[Android初级]NDK入门体验の方法调用

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如何在android应用的自定义底层C中调用java的代码？这是本次自我复习的计划。

首先，我得创建一个demo，结构如下：

1.创建一个接口类NativeDataProvider，他的作用是调用c方法，然后回调java方法

public class NativeDataProvider {static final String TAG = "NativeDataProvider";private Context context;public NativeDataProvider(Context context){this.context = context;}//调用C中的代码public native void callCCode();public native int callCAdd(int i,int j);public native void callCPrintString();//在C中调用java中的org.jan.ndk.example.NativeDataProvider无参方法helloFrmJavapublic void helloFromJava(){Log.i(TAG, "hello from java!");Toast.makeText(context, "hello from java", Toast.LENGTH_SHORT).show();}//在C中调用参数为string的方法public void printString(String str){Log.i(TAG, "[from java]:"+str);Toast.makeText(context, "printString->str:"+str, Toast.LENGTH_SHORT).show();}//在c中调用int参数的方法public int add(int i,int j){int result = i+j;Log.i(TAG, "result is"+result);Toast.makeText(context, "add->result="+result, Toast.LENGTH_SHORT).show();return result;}}

2.创建Android.mk

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE    := jni_demoLOCAL_SRC_FILES := jni_demo.c#增加log库LOCAL_LDLIBS += -llog include $(BUILD_SHARED_LIBRARY)

3.创建jni，也就是c代码，之前先创建NativeDataProvider的头文件，到项目目录/bin/classes下，调用javah org.jan.ndk.example.NativeDataProvider

就会生成org_jan_ndk_example_NativeDataProvider.h

/* DO NOT EDIT THIS FILE - it is machine generated */#include <jni.h>/* Header for class org_jan_ndk_example_NativeDataProvider */#ifndef _Included_org_jan_ndk_example_NativeDataProvider#define _Included_org_jan_ndk_example_NativeDataProvider#ifdef __cplusplusextern "C" {#endif/* * Class:     org_jan_ndk_example_NativeDataProvider * Method:    callCCode * Signature: ()V */JNIEXPORT void JNICALL Java_org_jan_ndk_example_NativeDataProvider_callCCode  (JNIEnv *, jobject);/* * Class:     org_jan_ndk_example_NativeDataProvider * Method:    callCAdd * Signature: (II)I */JNIEXPORT jint JNICALL Java_org_jan_ndk_example_NativeDataProvider_callCAdd  (JNIEnv *, jobject, jint, jint);/* * Class:     org_jan_ndk_example_NativeDataProvider * Method:    callCPrintString * Signature: ()V */JNIEXPORT void JNICALL Java_org_jan_ndk_example_NativeDataProvider_callCPrintString  (JNIEnv *, jobject);#ifdef __cplusplus}#endif#endif

其中看到有Signature:()V 这样类似的 就是 方法签名。

例如：

har* classname = "org/jan/ndk/example/NativeDataProvider";
jclass dpclazz = (*env)->FindClass(env, classname);

这是通过jnienv指针获取class对象。

jmethodID methodID = (*env)->GetMethodID(env, dpclazz, "helloFromJava", "()V");

GetMethodID指 从java中获取定义好的method方法，第二个参数是class对象，第三个参数是方法名，第四个是方法签名（可以从头文件中看Signature）

看方法签名的方法： 进入项目\bin\classes 下，命令：javap -s org.jan.ndk.example.NativeDataProvider

"()V"的大概意思就是 括号内没有参数，就是没有入参，V就是void无返回值。

下面是C主要代码(具体请看注释):

