韦东山视频第3课第2节_JNI_C调用JAVA_P【学习笔记】

C调JAVA方法主要步骤如下:

一、C代码调用java的静态方法

Hello.java

 public class Hello{
         public static void main(String args[]){
                 System.out.println("Hello World!");
         }

         public static void sayhello_to(String name){

         }

         public static void sayhello_to(){

         }

 }

caller.c

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

 jint 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;

     if (create_vm(&jvm, &env)) {
         printf("can not create jvm\n");
         return -;
     }

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

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

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

 destroy:

     (*jvm)->DestroyJavaVM(jvm);
     return ret;
 }

编译运行命令:

 javac Hello.java

 javap -p -s Hello.class 

 gcc -I/usr/lib/jvm/java-1.7.-openjdk-amd64/include/ -o caller caller.c -L /usr/lib/jvm/java--openjdk-amd64/jre/lib/amd64/server -ljvm

 LD_LIBRARY_PATH=/usr/lib/jvm/java--openjdk-amd64/jre/lib/amd64/server ./caller

 二、C调java非静态方法

Hello.java

 public class Hello{
     public static void main(String args[]){
         System.out.println("Hello World!");
     }

     public int sayhello_to(String name){
         System.out.println("java Hello World! " + name);
         return ;
     }

     public static void sayhello_to(){

     }

 }

caller.c

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

 jint 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;

     if (create_vm(&jvm, &env)) {
         printf("can not create jvm\n");
         return -;
     }

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

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

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

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

     jstr = (*env)->NewStringUTF(env, "[e-mail]");

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

 destroy:

     (*jvm)->DestroyJavaVM(jvm);
     return ret;
 }

运行结果:

 二、C调java非静态方法，并设置属性

Hello.java

 public class Hello{
     private String name;
     private int age;

     public static void main(String args[]){
         System.out.println("Hello World!");
     }

     public int sayhello_to(String name){
         System.out.println("java Hello , " + name + " I'm " + this.name + "," + age+"years old");
         return 123;
     }

     public static void sayhello_to(){

     }

 }

caller.c

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

 jint 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;

     jfieldID nameID;
     jfieldID ageID;

     int r;

     if (create_vm(&jvm, &env)) {
         printf("can not create jvm\n");
         return -;
     }

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

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

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

     nameID = (*env)->GetFieldID(env, cls, "name", "Ljava/lang/String;");
     if (nameID == NULL) {
         ret = -;
         printf("can not get field name");
         goto destroy;
     }
     jstr = (*env)->NewStringUTF(env, "Bill");
     (*env)->SetObjectField(env, jobj, nameID, jstr);

     ageID = (*env)->GetFieldID(env, cls, "age", "I");
     if (ageID == NULL) {
         ret = -;
         printf("can not get field age");
         goto destroy;
     }
     (*env)->SetIntField(env, jobj, ageID, );

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

     jstr = (*env)->NewStringUTF(env, "[e-mail]");

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

 destroy:

     (*jvm)->DestroyJavaVM(jvm);
     return ret;
 }

运行结果:

 相关代码可以在这里下载:

https://github.com/zzb2760715357/100ask/tree/master/java