Android NDK基础11：JNI数据类型_C调用Java

JNI数据类型

jclass和jobject

//每个native函数，都至少有两个参数（JNIEnv*, jclass或者jobject)
//1）当native方法为静态方法时：
//jclass 代表native方法所属类的class对象(JniTest.class)
//2）当native方法为非静态方法时：
//jobject 代表native方法所属的对象

基本数据类型

Java基本数据类型与JNI数据类型的映射关系

引用类型(对象)

//String jstring
//Object jobject
//Class jobject
//数组，基本数据类型的数组
//boolean[] jbooleanArray
//byte[] jbyteArray
//char[] jcharArray
//short[] jshortArray
//int[] jintArray
//long[] jlongArray
//float[] jfloatArray
//double[] jdoubleArray
//对象数组
//String[] jobjectArray
//object[] jobjectArray

C/C++访问Java的成员

类型签名

访问属性

#define _CRT_SECURE_NO_WARNINGS
#include "cn_appblog_jni_JniTest.h"
#include <string.h>

//修改属性key
JNIEXPORT jstring JNICALL Java_cn_appblog_jni_JniTest_accessField
  (JNIEnv *env, jobject jobj) {
    //jobj是t对象，JniTest.class
    jclass cls = (*env)->GetObjectClass(env, jobj);
    //jfieldID
    //属性名称，属性签名
    jfieldID fid = (*env)->GetFieldID(env, cls, "key", "Ljava/lang/String;");    

    //jason >> super jason
    //获取key属性的值
    //Get<Type>Field
    jstring jstr = (*env)->GetObjectField(env, jobj, fid);    
    printf("jstr:%#x\n", &jstr);

    //jstring -> c字符串
    //第三个参数isCopy 是否复制（true代表复制，false不复制）
    char *c_str = (*env)->GetStringUTFChars(env, jstr, JNI_FALSE);
    //拼接得到新的字符串
    char text[20] = "super ";
    strcat(text, c_str);

    //c字符串 ->jstring
    jstring new_jstr = (*env)->NewStringUTF(env, text);

    //修改key
    //Set<Type>Field
    (*env)->SetObjectField(env, jobj, fid, new_jstr);

    printf("new_jstr:%#x\n", &new_jstr);

    return new_jstr;
}

访问静态属性

JNIEXPORT void JNICALL Java_cn_appblog_jni_JniTest_accessStaticField
  (JNIEnv *env, jobject jobj) {
    //jclass
    jclass cls = (*env)->GetObjectClass(env, jobj);
    //jfieldID
    jfieldID fid = (*env)->GetStaticFieldID(env, cls, "count", "I");
    //GetStatic<Type>Field
    jint count = (*env)->GetStaticIntField(env, cls, fid);
    count++;
    //修改
    //SetStatic<Type>Field
    (*env)->SetStaticIntField(env, cls, fid, count);
}

访问对象方法

//获取指定类所有属性和方法的签名
> javap -s -p cn.appblog.jni.JniTest

JNIEXPORT void JNICALL Java_cn_appblog_jni_JniTest_accessMethod
  (JNIEnv *env, jobject jobj) {
    //jclass
    jclass cls = (*env)->GetObjectClass(env, jobj);
    //jmethodID
    jmethodID mid = (*env)->GetMethodID(env, cls, "genRandomInt", "(I)I");
    //调用
    //Call<Type>Method
    jint random = (*env)->CallIntMethod(env, jobj, mid, 200);
    printf("random num:%ld", random);

    //.....
}

访问静态方法

JNIEXPORT void JNICALL Java_cn_appblog_jni_JniTest_accessStaticMethod
  (JNIEnv *env, jobject jobj) {
    //jclass
    jclass cls = (*env)->GetObjectClass(env, jobj);
    //jmethodID    
    jmethodID mid = (*env)->GetStaticMethodID(env, cls, "getUUID", "()Ljava/lang/String;");

    //调用
    //CallStatic<Type>Method
    jstring uuid = (*env)->CallStaticObjectMethod(env, cls, mid);

    //随机文件名称 uuid.txt
    //jstring -> char*
    //isCopy JNI_FALSE，代表java和c操作的是同一个字符串
    char *uuid_str = (*env)->GetStringUTFChars(env, uuid, JNI_FALSE);
    //拼接
    char filename[100];
    sprintf(filename, "D://%s.txt", uuid_str);
    FILE *fp = fopen(filename,"w");
    fputs("I love Joe", fp);
    close(fp);
}

