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