How to call C code in Java-(implement the Java local method in Linux)

Original article: Java calls C (implementing Java local methods in Linux)

 

 

1. Declare local methods in the Java class and use the native keyword

 

1. Public class mynative {
2. Public void showparms (string S, int I, Boolean B ){
3. Showparms0 (S, I, B );
4. }
5. Public int hypotenuse (int A, int B ){
6. Return hypotenuse0 (A, B );
7. }
8. Public void setarray (Boolean [] BA ){
9. For (INT I = 0; I <ba. length; I ++)
10. Ba [I] = true;
11. Setarray0 (BA );
12. }
13. Public void showstrings (string [] SA ){
14. Showstrings0 (SA );
15. }
16. Public String [] getstrings (){
17. Return getstrings0 ();
18. }
19. Private native void showparms0 (string S, int I, Boolean B );
20. Private native int hypotenuse0 (int A, int B );
21. Private native void setarray0 (Boolean [] BA );
22. Private native void showstrings0 (string [] SA );
23. Private native string [] getstrings0 ();
24. Static {
25. System. loadlibrary ("mynative ");
26. }
27. Public static void main (string [] ARGs ){
28. Mynative OBJ = new mynative ();
29. OBJ. showparms ("hello", 23, true );
30. OBJ. showparms ("world", 34, false );
31. System. Out. println (obj. hypotenuse (3, 4 ));
32. System. Out. println (obj. hypotenuse (9, 12 ));
33. Boolean [] BA = new Boolean [5];
34. OBJ. setarray (BA );
35. For (INT I = 0; I <ba. length; I ++)
36. System. Out. println (BA [I]);
37. String [] SA = new string [] {"hello,", "World! "," JNI "," is "," Fun ."};
38. OBJ. showstrings (SA );
39. OBJ. showstrings (obj. getstrings ());
40. }
41. }

 

Loadlibrary

public static void loadLibrary(String libname)

Loads the system library specified by libnameArgument. The manner in which a library name is mapped to the actual system library is system dependent.

The callSystem.loadLibrary(name)Is required tively equivalent to the call

 Runtime.getRuntime().loadLibrary(name) 

Parameters:

libname-The name of the library.

Throws:

SecurityException -If a security manager exists and its checkLinkMethod doesn' t allow loading of the specified dynamic library

UnsatisfiedLinkError -If the library does not exist.

NullPointerException -If libnameIs null

See also:

Runtime.loadLibrary(java.lang.String), SecurityManager.checkLink(java.lang.String)

Ii. Generate. Class and. H files

1. Javac mynative. Class
2. Javah-JNI mynative

 

The generated. h file is as follows:

 

1. /* Do not edit this file-it is machine generated */
2. # Include <JNI. h>
3. /* Header for class mynative */
4. # Ifndef _ included_mynative
5. # DEFINE _ included_mynative
6. # Ifdef _ cplusplus
7. Extern "C "{
8. # Endif
9. /*
10. * Class: mynative
11. * Method: getstrings0
12. * Signature: () [ljava/lang/string;
13. */
14. Jniexport jobjectarray jnicall java_mynative_getstrings0
15. (Jnienv *, jobject );
16. /*
17. * Class: mynative
18. * Method: showstrings0
19. * Signature: ([ljava/lang/string;) V
20. */
21. Jniexport void jnicall java_mynative_showstrings0
22. (Jnienv *, jobject, jobjectarray );
23. /*
24. * Class: mynative
25. * Method: showparms0
26. * Signature: (ljava/lang/string; IZ) V
27. */
28. Jniexport void jnicall java_mynative_showparms0
29. (Jnienv *, jobject, jstring, jint, jboolean );
30. /*
31. * Class: mynative
32. * Method: hypotenuse0
33. * Signature: (ii) I
34. */
35. Jniexport jint jnicall java_mynative_hypotenuse0
36. (Jnienv *, jobject, jint, jint );
37. /*
38. * Class: mynative
39. * Method: setarray0
40. * Signature: ([Z) V
41. */
42. Jniexport void jnicall java_mynative_setarray0
43. (Jnienv *, jobject, jbooleanarray );
44. # Ifdef _ cplusplus
45. }
46. # Endif
47. # Endif

 

3. Implement the methods declared in. h file in C Language

Include the generated. h file in the header file and use the C language to implement these local methods. The Code is as follows:

 