访问构造方法

//使用java.util.Date产生一个当前的时间戳
JNIEXPORT jobject JNICALL Java_cn_appblog_jni_JniTest_accessConstructor
  (JNIEnv *env, jobject jobj) {
    jclass cls = (*env)->FindClass(env, "java/util/Date");
    //jmethodID
    jmethodID constructor_mid = (*env)->GetMethodID(env, cls, "<init>", "()V");
    //实例化一个Date对象
    jobject date_obj = (*env)->NewObject(env, cls, constructor_mid);
    //调用getTime方法
    jmethodID mid = (*env)->GetMethodID(env, cls, "getTime", "()J");
    jlong time = (*env)->CallLongMethod(env, date_obj, mid);

    printf("\ntime:%lld\n", time);

    return date_obj;
}

Java层调用

public class JniTest {

    public String key = "joe";

    public static int count = 9;

    //访问属性，返回修改之后的属性内容
    public native String accessField();

    public native void accessStaticField();

    public native void accessMethod();

    public native void accessStaticMethod();

    public native Date assessConstructor();

    public static void main(String[] args) {
        JniTest t = new JniTest();

        System.out.println("key修改前：" + t.key);
        t.accessField();
        System.out.println("key修改后：" + t.key);

        System.out.println("count修改前：" + count);
        t.accessStaticField();
        System.out.println("count修改后：" + count);

        t.accessMethod();
        t.accessStaticMethod();
        t.assessConstructor();
    }

    //产生指定范围的随机数
    public int genRandomInt(int max) {
        System.out.println("genRandomInt 执行了...");
        return new Random().nextInt(max); 
    }

    //产生UUID字符串
    public static String getUUID() {
        return UUID.randomUUID().toString();
    }

    //加载动态库
    static {    
        System.loadLibrary("JniTest");
    }

}

调用父类的方法

public class Human {

    @Override
    public void sayHi() {
        System.out.println("父类：sayHi");
    }

}

public class Man extends Human {

    @Override
    public void sayHi() {
        super.sayHi();
        System.out.println("子类：sayHi");
    }

}

public class JniTest {

    public Human human = new Man();

    public native Date assessNonvirtualMethod();

    public static void main(String[] args) {
        JniTest t = new JniTest();
        t.assessNonvirtualMethod();
    }

    //加载动态库
    static {    
        System.loadLibrary("JniTest");
    }

}

JNIEXPORT void JNICALL Java_cn_appblog_jni_JniTest_accessNonvirtualMethod
  (JNIEnv *env, jobject jobj) {
    jclass cls = (*env)->GetObjectClass(env, jobj);
    //获取man属性（对象）
    jfieldID fid = (*env)->GetFieldID(env, cls, "human", "Lcc/androidios/jni/Human;");
    //获取
    jobject human_obj = (*env)->GetObjectField(env, jobj, fid);

    //执行sayHi方法
    jclass human_cls = (*env)->FindClass(env, "cc/androidios/jni/Human"); //注意：传父类的名称
    jmethodID mid = (*env)->GetMethodID(env, human_cls, "sayHi", "()V");

    //执行
    //(*env)->CallObjectMethod(env, human_obj, mid);
    //调用的父类的方法
    (*env)->CallNonvirtualObjectMethod(env, human_obj, human_cls, mid);
}

中文问题

public class JniTest {

    public native String chineseChars(String content);

    public static void main(String[] args) {
        JniTest t = new JniTest();
        t.chineseChars("中文");
    }

    //加载动态库
    static {    
        System.loadLibrary("JniTest");
    }

}

JNIEXPORT jstring JNICALL Java_cn_appblog_jni_JniTest_chineseChars
  (JNIEnv *env, jobject jobj, jstring content) {
    //输出
    //char *c_str = (*env)->GetStringUTFChars(env, content, JNI_FALSE);
    //printf("%s\n", c_str);

    //c -> jstring
    char *c_str = "马蓉与宋江";
    //char c_str[] = "马蓉与宋喆";
    //jstring jstr = (*env)->NewStringUTF(env, c_str);
    //执行String(byte bytes[], String charsetName)构造方法需要的条件
    //1.jmethodID
    //2.byte数组
    //3.字符编码jstring

    jclass str_cls = (*env)->FindClass(env, "java/lang/String");
    jmethodID constructor_mid = (*env)->GetMethodID(env, str_cls, "<init>", "([BLjava/lang/String;)V");

    //jbyte -> char 
    //jbyteArray -> char[]
    jbyteArray bytes = (*env)->NewByteArray(env, strlen(c_str));
    //byte数组赋值
    //0->strlen(c_str)，从头到尾
    //对等于，从c_str这个字符数组，复制到bytes这个字符数组
    (*env)->SetByteArrayRegion(env, bytes, 0, strlen(c_str), c_str);

    //字符编码jstring
    jstring charsetName = (*env)->NewStringUTF(env, "GB2312");

    //调用构造函数，返回编码之后的jstring
    return (*env)->NewObject(env, str_cls, constructor_mid, bytes, charsetName);
}