code .jar} and {@code .apk} files containing a {@code classes.dex} entry.
//加载未安装到系统中的应用的类，动态加载的核心
public class DexClassLoader extends BaseDexClassLoader {

    public DexClassLoader(String dexPath, String optimizedDirectory,String libraryPath, ClassLoader parent) {
       super(dexPath, new File(optimizedDirectory), libraryPath, parent);
    }
}
//加载指定路径下的 class 文件 
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);
    }
}

public class BaseDexClassLoader extends ClassLoader {
    private final DexPathList pathList;

    public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        super(parent);
        this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
    }

    @Override
    protected Class findClass(String name) throws ClassNotFoundException {
        List suppressedExceptions = new ArrayList();
        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;
    }

    @Override
    protected URL findResource(String name) {
        return pathList.findResource(name);
    }

    @Override
    protected Enumeration findResources(String name) {
        return pathList.findResources(name);
    }

    @Override
    public String findLibrary(String name) {
        return pathList.findLibrary(name);
    }

    @Override
    protected synchronized Package getPackage(String name) {
        if (name != null && !name.isEmpty()) {
            Package pack = super.getPackage(name);

            if (pack == null) {
                pack = definePackage(name, "Unknown", "0.0", "Unknown",
                        "Unknown", "0.0", "Unknown", null);
            }

            return pack;
        }

        return null;
    }

    /**
     * @hide
     */
    public String getLdLibraryPath() {
        StringBuilder result = new StringBuilder();
        for (File directory : pathList.getNativeLibraryDirectories()) {
            if (result.length() > 0) {
                result.append(':');
            }
            result.append(directory);
        }

        return result.toString();
    }

    @Override public String toString() {
        return getClass().getName() + "[" + pathList + "]";
    }
}
final class DexPathList {
    private static final String DEX_SUFFIX = ".dex";
    private static final String zipSeparator = "!/";
    ...
        public Class findClass(String name, List 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;
    ...
    }
}

Android 中的动态加载难点

• 有许多组件类需要注册后才能使用
• 资源的动态加载很复杂
• Android 程序运行需要一个上下文环境