jni中对于类对象的传递

前言

jni部分系列文章

1. <a href="http://www.jianshu.com/p/476aae319808"> jni基本数据类型的传递</a>
2. <a href="http://www.jianshu.com/p/e397382ba810"> jni 中较为复杂的数据类型(String和数组)</a>

这篇博客承接上一篇,是系列中的第三篇，本文主要描述jni中java类对象的传递和操作，包括操作对象和新建对象两种操作

操作对象

1.建立一个简单的java类，用于操作

package model;

/**
 * Created by act64 on 2017/5/29.
 */

public class Screen {
    int height;
    int width;

    public Screen(int width,int height){
        this.height=height;
        this.width=width;
    }

    public int getHeight() {
        return height;
    }

    public void setHeight(int height) {
        this.height = height;
    }

    public int getWidth() {
        return width;
    }

    public void setWidth(int width) {
        this.width = width;
    }

    public void printInfo(){
        System.out.println(" screen height = "+ height+" width = "+width);
    }
}

2.创建java的jni和主程序入口

import model.Screen;

/**
 * Created by act64 on 2017/5/29.
 */

public class JniClassHello {

      static {
        System.loadLibrary("helloClass");
    }
    

    public static void main(String[] args){
        Screen s=new Screen(1440,720);
        s.printInfo();
        Screen screenBigger = changeScreen(s);
        screenBigger.printInfo();
        if (screenBigger.equals(s)){
            System.out.println("屏幕是同一块");
        }
    }

    public native static Screen changeScreen(Screen screen);
}

这两部分都是十分简单的，就不加以描述了
3.建立JNI c文件
文件名 hello.c

#include <stdio.h>
#include <jni.h>
#include <stdlib.h>
//jni映射java的方法，可以通过javah命令生成，需要的看系列文章1和2
jobject c_hello(JNIEnv * env, jobject mJobject,jobject jScreenObj){
    //获得java的 class对象，用于找到方法和构造函数等
    //对于java反射有所了解的话，操作起来会习惯点
    jclass screenCls = (*env)->GetObjectClass(env, jScreenObj);
    //获得函数“setHight ” ，描述符是(I)V，即参数为int 返回void
   //描述符都可以用javah生成头文件然后复制过来，很方便
    jmethodID setHight =
    (*env)->GetMethodID(env, screenCls, "setHeight", "(I)V");
    if (setHight == NULL) {
        return; /* method not found */
    }
    printf("ChangeScreenHeightTo 800\n");
//调用一个返回值为void的java方法，参数是va_list不定参数
    (*env)->CallVoidMethod(env, jScreenObj, setHight,800);
    return jScreenObj;
}

// 建立jni映射表，将c和java的函数关联起来
const JNINativeMethod methods[]={
    {"changeScreen","(Lmodel/Screen;)Lmodel/Screen;",(jobject *)c_hello},
};

//jni load时的初始化函数回调
JNIEXPORT jint JNICALL
JNI_OnLoad(JavaVM *jvm, void *reserved)
{
    JNIEnv *env;
    jclass cls;
    if ((*jvm)->GetEnv(jvm, (void **)&env, JNI_VERSION_1_4)) {
return JNI_ERR; /* JNI version not supported */
    }
    cls = (*env)->FindClass(env, "JniClassHello");
    if (cls == NULL) {
        return JNI_ERR;
    }
//注册映射表
    if((*env)->RegisterNatives(env,cls,methods,1)<0){
        return JNI_ERR;
    }
    return JNI_VERSION_1_4;
}

4.编译和运行

javac JniClassHello.java

gcc -fPIC -I /usr/lib/jvm/java-1.7.0-openjdk-amd64/include -shared -o libhelloClass.so  hello.c

export LD_LIBRARY_PATH=./

java JniClassHello

运行结果如下

运行结果.png

新建对象

新建对象的用法和操作对象很像，只不过操作的是构造函数
1.先改写 java文件,加入create1080Screen Native方法，并调用

import model.Screen;

/**
 * Created by act64 on 2017/5/29.
 */

public class JniClassHello {

      static {
        System.loadLibrary("helloClass");
    }
    

    public static void main(String[] args){
        Screen s=new Screen(1440,720);
        s.printInfo();
        Screen screenBigger = changeScreen(s);
        screenBigger.printInfo();
        if (screenBigger.equals(s)){
            System.out.println("屏幕是同一块");
        }
        create1080Screen().printInfo();
    }

    public native static Screen changeScreen(Screen screen);
    public native static Screen create1080Screen();
}

2.修改hello.c程序,加入对于create1080Screen的支持

#include <stdio.h>
#include <jni.h>
#include <stdlib.h>
jobject c_hello(JNIEnv * env, jobject mJobject,jobject jScreenObj){
    jclass screenCls = (*env)->GetObjectClass(env, jScreenObj);
    jmethodID setHight =
    (*env)->GetMethodID(env, screenCls, "setHeight", "(I)V");
    if (setHight == NULL) {
        return; /* method not found */
    }
    printf("ChangeScreenHeightTo 800\n");
    (*env)->CallVoidMethod(env, jScreenObj, setHight,800);
    return jScreenObj;
}

jobject c_hello_create(JNIEnv * env, jobject mJobject){
    //通过名称查找类
    jclass screenCls = (*env)->FindClass(env, "model/Screen");
    if (screenCls == NULL)
    {
        return ;
    }
//查找引用 Screen类的构造函数，参数为两个int
    jmethodID cid = (*env)->GetMethodID(env, screenCls,"<init>", "(II)V");

    if (cid == NULL)
    {
        return ;
    }
// 调用java 构造函数
//类似  new Screen(1920,1080)
    jobject jScreenObj=(*env)->NewObject(env, screenCls, cid, 1920,1080);

    
    printf("Create Screen 1920*1080 \n");
//free memory
    (*env)->DeleteLocalRef(env, screenCls);

    return jScreenObj;
}
//扩充jni的函数映射
//RegisterNatives的参数也要变
const JNINativeMethod methods[]={
    {"changeScreen","(Lmodel/Screen;)Lmodel/Screen;",(jobject *)c_hello},
    {"create1080Screen","()Lmodel/Screen;",(jobject *)c_hello_create},
};


JNIEXPORT jint JNICALL
JNI_OnLoad(JavaVM *jvm, void *reserved)
{
    JNIEnv *env;
    jclass cls;
    if ((*jvm)->GetEnv(jvm, (void **)&env, JNI_VERSION_1_4)) {
return JNI_ERR; /* JNI version not supported */
    }
    cls = (*env)->FindClass(env, "JniClassHello");
    if (cls == NULL) {
        return JNI_ERR;
    }

//加载的数量改为2
    if((*env)->RegisterNatives(env,cls,methods,2)<0){
        return JNI_ERR;
    }
    return JNI_VERSION_1_4;
}

3.编译运行

javac JniClassHello.java

gcc -fPIC -I /usr/lib/jvm/java-1.7.0-openjdk-amd64/include -shared -o libhelloClass.so  hello.c

export LD_LIBRARY_PATH=./

java JniClassHello

结果如下

运行结果

小结

系列三篇博客比较完整的叙述jni的入门部分，仔细阅读很快就能上手入门jni，剩下的技能提高只能在查询官方文档和实际项目中提升了，大家共同学习，共同进步^_