安卓classloader浅析

classloader 是采用双亲委派的方式加载所需要的类。

 protected Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException {
        synchronized(this.getClassLoadingLock(var1)) {
            Class var4 = this.findLoadedClass(var1);
            if(var4 == null) {
                ...
                try {
                    if(this.parent != null) {
                        var4 = this.parent.loadClass(var1, false);
                    } else {
                        var4 = this.findBootstrapClassOrNull(var1);
                    }
                } catch (ClassNotFoundException var10) {
                    ;
                }
                if(var4 == null) {
                    ...
                    var4 = this.findClass(var1);
                    ....
                }
            }
            if(var2) {
                this.resolveClass(var4);
            }
            return var4;
        }
    }

双亲委派:从classloader的源码分析,在加载类的时候,先在自己已加载的类中查找该类,加载过了就直接返回;如果没有找到就调用父classloader的loadClass方法,如果找到了就直接返回,以此类推,直到
从 bootstrapclass 中加载。如果仍未找到就调用自身的findClass查找相关的类,这是个待实现的方法,需要具体的 classloader 子类自己实现。

Android 中 classloader 大致分以下几种。
BootClassLoader ,PathClassLoader 和 DexClassLoader

// 位于类加载器链的头部,负责将请求委托给VM内部类加载机制
BootClassLoader (PathClassLoader的父classloader)
    // 系统类加载器,是新类加载器的 parent,通常用于启动应用程序时。
    ->PathClassLoader (在ApplicationLoaders.getClassLoader()中,调用ClassLoader.getSystemClassLoader()时返回)

1、PathClassLoader 、DexClassLoader的源码:

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

// PathClassLoader.java
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);
    }
}

从以上源码可以看到,两个类很相似,都集成自 BaseDexClassLoader,细微之处就是 PathClassLoader 没有传 optimizedDirectory 这个参数。

2、 BaseDexClassLoader 的源码:

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<Throwable> suppressedExceptions = new ArrayList<Throwable>();
        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;
    }
    ...
}

从以上代码发现 findClass 的实现是在 DexPathList 中查找相关的类。
DexPathList 的关键源码:

/*package*/ 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";
    ...
    /**
     * List of dex/resource (class path) elements.
     * Should be called pathElements, but the Facebook app uses reflection
     * to modify 'dexElements' (http://b/7726934).
     */
    private final Element[] dexElements;
    ....
    public DexPathList(ClassLoader definingContext, String dexPath,
            String libraryPath, File optimizedDirectory) {
        if (definingContext == null) {
            throw new NullPointerException("definingContext == null");
        }

        if (dexPath == null) {
            throw new NullPointerException("dexPath == null");
        }

        if (optimizedDirectory != null) {
            if (!optimizedDirectory.exists())  {
                throw new IllegalArgumentException(
                        "optimizedDirectory doesn't exist: "
                        + optimizedDirectory);
            }

            if (!(optimizedDirectory.canRead()
                            && optimizedDirectory.canWrite())) {
                throw new IllegalArgumentException(
                        "optimizedDirectory not readable/writable: "
                        + optimizedDirectory);
            }
        }

        this.definingContext = definingContext;
        ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
        this.dexElements = makeDexElements(splitDexPath(dexPath), optimizedDirectory,
                                           suppressedExceptions);
        if (suppressedExceptions.size() > 0) {
            this.dexElementsSuppressedExceptions =
                suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
        } else {
            dexElementsSuppressedExceptions = null;
        }
        this.nativeLibraryDirectories = splitLibraryPath(libraryPath);
    }
    ...
    /**
     * Splits the given dex path string into elements using the path
     * separator, pruning out any elements that do not refer to existing
     * and readable files. (That is, directories are not included in the
     * result.)
     */
    private static ArrayList<File> splitDexPath(String path) {
        return splitPaths(path, null, false);
    }
    /**
     * Helper for {@link #splitPaths}, which does the actual splitting
     * and filtering and adding to a result.
     */
    private static void splitAndAdd(String searchPath, boolean directoriesOnly,
            ArrayList<File> resultList) {
        if (searchPath == null) {
            return;
        }
        for (String path : searchPath.split(":")) {
            try {
                StructStat sb = Libcore.os.stat(path);
                if (!directoriesOnly || S_ISDIR(sb.st_mode)) {
                    resultList.add(new File(path));
                }
            } catch (ErrnoException ignored) {
            }
        }
    }
    ...
     /**
     * 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<IOException> suppressedExceptions) {
        ArrayList<Element> elements = new ArrayList<Element>();
        /*
         * Open all files and load the (direct or contained) dex files
         * up front.
         */
        for (File file : files) {
            File 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)) {
                zip = file;

                try {
                    dex = loadDexFile(file, optimizedDirectory);
                } catch (IOException suppressed) {
                    /*
                     * IOException might get thrown "legitimately" by the DexFile constructor if the
                     * zip file turns out to be resource-only (that is, no classes.dex file in it).
                     * Let dex == null and hang on to the exception to add to the tea-leaves for
                     * when findClass returns null.
                     */
                    suppressedExceptions.add(suppressed);
                }
            } else if (file.isDirectory()) {
                // We support directories for looking up resources.
                // This is only useful for running libcore tests.
                elements.add(new Element(file, true, null, null));
            } else {
                System.logW("Unknown file type for: " + file);
            }

            if ((zip != null) || (dex != null)) {
                elements.add(new Element(file, false, zip, dex));
            }
        }

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

    /**
     * Constructs a {@code DexFile} instance, as appropriate depending
     * on whether {@code optimizedDirectory} is {@code null}.
     */
    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);
        }
    }

很清楚,DexPathList 中 findClass 的策略是:把加载的dex,jar等文件转换为 Element 对象存放到ArrayList中,然后在这些 Elements 中依次查找所需要的类。

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