/*
* Copyright (C) 2015 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.android.tools.fd.runtime;
import android.util.Log;
import com.android.annotations.NonNull;
import dalvik.system.BaseDexClassLoader;
import java.io.File;
import java.lang.reflect.Field;
import java.util.List;
import static com.android.tools.fd.runtime.BootstrapApplication.LOG_TAG;
// This is based on com.google.devtools.build.android.incrementaldeployment.IncrementalClassLoader
// with some cleanup around path handling and made it compile on JDK 6 (e.g. removed multicatch
// etc)
// See
// https://github.com/google/bazel/blob/master/src/tools/android/java/com/google/devtools/build/android/incrementaldeployment/IncrementalClassLoader.java
// (May 11 revision, ca96e11)
/**
* A class loader that loads classes from any .dex file in a particular directory on the SD card.
* <p>
* <p>Used to implement incremental deployment to Android phones.
*
* IncrementalClassLoader 主要提供了静态方法 inject(ClassLoader classLoader,
* String nativeLibraryPath,
* String codeCacheDir,
* List<String> dexes)
*
* 方便将目标 classLoader 的 parent 替换为 IncrementalClassLoader
* 这样的话 parent 的父加载的 class 和 res 就走的是 IncrementalClassLoader 的加载
* BootClassLoader -> classLoader 就会变为 BootClassLoader -> IncrementalClassLoader -> classLoader
*
* 然而 IncrementalClassLoader 的加载的逻辑又靠 DelegateClassLoader
* DelegateClassLoader 是 BaseDexClassLoader 的子类
* 覆写了 findClass 方法，但是，只是为了打点 Log
*
* 可以理解 IncrementalClassLoader 就是 插件 class 的 classLoader
*/
public class IncrementalClassLoader extends ClassLoader {
/** When false, compiled out of runtime library */
public static final boolean DEBUG_CLASS_LOADING = false;
private final DelegateClassLoader delegateClassLoader;
/**
* 为了保证和 要 Hook 的 classLoader 保持一致的 父 classLoader，特意用了 super(original.getParent()) 去实例化
* 一个和 要 Hook 的 classLoader 一样的 父 classLoader
* 然后执行 静态工厂方法 createDelegateClassLoader 去 构造一个 DelegateClassLoader
*
* 比如 original 的关系如下：BootClassLoader -> original
* 正常的话，我们拿到 original 去实例化一个 classLoader 会如下情况：
* BootClassLoader -> original —> classLoader
* 但是这里执行的是 super(original.getParent()) 而不是 super(original)
*
* 所以这里得到的是 BootClassLoader -> classLoader
*
* @param original hook 的 classLoader
* @param nativeLibraryPath 本地 lib 路径
* @param codeCacheDir 项目 cache 文件夹路径
* @param dexes dex 的所有路径
*/
public IncrementalClassLoader(
ClassLoader original, String nativeLibraryPath, String codeCacheDir, List<String> dexes) {
super(original.getParent());
// TODO(bazel-team): For some mysterious reason, we need to use two class loaders so that
// everything works correctly. Investigate why that is the case so that the code can be
// simplified.
delegateClassLoader = createDelegateClassLoader(nativeLibraryPath, codeCacheDir, dexes,
original);
}
@Override
public Class<?> findClass(String className) throws ClassNotFoundException {
try {
Class<?> aClass = delegateClassLoader.findClass(className);
//noinspection PointlessBooleanExpression,ConstantConditions
if (DEBUG_CLASS_LOADING && Log.isLoggable(LOG_TAG, Log.VERBOSE)) {
Log.v(LOG_TAG,
"Incremental class loader: findClass(" + className + ") = " + aClass);
}
return aClass;
} catch (ClassNotFoundException e) {
//noinspection PointlessBooleanExpression,ConstantConditions
if (DEBUG_CLASS_LOADING && Log.isLoggable(LOG_TAG, Log.VERBOSE)) {
Log.v(LOG_TAG,
"Incremental class loader: findClass(" + className + ") : not found");
}
throw e;
}
}
/**
* 代理 ClassLoader
* 没有做任何功能上的扩展
* 主要为了想在 findClass 里打点 Log
*
* A class loader whose only purpose is to make {@code findClass()} public.
