我们先从Activity的启动流程开始切入：

//位于android/app/ActivityThread.java中
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
            // ........省略代码
			//通过反射创建activity
            java.lang.ClassLoader cl = appContext.getClassLoader();
            Activity activity = mInstrumentation.newActivity(
                         cl, component.getClassName(), r.intent);
            // ........省略代码
     }

//位于android/app/Instrumentation.java
/**
     * Perform instantiation of the process's {@link Activity} object.  The
     * default implementation provides the normal system behavior.
     * 
     * @param cl The ClassLoader with which to instantiate the object.
     * @param className The name of the class implementing the Activity
     *                  object.
     * @param intent The Intent object that specified the activity class being
     *               instantiated.
     * 
     * @return The newly instantiated Activity object.
     */
    public Activity newActivity(ClassLoader cl, String className,
            Intent intent)
            throws InstantiationException, IllegalAccessException,
            ClassNotFoundException {
        String pkg = intent != null && intent.getComponent() != null
                ? intent.getComponent().getPackageName() : null;
        return getFactory(pkg).instantiateActivity(cl, className, intent);
    }

public @NonNull Activity instantiateActivity(@NonNull ClassLoader cl, @NonNull String className,
            @Nullable Intent intent)
            throws InstantiationException, IllegalAccessException, ClassNotFoundException {
        return (Activity) cl.loadClass(className).newInstance();
    }

我们通过上面代码发现最终是通过classloader.loadClass(className).newInstance()来进行Activity类的实例化的。
classloader通过java.lang.ClassLoader cl = appContext.getClassLoader()构造。

//位于android/app/ContextImpl.java
public ClassLoader getClassLoader() {
        return mClassLoader != null ? mClassLoader : (mPackageInfo != null ? mPackageInfo.getClassLoader() : ClassLoader.getSystemClassLoader());
    }

//位于java/lang/ClassLoader.java
public static ClassLoader getSystemClassLoader() {
        return SystemClassLoader.loader;
    }

public abstract class ClassLoader {

    static private class SystemClassLoader {
        public static ClassLoader loader = ClassLoader.createSystemClassLoader();
    }

private static ClassLoader createSystemClassLoader() {
        String classPath = System.getProperty("java.class.path", ".");
        String librarySearchPath = System.getProperty("java.library.path", "");
        return new PathClassLoader(classPath, librarySearchPath, BootClassLoader.getInstance());
    }
}

我们看源码发现
Activity的ClassLoader我们仔细看源码是 PathClassLoader extends BaseDexClassLoader extends ClassLoader 而loadClass这个方法在ClassLoader 中：

所以cl.loadClass(className)调用的流程如下：

//位于java/lang/ClassLoader.java
　protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
            // First, check if the class has already been loaded
            Class c = findLoadedClass(name);
            if (c == null) {
                 // If still not found, then invoke findClass in order
                 // to find the class.
                 c = findClass(name);//这边调用findClass方法，发现这是一个protected类型方法，ClassLoader.java中没有其实现，那就是在子方法中实现了
                 // this is the defining class loader; record the stats
            }
            return c;
    }

BaseDexClassLoader部分源码：

public class BaseDexClassLoader extends ClassLoader {
    private final DexPathList pathList;

    /**
     * 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.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;
    }
}

DexPathList部分源码：

//位于/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java
/*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";

    /** class definition context */
    private final ClassLoader definingContext;

    /**
     * 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;

    /**
     * Finds the named class in one of the dex files pointed at by
     * this instance. This will find the one in the earliest listed
     * path element. If the class is found but has not yet been
     * defined, then this method will define it in the defining
     * context that this instance was constructed with.
     *
     * @param name of class to find
     * @param suppressed exceptions encountered whilst finding the class
     * @return the named class or {@code null} if the class is not
     * found in any of the dex files
     */
    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;
    }
}

从这段源码可以看出，dexElements是用来保存dex的数组，而每个dex文件其实就是DexFile对象。遍历dexElements，然后通过DexFile去加载class文件，加载成功就返回，否则返回null。