JNI 对象处理 (转)

JNI 的基本问题就是解决 Java 和 C++ 代码互相调用的通信问题，在 C++ 代码编写过程中最大的问题莫过于适应其中的代码编写规则，C++调用或是返回的内容必须遵守 JVM 和 C++ 代码的通信规则。

C++ 调用 Java 的一般步骤如下：

获得类：
- jclass cls = env->FindClass("com/ldq/Student");
- cls 可认为是类的句柄
- "com/ldq/Student" 就是类文件，注意不能用 "com.ldq.Student"
获得方法：
- jmethodID mid = env->GetMethodID(cls,"<init>","()V");
- 以上为构造函数，参数是 "<init>" "()V"
- jmethodID mid = env->GetMethodID(cls,"getAge","()I");
- 以上为类的方法，第一个参数是类句柄，第二个参数是方法名字，第三个参数是签名标识

Java类型	符号
boolean	Z
byte	B
char	C
short	S
int	I
long	L
float	F
double	D
void	V
objects对象	Lfully-qualified-class-name; L类名;
Arrays数组	[array-type [数组类型
methods方法	(argument-types)return-type(参数类型)返回类型

获得对象：
- jobject obj=env->NewObject(cls,mid);
- 以上便获得了一个对象的句柄
获得对象成员变量：
- jfieldID fid=env->GetFieldID(cls,"age","I");
- 以上和获得类方法差不多
操作成员变量：
- jint a=env->GetIntField(obj,mid);
- age=age+10;
- env->SetIntField(obj,fid,a);
返回：
- return obj;

下面是本人练习的例子

ExList.java

package com.ldq.list;

import java.util.List;

public class ExList {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("-------WifiManager.test()");
        System.out.println(WifiManager.test());

        System.out.println("-------WifiManager.testArray()");
        String[] s1 = WifiManager.testArray();
        for (int i = 0; i < s1.length; i++) {
            System.out.println(s1[i]);
        }

        System.out.println("-------WifiManager.testObject()");
        System.out.println(WifiManager.testObject().ssid);
        System.out.println(WifiManager.testObject().mac);
        System.out.println(WifiManager.testObject().level);

        System.out.println("-------WifiManager.getScanResultsA()");
        ScanResult[] s2 = WifiManager.getScanResultsA();
        for (int i = 0; i < s2.length; i++) {
            System.out.println(s2[i].ssid);
            System.out.println(s2[i].mac);
            System.out.println(s2[i].level);
        }

        System.out.println("-------WifiManager.getScanResults()");
        List<ScanResult> list = WifiManager.getScanResults();
        System.out.println(list.get(0).ssid);
        System.out.println(list.get(0).mac);
        System.out.println(list.get(0).level);

    }
}

ScanResult.java

package com.ldq.list;

public class ScanResult {
    String ssid;
    String mac;
    int level;

    public ScanResult() {
    }

    public ScanResult(String ssid, String mac, int level) {
        this.ssid = ssid;
        this.mac = mac;
        this.level = level;
    }
}

WifiManager.java

package com.ldq.list;

import java.util.List;

public class WifiManager {

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

    public native static String test();

    public native static String[] testArray();

    public native static ScanResult testObject();

    public native static ScanResult[] getScanResultsA();

    public native static List<ScanResult> getScanResults();
}

wifi.cpp

#include <jni.h>
#include "com_ldq_list_WifiManager.h"

/*
 * Class:     com_ldq_list_WifiManager
 * Method:    test
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_ldq_list_WifiManager_test (JNIEnv *env, jclass cls)
{
    return env->NewStringUTF("hello");
}

/*
 * Class:     com_ldq_list_WifiManager
 * Method:    testArray
 * Signature: ()[Ljava/lang/String;
 */
JNIEXPORT jobjectArray JNICALL Java_com_ldq_list_WifiManager_testArray (JNIEnv *env, jclass cls)
{
    jobjectArray ret;
    int i;

    char *message[5]= {"first", 
    "second", 
    "third", 
    "fourth", 
    "fifth"};

    ret= (jobjectArray)env->NewObjectArray(5,
         env->FindClass("java/lang/String"),
         env->NewStringUTF(""));

    for(i=0;i<5;i++) {
        env->SetObjectArrayElement(
        ret,i,env->NewStringUTF(message[i]));
    }
    return(ret);

