JNI——C调用JAVA

步骤：

1. 创建虚拟机

2. 获得class

3. 实例化对象：获得构造方法（方法名为“<init>”）,构造参数，调用方法

4. 调用方法：又分为获得方法，构造方法，调用方法

操作方法：

1. 编译：javac Hello.java

2. javap -p -s Hello.class：查看Signature

3. gcc -I/usr/lib/jvm/java-1.8.0-openjdk-amd64/include/ -I/usr/lib/jvm/java-1.8.0-openjdk-amd64/include/linux/ -o caller caller.c -L /usr/lib/jvm/java-8-openjdk-amd64/jre/lib/amd64/server -ljvm

4. 执行：LD_LIBRARY_PATH=/usr/lib/jvm/java-8-openjdk-amd64/jre/lib/amd64/server ./caller

举例：

（1）call_static_method

 #include <stdio.h>
 #include <jni.h>

 JNIEnv* create_vm(JavaVM** jvm, JNIEnv** env)
 {
         JavaVMInitArgs args;
         JavaVMOption options[];
         args.version = JNI_VERSION_1_6;
         args.nOptions = ;
         options[].optionString = "-Djava.class.path=./";
         args.options = options;
         args.ignoreUnrecognized = JNI_FALSE;
         return JNI_CreateJavaVM(jvm, (void **)env, &args);
 }  

 int main(int argc, char **argv)
 {
     JavaVM* jvm;
     JNIEnv* env;

     jclass cls;
     int ret = ;

     jmethodID mid;

     /* 1. create java virtual machine */
     if(create_vm(&jvm, &env))
     {
         printf("can not create jvm\n");
         return -;
     }

     /* 2. get class */
     cls = (*env)->FindClass(env, "Hello");
     if(cls == NULL)
     {
         printf("can not find hello class\n");
         ret = -;
         goto destory;
     }

     /* 3. create object */

     /* 4. call method
      *  4.1 get method
      *  4.2 create parameter
      *  4.3 call method
      */

     mid = (*env)->GetStaticMethodID(env, cls, "main", "([Ljava/lang/String;)V");
     if(mid == NULL)
     {
         ret = -;
         printf("can not get method\n");
         goto destory;
     }

     (*env)->CallStaticVoidMethod(env, cls, mid, NULL);

 destory:
     (*jvm)->DestroyJavaVM(jvm);

     return ret;
 }

（2）call_non_static_method

 #include <stdio.h>
 #include <jni.h>

 JNIEnv* create_vm(JavaVM** jvm, JNIEnv** env)
 {
         JavaVMInitArgs args;
         JavaVMOption options[];
         args.version = JNI_VERSION_1_6;
         args.nOptions = ;
         options[].optionString = "-Djava.class.path=./";
         args.options = options;
         args.ignoreUnrecognized = JNI_FALSE;
         return JNI_CreateJavaVM(jvm, (void **)env, &args);
 }  

 int main(int argc, char **argv)
 {
     JavaVM* jvm;
     JNIEnv* env;

     jclass cls;
     int ret = ;

     jmethodID mid;
     jmethodID cid;

     jobject jobj;
     jstring jstr;

     int r;

     /* 1. create java virtual machine */
     if(create_vm(&jvm, &env))
     {
         printf("can not create jvm\n");
         return -;
     }

     /* 2. get class */
     cls = (*env)->FindClass(env, "Hello");
     if(cls == NULL)
     {
         printf("can not find hello class\n");
         ret = -;
         goto destory;
     }

     /* 3. create object
      *
      */

     cid = (*env)->GetMethodID(env, cls, "<init>", "()V");
     if(cid == NULL)
     {
         printf("can not get construct method\n");
         ret = -;
         goto destory;
     }

     jobj = (*env)->NewObject(env, cls, cid);
     if(jobj == NULL)
     {
         printf("can not create object\n");
         ret = -;
         goto destory;
     }

     /* 4. call method
      *  4.1 get method
      *  4.2 create parameter
      *  4.3 call method
      */

     mid = (*env)->GetMethodID(env, cls, "sayhello_to", "(Ljava/lang/String;)I");
     if(mid == NULL)
     {
         ret = -;
         printf("can not get method\n");
         goto destory;
     }

     jstr = (*env)->NewStringUTF(env, "287787472@qq.com");

     r = (*env)->CallIntMethod(env, jobj, mid, jstr);
     printf("ret = %d\n", r);

 destory:
     (*jvm)->DestroyJavaVM(jvm);

     return ret;
 }