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

Source: Internet
Author: User

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

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