}

/*
 * Class:     com_ldq_list_WifiManager
 * Method:    testObject
 * Signature: ()Lcom/ldq/list/ScanResult;
 */
JNIEXPORT jobject JNICALL Java_com_ldq_list_WifiManager_testObject (JNIEnv *env, jclass cls)
{
    jclass m_cls  = env->FindClass("com/ldq/list/ScanResult");
    jmethodID mid = env->GetMethodID(m_cls,"<init>","()V");
    jobject obj   = env->NewObject(m_cls,mid);

    jfieldID fid_ssid  = env->GetFieldID(m_cls,"ssid","Ljava/lang/String;");
    jfieldID fid_mac   = env->GetFieldID(m_cls,"mac","Ljava/lang/String;");
    jfieldID fid_level = env->GetFieldID(m_cls,"level","I");

                         env->SetObjectField(obj,fid_ssid,env->NewStringUTF("AP1"));
                         env->SetObjectField(obj,fid_mac,env->NewStringUTF("00-11-22-33-44-55"));
                         env->SetIntField(obj,fid_level,-66);
    return obj;
}

/*
 * Class:     com_ldq_list_WifiManager
 * Method:    getScanResultsA
 * Signature: ()[Lcom/ldq/list/ScanResult;
 */
JNIEXPORT jobjectArray JNICALL Java_com_ldq_list_WifiManager_getScanResultsA (JNIEnv *env, jclass cls)
{
    jclass cls_array=env->FindClass("java/lang/Object");
    jobjectArray obj_array=env->NewObjectArray(2,cls_array,0);

    jclass cls_obj = env->FindClass("com/ldq/list/ScanResult");
    jmethodID m    = env->GetMethodID(cls_obj,"<init>","()V");

    jfieldID fid_ssid  = env->GetFieldID(cls_obj,"ssid","Ljava/lang/String;");
    jfieldID fid_mac   = env->GetFieldID(cls_obj,"mac","Ljava/lang/String;");
    jfieldID fid_level = env->GetFieldID(cls_obj,"level","I");

    for(int i=0;i<2;i++)
    {
        jobject obj=env->NewObject(cls_obj,m);

        jobject o1=env->NewStringUTF("AP2");
        env->SetObjectField(obj,fid_ssid,o1);

        jobject o2=env->NewStringUTF("22-22-22-22-22-22");
        env->SetObjectField(obj,fid_mac,o2);

        env->SetIntField(obj,fid_level,-66);

        env->SetObjectArrayElement(obj_array,i,obj);
    }
    return obj_array;
}

/*
 * Class:     com_ldq_list_WifiManager
 * Method:    getScanResults
 * Signature: ()Ljava/util/List;
 */
JNIEXPORT jobject JNICALL Java_com_ldq_list_WifiManager_getScanResults (JNIEnv *env, jclass cls)
{
    jclass m_cls_list    = env->FindClass("java/util/ArrayList");
    jmethodID m_mid_list = env->GetMethodID(m_cls_list,"<init>","()V");
    jobject m_obj_list   = env->NewObject(m_cls_list,m_mid_list);

    jmethodID m_mid_add  = env->GetMethodID(m_cls_list,"add","(Ljava/lang/Object;)Z");

    jclass m_cls_result    = env->FindClass("com/ldq/list/ScanResult");
    jmethodID m_mid_result = env->GetMethodID(m_cls_result,"<init>","()V");
    jobject m_obj_result   = env->NewObject(m_cls_result,m_mid_result);

    jfieldID m_fid_1 = env->GetFieldID(m_cls_result,"ssid","Ljava/lang/String;");
    jfieldID m_fid_2 = env->GetFieldID(m_cls_result,"mac","Ljava/lang/String;");
    jfieldID m_fid_3 = env->GetFieldID(m_cls_result,"level","I");

    env->SetObjectField(m_obj_result,m_fid_1,env->NewStringUTF("AP6"));
    env->SetObjectField(m_obj_result,m_fid_2,env->NewStringUTF("66-66-66-66-66-66"));
    env->SetIntField(m_obj_result,m_fid_3,-66);

    env->CallBooleanMethod(m_obj_list,m_mid_add,m_obj_result);

    return m_obj_list;
}

转自：http://leidiqiu.iteye.com/blog/720307

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原文地址：https://www.cnblogs.com/chensheng-zhou/p/JNI.html