解析android framework下利用app_process来调用java写的命令及示例
在android SDK的framework/base/cmds目录下了，有不少目录，这些目的最终都是build出一个bin文件，再存放到/system/bin目录下，对于C/CPP写的命令，我们还是比较好理解的，都有一个main函数作为入口，但是在cmds目录下还有一些原生代码是java的，比如input、settings，那么这种类型的命令是怎么实现的呢？
笔者研习了原生的命令实现，写了一个demo，抛砖引玉吧！暂时叫strong吧！我们都知道java写的文件最后都是编译成了class文件，java类里面也有很多接口，在android平台上cmds目录下的各模块的java文件都实现了一个共同的方法，还是叫main（），真是情有独钟啊！当然从技术角度看叫其他名字也是可以的。那我们就简单实现以下这个class吧！如下：
/* * Copyright (C) 2012 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.commands.strong;import android.app.ActivityManagerNative;import android.app.IActivityManager;import android.app.IActivityManager.ContentProviderHolder;import android.content.IContentProvider;import android.os.Binder;import android.os.Bundle;import android.os.IBinder;import android.os.RemoteException;import android.os.UserHandle;public final class strongcmd { static final String TAG = "strong"; static String[] mArgs; int mNextArg; static int value = 0; public static void main(String[] args) { int c; printUsage(); System.err.println("Wellcom strong test function!!"); try { new strongcmd().run(); } catch (Exception e) { System.err.println("Unable to run settings command"); } } public void run() { try { System.err.println("Now strong run() again"); } catch (Exception e) { System.err.println("Now strong run() Exception"); } } private String nextArg() { if (mNextArg >= mArgs.length) { return null; } String arg = mArgs[mNextArg]; mNextArg++; return arg; } private static void printUsage() { System.err.println("usage: strong -a -b -h"); System.err.println("'a' is for add"); System.err.println("-h for help"); }}/*****************************************************************************************************/
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写好Android.mk，编译好这个文件，会生成strong.jar包，包含这个class。那么，又怎么跟命令挂钩呢？先看看Android.mk，如下：
# Copyright 2011 The Android Open Source Project#LOCAL_PATH:= $(call my-dir)include $(CLEAR_VARS)LOCAL_SRC_FILES := $(call all-subdir-java-files)LOCAL_MODULE := strongLOCAL_MODULE_TAGS := optionalinclude $(BUILD_JAVA_LIBRARY)include $(CLEAR_VARS)LOCAL_MODULE := strongLOCAL_SRC_FILES := pre_strongLOCAL_MODULE_CLASS := EXECUTABLESLOCAL_MODULE_TAGS := optionalinclude $(BUILD_PREBUILT)
上一部分是BUILD_JAVA_LIBRARY，关键在下面，利用的是BUILD_PREBUILT，添加一个预编译好的应用程序，我们叫pre_strong，它有可执行的权限，看看它的具体实现吧！
# Script to start "strong" on the device#base=/systemexport CLASSPATH=$base/framework/strong.jarexec app_process $base/bin com.android.commands.strong.strongcmd "$@"
首先还是设置好这个java lib的路径，如何再调用app_process来执行，主要是把这个类名要给对，app_process其实也是个命令。在app_process里面，还是一样利用JNI技术，在java ENV里面查找传给app_process的class，找到这个class后再去找main函数接口的field，然后再call这个main接口，这样就call到java里面去了。
下面简要看看app_process的关键代码吧！
virtual void onVmCreated(JNIEnv* env) { if (mClassName == NULL) { return; // Zygote. Nothing to do here. } /* * This is a little awkward because the JNI FindClass call uses the * class loader associated with the native method we're executing in. * If called in onStarted (from RuntimeInit.finishInit because we're * launching "am", for example), FindClass would see that we're calling * from a boot class' native method, and so wouldn't look for the class * we're trying to look up in CLASSPATH. Unfortunately it needs to, * because the "am" classes are not boot classes. * * The easiest fix is to call FindClass here, early on before we start * executing boot class Java code and thereby deny ourselves access to * non-boot classes. */ char* slashClassName = toSlashClassName(mClassName); mClass = env->FindClass(slashClassName); if (mClass == NULL) { ALOGE("ERROR: could not find class '%s'\n", mClassName); } free(slashClassName); mClass = reinterpret_cast<jclass>(env->NewGlobalRef(mClass)); } virtual void onStarted() { sp<ProcessState> proc = ProcessState::self(); ALOGV("App process: starting thread pool.\n"); proc->startThreadPool(); AndroidRuntime* ar = AndroidRuntime::getRuntime(); ar->callMain(mClassName, mClass, mArgC, mArgV); IPCThreadState::self()->stopProcess(); }if (className) { // Remainder of args get passed to startup class main() runtime.mClassName = className; runtime.mArgC = argc - i; runtime.mArgV = argv + i; runtime.start("com.android.internal.os.RuntimeInit", application ? "application" : "tool"); }Android平台提供的app_process，还是相当不错的，比较实用，利用好app_process还是可以写成很多供我们自己开发、测试、定制一些特殊的程序，给开发带来了很大的便利。