JNI 对象处理

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;
}

 

 

评论

3 楼 zhangdong92 2016-05-26  

ihopethatwell 写道

楼主，直接return env->NewStringUTF("hello");   不会有内存溢出？

内存溢出？是指内存泄露吗？
我刚才也有这个疑问，参考这个
jni内存泄露 - 饭特稀 - BlogJava
http://www.blogjava.net/neumqp/archive/2006/03/02/33152.html
返回java的env->New...的对象不需要手动释放，其它的都需要，包括在c中env->NewObjectClass生成的没有返回java的对象

2 楼 ihopethatwell 2013-04-03  

楼主，直接return env->NewStringUTF("hello");   不会有内存溢出？

1 楼 kissdarkwind 2011-11-21  

如何释放这个对象呢，仅仅从java端置为null可以吗？还是从jni这里也要释放？jni这边new出来的是一个local reference呢还是一个global reference？ 亦或是一个weak reference？