如何在C/C++中调用Java （1）

package jni.test;
/**
 * 该类是为了演示JNI如何访问各种对象属性等
 * @author liudong
 */
public class Demo {
 //用于演示如何访问静态的基本类型属性
 public static int COUNT = 8;
 //演示对象型属性
 public String msg;
 private int[] counts;
 public Demo() {
 this("缺省构造函数");
 }
 /**
  * 演示如何访问构造器
  */
 public Demo(String msg) {
  System.out.println("<init>:" + msg);
  this.msg = msg;
  this.counts = null;
 }
 /**
  * 该方法演示如何访问一个访问以及中文字符的处理
  */
 public String getMessage() {
  return msg;
 }
 /**
  * 演示数组对象的访问
  */
 public int[] getCounts() {
  return counts;
 }
 /**
  * 演示如何构造一个数组对象
 */
 public void setCounts(int[] counts) {
  this.counts = counts;
 }
 /**
  * 演示异常的捕捉
 */
 public void throwExcp() throws IllegalAccessException {
  throw new IllegalAccessException("exception occur.");
 }
}

#include <jni.h>
int main() {
 JNIEnv *env;
 JavaVM *jvm;
 JDK1_1InitArgs vm_args;
 jint res;
 /* IMPORTANT: 版本号设置一定不能漏 */
 vm_args.version = 0x00010001;
 /*获取缺省的虚拟机初始化参数*/
 JNI_GetDefaultJavaVMInitArgs(&vm_args);
 /* 添加自定义的类路径 */
 sprintf(classpath, "%s%c%s",
 vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH);
 vm_args.classpath = classpath;
 /*设置一些其他的初始化参数*/
 /* 创建虚拟机 */
 res = JNI_CreateJavaVM(&jvm,&env,&vm_args);
 if (res < 0) {
  fprintf(stderr, "Can't create Java VM\n");
  exit(1);
 }
 /*释放虚拟机资源*/
 (*jvm)->DestroyJavaVM(jvm);
}

/* invoke2.c */
#include <jni.h>
int main() {
 int res;
 JavaVM *jvm;
 JNIEnv *env;
 JavaVMInitArgs vm_args;
 JavaVMOption options[3];
 vm_args.version=JNI_VERSION_1_2;//这个字段必须设置为该值
 /*设置初始化参数*/
 options[0].optionString = "-Djava.compiler=NONE"; 
 options[1].optionString = "-Djava.class.path=."; 
 options[2].optionString = "-verbose:jni";	//用于跟踪运行时的信息
 /*版本号设置不能漏*/
 vm_args.version = JNI_VERSION_1_2;
 vm_args.nOptions = 3;
 vm_args.options = options;
 vm_args.ignoreUnrecognized = JNI_TRUE;
 res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
 if (res < 0) {
  fprintf(stderr, "Can't create Java VM\n");
  exit(1);
 }   
 (*jvm)->DestroyJavaVM(jvm);
 fprintf(stdout, "Java VM destory.\n");
}

jclass cls = (*env)->FindClass(env, "jni/test/Demo");	//把点号换成斜杠

jclass cls = (*env)-> GetObjectClass(env, obj);
//其中obj是要引用的对象，类型是jobject

jmethodID (JNICALL *GetMethodID)(JNIEnv *env, jclass clazz, const char *name, 
const char *sig);
jmethodID (JNICALL *GetStaticMethodID)(JNIEnv *env, jclass class, const char 
*name, const char *sig);