jni c回调java

使用头文件将C语言中的标准实现通过预编译映射到自己定义的函数中，并在自己定义的函数中调用java函数来实现，这一思路怎么实现呢

首先，通过.h定义函数替换C库中的函数，比如将C中的fopen替换为test_print，新建test.h 文件

//
// Created by crow on 2022/6/1.
//

#ifndef CCDEMO_TEST_H
#define CCDEMO_TEST_H

#endif //CCDEMO_TEST_H

#include 

#define fopen test_print

 新建test.c实现定义的test_print方法，在c里面通过jni调用java代码，要获取JavaVM，然后通过JavaVM获取到当前线程的JNIEnv，代码如下

//
// Created by crow on 2022/6/1.
//

#include "test.h"
#include 
#include 

JavaVM *javaVm;


jstring charToJstring(JNIEnv *env, const char *pat) {
    jsize len = strlen(pat);

    jclass strClass = (*env)->FindClass(env, "java/lang/String");
    jstring encoding = (*env)->NewStringUTF(env, "utf-8");

    jmethodID jmethodId = (*env)->GetMethodID(env, strClass, "","([BLjava/lang/String;)V");
    jbyteArray bytes = (*env)->NewByteArray(env, len);

    (*env)->SetByteArrayRegion(env, bytes, 0, len, (jbyte *) pat);
    return (jstring) (*env)->NewObject(env, strClass, jmethodId, bytes, encoding);
}

jstring test_print(const char* filename, const char* mode) {
    JNIEnv *env = NULL;
    if ((*javaVm)->AttachCurrentThread(javaVm, &env, NULL) != JNI_OK) {
        return NULL;
    }

    jclass clazz = (*env)->FindClass(env, "com/sun/android/ccdemo/MainActivity");
    jobject jobj = (*env)->AllocObject(env, clazz);
    jmethodID  jmethodId = (*env)->GetMethodID(env, clazz, "printLine",
                                               "(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;");
    jstring jFileName = charToJstring(env, filename);
    jstring jMode = charToJstring(env, mode);
    return (*env)->CallObjectMethod(env, jobj, jmethodId, jFileName, jMode);
}

JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* reserved) {
    javaVm = vm;
    JNIEnv *env = NULL;
    jint result;

    if ((*vm)->GetEnv(vm, (void **) &env, JNI_VERSION_1_6) == JNI_OK) {
        result = JNI_VERSION_1_6;
    }
    else if ((*vm)->GetEnv(vm, (void **)&env, JNI_VERSION_1_4) == JNI_OK) {
        result = JNI_VERSION_1_4;
    }
    else {
        result = JNI_VERSION_1_2;
    }

    return result;
}

然后通过调用fopen即可实现调用test_print，如在native方法的实现中进行如下调用

 在调用native方法的地方进行调用即可如在MainActivity里面进行调用，如下

   private void readFile() {
        String path = "/storage/emulated/0/document/example2.txt";
        Log.e("crow", printFirstLine(path, "r"));
         
   }
   

    public native String printFirstLine(String fileName, String mode);

    public String printLine(String fileName, String mode) {
        RandomAccessFile randomAccessFile = null;
        try {
            randomAccessFile = new RandomAccessFile(fileName, mode);
            return randomAccessFile.readLine();
        } catch (IOException e) {
            return null;
        } finally {
            try {
                if (null != randomAccessFile)
                    randomAccessFile.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            Log.e("crow", "printLine: finally " );
        }
    }