Java JNI由浅入深(包括：Java和C++互传ArrayList泛型对象参数)

Java JNI由浅入深(包括：Java和C++互传ArrayList泛型对象参数)

 

         我们知道Java是一个运行在虚拟机里面的高级的编程语言，如果要调用系统的动态链接库的话，就要先声明native修饰的方法(类似接口里面的方法)，再由C/C++程序来实现(类似实现接口里的方法)。这样Java调用这些native方法就相当于调用了C/C++里面实现了的方法。通常我们把这种机制叫做JNI(Java NativeInterfac)，即Java 本地编程接口

          Android也同理，要学会在Android上进行NDK开发，首先我们到打好java JNI的基础。现在我们暂时把Android开发丢到一边先，试试在Java之下编译一个C动态链接库，再用Java程序调用。

 

1)先来个最简单的打印HelloWorld例子：

Java代码(HelloJni.java)：

import java.util.*;
public class HelloJni{

    static{
        System.loadLibrary("hello");
    }
    public native static void sayHello();
    public static void main(String [] args)
    {
                HelloJni.sayHello();
    }
}

 

生成头文件(HelloJni.h)：

先javac HelloJni.java编译你的Java源码，再javah–jni HelloJni生成所需的头文件

头文件内容是这样的：

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloJni */
#ifndef _Included_HelloJni
#define _Included_HelloJni
#ifdef __cplusplus
extern "C" {
#endif
JNIEXPORT void JNICALL Java_HelloJni_sayHello (JNIEnv *, jclass);
#ifdef __cplusplus
}
#endif
#endif

 

实现头文件声明的方法(HelloJni.cpp)

#include "HelloJni.h"
JNIEXPORT void JNICALL Java_HelloJni_sayHello(JNIEnv *env, jclass cls)
{
       printf("HelloWorld");
}

 

 

编译命令: g++ -I$JAVA_HOME/include-I$JAVA_HOME/include/linux HelloJni.cpp -shared -o libhello.so

命令参数解析：-I 是指引入java虚拟机的库的路径，-shared 是指编译成动态链接库(共享库) –o 输出文件名(注意，在Linux平台下的动态链接库有一个命名格式：“lib+库名+.so”在java代码里面loadLibrary的时候不要加lib前缀和.so后缀)

由于我这里把这个动态链接库编译放在当前目录下，所以还要设置环境变量LD_LIBRARY_PATH=该so动态链接库所在的目录，才能正常运行

 

#############################+++华丽的分割线+++###########################

2)好了，可以打印HelloWorld出来后，我们再深入一点点，传入一个int的数，在C/C++代码里面加1后返回。

 

Java代码(HelloJni.java)：

public class HelloJni{
        static{
                   System.loadLibrary("hello");
        }
        public static native void sayHello();
        public native int getInt();
        public native void setInt(int i);
        public static void main(String args[]){
        //      HelloJni.sayHello();
                 HelloJni hello = new HelloJni();
                 hello.setInt(2);
                 System.out.println(hello.getInt());
        }
}

 

生成头文件(HelloJni.h)：

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloJni */
#ifndef _Included_HelloJni
#define _Included_HelloJni
#ifdef __cplusplus
extern "C" {
#endif
JNIEXPORT void JNICALL Java_HelloJni_sayHello (JNIEnv *, jclass);
JNIEXPORT jint JNICALL Java_HelloJni_getInt (JNIEnv *, jobject);
JNIEXPORT void JNICALL Java_HelloJni_setInt (JNIEnv *, jobject, jint);
#ifdef __cplusplus
}
#endif
#endif

 

实现头文件声明的方法(HelloJni.cpp)

#include "HelloJni.h"
int i=1;
JNIEXPORT void JNICALL Java_HelloJni_sayHello
  (JNIEnv *env, jclass cls){
        printf("HelloWorld/n");
}
JNIEXPORT jint JNICALL Java_HelloJni_getInt
  (JNIEnv *env, jobject thiz){
        return i;
}
JNIEXPORT void JNICALL Java_HelloJni_setInt
  (JNIEnv *env, jobject thiz,jint ji){
        i = ji+1;
}

