[Android] 解析android framework下利用app_process来调用java写的命令及示例

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

[Android] 解析android framework下利用app_process来调用java写的命令及示例
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");
    }
}
[Android] 解析android framework下利用app_process来调用java写的命令及示例

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:

[Android] 解析android framework下利用app_process来调用java写的命令及示例
    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) 
[Android] 解析android framework下利用app_process来调用java写的命令及示例

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!

[Android] 解析android framework下利用app_process来调用java写的命令及示例
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] 解析android framework下利用app_process来调用java写的命令及示例

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。

分类: Android Pro
本文转自demoblog博客园博客,原文链接http://www.cnblogs.com/0616--ataozhijia/p/4993275.html如需转载请自行联系原作者

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