java - 从 C++ 调用 Java

Question

我正在尝试按照找到的示例从 C++ 调用 Java 虚拟机 here :

基本上我有一个小的Java程序:

public class TestJNIInvoke
{
    public static void main(String[] args)
    {
    System.out.println(args[0]);
    }
}

然后我有一个 C++ 程序，我想创建一个 JVM 并调用 TestJNIInvoke 类:

#include <jni.h>
#include <cstdlib>
#define PATH_SEPARATOR ';' /* define it to be ':' on Solaris */
#define USER_CLASSPATH "." /* where Prog.class is */

using namespace std;

int main() {
     JNIEnv *env;
     JavaVM *jvm;
     jint res;
     jclass cls;
     jmethodID mid;
     jstring jstr;
     jclass stringClass;
     jobjectArray args;

 #ifdef JNI_VERSION_1_2
     JavaVMInitArgs vm_args;
     JavaVMOption options[1];
     options[0].optionString =
         "-Djava.class.path=" USER_CLASSPATH;
     vm_args.version = 0x00010002;
     vm_args.options = options;
     vm_args.nOptions = 1;
     vm_args.ignoreUnrecognized = JNI_TRUE;
     /* Create the Java VM */
     res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
 #else
     JDK1_1InitArgs vm_args;
     char classpath[1024];
     vm_args.version = 0x00010001;
     JNI_GetDefaultJavaVMInitArgs(&vm_args);
     /* Append USER_CLASSPATH to the default system class path */
     sprintf(classpath, "%s%c%s",
             vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH);
     vm_args.classpath = classpath;
     /* Create the Java VM */
     res = JNI_CreateJavaVM(&jvm, &env, &vm_args);
 #endif /* JNI_VERSION_1_2 */

     if (res < 0) {
         fprintf(stderr, "Can't create Java VM\n");
         exit(1);
     }
     cls = (*env)->FindClass(env, "TestJNIInvoke");
     if (cls == NULL) {
         goto destroy;
     }

     mid = (*env)->GetStaticMethodID(env, cls, "main",
                                     "([Ljava/lang/String;)V");
     if (mid == NULL) {
         goto destroy;
     }
     jstr = (*env)->NewStringUTF(env, " from CPP!");
     if (jstr == NULL) {
         goto destroy;
     }
     stringClass = (*env)->FindClass(env, "java/lang/String");
     args = (*env)->NewObjectArray(env, 1, stringClass, jstr);
     if (args == NULL) {
         goto destroy;
     }
     (*env)->CallStaticVoidMethod(env, cls, mid, args);

 destroy:
     if ((*env)->ExceptionOccurred(env)) {
         (*env)->ExceptionDescribe(env);
     }
     (*jvm)->DestroyJavaVM(jvm);
 }

但是当我尝试编译 C++ 程序时，我得到了这个错误:

c:\java\JNI> g++ -I"c:\Program Files\Java\jdk1.7.0\include"-I"c:\ProgramFiles\Java\jdk1.7.0\include\win32" -c TestJNIInvoke.cpp


TestJNIInvoke.cpp: In function 'int main()':
TestJNIInvoke.cpp:20:31: warning: deprecated conversion from string constant to
'char*'
TestJNIInvoke.cpp:44:18: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:49:18: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:54:19: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:58:26: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:59:19: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:63:12: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:66:16: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:67:16: error: base operand of '->' has non-pointer type 'JNIEn
v'
TestJNIInvoke.cpp:69:12: error: base operand of '->' has non-pointer type 'JavaVM'

有什么想法吗？

谢谢

Answer 1

即使您包含相同的头文件，Java 原生接口(interface)也为 C 和 C++ 使用两个不同的接口(interface)。

在 C++ 中，它是:

jclass cls = env->FindClass("java/lang/String");

代替(对于C):

jclass cls = (*env)->FindClass(env, "java/lang/String");

所以在两个地方都需要 env 的 C 函数调用变成了 C++ 中方便的成员函数调用。

这在 Native Method Arguments 中有所提及Java Native Interface 6.0 Specification 的部分.