在 Android 中从 c + + 调用 java 方法

我试图从 C + + 获得一个简单的 Java 方法调用，而 Java 调用本机方法:

public class MainActivity extends Activity {
private static String LIB_NAME = "name";


static {
System.loadLibrary(LIB_NAME);
}


/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
TextView tv = (TextView) findViewById(R.id.textview);
tv.setText(this.getJniString());
}


public void messageMe(String text) {
System.out.println(text);
}


public native String getJniString();
}

在从 Java 到本机的 getJniString*方法调用过程中，我试图从本机代码调用 messageMe方法。

Cpp:

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


jstring Java_the_package_MainActivity_getJniString( JNIEnv* env, jobject obj, jint depth ){


//    JavaVM *vm;
//    JNIEnv *env;
//    JavaVMInitArgs vm_args;
//    vm_args.version = JNI_VERSION_1_2;
//    vm_args.nOptions = 0;
//    vm_args.ignoreUnrecognized = 1;
//
//    // Construct a VM
//    jint res = JNI_CreateJavaVM(&vm, (void **)&env, &vm_args);


// Construct a String
jstring jstr = env->NewStringUTF("This string comes from JNI");
// First get the class that contains the method you need to call
jclass clazz = env->FindClass("the/package/MainActivity");
// Get the method that you want to call
jmethodID messageMe = env->GetMethodID(clazz, "messageMe", "(Ljava/lang/String;)V");
// Call the method on the object
jobject result = env->CallObjectMethod(jstr, messageMe);
// Get a C-style string
const char* str = env->GetStringUTFChars((jstring) result, NULL);
printf("%s\n", str);
// Clean up
env->ReleaseStringUTFChars(jstr, str);


//    // Shutdown the VM.
//    vm->DestroyJavaVM();


return env->NewStringUTF("Hello from JNI!");
}

编译完成后，应用程序停止下一条消息:

ERROR/AndroidRuntime(742): FATAL EXCEPTION: main
java.lang.NoSuchMethodError: messageMe
at *.android.t3d.MainActivity.getJniString(Native Method)
at *.android.t3d.MainActivity.onCreate(MainActivity.java:22)

显然，这意味着方法名是错误的，但在我看来没问题。

115670

小开

Solution posted by Denys S. in the question post:

I quite messed it up with c to c++ conversion (basically env variable stuff), but I got it working with the following code for C++:

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


jstring Java_the_package_MainActivity_getJniString( JNIEnv* env, jobject obj){


jstring jstr = (*env)->NewStringUTF(env, "This comes from jni.");
jclass clazz = (*env)->FindClass(env, "com/inceptix/android/t3d/MainActivity");
jmethodID messageMe = (*env)->GetMethodID(env, clazz, "messageMe", "(Ljava/lang/String;)Ljava/lang/String;");
jobject result = (*env)->CallObjectMethod(env, obj, messageMe, jstr);


const char* str = (*env)->GetStringUTFChars(env,(jstring) result, NULL); // should be released but what a heck, it's a tutorial :)
printf("%s\n", str);


return (*env)->NewStringUTF(env, str);
}

And next code for java methods:

    public class MainActivity extends Activity {
private static String LIB_NAME = "thelib";


static {
System.loadLibrary(LIB_NAME);
}


/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
TextView tv = (TextView) findViewById(R.id.textview);
tv.setText(this.getJniString());
}


// please, let me live even though I used this dark programming technique
public String messageMe(String text) {
System.out.println(text);
return text;
}


public native String getJniString();
}

最佳答案

If it's an object method, you need to pass the object to CallObjectMethod:

jobject result = env->CallObjectMethod(obj, messageMe, jstr);

What you were doing was the equivalent of jstr.messageMe().

Since your is a void method, you should call:

env->CallVoidMethod(obj, messageMe, jstr);

If you want to return a result, you need to change your JNI signature (the ()V means a method of void return type) and also the return type in your Java code.