Android中的ClassLoader(PathClassLoader和DexClassLoder)

最近在研究插件动态加载,记录一下其中ClassLoader部分的内容.

Android中的ClassLoader与Java有些不同,Android中ClassLoader加载的是dex文件,而Java中加载的是jar文件.相同的是两者都采用了双亲委派模型.ClassLoaderloadClass函数体现了这个模型:

protected Class<?> loadClass(String className, boolean resolve) 
    throws ClassNotFoundException {
    Class<?> clazz = findLoadedClass(className);// 首先检查Class是否已经加载过
    if (clazz == null) {
        ClassNotFoundException suppressed = null;
        try {
            clazz = parent.loadClass(className, false);//尝试使用parent ClassLoader去加载Class
        } catch (ClassNotFoundException e) {
            suppressed = e;
        }

        if (clazz == null) {
            try {
                clazz = findClass(className);// 如果加载不成功,调用findClass函数来获取class对象.
            } catch (ClassNotFoundException e) {
                e.addSuppressed(suppressed);
                throw e;
            }
        }
    }
    return clazz;
}

其中findClass()为protected,直接抛出异常,需要子类来实现具体的find过程.Android中ClassLoader的子类是BaseDexClassLoder,同时有PathClassLoaderDexClassLoder两个类直接继承自BaseDexClassLoder.

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

DexClassLoaderPathClassLoader都直接调用BaseDexClassLoader的构造函数,区别是传入的参数不一样,PathClassLoader的super调用中,传入的optimizedDirectory一直为null.BaseDexClassLoder的构造函数使用入参的三个路径构造了一个DexPathList对象.同时findClass()直接使用DexPathListfindClass()函数.看一下DexPathList的源码:

/*package*/ final class DexPathList {
    /** list of dex/resource (class path) elements */
    private final Element[] dexElements;

    public DexPathList(ClassLoader definingContext, String dexPath,
            String libraryPath, File optimizedDirectory) {
        // some error checking
        this.definingContext = definingContext;
        this.dexElements =
            makeDexElements(splitDexPath(dexPath), optimizedDirectory);
        this.nativeLibraryDirectories = splitLibraryPath(libraryPath);
    }
    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;
    }
    /*package*/ static class Element {
        public final File file;
        public final ZipFile zipFile;
        public final DexFile dexFile;
    }
}

DexPathList的构造函数将传入的路径转化为Element数组,findClass()中遍历Element中的DexFile来加载Class,看一下Element数组的生成过程:

/**
 * Makes an array of dex/resource path elements, one per element of
 * the given array.
 */
private static Element[] makeDexElements(ArrayList<File> files,
        File optimizedDirectory) {
    ArrayList<Element> elements = new ArrayList<Element>();
    for (File file : files) {
        ZipFile zip = null;
        DexFile dex = null;
        String name = file.getName();
        if (name.endsWith(DEX_SUFFIX)) { // 文件后缀为.dex
            try {
                dex = loadDexFile(file, optimizedDirectory);//1.加载dex文件
            } 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)) {// 文件为压缩包
            try {
                zip = new ZipFile(file);//构造zip对象
            } catch (IOException ex) {
                System.logE("Unable to open zip file: " + file, ex);
            }
            try {
                dex = loadDexFile(file, optimizedDirectory);//2.加载dex文件
            } catch (IOException ignored) {
                 // 如果压缩文件中没有dex文件,抛出这个异常,可以直接无视
            }
        } else {
            System.logW("Unknown file type for: " + file);
        }
        if ((zip != null) || (dex != null)) {//如果同时有zip和dex文件,就构造对应的Element
            elements.add(new Element(file, zip, dex)); 
        }
    }
    return elements.toArray(new Element[elements.size()]);
}

调用了两次loadDexFile()来生成dex对象,第一次的file为dex文件,第二次的file是压缩文件(zip,jar,apk).看一下对应的代码:

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);
    }
}

根据optimizedDirectory是否为null调用不同的方法来构造DexFile.这两种构造方法最后都会调用openDexFile()这个native函数.其中outputName就是optimizedDirectory.回到前面看,PathClassLoaderDexClassLoader的区别是optimizedDirectory是否为null,对应openDexFile()outputName是否为null.

// java code
native private static int openDexFile(String sourceName, String outputName,
    int flags) throws IOException;

// native code
static void Dalvik_dalvik_system_DexFile_openDexFile(const u4* args,
    JValue* pResult)
{
    // ....
    if (hasDexExtension(sourceName)
            && dvmRawDexFileOpen(sourceName, outputName, &pRawDexFile, false) == 0) {
        ALOGV("Opening DEX file '%s' (DEX)", sourceName);
        pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
        pDexOrJar->isDex = true;
        pDexOrJar->pRawDexFile = pRawDexFile;
        pDexOrJar->pDexMemory = NULL;
    } else if (dvmJarFileOpen(sourceName, outputName, &pJarFile, false) == 0) {
        ALOGV("Opening DEX file '%s' (Jar)", sourceName);
        pDexOrJar = (DexOrJar*) malloc(sizeof(DexOrJar));
        pDexOrJar->isDex = false;
        pDexOrJar->pJarFile = pJarFile;
        pDexOrJar->pDexMemory = NULL;
    } else {
        ALOGV("Unable to open DEX file '%s'", sourceName);
        dvmThrowIOException("unable to open DEX file");
    }
    //....
}

加载dex文件的过程在dvmRawDexFileOpen()dvmJarFileOpen()方法中完成.看一下dvmJarFileOpen()的代码:

int dvmJarFileOpen(const char* fileName, const char* odexOutputName,
    JarFile** ppJarFile, bool isBootstrap)
{
tryArchive:
        entry = dexZipFindEntry(&archive, kDexInJarName);
        if (entry != NULL) {
            if (odexOutputName == NULL) {
                cachedName = dexOptGenerateCacheFileName(fileName,
                                kDexInJarName);
                if (cachedName == NULL)
                    goto bail;
            } else {
                cachedName = strdup(odexOutputName);
            }
        }
bail:
        /* clean up, closing the open file */
        if (archiveOpen && result != 0)
            dexZipCloseArchive(&archive);
        free(cachedName);
        if (fd >= 0) {
            if (locked)
                (void) dvmUnlockCachedDexFile(fd);
            close(fd);
        }
        return result;
    }

odexOutputNamenull的情况下,会直接返回一个ERROR,表示加载失败。

所以PathClassLoader只能加载dex格式的文件,而DexClassLoader可以加载dex文件,对于apk等压缩文件,加载过程会将压缩文件中的class.dex解压到optimizedDirectory目录中进行加载。

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