1. # Include <stdio. h>
2. # Include <math. h>
3. # Include "mynative. H"
4. Jniexport void jnicall java_mynative_showparms0
5. (Jnienv * ENV, jobject OBJ, jstring S, jint I, jboolean B)
6. {
7. Const char * szstr = (* env)-> getstringutfchars (ENV, S, 0 );
8. Printf ("string = [% s]/n", szstr );
9. Printf ("Int = % d/N", I );
10. Printf ("Boolean = % s/n", (B = jni_true? "True": "false "));
11. (* Env)-> releasestringutfchars (ENV, S, szstr );
12. }
13. Jniexport jint jnicall java_mynative_hypotenuse0
14. (Jnienv * ENV, jobject OBJ, jint A, jint B)
15. {
16. Int RTN = (INT) SQRT (double) (A * A) + (B * B )));
17. Return (jint) RTN;
18. }
19. Jniexport void jnicall java_mynative_setarray0
20. (Jnienv * ENV, jclass CLs, jbooleanarray BA)
21. {
22. Jboolean * PBA = (* env)-> getbooleanarrayelements (ENV, Ba, 0 );
23. Jsize Len = (* env)-> getarraylength (ENV, BA );
24. Int I = 0;
25. // Change the even array element
26. For (I = 0; I <Len; I + = 2)
27. PBA [I] = jni_false;
28. (* Env)-> releasebooleanarrayelements (ENV, Ba, PBA, 0 );
29. }
30. Jniexport void jnicall java_mynative_showstrings0
31. (Jnienv * ENV, jclass CLs, jobjectarray SA)
32. {
33. Int Len = (* env)-> getarraylength (ENV, SA );
34. Int I = 0;
35. For (I = 0; I <Len; I ++)
36. {
37. Jobject OBJ = (* env)-> getobjectarrayelement (ENV, SA, I );
38. Jstring STR = (jstring) OBJ;
39. Const char * szstr = (* env)-> getstringutfchars (ENV, STR, 0 );
40. Printf ("% s", szstr );
41. (* Env)-> releasestringutfchars (ENV, STR, szstr );
42. }
43. Printf ("/N ");
44. }
45. Jniexport jobjectarray jnicall java_mynative_getstrings0
46. (Jnienv * ENV, jclass CLs)
47. {
48. Jstring STR;
49. Jobjectarray ARGs = 0;
50. Jsize Len = 5;
51. Char * Sa [] = {"hello,", "World! "," JNI "," is "," fun "};
52. Int I = 0;
53. ARGs = (* env)-> newobjectarray (ENV, Len, (* env)-> findclass (ENV, "Java/lang/string"), 0 );
54. For (I = 0; I <Len; I ++)
55. {
56. STR = (* env)-> newstringutf (ENV, sa [I]);
57. (* Env)-> setobjectarrayelement (ENV, argS, I, STR );
58. }
59. Return ARGs;
60. }

 

4. compile C language files

Write the MAKEFILE file, compile the C code you just wrote, and generate the. So file. in Linux (Windows, DLL files)

 

1. Libmynative. So: mynative. O makefile
2. Gcc-wall-rdynamic-shared-O libmynative. So mynative. o
3. Mynative. O: mynative. c mynative. h
4. Gcc-wall-C mynative. C-I./-I/sandbox/java2s/jdk1.6.0 _ 16/include-I/sandbox/java2s/jdk1.6.0 _ 16/include/Linux
5. CL:
6. Rm-RF *. O *. So

 

"/Sandbox/java2s/jdk1.6.0 _ 16" is the JDK installation directory.

"Libmynative. So" is in the Java class

Static {
System. loadlibrary ("mynative ");
}

Parameters + Lib in static methods

 

5. Configure Environment Variables

Modify the. bash_profile file and configure the environment variables. Because the generated. So file is in the current directory

The. bash_profile file is set as follows:

Export LD_LIBRARY_PATH = ./

 

6. Execute the target file

Run the Java mynative command. The result is as follows:

 

 

In Linux, Java local method (Java calls C) is implemented.

References:

Http://www.ibm.com/developerworks/cn/java/jnimthds/index.html

 

 

 

Java is used in combination with C and C ++, Java calls C language, Java calls C ++ language, Java JNI implementation, and Java local call method.

 

From: http://blog.csdn.net/kangkanglou/archive/2010/08/13/5807787.aspx

Author: kangkang_kevin