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.c
LOCAL_SRC_FILES :=CallbackJava.c Hello.c
LOCAL_LDLIBS += -llog
include $(BUILD_SHARED_LIBRARY)

参考http://www.open-open.com/lib/view/open1359700820369.html