*/
private static class DelegateClassLoader extends BaseDexClassLoader {
private DelegateClassLoader(
String dexPath, File optimizedDirectory, String libraryPath, ClassLoader parent) {
super(dexPath, optimizedDirectory, libraryPath, parent);
}
/**
* 代理 BaseDexClassLoader 的 findClass(String name) 方法
* 主要就是想打点 Log
*
* @param name 类名
* @return Class
* @throws ClassNotFoundException
*/
@Override
public Class<?> findClass(String name) throws ClassNotFoundException {
try {
Class<?> aClass = super.findClass(name);
//noinspection PointlessBooleanExpression,ConstantConditions
if (DEBUG_CLASS_LOADING && Log.isLoggable(LOG_TAG, Log.VERBOSE)) {
Log.v(LOG_TAG, "Delegate class loader: findClass(" + name + ") = " + aClass);
}
return aClass;
} catch (ClassNotFoundException e) {
//noinspection PointlessBooleanExpression,ConstantConditions
if (DEBUG_CLASS_LOADING && Log.isLoggable(LOG_TAG, Log.VERBOSE)) {
Log.v(LOG_TAG, "Delegate class loader: findClass(" + name + ") : not found");
}
throw e;
}
}
}
/**
* 静态工厂方法，构建一个 DelegateClassLoader 实例
*
* @param nativeLibraryPath 本地 lib 路径
* @param codeCacheDir 项目 cache 文件夹路径
* @param dexes dex 的所有路径
* @param original 父 classloader（双亲机制需要）
* @return DelegateClassLoader
*/
private static DelegateClassLoader createDelegateClassLoader(
String nativeLibraryPath, String codeCacheDir, List<String> dexes,
ClassLoader original) {
String pathBuilder = createDexPath(dexes);
return new DelegateClassLoader(pathBuilder, new File(codeCacheDir),
nativeLibraryPath, original);
}
/**
* 拿到一组 dex 的路径，合成一个 String 数据
*
* @param dexes 一组 dex 的路径
* @return String
*/
@NonNull
private static String createDexPath(List<String> dexes) {
StringBuilder pathBuilder = new StringBuilder();
boolean first = true;
for (String dex : dexes) {
// 最后一个不加分隔符 "/"
if (first) {
first = false;
} else {
pathBuilder.append(File.pathSeparator);
}
pathBuilder.append(dex);
}
if (Log.isLoggable(LOG_TAG, Log.VERBOSE)) {
Log.v(LOG_TAG, "Incremental dex path is "
+ BootstrapApplication.join('\n', dexes));
}
return pathBuilder.toString();
}
/**
* Hook ClassLoader 的 parent，由于 ClassLoader 的 parent 的只涉及到加载 class 和 res，所有 hook 了加载机制
* newParent 一般都会是通过 BootClassloader 实例化出来的，所以 BootClassloader -> newParent
* Hook 之后变为
* BootClassloader -> classLoader >>>>>> BootClassloader -> newParent -> classLoader
*
* @param classLoader hook 目标 classLoader
* @param newParent 需要作为 classLoader 的 parent classLoader 去加载 class 和 res
*/
private static void setParent(ClassLoader classLoader, ClassLoader newParent) {
try {
Field parent = ClassLoader.class.getDeclaredField("parent");
parent.setAccessible(true);
parent.set(classLoader, newParent);
} catch (IllegalArgumentException e) {
throw new RuntimeException(e);
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
} catch (NoSuchFieldException e) {
throw new RuntimeException(e);
}
}
/**
* 静态工厂方法，通过 classLoader + nativeLibraryPath + codeCacheDir + dexes = incrementalClassLoader
* incrementalClassLoader 的构造方法指明了，会拿到 classLoader 的 parent 去作为 incrementalClassLoader 的 parent
* 去实例化 incrementalClassLoader。所以是 BootClassLoader -> incrementalClassLoader。
* 然后 classLoader 的关系是 BootClassLoader -> classLoader
* 执行 setParent 后：BootClassLoader -> incrementalClassLoader -> classLoader
*
* @param classLoader 需要 hook 的 classLoader
* @param nativeLibraryPath 本地 lib 路径
* @param codeCacheDir 项目 cache 文件夹路径
* @param dexes dex 的所有路径
* @return hook 后的 classLoader 的 parent（incrementalClassLoader）
*/
public static ClassLoader inject(
ClassLoader classLoader, String nativeLibraryPath, String codeCacheDir,
List<String> dexes) {
IncrementalClassLoader incrementalClassLoader =
new IncrementalClassLoader(classLoader, nativeLibraryPath, codeCacheDir, dexes);
setParent(classLoader, incrementalClassLoader);
// This works as follows:
// We're given the current class loader that's used to load the bootstrap application.
// We have a new class loader which reads patches/overrides from the data directory
// instead. We want *that* class loader to have the bootstrap class loader's parent
// as its parent, and then we make the bootstrap class loader parented by our
// class loader.
//
// In other words, we have this:
// BootstrapApplication.classLoader = ClassLoader1, parent=ClassLoader2
// We create ClassLoader3 from the .dex files in the data directory, and arrange for
// the hierarchy to be like this:
// BootstrapApplication.classLoader = ClassLoader1, parent=ClassLoader3, parent=ClassLoader2
// With this approach, a class find (which should always look at the parents first) should
// find anything from ClassLoader3 before they get them from ClassLoader1.
// (Note that ClassLoader2 in the above is generally the BootClassLoader, not containing
// any classes we care about.)
return incrementalClassLoader;
}
}