"Go" JNI object processing

Original URL: http://leidiqiu.iteye.com/blog/720307

The basic problem of JNI is to solve the communication problem that the Java and C + + code call each other, the biggest problem in the writing process of C + + code is to adapt to the code writing rules, C + + call or return content must obey the rules of communication between JVM and C + + code.

The general procedure for calling Java in C + + is as follows:

• Get class:
  • Jclass cls = Env->findclass ("com/ldq/student");
  • The CLS can be considered a handle to a class
  • "Com/ldq/student" is a class file, note that "com.ldq.Student" cannot be used
• How to obtain:
  • Jmethodid mid = Env->getmethodid (CLS, "<init>", "() V");
  • The above is a constructor, the parameter is "<init>" "() V"
  • Jmethodid mid = Env->getmethodid (CLS, "Getage", "() I");
  • The above is the method of the class, the first argument is the class handle, the second argument is the method name, the third parameter is the signature identity

td>z 

java type	symbol
boolean
Byte	b
Char	C &N BSP;
short	s
int	i
long	l
float	f
Double	d
Void	v
Objects Object	lfully-qualified-class-name;        L class name;
Arrays array	[array-type [Array type
Methods Method     &nb sp;	(Argument-types) Return-type (parameter type) return type

• Get the object:
  • Jobject Obj=env->newobject (Cls,mid);
  • The above gets the handle of an object
• To get an object member variable:
  • Jfieldid Fid=env->getfieldid (CLS, "Age", "I");
  • The above and the obtained class method are almost
• Action member Variable:
  • Jint A=env->getintfield (Obj,mid);
  • age=age+10;
  • Env->setintfield (Obj,fid,a);
• Return:
  • return obj;

Here is an example of my practice

• Exlist.java

Java code

2. Package com.ldq.list;
6. Import java.util.List;
10. public class Exlist {
14. /**
16. * @param args
18. */
20. public static void Main (string[] args) {
22. TODO auto-generated Method Stub
24. System.out.println ("-------wifimanager.test ()");
26. System.out.println (Wifimanager.test ());
30. System.out.println ("-------Wifimanager.testarray ()");
32. string[] S1 = Wifimanager.testarray ();
34. for (int i = 0; i < s1.length; i++) {
36. System.out.println (S1[i]);
38. }
42. System.out.println ("-------Wifimanager.testobject ()");
44. System.out.println (Wifimanager.testobject (). SSID);
46. System.out.println (Wifimanager.testobject (). Mac);
48. System.out.println (Wifimanager.testobject (). level);
52. System.out.println ("-------Wifimanager.getscanresultsa ()");
54. scanresult[] s2 = Wifimanager.getscanresultsa ();
56. for (int i = 0; i < s2.length; i++) {
58. System.out.println (S2[I].SSID);
60. System.out.println (S2[I].MAC);
62. System.out.println (S2[i].level);
64. }
68. System.out.println ("-------wifimanager.getscanresults ()");
70. list<scanresult> list = Wifimanager.getscanresults ();
72. System.out.println (List.get (0). SSID);
74. System.out.println (List.get (0). Mac);
76. System.out.println (List.get (0). level);
80. }
82. }

• Scanresult.java

Java code

2. Package com.ldq.list;
6. public class Scanresult {
8. String SSID;
10. String mac;
12. int level;
16. Public Scanresult () {
18. }
22. Public Scanresult (String SSID, String Mac, int. level) {
24. This.ssid = SSID;
26. This.mac = Mac;
28. This.level = level;
30. }
32. }

• Wifimanager.java

Java code

2. Package com.ldq.list;
6. Import java.util.List;
10. public class Wifimanager {
14. static {
16. System.loadlibrary ("WiFi");
18. }
22. Public native static String test ();
26. Public native static string[] Testarray ();
30. Public native static Scanresult Testobject ();
34. Public native static scanresult[] Getscanresultsa ();
38. Public native static list<scanresult> getscanresults ();
40. }