 

运行结果：打印2

3)只是传入简单的数据类型不爽，这次让C/C++生成个Java对象返回

 

Java代码：pojo实体类(User.java)

public class User{
        private long id;
        private String userName;
        private boolean isMan;
        private int age;
        public User(){}
        public User(long id, String userName, boolean isMan, int age) {
                super();
                this.id = id;
                this.userName = userName;
                this.isMan = isMan;
                this.age = age;
        }
        public long getId() {
                return id;
        }
        public void setId(long id) {
                this.id = id;
        }
        public String getUserName() {
                return userName;
        }
        public void setUserName(String userName) {
                this.userName = userName;
        }
        public boolean isMan() {
                return isMan;
        }
        public void setMan(boolean isMan) {
                this.isMan = isMan;
        }
        public int getAge() {
                return age;
        }
        public void setAge(int age) {
                this.age = age;
        }
}

 

Java代码(HelloJni.java)：

public class HelloJni{
        static{
                System.loadLibrary("userbean");
        }
        public static native void sayHello();
        public native int getInt();
        public native void setInt(int i);
        public native void setUser(String userName);
        public native User getUser();
        public static void main(String args[]){
        //      HelloJni.sayHello();
                HelloJni hello = new HelloJni();
        //      hello.setInt(2);
        //      System.out.println(hello.getInt());
                hello.setUser("LiangYaotian");
                User user = hello.getUser();
                System.out.println("user from c/c++");
                System.out.println("name:"+user.getUserName());
                System.out.println("isMan?:"+user.isMan());
                System.out.println("age:"+user.getAge());
        }
}

 

生成头文件(HelloJni.h)：

#include <jni.h>
/* Header for class HelloJni */
#ifndef _Included_HelloJni
#define _Included_HelloJni
#ifdef __cplusplus
extern "C" {
#endif
JNIEXPORT void JNICALL Java_HelloJni_sayHello (JNIEnv *, jclass);
JNIEXPORT jint JNICALL Java_HelloJni_getInt (JNIEnv *, jobject);
JNIEXPORT void JNICALL Java_HelloJni_setInt (JNIEnv *, jobject, jint);
JNIEXPORT void JNICALL Java_HelloJni_setUser (JNIEnv *, jobject, jstring);
JNIEXPORT jobject JNICALL Java_HelloJni_getUser  (JNIEnv *, jobject);
#ifdef __cplusplus
}
#endif
#endif

 

实现头文件声明的方法(HelloJni.cpp)

#include <iostream>
#include <stdio.h>
using namespace std;
int i=1;
jobject user;
JNIEXPORT void JNICALL Java_HelloJni_sayHello
  (JNIEnv *env, jclass cls){
        printf("HelloWorld/n");
}
JNIEXPORT jint JNICALL Java_HelloJni_getInt
  (JNIEnv *env, jobject thiz){
        return i;
}
JNIEXPORT void JNICALL Java_HelloJni_setInt
  (JNIEnv *env, jobject thiz,jint ji){
        i = ji+1;
}
JNIEXPORT void JNICALL Java_HelloJni_setUser
  (JNIEnv *env, jobject thiz, jstring name){
        jclass userClass = env->FindClass("User");
        jmethodID userMethod = env->GetMethodID(userClass,"<init>","()V");
        jfieldID mId = env->GetFieldID(userClass,"id","J");
        jfieldID mUserName = env->GetFieldID(userClass,"userName","Ljava/lang/String;");
        jfieldID mIsMan = env->GetFieldID(userClass,"isMan","Z");
        jfieldID mAge = env->GetFieldID(userClass,"age","I");
        jobject userObject = env->NewObject(userClass,userMethod);
        env->SetObjectField(userObject,mUserName,name);
        env->SetLongField(userObject,mId,1001);
        env->SetBooleanField(userObject,mIsMan,1);
        env->SetIntField(userObject,mAge,21);
        user = userObject;
}

 

 

 

4)有些同学可能会说：“返回个Java对象算什么啊，C/C++和Java之间互传复杂对象才好玩呢！”正所谓“不怕做不到，就怕想不到”，我们接着重构一下上面那个例子!

