reference to : http://bbs.9ria.com/thread-253058-1-1.html
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:
java代码
public final class strongcmd { static final String TAG = "strong"; static String[] mArgs; int mNextArg; public static void main(String[] args) { 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 static void printUsage() { System.err.println("usage: strong -a -b -h"); System.err.println("'a' is for add"); System.err.println("-h for help"); } }
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:
LOCAL_PATH:= $(call my-dir) include $(CLEAR_VARS) LOCAL_SRC_FILES := $(call all-subdir-java-files) LOCAL_MODULE := strong LOCAL_MODULE_TAGS := optional include $(BUILD_JAVA_LIBRARY) include $(CLEAR_VARS) LOCAL_MODULE := strong LOCAL_SRC_FILES := pre_strong LOCAL_MODULE_CLASS := EXECUTABLES LOCAL_MODULE_TAGS := optional include $(BUILD_PREBUILT)
上一部分是BUILD_JAVA_LIBRARY,关键在下面,利用的是BUILD_PREBUILT,添加一个预编译好的应用程序,我们叫pre_strong,它有可执行的权限,看看它的具体实现吧!
base=/system export CLASSPATH=$base/framework/strong.jar exec 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(env -> NewGlobalRef(mClass)); } virtual void onStarted() { sp 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(); } 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。