• Wifi.cpp

CPP Code

2. #include <jni.h>
4. #include "Com_ldq_list_wifimanager.h"
8. /*
10. * Class:com_ldq_list_wifimanager
12. * Method:test
14. * Signature: () ljava/lang/string;
16. */
18. Jniexport jstring jnicall java_com_ldq_list_wifimanager_test (jnienv *env, Jclass CLS)
20. {
22. return Env->newstringutf ("Hello");
24. }
28. /*
30. * Class:com_ldq_list_wifimanager
32. * Method:testarray
34. * Signature: () [ljava/lang/string;
36. */
38. Jniexport jobjectarray jnicall Java_com_ldq_list_wifimanager_testarray (jnienv *env, Jclass CLS)
40. {
42. Jobjectarray ret;
44. int i;
48. Char *message[5]= {"First",
50. "Second",
52. "Third",
54. "Fourth",
56. "Fifth"};
60. ret= (Jobjectarray) Env->newobjectarray (5,
62. Env->findclass ("java/lang/string"),
64. Env->newstringutf (""));
68. for (i=0;i<5;i++) {
70. Env->setobjectarrayelement (
72. Ret,i,env->newstringutf (Message[i]));
74. }
76. return (ret);
80. }
84. /*
86. * Class:com_ldq_list_wifimanager
88. * Method:testobject
90. * Signature: () Lcom/ldq/list/scanresult;
92. */
94. Jniexport jobject jnicall java_com_ldq_list_wifimanager_testobject (jnienv *env, Jclass CLS)
96. {
98. Jclass m_cls = Env->findclass ("Com/ldq/list/scanresult");
100. Jmethodid mid = Env->getmethodid (M_cls, "<init>", "() V");
102. Jobject obj = Env->newobject (m_cls,mid);
106. Jfieldid Fid_ssid = Env->getfieldid (M_cls, "SSID", "ljava/lang/string;");
108. Jfieldid Fid_mac = Env->getfieldid (M_cls, "Mac", "ljava/lang/string;");
110. Jfieldid fid_level = Env->getfieldid (M_cls, "level", "I");
114. Env->setobjectfield (Obj,fid_ssid,env->newstringutf ("AP1"));
116. Env->setobjectfield (Obj,fid_mac,env->newstringutf ("00-11-22-33-44-55"));
118. Env->setintfield (obj,fid_level,-66);
120. return obj;
122. }
126. /*
128. * Class:com_ldq_list_wifimanager
130. * Method:getscanresultsa
132. * Signature: () [Lcom/ldq/list/scanresult;
134. */
136. Jniexport jobjectarray jnicall Java_com_ldq_list_wifimanager_getscanresultsa (jnienv *env, Jclass CLS)
138. {
140. Jclass Cls_array=env->findclass ("Java/lang/object");
142. Jobjectarray Obj_array=env->newobjectarray (2,cls_array,0);
146. Jclass cls_obj = Env->findclass ("Com/ldq/list/scanresult");
148. Jmethodid m = Env->getmethodid (Cls_obj, "<init>", "() V");
152. Jfieldid Fid_ssid = Env->getfieldid (Cls_obj, "SSID", "ljava/lang/string;");
154. Jfieldid Fid_mac = Env->getfieldid (Cls_obj, "Mac", "ljava/lang/string;");
156. Jfieldid fid_level = Env->getfieldid (Cls_obj, "level", "I");
160. for (int i=0;i<2;i++)
162. {
164. Jobject Obj=env->newobject (cls_obj,m);
168. Jobject O1=env->newstringutf ("AP2");
170. Env->setobjectfield (OBJ,FID_SSID,O1);
174. Jobject O2=env->newstringutf ("22-22-22-22-22-22");
176. Env->setobjectfield (OBJ,FID_MAC,O2);
180. Env->setintfield (obj,fid_level,-66);
184. Env->setobjectarrayelement (Obj_array,i,obj);
186. }
188. return obj_array;
190. }
194. /*
196. * Class:com_ldq_list_wifimanager
198. * Method:getscanresults
200. * Signature: () ljava/util/list;
202. */
204. Jniexport jobject jnicall java_com_ldq_list_wifimanager_getscanresults (jnienv *env, Jclass CLS)
206. {
208. Jclass m_cls_list = Env->findclass ("Java/util/arraylist");
210. Jmethodid m_mid_list = Env->getmethodid (m_cls_list, "<init>", "() V");
212. Jobject m_obj_list = Env->newobject (m_cls_list,m_mid_list);
216. Jmethodid M_mid_add = Env->getmethodid (m_cls_list, "Add", "(Ljava/lang/object;) Z");
220. Jclass M_cls_result = Env->findclass ("Com/ldq/list/scanresult");
222. Jmethodid M_mid_result = Env->getmethodid (M_cls_result, "<init>", "() V");
224. Jobject M_obj_result = Env->newobject (M_cls_result,m_mid_result);
228. Jfieldid m_fid_1 = Env->getfieldid (M_cls_result, "SSID", "ljava/lang/string;");
230. Jfieldid m_fid_2 = Env->getfieldid (M_cls_result, "Mac", "ljava/lang/string;");
232. Jfieldid m_fid_3 = Env->getfieldid (M_cls_result, "level", "I");
236. Env->setobjectfield (M_obj_result,m_fid_1,env->newstringutf ("AP6"));
238. Env->setobjectfield (M_obj_result,m_fid_2,env->newstringutf ("66-66-66-66-66-66"));
240. Env->setintfield (m_obj_result,m_fid_3,-66);
244. Env->callbooleanmethod (M_obj_list,m_mid_add,m_obj_result);
248. return m_obj_list;
250. }

"Go" JNI object processing