这次我们传包含有User对象的List到C/C++程序里面

Java代码(User.java)

同上。

Java代码(HelloJni.java)

import java.util.*;
public class HelloJni{
        static{
                System.loadLibrary("userbean");
        }
        public native int get();
        public native void set(int i);
        public native void setUser(String userName);
        public native User getUser();
        public native void setUserList(ArrayList<User> userList);
        public native ArrayList<User> getUserList();
        public static void main(String [] args)
        {
                HelloJni hello = new HelloJni();
        //      hello.set(2);
        //      System.out.println(hello.get());
        /*
                hello.setUser("LiangYaotian");
                User user = hello.getUser();
                System.out.println("user from c/c++");
                System.out.println("name:"+user.getUserName());
                System.out.println("isMan?:"+user.isMan());
                System.out.println("age:"+user.getAge());
        */
                ArrayList<User> userList = new ArrayList<User>();
                for(int i=0;i<10;i++){
                        User u = new User((long)(1000+i),"LiangYaotian"+i,true,21);
                        userList.add(u);
                }
                hello.setUserList(userList);
                userList = null;
                userList = hello.getUserList();
                System.out.println("ArrayList<User> construct from C++,then Java print it.....");
                for(User u : userList){
                        System.out.println("id:"+u.getId()+"; name:"+u.getUserName());
                }
        }
}

 

C头文件(HelloJni.h)

#include <jni.h>
/* Header for class HelloJni */
#ifndef _Included_HelloJni
#define _Included_HelloJni
#ifdef __cplusplus
extern "C" {
#endif
JNIEXPORT void JNICALL Java_HelloJni_sayHello (JNIEnv *, jclass);
JNIEXPORT jint JNICALL Java_HelloJni_getInt (JNIEnv *, jobject);
JNIEXPORT void JNICALL Java_HelloJni_setInt (JNIEnv *, jobject, jint);
JNIEXPORT void JNICALL Java_HelloJni_setUser (JNIEnv *, jobject, jstring);
JNIEXPORT jobject JNICALL Java_HelloJni_getUser  (JNIEnv *, jobject);

 

头文件的实现(HelloJni.cpp)

