六、Android学习笔记_JNI_c调用java代码

1、编写native方法（java2c）和非native方法（c2java）:

package com.example.provider;public class CallbackJava {    // C调用java空方法    public void helloFromJava() {        System.out.println("hello from java");    }    // C调用java中的带两个int参数的方法    public int Add(int x, int y) {        int result = x + y;        System.out.println("java result=" + result);        return result;    }    // C调用java中参数为string的方法    public void printString(String s) {        System.out.println("java " + s);    }    //静态方法    public static void printStaticStr(String s) {        System.out.println("java static-> " + s);    }    // 让c代码调用对应的java代码    public native void callHelloFromJava();    public native void callAdd();    public native void callPrintString();    // 调用一个静态的c代码    public native void callPrintStaticStr();}

2、通过javah命令生成签名，通过javap命令生产native方法签名：

　　

3、c代码

FindClass：找到java中的类文件

GetMethodID：根据类和方法签名找到要调用的方法。

#include <stdio.h>#include <jni.h>#include <malloc.h>#include "com_example_provider_CallbackJava.h";#include "com_example_jni_MainActivity.h";#include <android/log.h>#define LOG_TAG "System.out.c"#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)jmethodID getMethodId(JNIEnv * env, char* methodname, char* signname) {    jclass jclazz = (*env)->FindClass(env, "com/example/provider/CallbackJava");    if (jclazz == 0) {        LOGD("class not find ");        return;    }    //2、找class中方法的id,第三个参数方法名称，第四个参数该方法对应的签名    jmethodID methodid = (*env)->GetMethodID(env, jclazz, methodname, signname);    if (methodid == 0) {        LOGD("method not find ");        return;    }    return methodid;}JNIEXPORT void JNICALL Java_com_example_provider_CallbackJava_callHelloFromJava(JNIEnv * env, jobject obj) {    //1、 找java中的class文件    //第二个参数是类名的（包名）    jclass jclazz = (*env)->FindClass(env,"com/example/provider/CallbackJava");    if(jclazz==0) {        LOGD("class not find ");        return;    }    //2、找class中方法的id,第三个参数方法名称，第四个参数该方法对应的签名    jmethodID methodid = (*env)->GetMethodID(env, jclazz,"helloFromJava","()V");    if(methodid==0) {        LOGD("helloFromJava method not find ");        return;    }    //3、调用方法    //void (*CallVoidMethod)(JNIEnv*, jobject, jmethodID, ...);    (*env)->CallVoidMethod(env,obj,methodid);}JNIEXPORT void JNICALL Java_com_example_provider_CallbackJava_callAdd(JNIEnv *env, jobject obj) {    jmethodID mid = getMethodId(env,"Add","(II)I");    int result = (*env)->CallIntMethod(env, obj, mid,3,6);    LOGD("c to java add: d%",result);}JNIEXPORT void JNICALL Java_com_example_provider_CallbackJava_callPrintString(JNIEnv *env, jobject obj) {    LOGD("c to java callPrintString start ");    jmethodID mid = getMethodId(env,"printString","(Ljava/lang/String;)V");    (*env)->CallVoidMethod(env, obj, mid,(*env)->NewStringUTF(env," call printString finish"));    LOGD("c to java callPrintString finish ");}JNIEXPORT void JNICALL Java_com_example_provider_CallbackJava_callPrintStaticStr(JNIEnv *env, jobject obj) {    jclass jclazz = (*env)->FindClass(env, "com/example/provider/CallbackJava");    if (jclazz == 0) {        LOGD("class not find ");        return;    }    //静态方法调用GetStaticMethodID 和 CallStaticXXXMethod    jmethodID methodid = (*env)->GetStaticMethodID(env, jclazz, "printStaticStr", "(Ljava/lang/String;)V");    if (methodid == 0) {        LOGD("printStaticStr method not find ");        return;    }    (*env)->CallStaticVoidMethod(env, jclazz, methodid,(*env)->NewStringUTF(env,"static hello java"));}JNIEXPORT void JNICALL Java_com_example_jni_MainActivity_callProvider  (JNIEnv *env, jobject obj){    jclass jclazz = (*env)->FindClass(env,"com/example/provider/CallbackJava");    if(jclazz==0) {        LOGD("class not find ");        return;    }    jmethodID methodid = (*env)->GetMethodID(env, jclazz,"helloFromJava","()V");    if(methodid==0) {        LOGD("helloFromJava method not find ");        return;    }    //创建一个CallbackJava对象    jobject jobj =(*env)->AllocObject(env, jclazz);    (*env)->CallVoidMethod(env,jobj,methodid);}

    4、编译多个c文件可以用空格分开

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE    :=Hello#LOCAL_SRC_FILES :=Hello.cLOCAL_SRC_FILES :=CallbackJava.c Hello.cLOCAL_LDLIBS += -lloginclude $(BUILD_SHARED_LIBRARY)