#include "org_jan_ndk_example_NativeDataProvider.h"#include <string.h>#include <android/log.h>#define LOG_TAG "System.out"#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)JNIEXPORT void JNICALL Java_org_jan_ndk_example_NativeDataProvider_callCCode  (JNIEnv * env, jobject obj){//调用NativeDataProvider对象中的helloFromJava()方法//获取到某个对象, 获取对象中的方法, 调用获取到的方法LOGI("in code");//NativeDataProvider完整类名 org.jan.ndk.example.NativeDataProviderchar* classname = "org/jan/ndk/example/NativeDataProvider";jclass dpclazz = (*env)->FindClass(env, classname);if(dpclazz == 0)LOGI("org.jan.ndk.example.NativeDataProvider.class not find !!!");elseLOGI("org.jan.ndk.example.NativeDataProvider.class find !!!");//使用jni.h中提供的GetMethodID方法, 获取jmethodID, 传入参数 ①JNIEnv指针 ②Class对象 ③ 方法名 ④方法签名, 在这里方法名和方法签名确定一个方法, 方法签名就是方法的返回值 与 参数的唯一标示;//参数介绍 : 第二个参数是Class对象, 第三个参数是方法名,第四个参数是方法的签名, 获取到调用的methodjmethodID methodID = (*env)->GetMethodID(env, dpclazz, "helloFromJava", "()V");if(methodID == 0)LOGI("method helloFromJava not find !!!");elseLOGI("method helloFromJava find !!!");/* * 调用方法 void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...); * 参数介绍 : 后面的 ... 是可变参数, 如果该返回值void的方法有参数, 就将参数按照次序排列 */LOGI("before call method");(*env)->CallVoidMethod(env, obj, methodID);LOGI("after call method");}JNIEXPORT void JNICALL Java_org_jan_ndk_example_NativeDataProvider_callCPrintString  (JNIEnv *env, jobject obj){//调用DataProvider对象中的helloFromJava()方法//获取到某个对象, 获取对象中的方法, 调用获取到的方法LOGI("in code");//NativeDataProvider完整类名 org.jan.ndk.example.NativeDataProviderchar* classname = "org/jan/ndk/example/NativeDataProvider";jclass dpclazz = (*env)->FindClass(env, classname);if(dpclazz == 0)LOGI("class not find !!!");elseLOGI("class find !!!");//参数介绍 : 第二个参数是Class对象, 第三个参数是方法名,第四个参数是方法的签名, 获取到调用的methodjmethodID methodID = (*env)->GetMethodID(env, dpclazz, "printString", "(Ljava/lang/String;)V");if(methodID == 0)LOGI("method not find !!!");elseLOGI("method find !!!");/* * 调用方法 void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...); * 参数介绍 : 后面的 ... 是可变参数, 如果该返回值void的方法有参数, 就将参数按照次序排列 */LOGI("before call method");(*env)->CallVoidMethod(env, obj, methodID, (*env)->NewStringUTF(env, "printString method callback success!!"));LOGI("after call method");}/* * 实际开发的情况 * C代码工程师给我们 first.h first.c , 我们只需要将first.h引入, 然后就可以使用其中的方法了 */JNIEXPORT jint JNICALL Java_org_jan_ndk_example_NativeDataProvider_callCAdd(JNIEnv *env, jobject obj, jint a, jint b){//调用DataProvider对象中的helloFromJava()方法//获取到某个对象, 获取对象中的方法, 调用获取到的方法LOGI("in code test_add=>");//NativeDataProvider完整类名 org.jan.ndk.example.NativeDataProviderchar* classname = "org/jan/ndk/example/NativeDataProvider";jclass dpclazz = (*env)->FindClass(env, classname);if(dpclazz == 0)LOGI("class not find !!!");elseLOGI("class find !!!");//参数介绍 : 第二个参数是Class对象, 第三个参数是方法名,第四个参数是方法的签名, 获取到调用的methodjmethodID methodID = (*env)->GetMethodID(env, dpclazz, "add", "(II)I");if(methodID == 0)LOGI("method not find !!!");elseLOGI("method find !!!");/* * 调用方法 void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...); * 参数介绍 : 后面的 ... 是可变参数, 如果该返回值void的方法有参数, 就将参数按照次序排列 */return (*env)->CallIntMethod(env, obj, methodID, a,b);}

4.编译jni，在demo目录下手动执行ndk-build

5.android的代码：

public class MainActivity extends ActionBarActivity {private static final String TAG = "MainActivity";private static NativeDataProvider ndp;static{System.loadLibrary("jni_demo");}@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);ndp = new NativeDataProvider(this);if (savedInstanceState == null) {getSupportFragmentManager().beginTransaction().add(R.id.container, new PlaceholderFragment()).commit();}}@Overridepublic boolean onCreateOptionsMenu(Menu menu) {// Inflate the menu; this adds items to the action bar if it is present.getMenuInflater().inflate(R.menu.main, menu);return true;}@Overridepublic boolean onOptionsItemSelected(MenuItem item) {// Handle action bar item clicks here. The action bar will// automatically handle clicks on the Home/Up button, so long// as you specify a parent activity in AndroidManifest.xml.int id = item.getItemId();if (id == R.id.action_settings) {return true;}return super.onOptionsItemSelected(item);}/** * A placeholder fragment containing a simple view. */public static class PlaceholderFragment extends Fragment implements OnClickListener {private Button callVoidMethod, callStringMethod, callIntMethod;public PlaceholderFragment() {}@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container,Bundle savedInstanceState) {View rootView = inflater.inflate(R.layout.fragment_main, container,false);callVoidMethod = (Button) rootView.findViewById(R.id.call_void_method_btn);callStringMethod = (Button) rootView.findViewById(R.id.call_string_parameter_method);callIntMethod = (Button) rootView.findViewById(R.id.call_int_parameter_method);callVoidMethod.setOnClickListener(this);callStringMethod.setOnClickListener(this);callIntMethod.setOnClickListener(this);return rootView;}@Overridepublic void onClick(View v) {switch (v.getId()) {case R.id.call_void_method_btn:ndp.callCCode();break;case R.id.call_string_parameter_method:ndp.callCPrintString();break;case R.id.call_int_parameter_method:ndp.callCAdd(2,3);break;}}}}

实现结果如下：

代码示例下载：请打开如意门下载