JNI学習ノート

1.xxx.JAvaはxxxを生成する.classファイルと専用です.hファイルコマンド

javac - h . xxx.java

2.現在のxxxを表示します.JAvaファイルのすべての署名情報

//    xxx.class  
  ->javap -s -p xxx.class
//      ，descriptor        （）     ；V    
public class com.example.cb.test.native1.FieldTest {
  public int a;
    descriptor: I
  public static int b;
    descriptor: I
  public com.example.cb.test.native1.FieldTest();
    descriptor: ()V

  public native void testField();
    descriptor: ()V

  public native void testStaticField();
    descriptor: ()V

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V

  static {};
    descriptor: ()V
}

3.Java調CはStringに入り、つづりStringは戻る

//************************************java  ************************************
package com.example.cb.test.native1;

public class StringTest {

    static {
    //      
        System.load("E:\\CPlae\\NativeTest\\cmake-build-debug\\libstringLib.dll");
    }

    public native static String sayHello(String txt);

    public static void main(String[] args) {
        String txt = sayHello("  ");
        System.err.println(txt);
    }
}

//************************************C  ************************************
#include "jni.h"
#include 
#include 


JNIEXPORT jstring JNICALL Java_com_example_cb_test_native1_StringTest_sayHello
        (JNIEnv *env, jobject jobject, jstring jstr) {
    const char *c_str = NULL;
    jboolean isCopy;
    c_str = (*env)->GetStringUTFChars(env, jstr, &isCopy);
    if (c_str == NULL)return NULL;

//    //      char *  
//    //1.malloc     
//    //2.   
//    //3.  
//    char buff[128] = {0};
//    sprintf(buff, "Hello   %s", c_str);

    //        string.h    API
    char hello[] = "Hello       ";
    char *p_str = strcat(hello, c_str);

    //    
    (*env)->ReleaseStringUTFChars(env, jstr, c_str);
    return (*env)->NewStringUTF(env, p_str);
}

//************************************    ************************************
Hello         

Process finished with exit code 0

4.C javaを呼び出す変数

//************************************java  ************************************
package com.example.cb.test.native1;

public class FieldTest {


    public int a = 1;

    public static int b = 2;

    public native void testField();

    public native void testStaticField();

    static {
        System.load("E:\\CPlae\\NativeTest\\cmake-build-debug\\libfield.dll");
    }

    public static void main(String[] args) {
        FieldTest fieldTest = new FieldTest();
        fieldTest.testField();
        fieldTest.testStaticField();

        System.err.println("1->" + fieldTest.a);
        System.err.println("2->" + fieldTest.b);
    }
}

//************************************C  ************************************
//C    java  private public
//    static  static

/*
 *   JAVA   ，    
 * Class:     com_example_cb_test_native1_FieldTest
 * Method:    testField
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_com_example_cb_test_native1_FieldTest_testField
        (JNIEnv *env, jobject jobject) {
    //   ，     
    jclass clz = (*env)->GetObjectClass(env, jobject);
    //   ,  fieldID
    jfieldID jfid = (*env)->GetFieldID(env, clz, "a", "I");
    //   ,  a  
    jint a = (*env)->GetIntField(env, jobject, jfid);
    printf("C   JAVA    a=%d
", a);
    (*env)->SetIntField(env, jobject, jfid, a + 100);
}

/*
 *   JAVA     ，    
 * Class:     com_example_cb_test_native1_FieldTest
 * Method:    testStaticField
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_com_example_cb_test_native1_FieldTest_testStaticField
        (JNIEnv *env, jobject jobject) {
    jclass clz = (*env)->GetObjectClass(env, jobject);
    jfieldID fid = (*env)->GetStaticFieldID(env, clz, "b", "I");
    jint b = (*env)->GetStaticIntField(env, clz, fid);
    printf("C      b=%d
", b);
    (*env)->SetStaticIntField(env, clz, fid, b + 66);
}

//************************************    ************************************
1->101
2->101
C   JAVA    a=1
C      b=2

Process finished with exit code 0