前言
jni部分系列文章
- <a href="http://www.jianshu.com/p/476aae319808"> jni基本数据类型的传递</a>
- <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_
