JNI 的基本問題就是解決 Java 和 C++ 代碼互相調用的通信問題,在 C++ 代碼編寫過程中最大的問題莫過於適應其中的代碼編寫規則,C++調用或是返回的內容必須遵守 JVM 和 C++ 代碼的通信規則。
C++ 調用 Java 的一般步驟如下:
-
獲得類:
- jclass cls = env->FindClass("com/ldq/Student");
- cls 可認爲是類的句柄
- "com/ldq/Student" 就是類文件,注意不能用 "com.ldq.Student"
-
獲得方法:
- jmethodID mid = env->GetMethodID(cls,"<init>","()V");
- 以上爲構造函數,參數是 "<init>" "()V"
- jmethodID mid = env->GetMethodID(cls,"getAge","()I");
- 以上爲類的方法,第一個參數是類句柄,第二個參數是方法名字,第三個參數是簽名標識
Java類型 |
符號 |
boolean | Z |
byte |
B |
char |
C |
short | S |
int | I |
long | L |
float | F |
double |
D |
void |
V |
objects對象 | Lfully-qualified-class-name; L類名; |
Arrays數組 | [array-type [數組類型 |
methods方法 | (argument-types)return-type(參數類型)返回類型 |
-
獲得對象:
- jobject obj=env->NewObject(cls,mid);
- 以上便獲得了一個對象的句柄
-
獲得對象成員變量:
- jfieldID fid=env->GetFieldID(cls,"age","I");
- 以上和獲得類方法差不多
-
操作成員變量:
- jint a=env->GetIntField(obj,mid);
- age=age+10;
- env->SetIntField(obj,fid,a);
-
返回:
- return obj;
下面是本人練習的例子
- 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;
}