#include "HelloJni.h"
int i=1;
jobject user;
JNIEXPORT jint JNICALL Java_HelloJni_get
  (JNIEnv *env, jobject jthiz){
        printf("HelloWorld/n");
}
JNIEXPORT void JNICALL Java_HelloJni_set
  (JNIEnv *env, jobject jthiz, jint ji){
        i = ji+1;
}
JNIEXPORT void JNICALL Java_HelloJni_setUser
  (JNIEnv *env, jobject jthiz, jstring name){
        jclass userClass = env->FindClass("User");
        jmethodID userMethod = env->GetMethodID(userClass,"<init>","()V");
        jfieldID mId = env->GetFieldID(userClass,"id","J");
        jfieldID mUserName = env->GetFieldID(userClass,"userName","Ljava/lang/String;");
        jfieldID mIsMan = env->GetFieldID(userClass,"isMan","Z");
        jfieldID mAge = env->GetFieldID(userClass,"age","I");
        jobject userObject = env->NewObject(userClass,userMethod);
        env->SetObjectField(userObject,mUserName,name);
        env->SetLongField(userObject,mId,1001);
        env->SetBooleanField(userObject,mIsMan,1);
        env->SetIntField(userObject,mAge,21);
        user = userObject;
}
JNIEXPORT jobject JNICALL Java_HelloJni_getUser
  (JNIEnv *env, jobject jthiz){
        return user;
}
JNIEXPORT void JNICALL Java_HelloJni_setUserList
  (JNIEnv *env, jobject jthiz, jobject userList){
        int i;
        //class ArrayList
        jclass cls_arraylist = env->GetObjectClass(userList);
        //method in class ArrayList
        jmethodID arraylist_get = env->GetMethodID(cls_arraylist,"get","(I)Ljava/lang/Object;");
        jmethodID arraylist_size = env->GetMethodID(cls_arraylist,"size","()I");
        jint len = env->CallIntMethod(userList,arraylist_size);
        printf("get java ArrayList<User> object by C++ , then print it...../n");
        for(i=0;i<len;i++){
                jobject obj_user = env->CallObjectMethod(userList,arraylist_get,i);
                jclass cls_user = env->GetObjectClass(obj_user);
                jmethodID user_getId = env->GetMethodID(cls_user,"getId","()J");
                jmethodID user_getUserName = env->GetMethodID(cls_user,"getUserName","()Ljava/lang/String;");
                jmethodID user_isMan = env->GetMethodID(cls_user,"isMan","()Z");
                jmethodID user_getAge = env->GetMethodID(cls_user,"getAge","()I");
                jstring name = (jstring)env->CallObjectMethod(obj_user,user_getUserName);
                jboolean b = true;
                const char *namePtr = env->GetStringUTFChars(name,&b);
                jlong id = env->CallLongMethod(obj_user,user_getId);
                jboolean sex = env->CallBooleanMethod(obj_user,user_isMan);
                jint age = env->CallIntMethod(obj_user,user_getAge);
                printf("Id:%d; ",id);
                printf("Name:%s; ",namePtr);
                printf("isMan? %d; ",sex);
                printf("Age:%d /n ",age);
        }
}
JNIEXPORT jobject JNICALL Java_HelloJni_getUserList
  (JNIEnv *env, jobject jthiz){
        //ArrayList Object
        jclass cls_ArrayList = env->FindClass("java/util/ArrayList");
        jmethodID construct = env->GetMethodID(cls_ArrayList,"<init>","()V");
        jobject obj_ArrayList = env->NewObject(cls_ArrayList,construct,"");
        jmethodID arrayList_add = env->GetMethodID(cls_ArrayList,"add","(Ljava/lang/Object;)Z");
        //User Object
        jclass cls_user = env->FindClass("User");
        //none argument construct function
        jmethodID construct_user = env->GetMethodID(cls_user,"<init>","()V");
        //new a object
        jobject obj_user = env->NewObject(cls_user,construct_user,"");
        //get method id
        /*
        jmethodID user_setId = env->GetMethodID(cls_user,"setId","(J)V");
        jmethodID user_setUserName = env->GetMethodID(cls_user,"setUserName","(Ljava/lang/String;)V");
        jmethodID user_setMan = env->GetMethodID(cls_user,"setMan","(Z)V");
        jmethodID user_setAge = env->GetMethodID(cls_user,"setAge","(I)V");
        */
        int i;
        for(i=0;i<10;i++){
        //new a object
        jobject obj_user = env->NewObject(cls_user,construct_user,"");
        //get field id
        jfieldID user_id = env->GetFieldID(cls_user,"id","J");
        jfieldID user_name = env->GetFieldID(cls_user,"userName","Ljava/lang/String;");
        jfieldID user_isMan = env->GetFieldID(cls_user,"isMan","Z");
        jfieldID user_age = env->GetFieldID(cls_user,"age","I");
        env->SetLongField(obj_user,user_id,i);
        env->SetObjectField(obj_user,user_name,env->NewStringUTF("LiangYaoTian"));
        env->SetBooleanField(obj_user,user_isMan,1);
        env->SetIntField(obj_user,user_age,21);
                env->CallObjectMethod(obj_ArrayList,arrayList_add,obj_user);
        }
        return obj_ArrayList;
}

 

运行结果：

 

 

源码：E-Mail我：[email protected]

原创作品，转载需注明出处...http://blog.csdn.net/u_xtian/archive/2010/11/25/6033963.aspx