类加载器ClassLoader和双亲委托模型

@Override
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ClassLoader classLoader = getClassLoader();
        if (classLoader != null){
            Log.i("ClassLoader", "classLoader " + " : " + classLoader.toString());
            while (classLoader.getParent()!=null){
                classLoader = classLoader.getParent();
                Log.i("ClassLoader" ,"classLoader " + " : " + classLoader.toString());
            }
        }
}

classLoader : dalvik.system.PathClassLoader[DexPathList[[zip file "/data/app/com.rocka.classloader/base.apk"],nativeLibraryDirectories=[/vendor/lib, /system/lib]]]
classLoader : java.lang.BootClassLoader@36af4e44

protected ClassLoader(ClassLoader parent) {
   this(checkCreateClassLoader(), parent);
}

// 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);
    }
}

/**
 * dexPath: 需要被加载的dex文件地址，可以多个，用File.pathSeparator分割
 * optimizedDirectory: 优化之后的dex存放路径,不可以为null。
 * libraryPath：包含libraries的目录列表，同样用File.pathSeparator分割，如果没有则传null就行了
 * parent: 父类的类加载器
 */
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
            String libraryPath, ClassLoader parent) {
        super(parent);
        this.originalPath = dexPath;
        this.pathList = new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}

/**
 * @param optimizedDirectory optimizedDirectory 如果为空，那么使用系统默认的文件夹
 */
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;
        //赋值了一个dexElements的数组，存放我们dex文件的数组
        this.dexElements =
            makeDexElements(splitDexPath(dexPath), optimizedDirectory);
        this.nativeLibraryDirectories = splitLibraryPath(libraryPath);
}

/**
  * Makes an array of dex/resource path elements, one per element of
  * the given array.
  */
private static Element[] makePathElements(List<File> files, File optimizedDirectory,List<IOException> suppressedExceptions) {
        List<Element> elements = new ArrayList<>();
        /*
         * Open all files and load the (direct or contained) dex files
         * up front.
         */
        for (File file : files) {
            File zip = null;
            File dir = new File("");
            DexFile dex = null;
            String path = file.getPath();
            String name = file.getName();

            if (path.contains(zipSeparator)) {
                String split[] = path.split(zipSeparator, 2);
                zip = new File(split[0]);
                dir = new File(split[1]);
            } else if (file.isDirectory()) {
                // We support directories for looking up resources and native libraries.
                // Looking up resources in directories is useful for running libcore tests.
                elements.add(new Element(file, true, null, null));
            } 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) {
                        /*
                         * 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  the tea-leaves for
                         * when findClass returns null.
                         */
                        suppressedExceptions.add(suppressed);
                    }
                }
            } else {
                System.logW("ClassLoader referenced unknown path: " + file);
            }

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

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

public Class findClass(String name, List<Throwable> 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;
}

public Class loadClassBinaryName(String name, ClassLoader loader) {
   return defineClass(name, loader, mCookie);
}
private native static Class defineClass(String name, ClassLoader loader, int cookie);