JNI由浅入深

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)：

[java] view
plaincopy

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：

[java] view
plaincopy

/* 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)

[c-sharp] view
plaincopy

#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)：

[java] view
plaincopy

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)：

[c-sharp] view
plaincopy

/* 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)

[java] view
plaincopy

#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)

[c-sharp] view
plaincopy

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)：

[java] view
plaincopy

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)：

[c-sharp] view
plaincopy

#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)

[c-sharp] view
plaincopy

#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)

[java] view
plaincopy

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)

[java] view
plaincopy

#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)

[java] view
plaincopy

#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：u_xtian@qq.com

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